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Notes on the distribution, ecology and open sea mariculture trials of the Philippine blue sponge, Xestospongia sp. (Porifera)

Notes on the distribution, ecology and open sea mariculture trials of the Philippine blue sponge,... INTRODUCTIONSponges are major benthic components of coral reefs that have significant roles in coral reef community dynamics (de Goeij et al., 2017). However, various aspects of the sponges such as their biology, ecology and distribution are still understudied. Apart from their ecological importance, sponges have numerous economic benefits, including as a potential source of biomedical products. A review by Leal et al. (2012) highlighted that among the marine natural products collected from the marine environment, the majority come from sponges. However, due to the large biomass needed to supply a sizable amount of chemicals, collecting biomass from wild sponge populations can be harmful to the ecosystem. As a result, continuous harvest from the wild is not economically and environmentally sustainable (Duckworth, 2009).Mariculture provides a sustainable solution to the ‘supply problem’ (Duckworth, 2009). The culture of sponges occurred for over a 100 years, where the most common method used is cutting explants from wild sponge colonies, since explants are potentially highly capable of regenerating after fragmentation (Kinne, 1977; Decaralt et al., 2003; Koopsman et al., 2010). In situ or sea‐based mariculture is the most promising and applicable method for biomass production among sponges. Ex situ or land‐based mariculture also has various advantages. However, sea‐based cultures may be cheaper than their land‐based counterparts because of the infrastructure needed to manipulate the culture environment ex situ (Duckworth, 2009). Although many species have been cultivated successfully in various parts of the world – from Mediterranean countries such as Egypt and Greece (Maslin et al., 2021), to the Caribbean (Ruiz, et al., 2013) and to tropical regions of the western Atlantic and Pacific Oceans (Duckworth, et al., 2009; Bierwirth et al., 2022), a crucial factor limiting the success of mariculture efforts is the lack of knowledge on the biology and ecology of different species. Thus, there is a need to further investigate the various aspects affecting their growth and survival.One of the sponges identified as a potential source of natural products for pharmaceutical use is the Philippine blue sponge, Xestospongia sp. (Tatsukawa et al., 2012), which can be observed in shallow coral reefs, growing on various substrates (e.g., corals, rocks, sand). The Philippine blue sponge has been identified as a source of renieramycins, which are compounds that exhibit antimicrobial and antiproliferative activity against certain types of cancer cell lines (Halim et al., 2011). Santiago et al. (2019) have conducted initial mariculture experiments to assess the cultivability of the blue sponge, specifically to assess effects of different factors (i.e., harvesting regime, culture period, translocation, farming method) on different parameters (e.g., sponge growth, survival, chemistry). However, biomass production may still be improved, and several aspects of its biology and ecology are still unknown.The objectives of this study were to (1) assess the distribution of the Philippine blue sponge through comparing the abundance and size–frequency distribution among different stations with varying environmental conditions (i.e., water quality and exposure to monsoons), (2) develop improved in situ mariculture methods, (3) observe the different factors that may affect cultured and wild sponges (i.e., predation, disease, maintenance, dislodgement) and (4) identify and compare the growth rates of wild and transplanted blue sponges. Overall, this study provides additional information on the biology and ecology of the Philippine blue sponge, Xestospongia sp., and highlights the process in developing a mariculture method. Additionally, the development of the improved farming protocols is an important step to have a more sustainable source of biomass for secondary metabolite production that is useful to the pharmaceutical industry. This resource can also be a potential source of alternative income for coastal communities (Duckworth, 2009).MATERIALS AND METHODSStudy sitePrevious reconnaissance activities in different coral reef sites in the Philippines revealed the high abundance of the Philippine blue sponge, Xestospongia sp., in the fringing reefs of Puerto Galera, Oriental Mindoro province (Figure 1). Puerto Galera is part of the Verde Island Passage, an area known to have very high marine biodiversity (Asaad et al., 2018). It is one of the major tourist and SCUBA diving destinations in the Philippines, making it a large contributor to the local economy (Roche et al., 2016). It was also designated by the United Nations Educational, Scientific and Cultural Organization (UNESCO) as a Man and Biosphere Reserve in 1977 (Iizuka et al., 2009; Dagamac et al., 2015).1FIGUREMap of the study area in Puerto Galera, Oriental Mindoro, Philippines. The map shows the mariculture area and the nine survey stations (SN, Sto Niño; MC, Manila Channel; LB, Long Beach; CG, Coral Garden; M, Muelle; GC, Giant Clam; MB, Monkey Beach; E, Escarceo; CC, Coral Cove).In this study, surveys were conducted in stations inside and outside Puerto Galera Bay, which are areas with variations in environmental conditions (Figure 1). The coral reefs inside the bay are exposed to waters with high turbidity and high nutrient levels (San Diego‐McGlone et al., 1995). The reefs inside the bay are also relatively unexposed to waves due to embayment, weak tidal current and associated flow, while reefs outside the bay are highly exposed to waves with stronger currents (Iizuka et al., 2009).Size–frequency distribution and abundanceSurveys were conducted via SCUBA diving at depths of 3–12 m to search for blue sponge colonies at nine stations in the coral reefs of Puerto Galera (Figure 1). Three stations were surveyed inside Puerto Galera Bay (Coral Garden, Giant Clams, Muelle) and six stations outside the bay (Sto. Niño, Manila Channel, Long Beach, Monkey Beach, Escarceo, Coral Cove) (Figure 1). Maximum height and greatest length of each colony encountered were measured and recorded for size–frequency distribution analysis. Ecological volume for each colony was calculated using the formula of the volume of a cylinder (Volume = πr$\pi r$2h) where the greatest width of the colony was assumed to be the diameter. Studies on sponge size–frequency mostly used colony length (e.g., Duckworth and Wolff, 2007) and area (e.g., Baquiran et al., 2020) to measure sizes. However, to take into account both the horizontal and vertical growth of blue sponge colonies, the use of volume was more ideal for this study (Wulff, 2001).Abundance surveys were conducted in the nine stations. At each station, a 75 m × 25 m area was established at 3–6 m depth. Five fifty‐metre transects were then randomly laid inside the 75 m × 25 m area. All blue sponge colonies observed inside the area 1 m on the shallow side of each transect were then counted. Thus, each transect was represented by the number of colonies per 50 m2 and each station was represented by the mean of the abundance of the transects (n = 5).The Shapiro–Wilk's test for normality and the Levene's test for homogeneity of variances were performed for the abundance dataset. The Kruskal–Wallis and Mann–Whitney pairwise tests were then performed to determine the significance of variations of the blue sponge mean abundance among stations. Descriptive statistics and histograms were then generated using log‐transformed ecological volumes of blue sponge colonies to compare the size–frequency distribution between colonies inside and outside the bay. The 2‐sample Kolmogorov–Smirnov (KS) test was then performed to evaluate the significance of the variation. Tests were performed using the PAST software (Hammer et al., 2001). The results of the abundance and size–frequency distribution surveys were then used as the main basis for the selection of the mariculture and collection sites for blue sponge fragments that were used in the mariculture trials.Monitoring of wild coloniesTen haphazardly selected blue sponge colonies in the Giant Clam station (Figure 1) were tagged to monitor factors that may affect the survival of natural blue sponge colonies. Detailed photos of each colony were taken monthly for 6 months to monitor possible changes. Factors that were targeted were mainly predation, competition with other benthic organisms, dislodgement and presence of disease.Development of open sea mariculture methodsSeed stock collection and preparationThe Giant Clam station was established as the collection site of fragments to be used for the open sea mariculture trials since it was identified to have the most abundant colonies among the study stations (Figure 1). Sponge fragments were collected by cutting 1/3 of the colony using a cutter to make sure that the colony left would still survive (Santiago et al., 2019). Fragments collected were then put in a meshed bag to be processed above water. Collected blue sponge fragments were then put in a 155 l container with seawater or were hung on the side of the boat to keep them underwater to lessen stress. From the collected samples, fragments for transplantation in the mariculture setups were prepared through slicing using a cutter. Each explant was at least 20 g in wet weight and was approximately 250 cm3 in volume and was assigned to one of the different treatments detailed in the Subsection 2.5.3. The wet weight of each explant was measured and recorded, and each explant was uniquely labelled for monitoring.Establishment of underwater mariculture setupsMariculture setups were established in an area adjacent to the collection site at about 5 m depth (Figure 1). The location was mainly chosen based on the presence of abundant blue sponges, which may indicate ideal environmental conditions for the blue sponge. The location was inside Puerto Galera Bay, characterised by low exposure to waves with high nutrient levels. Its relatively low exposure to the monsoons also makes it an ideal area to do mariculture since it has a lower chance of being severely hit by typhoons, thus, avoiding destruction of the mariculture setups. The benthos of the area was characterized by high sand and rubble cover with some coral patches, thus, damages on communities, such as corals, during the different activities of this study were minimized. Mariculture setups were assembled underwater through SCUBA diving. A setup was defined as a hollow cube‐like structure with frames made up of polyvinyl chloride (PVC) pipes (Figure 2). Width, length and height of each set up was 2 m. The four corners of the bottom part of structure were also connected to staked cement blocks for stability. Three setups (setups A, B and C) were established in spots that had flat benthos to make the structures more stable.2FIGUREUnderwater structure used for attaching the different mariculture media tested. This structure was made of polyvinyl chloride (PVC) pipes and attached to the substrate using cement blocks.Phase 1 mariculture trials: comparison of farming methodsMariculture trials were conducted to compare wet weight increase (%) and survival (%) of explants using three farming methods (bag, tube and string) and different number of oscula (0, 1, 2, ≥3). An osculum is part of the sponge aquiferous system that acts as the exhalant canal of sponges where the filtered seawater passes through. For the bag method, explants were attached inside a bag made of polyethylene plastic screen (2 cm mesh size, with ∼15 cm diameter at the base, and 30 cm in height), which were hung on the PVC frames of the mariculture setups (Figure 3a). In the tube method, an explant was attached to a 6‐in. PVC pipe using a plastic zip tie. These where then attached to a polyethylene plastic screen covering a side of the mariculture setup (Figure 3b). In the string method, explants were attached to nylon strings (5 mm diameter) using zip ties and were hung from the PVC frames of the setups (Figure 3c). The bag and tube methods were designed according to Santiago et al. (2019). The string was designed and tested as an alternative method that was simpler with minimal logistical effort in the deployment, maintenance and collection. A total of 48 replicate explants were deployed per farming method and 36 replicate explants per oscula number (see experimental design on Figure S1).3FIGUREFarming methods used in the phase 1 mariculture trials. (a) Bag method, (b) tube method, and (c) string method.Explants were transplanted in January 2020. We originally planned to conduct monthly monitoring for at least 3 months. However, monitoring and maintenance activities were paused in March 2020 due to travel restrictions brought by the COVID‐19 pandemic. The mariculture site was only revisited in December 2020, 11 months post‐transplantation of the explants.Using the remaining explants after 11 months, growth for each explant was computed using the wet‐weight method through the equation,1Growth%=Wn−WiWn×100$$\begin{equation}{\rm{Growth}}\;\left( \% \right)\; = \frac{{{W_{\rm{n}}} - {W_{\rm{i}}}}}{{{W_{\rm{n}}}}} \times 100\end{equation}$$where Wn is the wet weight of an explant at the end of the experiment, and Wi is the weight on an explant during transplantation. The Shapiro–Wilk's test for normality and the Levene's test for homogeneity of variances were then performed for the datasets. The analysis of variance (ANOVA) and Tukey's test were then performed to determine significance of the difference in the mean growth among farming methods and oscula numbers. Tests were performed using the PAST software (Hammer et al., 2001).Survival of explants per treatment was then calculated as the percentage of live fragments after the end of the experiment. This was calculated using the equation,2Survival%survival=NtN0×100$$\begin{equation}{\rm{Survival}}\;\;\left( {\% {\rm{survival}}} \right) = \frac{{{N_{\rm{t}}}}}{{{N_0}}} \times 100\end{equation}$$where Nt is the number of explants that have survived after 11 months, and N0 is the number of explants during the start of the experiment.Phase 2 mariculture trials: ecological observations using the ‘improved string method’Based on the observations in the Phase 1 mariculture trials, an improved farming method was developed for a scaled‐up Phase 2. The three different farming methods in Phase 1 were no longer used. Instead, an improved string method was developed, where explants were attached to nylon ropes (5 mm diameter) that were vertically tied to the PVC frames of the setup (Figure 4). To increase stability and chances of attachment, it was made sure that each explant was attached to the PVC frames, which served as the main artificial substrate. All pre‐labelled and pre‐weighed (at least 20 g wet weight) explants transplanted in this phase were also explants with three or more oscula, as the result of the first mariculture trials phase indicated that fragments with more than 3 oscula had higher survival. Forty‐seven replicate explants were transplanted in December 2020. This method is hereunto referred as the ‘improved string method’ (see experimental design on Figure S1).4FIGUREThe improved string method. Sponge explants are attached to a nylon string tightly ties to the underwater structure. Sponge explants were attached to the string and PVC pipes using plastic zip ties.Maintenance work and monitoring of the health conditions of the explants were done weekly by cleaning the setups and explants from biofouling organisms. Organisms growing on or adjacent to the explants were recorded and taken out to prevent possible stress for competition for space. Observed predators were also recorded and were taken out from the explants. Health monitoring of each explant was also performed (i.e., attachment to substrate, wound healing, presence of disease and biofoulers, associated organisms on colony surface and predator bite marks) once a week for 6 months. After 6 months, five explants that were not affected by predation were collected and weighed for wet weights and potential growth rate was calculated using the equation,3Growthrateg/month=Wn−WiWn×100/6months$$\begin{equation}{\rm{Growth\;rate}}\;\left( {{\rm{g}}/{\rm{month}}} \right)\; = \;\left( {\frac{{{W_{\rm{n}}} - {W_{\rm{i}}}}}{{{W_{\rm{n}}}}} \times 100} \right)/\;6\;{\rm{months}}\end{equation}$$where Wn is the wet weight of an explant at the end of the experiment, and Wi is the weight on an explant during transplantation.Effect of adjacent coral cover and fish abundance on biofouling intensityTo initially investigate the effect of adjacent coral cover and fish abundance on the biofouling of the mariculture setups, covers of live hard corals in the vicinity of the three setups established (Setups A, B, and C) were measured by analysing top view photos of the setups. To ensure uniformity of the photos, the benthos within 2 m from the edge of a setup was consistently made visible in each photo. A 1‐m tape was also always visible in the photos to serve as a length scale to aid in the photo analysis. Photos were taken using a camera (Sony RX100) with underwater housing (Ikelite) with wide‐angle lens (Inon). The photos were then analysed using the Coral Point Count with Excel extensions (Kohler and Gill, 2006) to generate percentage covers of adjacent hard corals per mariculture setup.Fishes were also assessed via point count visual census. This was done by estimating the maximum count of individuals of a fish family at one time observed in a certain duration. Fish abundance was estimated by observing the fishes inside a mariculture setup and 1 m from the edge of a setup for 10 min. Visual census of the setups was done in the morning (9AM–11AM) on four different dates – once in June, once in July and twice in August. A survey for each date for a setup was considered one replicate for each mariculture setup (n = 4 per setup). Mean abundance of fish families per setup was computed and compared using the Kruskal–Wallis and Mann–Whitney pairwise tests after testing for normality and homogeneity of variances using the Shapiro–Wilk's test and the Levene's test, respectively.Mariculture trials (Phase 3): floating methodTo test if predation by the nudibranch, Jorunna funebris, can be minimised, a floating mariculture setup was designed, built, and deployed. The floating mariculture setup, herein after referred to as the ‘floating method’, was made up of a floating device made of buoys and bamboo sticks where the hanging artificial substrate, made of nylon ropes and PVC pipes, were attached to (Figure 5). Weights were used to maintain the depth where the blue sponge fragments were suspended. The floating device was also attached to the substrate using a nylon rope and a mooring block staked on the substrate. A total of 12 explants were deployed using the floating method. We also we hypothesized that depth influences sponge health and survival. To test this hypothesis, the floating setup was used to deploy blue sponge fragments at two depth ranges – near surface (1 m) (n = 6) and near substrate (5 m) (n = 6). Monitoring of the fragments deployed using the floating method was done weekly for 4 months (February–May, 2021) (see Figure S1 for experimental design).5FIGUREThe floating mariculture method. Sponge explants were attached to PVC substrates that were hanged to a floater at the surface. The floater was securely attached to the substrate using cement mooring blocksGrowth (wild vs. cultured explants)To compare growth rates of the explants from wild colonies, branch growth was monitored. Branch growth for a colony was measured by tying plastic zip ties to colony branches. To identify the initial length, the distance of the zip tie from the tip of the branch was measured using a caliper. Initial measurement was done in June 1, 2021 and monitored in July 30, 2021. Growth rate (cm per month) of a branch was calculated through the equation,4Growthratecm/month=Finallengthcm−Initiallengthcm2months$$\begin{equation}{\rm{\;Growth}}\,{\rm{rate}}\left( {{\rm{cm}}/{\rm{month}}} \right) = \frac{{{\rm{Final}}\,{\rm{length}}\left( {{\rm{cm}}} \right) - {\rm{Initial}}\,{\rm{length}}\left( {{\rm{cm}}} \right)}}{{2\,{\rm{months}}}}\end{equation}$$Branches of three wild colonies and three explants were monitored, with three branches from each colony or explant. Mean growth rates (n = 3) of wild colonies and explants were then computed and compared. The Shapiro–Wilk's test for normality and the Levene's test for homogeneity of variances were performed for the dataset. Since the dataset was nonparametric, the Kruskal–Wallis test was performed to determine the significance of the difference of the mean growth rates between the two treatments. The test was performed using the PAST software (Hammer et al., 2001).RESULTSSize–frequency distribution and abundanceDescriptive statistics revealed variation in the size–frequency distribution of blue sponge colonies inside and outside the bay (Table 1). Colonies outside the bay were positively skewed, suggesting dominance of small‐sized colonies (Figure 6b). Colonies inside the bay were slightly negatively skewed, which suggests that large colonies were more abundant compared with outside the bay (Figure 6a). The size class mode of colonies inside the bay was also larger than outside the bay. The KS test also revealed significant variation in the size–frequency distribution (KS test, p < 0.05).1TABLEDescriptive statistics of sizes of the blue sponge colonies outside and inside Puerto Galera BayOutsideInsideMean1.672.17Standard error0.150.08Median1.702.24Mode range>1.60, ≤1.80>2, ≤2.4Standard deviation0.780.91Sample variance0.620.83Kurtosis1.15‐0.76Skewness0.56‐0.03Range3.543.87Minimum0.260.44Maximum3.804.31Sum43.48270.88Count261256FIGUREHistogram of the size–frequency distribution of blue sponge colonies (A) inside and (B) outside the bay. For comparison, the figures also show normal bell curves generated using the mean and standard deviations of the datasets. Different size classes inside the bay were generally well represented while colonies outside the bay were generally dominated by small colonies.Significant variations in the abundance of blue sponge colonies were observed among the nine stations surveyed (Kruskal–Wallis, p < 0.05) (Figure 7). Mean abundance was highest in the Giant Clam station, while no colonies were observed in Coral Cove, Coral Garden, Long Beach and Sto. Niño. The Mann–Whitney pairwise tests revealed no significant difference between Giant Clam and Muelle (Mann–Whitney, p > 0.05), both of which were located inside the Bay. The two stations, however, were significantly different to the rest of the stations where blue sponge colonies were observed, all of which were all outside the bay (Mann‐Whitney, p < 0.05).7FIGUREMean abundance (±SD) of blue sponge colonies (density of colonies per 50 m2) in the surveyed nine stations in Puerto Galera, Oriental Mindoro.Monitoring of wild coloniesAfter six months of monitoring, there were no changes in the adjacent benthic assemblages that could potentially affect the ten tagged wild blue sponge colonies (e.g., competition for space). Observations for predation revealed that the nudibranch, J. funebris, was the only predator of the blue sponge (Figure 8a). In some cases, the bite marks previously observed healed after a month. Some of the colonies also had partial mortality, with the presence of white colouration on the tissue, which eventually disintegrated (Figure 8b). The cause for this discolouration is still unknown. Three out of ten colonies were also observed to be dislodged and detached from the substrate. Also, three colonies were lost after 6 months of monitoring, which was likely due to predation or dislodgement.8FIGUREMajor observations of wild blue sponge colonies. (a) Jorunna funebris feeding on a wild blue sponge. (b) White colouration on parts of a blue sponge which eventually leads to partial mortality.In situ mariculture trialsPhase 1 mariculture trials: comparison of farming methodsThe survival of the sponge explants was highest (65%) when the string method was employed (Figure 9a) and when explants had three or more oscula (Figure 9b). Explants with more than three or more oscula had a 100% survival. The number of survivors were lowest (44%) when explants were transplanted with no osculum and when using the bag method. In terms of variation in the mean % wet weight increase, no obvious patterns were observed (Figure 10). The ANOVA also revealed no significant differences among treatments in terms of % wet weight increase (Kruskal–Wallis test, p > 0.05).9FIGUREPercentage survival of explants among (a) culture methods and (b) among explants with different number of oscula after 11 months in the first phase of the trials.10FIGUREMean percentage weight increase (±SD) of explants among different farming method after 11 months in the first phase of the trials.Descriptive observations are also reported here. For the tube, explants that have survived or were not totally outcompeted by biofoulers overgrew and encrusted the PVC pipes and screens that they were attached to (Figure S2a). This made it more challenging to collect them. For the bag method, all the bags were heavily overgrown or biofouled by different organisms, such as invertebrates and algae (Figure S2b). When the bags were opened, we observed other organisms (e.g., mollusks, algae, tunicates) that grew inside and outcompeted the initially established blue sponge explants (Figure S2c). For the string method, growth was not observed on most of the explants that survived (Figure S2d). However, a few explants from the string method grew large and healthy. The said healthy string explants were able to attach to the PVC frames because the nylon ropes were long, which enabled them to be in contact with an artificial substrate (Figure S2e). These observations became the basis for the improvement of the design of the mariculture method used in second phase of the mariculture trials.Phase 2 mariculture trials: ecological observations using the improved string methodMajority of the explants attached to the PVC substrate after a week from transplantation (Figure S3). Some explants were observed to have partial mortality after a week from transplantation. Partial mortality was visible when a part of an explant became white (Figure S4a), similar to observations in wild colonies (Figure 8b). This type of partial mortality was only observed in the first month from transplantation. However, in March 2021, some explants that were severely biofouled by algae also had partial mortality, possibly caused by stress due to suffocation.Predation on the explants was already observed even after a week from transplantation. Only predation by J. funebris was observed every monitoring (Figure 11) while predation of fish was not observed. Bite marks on a number of colonies were also observed during monitoring. After 6 months, 70% of the explants survived and were healthy. Mean growth rate of healthy colonies collected after 6 months was 17.5 g per month (SD ± 6.4).11FIGUREJorunna funebris feeding on a blue sponge explant using the type 2 method.Biofouling on the PVC frames and on the surface of some explants were observed immediately a week from deployment (Figure S4b and c). However, a major observation was an increased level of biofouling intensity in March that lasted up to the 2nd week of June. Because of that, weekly cleanup activities became necessary. Minimal biofouling was observed again in the 3rd week of July and in the last week of August. During this time, cleanup effort became less. Another major observation was the difference in the biofouling intensity among the mariculture setups established in areas with different adjacent substrate types and abundance of fishes, which were observed to feed directly on fouling algae (Figure S5). This prompted us to measure adjacent coral cover and fish abundance, which results are presented in the next section.Adjacent coral cover and fish abundance on biofouling intensitySetup B, which was established in a sandy area with the lowest adjacent coral cover (Figure 12), always had the highest biofouling intensity every monitoring (Figure S5a). Setups A and C, which had higher adjacent coral covers, always had less biofouling (Figure S5b). In terms of fish abundance, setup A and C had significantly higher fish abundance than setup B (ANOVA, Tukey's test, p < 0.05) (Figure 13). Among the fishes observed, damselfishes (Pomacentridae) differed significantly among setups (ANOVA, p < 0.05) with setup B having a significantly lower damselfish abundance (Tukey's test, p < 0.05). Damselfishes were observed to frequent the setups and feed on algae attached to the PVC frames and explants, and were obviously residents of the adjacent coral colonies (Figure S6).12FIGUREPercentage adjacent (within 2 m from each setup) coral cover of the three mariculture setups.13FIGUREMean abundance (±SD) of the different fish families observed in the mariculture setups computed using four replicates.Phase 3 mariculture trials: floating methodAll explants in the floating method survived after 4 months. However, explants that were deployed near the surface (1 m depth) were severely fouled by algae days after transplantation (Figure 14a). Unlike fouling on the explants transplanted in the improved string method, which can be taken off via fanning (Figure S7), algae that fouled on the floating method attached better and were hard to remove without scraping the surface of the explants. However, explants that were deployed at 5 m depth were all healthy and were not severely biofouled as compared with those deployed at 1 m depth (Figure 14b). In the 4‐month monitoring done using the floating method, predation by J. funebris was not observed.14FIGUREDifference between the biofouling intensity of explants transplanted (a) near the surface and (b) at 5‐m depth.Growth (wild vs. mariculture)After two months, one of the three wild colonies tagged for branch growth measurement was lost probably due to predation or dislodgement. Three of the remaining six tagged branches were obviously preyed by J. funebris. All the tagged explants in the mariculture setups were still alive with all tagged branches still present. Branch growth was higher for the explants (2.05 cm ± SD 0.23/2 months) compared with wild colonies (0.25 cm ± SD 0.21/2 months) (Figure 15). ANOVA revealed significant difference (ANOVA, p < 0.05).15FIGUREMean tip growth (±SD) of colonies in the wild and explants in the mariculture setup, generated using tip growth of three mariculture and two wild colonies with three tips per colony. The Kruskal–Wallis test showed significant difference between the treatments (p < 0.05).DISCUSSIONSize–frequency distribution and abundanceThe variation in the size–frequency distribution and abundance may indicate variation in population health. Blue sponge colonies were more abundant in the embayed area and were generally larger compared with colonies outside that bay. In other colonial invertebrates (e.g., corals), size–frequency distributions are often used to assess the health of a population in an area (e.g., Adjeroud et al., 2015; Lalas et al., 2021). Variations in size–frequency distribution of populations may indicate variations in historical events in different areas that have implications on juvenile input and colony longevity (Bak and Meesters, 1998). Lower abundance and dominance of small colonies outside the bay may indicate that colonies were not able to grow and survive under the given environmental conditions. Inside the bay, the slight negative skewness of the size–frequency distribution suggests that the population is generally represented by large colonies, but with small colonies still having representatives. This indicates that colonies produce juveniles with higher colony longevity. Thus, this can also indicate a healthier blue sponge population in stations inside the bay.Differences in the abundance and size–frequency distribution of the blue sponge colonies among stations can be attributed to a mix of different biotic and abiotic factors (Duckworth and Wolff, 2007). In the study of Duckworth et al. (2009), abundance of the sponge, Coscinoderma matthewsi, differed at different spatial scales, including sites that are 200 m apart, which was less than the distance between our survey stations. Since the nineties, studies have highlighted pollution inside Puerto Galera Bay due to domestic wastes and restricted circulation causing low flushing rates (Gomez et al., 1994; San Diego‐McGlone et al., 1995). The study by San Diego et al. (1995) showed the formation of high nutrient pool in the area because of low flushing rates and high nutrient input from freshwater runoff and wastewater discharge from land‐based sources. In 2009, Iizuka and colleagues highlighted contaminated coastal waters due to poorly constructed sanitation and household facilities and runoff from the hillsides associated with the tourism development. Many sponges have been shown to benefit from polluted waters (Rose and Risk, 1985) and the blue sponge may be one of them. Furthermore, sponge microbiomes are said to be stable under high nutrient conditions, which means that they are able to withstand eutrophication pressure (Baquiran and Conaco, 2018). Iizuka et al.’s (2009) study also highlighted weak tidal currents and associated flow in the bay area. As with many filter‐feeding invertebrates, the blue sponge may benefit from these conditions making them more abundant and grow faster (see Gokalp et al., 2021). The blue sponge may also benefit from the relatively low turbulence inside the bay. High exposure of reefs outside the bay to waves and typhoons passing the area (Licuanan, 1991) may prevent colonies from growing tall, similar to tall branching formed colonies inside the bay. In Panama, Wulff (1995) highlighted the loss of sponge biomass and an increase in the abundance of small colonies after a storm. The dominance of small‐sized colonies of tetillid sponges in the northwestern Philippines was also hypothesized to be caused by strong wave action, which can limit growth or cause tissue loss and mortality (Baquiran et al., 2020). The weaker water movement inside the bay may also give more chance to dislodged colonies or detached fragments to attach to an available substrate, which is a common dispersal strategy of sessile organisms (Crisp, 1955; Rodriguez et al., 1993). High water flow can also damage sponges, remove tissue and decrease size (Trautman et al., 2000). The greater available space, such as rocks and dead corals, may also provide a space for settlement that has lesser chance of competition with other sessile organisms (i.e., corals). For the in situ mariculture trials that aim to increase biomass production of any organism, the most practical choice is to establish mariculture setups in areas where members of a population grow larger and more abundant.Observations of wild coloniesMajor factors observed that affected the survival and mortality of the blue sponge were predation, dislodgement and an unknown cause of partial mortality that was disease‐like in appearance (Figure 8b). The blue sponge was observed to have a lone predator, the nudibranch, J. funebris (Figure 8a). It was also observed to be the main predator of other blue sponges in other regions, such as in Hainan, China (Wu et al., 2021) and India (Fontana et al., 2000). Dislodgement was also observed in the study site. Whole colonies or fragments that were unattached to the substrate, which were obviously dislodged, were observed. This can be attributed to the relatively fragile morphology of the blue sponge colony, which can easily break when there is high water movement, such as caused by waves and underwater currents. This might also explain why blue sponges are not abundant or even absent in stations outside the bay, which are more exposed to waves, especially when there are typhoons passing the area. Another factor that contributed to whole and partial colony mortality was the occurrence of white colouration on parts of the colonies that eventually disintegrated. One of the possible causes of this is disease, which stresses out colonies and eventually kills a part or the whole colony. A similar discolouration was also reported in the Caribbean sponge, Amphimedon compressa (Angermeier et al., 2012). The discolouration, called ‘sponge white patch’, was characterized by distinctive white patches found irregularly throughout the diseased sponges and showed severe degradation of tissues. Meanwhile, the barrel sponge Xestospongia muta, a congener of the target blue sponge, was observed to be affected by the sponge orange band disease, characterized by a reddish‐brown colouration, which causes a destruction of the pinacoderm (Angermeier, et al., 2010). However, the cause of these sponge diseases remains unidentified. The white colouration that was observed was the same with the appearance when fragments are stressed due to suffocation when not aerated properly or when they were removed underwater and exposed to air for a long time. Although the factors listed were major factors that contributed to blue sponge survival and growth, information on the different aspects of its life history is still unknown (e.g., reproduction, eco‐physiology). Thus, more studies can be conducted to further understand blue sponge biology and ecology.Development of an open sea mariculture protocolAn ideal mariculture setup should be easy to construct, deploy, maintain, where explant mortality is reduced, growth is increased, and can easily be retrieved. From a general perspective, the string method had the highest number of explant survivors among the methods used. However, there was no clear pattern on the variations in the wet weight increase among treatments. This can be attributed to the fact that the effect of other factors (e.g., predation, biofouling and competition) were not minimized due to the lack of maintenance activities for 11 months because of travel restrictions caused by the COVID‐19 pandemic. However, descriptive observations on the status of the setups after 11 months gave us important information on blue sponge ecology, as well as ideas for improvement on the mariculture method. The bag method, which was also used in previous mariculture trials of the blue sponge (Santiago et al., 2019), may be protected from predators. However, the greater surface area of the bags where algae can attach to resulted to heavy biofouling, thus requiring more maintenance effort. The explants inside the bags also had the tendency to be overgrown by other organisms (e.g., tunicates, bivalves) that might have entered the bags through settling during their larval phase. The tube method performed better, but the limited substrate available may have limited explant growth. Also, the explants had a tendency to grow beyond the tubes they were attached to and encrust on the polyethylene screen. Encrusting blue sponges in the polyethylene screen were challenging to collect. Aside from the challenges in collection, the use of polyethylene plastic screen and additional PVC pipes to be used as sponge substrate would require additional expenses. The simplest method among the three methods tested was the string method. However, an explant with no substrate to grow on might limit its growth, since blue sponge colonies are observed to grow by encrusting substrate. However, unintentionally, some explants that were transplanted using the string method had strings that were long enough, which allowed them to attach to the PVC frames that resulted to having an artificial substrate, resulting to healthier explants with apparent high growth rates. Thus, it was decided to develop an improved string method, which was the ‘improved string method’ used in the 2nd phase of the mariculture trials. Even though the farming methods tested in the 1st phase were rejected, the initial trials were very essential to the improvement of the later versions used.The number of oscula during transplantation may also be a factor in the survival of the explants. Results show that number of survivors were higher on explants transplanted with three or more oscula compared with explants transplanted with less than three oscula, regardless of farming method used (Figure 9). The osculum is the most obvious feature in sponges, which is a chimney‐like structure where water is pumped out after being filtered by the sponge body (Leys et al., 2011). The osculum is also a very important structure in a sponge because it was shown to induce development of colony's aquiferous system (Windsor and Leys, 2010). Ludeman et al. (2014) suggested that non‐motile cilia in the osculum serve as sensory system to detect changes in flow and control the responses of the sponge colony as a whole. Thus, a blue sponge fragment collected with no osculum may have a lower growth rate and even a lesser chance of survival.The ‘improved string method’ was suitable for blue sponge farming. One criterion for usable transplantation methods is how fast explants can attach to an artificial substrate. In our trials, explants were observed to be attached to the PVC substrate 7 days from transplantation and have remained attached during the last monitoring activity. This indicates that the PVC pipes used are suitable as artificial substrates. Factors that were observed to be a potential source of stress of explants were fouling organisms and organisms that can attach on the artificial substrate and may compete for space. However, biofouling and attachment of space competitors were minimised by conducting more frequent monitoring and maintenance works. One of the challenges when using the improved string method was predation by J. funebris. Even though survival was high, there was still a need to improve the design of the farming medium.The ‘floating method’ was an improvised version of the improved string method. Since J. funebris can only go to one spot to another through crawling, it cannot reach the hanging artificial substrate. Thus, this design prevented predation, which was the main challenge in the improved string method. An advantage of the floating setup is also that deployment, maintenance, and collection can be done without SCUBA diving. Thus, may also have an advantage through lower costs. However, just like other methods used in sponge farming (Duckworth, 2009), the floating method has its disadvantage, which is mainly its higher exposure to winds and waves that can destroy the floating setup. Thus, it is important to consider different factors (i.e., ecological, logistical, and financial) when selecting what method to use in sponge farming. Also, it might be worth maintaining setups using multiple methods so there are backups when unfavourable situations occur.Biofouling intensity of algae varied temporally and spatially. One of the major observations during the monitoring is the increased intensity of biofouling in March and decreased after the 2nd week of June. During this time, more frequent cleanups were needed. This increase in biofouling intensity was in line with the increased in temperature in March to June (Figure S8). Thus, biofouling intensity, which affects maintenance effort needed, was affected by seasonal variation in temperature, as also observed in studies that have shown seasonal algal blooms (Lüning and Dieck, 1989). This should be factored‐in in maintenance planning since biofouling on the surface of the sponges may be a cause for colony stress that can lead to mortality. Some parts of the explants that were observed to be heavily fouled were observed to eventually disintegrate. Another major observation is the variation in the biofouling intensity among setups established in different areas with different benthic assemblages. Generally, the setup established in the area with the lowest adjacent coral cover always had the highest biofouling intensity. It was observed that browsing fishes, which resided in adjacent coral colonies, can aid in minimising biofouling by algae. Browsing fishes have also been utilized by other studies to minimize biofouling (Bannister et al., 2019). The condition of the mariculture setup may also be affected by depth. In the floating method, the explants and artificial substrate that were hung near the water surface showed very severe biofouling by algae as compared with those hanged in deeper water. This can be indirectly attributed to differences in depth, which may vary in light intensity (Mundy and Bobcock, 1998) and the browser abundance (Ferrari et al., 2018). Higher light intensity promotes algal growth as it enhances algal photosynthesis (Lüning and Dieck, 1989). The shallow part of the water column was also far from fish browsers. Fish browsers observed in the area were all residential and were observed not to move far from their territories. In the monitored wild colonies at deeper depth, heavy biofouling by algae was never observed, providing support for the influence of depth to the level of biofouling. This highlights proper depth selection when using hanging setups for the cultivation of sponges. High light levels on the shallower depths may be ideal for algae fouling than low‐light conditions at deeper water (Lewbel et al., 1987). Thus, careful planning on site selection for a mariculture area should be done and depth where explants should be suspended to increase chances of producing healthier and fast‐growing explants.Growth comparison (mariculture vs. wild)Transplanted fragments grew faster than colonies in the wild. This is one of the positive attributes of collecting fragments from the wild. The result was also similar to other species, where transplanted fragments had faster growth rates and often doubled their sizes after few months from transplantation (Duckworth, 2009). The difference of the growth rates might also be caused by the difference in the growth rates of sponges at different size classes. A number of studies highlighted decreasing growth rates as sponge age or as size increases (Cardone et al., 2010; Singh and Thakur, 2015). This needs to be further investigated. However, the results in the present study shows the advantage of sponge mariculture through transplantation.General criteria for the mariculture establishment site selectionBased on the observations, we came up with a protocol in site selection to increase the chances of success in the open sea farming. In selecting a site for mariculture establishment, different factors should be considered. (1) A mariculture site should be established in an area or adjacent to an area where a natural population of the selected species are abundant. This is to ensure that the environmental condition of an area is ideal for the species, which will increase the chances of survival and growth. It is also important to note that although renieramycin content was not measured here, previous work (Santiago et al., 2019) has shown that renieramycin content does not vary among setups, as long as mariculture is done in the same area. (2) A mariculture area should be easily accessed by maintenance workers. This is to minimise time of travel and maintenance cost. (3) A mariculture site should be established in an area with minimal exposure to wind and waves, such as in embayed areas and lagoons. This is to lessen chances of destruction, especially during typhoon season. However, this will depend on the environmental preferences of a species. (4) A mariculture setup should also be established at a depth similar to the depth range where natural populations of the selected species are abundant to ensure ideal environmental conditions. Ideally, setups should be established at the shallowest depth suitable for the species for easier maintenance. A greater depth would mean faster air consumption of SCUBA divers. Thus, working at a shallower depth would mean relatively more time for SCUBA divers to do maintenance work underwater. Last, (5) a mariculture setup should be established in a specific area where damage to benthic communities (e.g., corals) will be avoided (e.g., sandy, rocky). However, it is also important to establish setups in an area with adjacent coral colonies. Coral colonies usually have resident browsers (e.g., fishes) or can attract browsers that can help minimize biofouling by algae on the setups.ConclusionsThis study has shown insights on the distribution, ecology and the mariculture potential of the Philippine blue sponge, Xestospongia sp. We conclude that blue sponge colonies were more abundant and had bigger sizes in an area characterized by high nutrient levels and low exposure, indicating ideal environmental conditions for blue sponge growth. Higher growth rate of transplanted explants also justified the benefits of sponge transplantation. In general, this study has shown the feasibility of the use of PVC pipes as an artificial substrate for blue sponge transplantation, as long as proper site selection and frequent monitoring and maintenance works are done.AUTHOR CONTRIBUTIONJue Alef Lalas: Conceptualization; Data curation; Formal analysis; Investigation; Methodology; Writing – original draft; Writing – review & editing. Geminne Manzano: Conceptualization; Data curation; Investigation; Methodology; Writing – original draft; Writing – review & editing. Lee Arraby Desabelle: Data curation; Investigation; Methodology; Writing – original draft; Writing – review & editing. Lilibeth Salvador‐Reyes: Conceptualization; Funding acquisition; Project administration; Resources; Supervision; Validation; Writing – original draft; Writing – review & editing. Porfirio Aliño: Conceptualization; Methodology; Project administration; Supervision. Maria Vanessa Rodriguez: Conceptualization; Methodology; Project administration; Resources; Supervision; Validation; Writing – original draft; Writing – review & editing.ACKNOWLEDGEMENTSThis study was funded by the Philippine Council for Health Research and Development of the Department of Science and Technology (DOST‐PCRHD) through the ‘Anti‐infective and Anticancer Drug Candidates from Marine Microorganisms and Sponges: Discovery and Development’ project of the ‘Discovery and Development of Health Products’ program, which was implemented by the Marine Science Institute of the University of the Philippines. Samples were collected with the permission of the local government of the Municipality of Puerto Galera and the Bureau of Fisheries and Aquatic Resources of the Department of Agriculture of the Philippines through Gratuitous Permit no. 0196‐20. We thank the researchers and volunteers that helped establish the initial setups, namely, Joey Cabasan, Daryll Valino, Maxine Prado, Romina Lim, Marc Punzalan and Czarmayne Escoro. We also thank the people of the Municipality of Puerto Galera for their continuous support for research projects in the area.CONFLICT OF INTERESTThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.FUNDING INFORMATIONPhilippine Council for Health Research and Development of the Department of Science and Technology (DOST‐PCRHD) through the ‘Anti‐infective and Anticancer Drug Candidates from Marine Microorganisms and Sponges: Discovery and Development’ project of the ‘Discovery and Development of Health Products’ program.DATA AVAILABILITY STATEMENTData inquiries can be directed to the authors.ETHICS STATEMENTThe activities and specimen collections for this study were approved through the Prior Informed Consent Certificate issued through Municipal Resolution No. 2019 by the Office of the Municipal Mayor of Puerto Galera, and the Gratuitous Permit (No. 0196‐20) issued by the Department of Agriculture of the Republic of the Philippines.PEER REVIEWThe peer review history for this article is available at: https://publons.com/publon/10.1002/aff2.98REFERENCESAdjeroud, M., Mauguit, Q., Penin, L. 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Notes on the distribution, ecology and open sea mariculture trials of the Philippine blue sponge, Xestospongia sp. (Porifera)

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Abstract

INTRODUCTIONSponges are major benthic components of coral reefs that have significant roles in coral reef community dynamics (de Goeij et al., 2017). However, various aspects of the sponges such as their biology, ecology and distribution are still understudied. Apart from their ecological importance, sponges have numerous economic benefits, including as a potential source of biomedical products. A review by Leal et al. (2012) highlighted that among the marine natural products collected from the marine environment, the majority come from sponges. However, due to the large biomass needed to supply a sizable amount of chemicals, collecting biomass from wild sponge populations can be harmful to the ecosystem. As a result, continuous harvest from the wild is not economically and environmentally sustainable (Duckworth, 2009).Mariculture provides a sustainable solution to the ‘supply problem’ (Duckworth, 2009). The culture of sponges occurred for over a 100 years, where the most common method used is cutting explants from wild sponge colonies, since explants are potentially highly capable of regenerating after fragmentation (Kinne, 1977; Decaralt et al., 2003; Koopsman et al., 2010). In situ or sea‐based mariculture is the most promising and applicable method for biomass production among sponges. Ex situ or land‐based mariculture also has various advantages. However, sea‐based cultures may be cheaper than their land‐based counterparts because of the infrastructure needed to manipulate the culture environment ex situ (Duckworth, 2009). Although many species have been cultivated successfully in various parts of the world – from Mediterranean countries such as Egypt and Greece (Maslin et al., 2021), to the Caribbean (Ruiz, et al., 2013) and to tropical regions of the western Atlantic and Pacific Oceans (Duckworth, et al., 2009; Bierwirth et al., 2022), a crucial factor limiting the success of mariculture efforts is the lack of knowledge on the biology and ecology of different species. Thus, there is a need to further investigate the various aspects affecting their growth and survival.One of the sponges identified as a potential source of natural products for pharmaceutical use is the Philippine blue sponge, Xestospongia sp. (Tatsukawa et al., 2012), which can be observed in shallow coral reefs, growing on various substrates (e.g., corals, rocks, sand). The Philippine blue sponge has been identified as a source of renieramycins, which are compounds that exhibit antimicrobial and antiproliferative activity against certain types of cancer cell lines (Halim et al., 2011). Santiago et al. (2019) have conducted initial mariculture experiments to assess the cultivability of the blue sponge, specifically to assess effects of different factors (i.e., harvesting regime, culture period, translocation, farming method) on different parameters (e.g., sponge growth, survival, chemistry). However, biomass production may still be improved, and several aspects of its biology and ecology are still unknown.The objectives of this study were to (1) assess the distribution of the Philippine blue sponge through comparing the abundance and size–frequency distribution among different stations with varying environmental conditions (i.e., water quality and exposure to monsoons), (2) develop improved in situ mariculture methods, (3) observe the different factors that may affect cultured and wild sponges (i.e., predation, disease, maintenance, dislodgement) and (4) identify and compare the growth rates of wild and transplanted blue sponges. Overall, this study provides additional information on the biology and ecology of the Philippine blue sponge, Xestospongia sp., and highlights the process in developing a mariculture method. Additionally, the development of the improved farming protocols is an important step to have a more sustainable source of biomass for secondary metabolite production that is useful to the pharmaceutical industry. This resource can also be a potential source of alternative income for coastal communities (Duckworth, 2009).MATERIALS AND METHODSStudy sitePrevious reconnaissance activities in different coral reef sites in the Philippines revealed the high abundance of the Philippine blue sponge, Xestospongia sp., in the fringing reefs of Puerto Galera, Oriental Mindoro province (Figure 1). Puerto Galera is part of the Verde Island Passage, an area known to have very high marine biodiversity (Asaad et al., 2018). It is one of the major tourist and SCUBA diving destinations in the Philippines, making it a large contributor to the local economy (Roche et al., 2016). It was also designated by the United Nations Educational, Scientific and Cultural Organization (UNESCO) as a Man and Biosphere Reserve in 1977 (Iizuka et al., 2009; Dagamac et al., 2015).1FIGUREMap of the study area in Puerto Galera, Oriental Mindoro, Philippines. The map shows the mariculture area and the nine survey stations (SN, Sto Niño; MC, Manila Channel; LB, Long Beach; CG, Coral Garden; M, Muelle; GC, Giant Clam; MB, Monkey Beach; E, Escarceo; CC, Coral Cove).In this study, surveys were conducted in stations inside and outside Puerto Galera Bay, which are areas with variations in environmental conditions (Figure 1). The coral reefs inside the bay are exposed to waters with high turbidity and high nutrient levels (San Diego‐McGlone et al., 1995). The reefs inside the bay are also relatively unexposed to waves due to embayment, weak tidal current and associated flow, while reefs outside the bay are highly exposed to waves with stronger currents (Iizuka et al., 2009).Size–frequency distribution and abundanceSurveys were conducted via SCUBA diving at depths of 3–12 m to search for blue sponge colonies at nine stations in the coral reefs of Puerto Galera (Figure 1). Three stations were surveyed inside Puerto Galera Bay (Coral Garden, Giant Clams, Muelle) and six stations outside the bay (Sto. Niño, Manila Channel, Long Beach, Monkey Beach, Escarceo, Coral Cove) (Figure 1). Maximum height and greatest length of each colony encountered were measured and recorded for size–frequency distribution analysis. Ecological volume for each colony was calculated using the formula of the volume of a cylinder (Volume = πr$\pi r$2h) where the greatest width of the colony was assumed to be the diameter. Studies on sponge size–frequency mostly used colony length (e.g., Duckworth and Wolff, 2007) and area (e.g., Baquiran et al., 2020) to measure sizes. However, to take into account both the horizontal and vertical growth of blue sponge colonies, the use of volume was more ideal for this study (Wulff, 2001).Abundance surveys were conducted in the nine stations. At each station, a 75 m × 25 m area was established at 3–6 m depth. Five fifty‐metre transects were then randomly laid inside the 75 m × 25 m area. All blue sponge colonies observed inside the area 1 m on the shallow side of each transect were then counted. Thus, each transect was represented by the number of colonies per 50 m2 and each station was represented by the mean of the abundance of the transects (n = 5).The Shapiro–Wilk's test for normality and the Levene's test for homogeneity of variances were performed for the abundance dataset. The Kruskal–Wallis and Mann–Whitney pairwise tests were then performed to determine the significance of variations of the blue sponge mean abundance among stations. Descriptive statistics and histograms were then generated using log‐transformed ecological volumes of blue sponge colonies to compare the size–frequency distribution between colonies inside and outside the bay. The 2‐sample Kolmogorov–Smirnov (KS) test was then performed to evaluate the significance of the variation. Tests were performed using the PAST software (Hammer et al., 2001). The results of the abundance and size–frequency distribution surveys were then used as the main basis for the selection of the mariculture and collection sites for blue sponge fragments that were used in the mariculture trials.Monitoring of wild coloniesTen haphazardly selected blue sponge colonies in the Giant Clam station (Figure 1) were tagged to monitor factors that may affect the survival of natural blue sponge colonies. Detailed photos of each colony were taken monthly for 6 months to monitor possible changes. Factors that were targeted were mainly predation, competition with other benthic organisms, dislodgement and presence of disease.Development of open sea mariculture methodsSeed stock collection and preparationThe Giant Clam station was established as the collection site of fragments to be used for the open sea mariculture trials since it was identified to have the most abundant colonies among the study stations (Figure 1). Sponge fragments were collected by cutting 1/3 of the colony using a cutter to make sure that the colony left would still survive (Santiago et al., 2019). Fragments collected were then put in a meshed bag to be processed above water. Collected blue sponge fragments were then put in a 155 l container with seawater or were hung on the side of the boat to keep them underwater to lessen stress. From the collected samples, fragments for transplantation in the mariculture setups were prepared through slicing using a cutter. Each explant was at least 20 g in wet weight and was approximately 250 cm3 in volume and was assigned to one of the different treatments detailed in the Subsection 2.5.3. The wet weight of each explant was measured and recorded, and each explant was uniquely labelled for monitoring.Establishment of underwater mariculture setupsMariculture setups were established in an area adjacent to the collection site at about 5 m depth (Figure 1). The location was mainly chosen based on the presence of abundant blue sponges, which may indicate ideal environmental conditions for the blue sponge. The location was inside Puerto Galera Bay, characterised by low exposure to waves with high nutrient levels. Its relatively low exposure to the monsoons also makes it an ideal area to do mariculture since it has a lower chance of being severely hit by typhoons, thus, avoiding destruction of the mariculture setups. The benthos of the area was characterized by high sand and rubble cover with some coral patches, thus, damages on communities, such as corals, during the different activities of this study were minimized. Mariculture setups were assembled underwater through SCUBA diving. A setup was defined as a hollow cube‐like structure with frames made up of polyvinyl chloride (PVC) pipes (Figure 2). Width, length and height of each set up was 2 m. The four corners of the bottom part of structure were also connected to staked cement blocks for stability. Three setups (setups A, B and C) were established in spots that had flat benthos to make the structures more stable.2FIGUREUnderwater structure used for attaching the different mariculture media tested. This structure was made of polyvinyl chloride (PVC) pipes and attached to the substrate using cement blocks.Phase 1 mariculture trials: comparison of farming methodsMariculture trials were conducted to compare wet weight increase (%) and survival (%) of explants using three farming methods (bag, tube and string) and different number of oscula (0, 1, 2, ≥3). An osculum is part of the sponge aquiferous system that acts as the exhalant canal of sponges where the filtered seawater passes through. For the bag method, explants were attached inside a bag made of polyethylene plastic screen (2 cm mesh size, with ∼15 cm diameter at the base, and 30 cm in height), which were hung on the PVC frames of the mariculture setups (Figure 3a). In the tube method, an explant was attached to a 6‐in. PVC pipe using a plastic zip tie. These where then attached to a polyethylene plastic screen covering a side of the mariculture setup (Figure 3b). In the string method, explants were attached to nylon strings (5 mm diameter) using zip ties and were hung from the PVC frames of the setups (Figure 3c). The bag and tube methods were designed according to Santiago et al. (2019). The string was designed and tested as an alternative method that was simpler with minimal logistical effort in the deployment, maintenance and collection. A total of 48 replicate explants were deployed per farming method and 36 replicate explants per oscula number (see experimental design on Figure S1).3FIGUREFarming methods used in the phase 1 mariculture trials. (a) Bag method, (b) tube method, and (c) string method.Explants were transplanted in January 2020. We originally planned to conduct monthly monitoring for at least 3 months. However, monitoring and maintenance activities were paused in March 2020 due to travel restrictions brought by the COVID‐19 pandemic. The mariculture site was only revisited in December 2020, 11 months post‐transplantation of the explants.Using the remaining explants after 11 months, growth for each explant was computed using the wet‐weight method through the equation,1Growth%=Wn−WiWn×100$$\begin{equation}{\rm{Growth}}\;\left( \% \right)\; = \frac{{{W_{\rm{n}}} - {W_{\rm{i}}}}}{{{W_{\rm{n}}}}} \times 100\end{equation}$$where Wn is the wet weight of an explant at the end of the experiment, and Wi is the weight on an explant during transplantation. The Shapiro–Wilk's test for normality and the Levene's test for homogeneity of variances were then performed for the datasets. The analysis of variance (ANOVA) and Tukey's test were then performed to determine significance of the difference in the mean growth among farming methods and oscula numbers. Tests were performed using the PAST software (Hammer et al., 2001).Survival of explants per treatment was then calculated as the percentage of live fragments after the end of the experiment. This was calculated using the equation,2Survival%survival=NtN0×100$$\begin{equation}{\rm{Survival}}\;\;\left( {\% {\rm{survival}}} \right) = \frac{{{N_{\rm{t}}}}}{{{N_0}}} \times 100\end{equation}$$where Nt is the number of explants that have survived after 11 months, and N0 is the number of explants during the start of the experiment.Phase 2 mariculture trials: ecological observations using the ‘improved string method’Based on the observations in the Phase 1 mariculture trials, an improved farming method was developed for a scaled‐up Phase 2. The three different farming methods in Phase 1 were no longer used. Instead, an improved string method was developed, where explants were attached to nylon ropes (5 mm diameter) that were vertically tied to the PVC frames of the setup (Figure 4). To increase stability and chances of attachment, it was made sure that each explant was attached to the PVC frames, which served as the main artificial substrate. All pre‐labelled and pre‐weighed (at least 20 g wet weight) explants transplanted in this phase were also explants with three or more oscula, as the result of the first mariculture trials phase indicated that fragments with more than 3 oscula had higher survival. Forty‐seven replicate explants were transplanted in December 2020. This method is hereunto referred as the ‘improved string method’ (see experimental design on Figure S1).4FIGUREThe improved string method. Sponge explants are attached to a nylon string tightly ties to the underwater structure. Sponge explants were attached to the string and PVC pipes using plastic zip ties.Maintenance work and monitoring of the health conditions of the explants were done weekly by cleaning the setups and explants from biofouling organisms. Organisms growing on or adjacent to the explants were recorded and taken out to prevent possible stress for competition for space. Observed predators were also recorded and were taken out from the explants. Health monitoring of each explant was also performed (i.e., attachment to substrate, wound healing, presence of disease and biofoulers, associated organisms on colony surface and predator bite marks) once a week for 6 months. After 6 months, five explants that were not affected by predation were collected and weighed for wet weights and potential growth rate was calculated using the equation,3Growthrateg/month=Wn−WiWn×100/6months$$\begin{equation}{\rm{Growth\;rate}}\;\left( {{\rm{g}}/{\rm{month}}} \right)\; = \;\left( {\frac{{{W_{\rm{n}}} - {W_{\rm{i}}}}}{{{W_{\rm{n}}}}} \times 100} \right)/\;6\;{\rm{months}}\end{equation}$$where Wn is the wet weight of an explant at the end of the experiment, and Wi is the weight on an explant during transplantation.Effect of adjacent coral cover and fish abundance on biofouling intensityTo initially investigate the effect of adjacent coral cover and fish abundance on the biofouling of the mariculture setups, covers of live hard corals in the vicinity of the three setups established (Setups A, B, and C) were measured by analysing top view photos of the setups. To ensure uniformity of the photos, the benthos within 2 m from the edge of a setup was consistently made visible in each photo. A 1‐m tape was also always visible in the photos to serve as a length scale to aid in the photo analysis. Photos were taken using a camera (Sony RX100) with underwater housing (Ikelite) with wide‐angle lens (Inon). The photos were then analysed using the Coral Point Count with Excel extensions (Kohler and Gill, 2006) to generate percentage covers of adjacent hard corals per mariculture setup.Fishes were also assessed via point count visual census. This was done by estimating the maximum count of individuals of a fish family at one time observed in a certain duration. Fish abundance was estimated by observing the fishes inside a mariculture setup and 1 m from the edge of a setup for 10 min. Visual census of the setups was done in the morning (9AM–11AM) on four different dates – once in June, once in July and twice in August. A survey for each date for a setup was considered one replicate for each mariculture setup (n = 4 per setup). Mean abundance of fish families per setup was computed and compared using the Kruskal–Wallis and Mann–Whitney pairwise tests after testing for normality and homogeneity of variances using the Shapiro–Wilk's test and the Levene's test, respectively.Mariculture trials (Phase 3): floating methodTo test if predation by the nudibranch, Jorunna funebris, can be minimised, a floating mariculture setup was designed, built, and deployed. The floating mariculture setup, herein after referred to as the ‘floating method’, was made up of a floating device made of buoys and bamboo sticks where the hanging artificial substrate, made of nylon ropes and PVC pipes, were attached to (Figure 5). Weights were used to maintain the depth where the blue sponge fragments were suspended. The floating device was also attached to the substrate using a nylon rope and a mooring block staked on the substrate. A total of 12 explants were deployed using the floating method. We also we hypothesized that depth influences sponge health and survival. To test this hypothesis, the floating setup was used to deploy blue sponge fragments at two depth ranges – near surface (1 m) (n = 6) and near substrate (5 m) (n = 6). Monitoring of the fragments deployed using the floating method was done weekly for 4 months (February–May, 2021) (see Figure S1 for experimental design).5FIGUREThe floating mariculture method. Sponge explants were attached to PVC substrates that were hanged to a floater at the surface. The floater was securely attached to the substrate using cement mooring blocksGrowth (wild vs. cultured explants)To compare growth rates of the explants from wild colonies, branch growth was monitored. Branch growth for a colony was measured by tying plastic zip ties to colony branches. To identify the initial length, the distance of the zip tie from the tip of the branch was measured using a caliper. Initial measurement was done in June 1, 2021 and monitored in July 30, 2021. Growth rate (cm per month) of a branch was calculated through the equation,4Growthratecm/month=Finallengthcm−Initiallengthcm2months$$\begin{equation}{\rm{\;Growth}}\,{\rm{rate}}\left( {{\rm{cm}}/{\rm{month}}} \right) = \frac{{{\rm{Final}}\,{\rm{length}}\left( {{\rm{cm}}} \right) - {\rm{Initial}}\,{\rm{length}}\left( {{\rm{cm}}} \right)}}{{2\,{\rm{months}}}}\end{equation}$$Branches of three wild colonies and three explants were monitored, with three branches from each colony or explant. Mean growth rates (n = 3) of wild colonies and explants were then computed and compared. The Shapiro–Wilk's test for normality and the Levene's test for homogeneity of variances were performed for the dataset. Since the dataset was nonparametric, the Kruskal–Wallis test was performed to determine the significance of the difference of the mean growth rates between the two treatments. The test was performed using the PAST software (Hammer et al., 2001).RESULTSSize–frequency distribution and abundanceDescriptive statistics revealed variation in the size–frequency distribution of blue sponge colonies inside and outside the bay (Table 1). Colonies outside the bay were positively skewed, suggesting dominance of small‐sized colonies (Figure 6b). Colonies inside the bay were slightly negatively skewed, which suggests that large colonies were more abundant compared with outside the bay (Figure 6a). The size class mode of colonies inside the bay was also larger than outside the bay. The KS test also revealed significant variation in the size–frequency distribution (KS test, p < 0.05).1TABLEDescriptive statistics of sizes of the blue sponge colonies outside and inside Puerto Galera BayOutsideInsideMean1.672.17Standard error0.150.08Median1.702.24Mode range>1.60, ≤1.80>2, ≤2.4Standard deviation0.780.91Sample variance0.620.83Kurtosis1.15‐0.76Skewness0.56‐0.03Range3.543.87Minimum0.260.44Maximum3.804.31Sum43.48270.88Count261256FIGUREHistogram of the size–frequency distribution of blue sponge colonies (A) inside and (B) outside the bay. For comparison, the figures also show normal bell curves generated using the mean and standard deviations of the datasets. Different size classes inside the bay were generally well represented while colonies outside the bay were generally dominated by small colonies.Significant variations in the abundance of blue sponge colonies were observed among the nine stations surveyed (Kruskal–Wallis, p < 0.05) (Figure 7). Mean abundance was highest in the Giant Clam station, while no colonies were observed in Coral Cove, Coral Garden, Long Beach and Sto. Niño. The Mann–Whitney pairwise tests revealed no significant difference between Giant Clam and Muelle (Mann–Whitney, p > 0.05), both of which were located inside the Bay. The two stations, however, were significantly different to the rest of the stations where blue sponge colonies were observed, all of which were all outside the bay (Mann‐Whitney, p < 0.05).7FIGUREMean abundance (±SD) of blue sponge colonies (density of colonies per 50 m2) in the surveyed nine stations in Puerto Galera, Oriental Mindoro.Monitoring of wild coloniesAfter six months of monitoring, there were no changes in the adjacent benthic assemblages that could potentially affect the ten tagged wild blue sponge colonies (e.g., competition for space). Observations for predation revealed that the nudibranch, J. funebris, was the only predator of the blue sponge (Figure 8a). In some cases, the bite marks previously observed healed after a month. Some of the colonies also had partial mortality, with the presence of white colouration on the tissue, which eventually disintegrated (Figure 8b). The cause for this discolouration is still unknown. Three out of ten colonies were also observed to be dislodged and detached from the substrate. Also, three colonies were lost after 6 months of monitoring, which was likely due to predation or dislodgement.8FIGUREMajor observations of wild blue sponge colonies. (a) Jorunna funebris feeding on a wild blue sponge. (b) White colouration on parts of a blue sponge which eventually leads to partial mortality.In situ mariculture trialsPhase 1 mariculture trials: comparison of farming methodsThe survival of the sponge explants was highest (65%) when the string method was employed (Figure 9a) and when explants had three or more oscula (Figure 9b). Explants with more than three or more oscula had a 100% survival. The number of survivors were lowest (44%) when explants were transplanted with no osculum and when using the bag method. In terms of variation in the mean % wet weight increase, no obvious patterns were observed (Figure 10). The ANOVA also revealed no significant differences among treatments in terms of % wet weight increase (Kruskal–Wallis test, p > 0.05).9FIGUREPercentage survival of explants among (a) culture methods and (b) among explants with different number of oscula after 11 months in the first phase of the trials.10FIGUREMean percentage weight increase (±SD) of explants among different farming method after 11 months in the first phase of the trials.Descriptive observations are also reported here. For the tube, explants that have survived or were not totally outcompeted by biofoulers overgrew and encrusted the PVC pipes and screens that they were attached to (Figure S2a). This made it more challenging to collect them. For the bag method, all the bags were heavily overgrown or biofouled by different organisms, such as invertebrates and algae (Figure S2b). When the bags were opened, we observed other organisms (e.g., mollusks, algae, tunicates) that grew inside and outcompeted the initially established blue sponge explants (Figure S2c). For the string method, growth was not observed on most of the explants that survived (Figure S2d). However, a few explants from the string method grew large and healthy. The said healthy string explants were able to attach to the PVC frames because the nylon ropes were long, which enabled them to be in contact with an artificial substrate (Figure S2e). These observations became the basis for the improvement of the design of the mariculture method used in second phase of the mariculture trials.Phase 2 mariculture trials: ecological observations using the improved string methodMajority of the explants attached to the PVC substrate after a week from transplantation (Figure S3). Some explants were observed to have partial mortality after a week from transplantation. Partial mortality was visible when a part of an explant became white (Figure S4a), similar to observations in wild colonies (Figure 8b). This type of partial mortality was only observed in the first month from transplantation. However, in March 2021, some explants that were severely biofouled by algae also had partial mortality, possibly caused by stress due to suffocation.Predation on the explants was already observed even after a week from transplantation. Only predation by J. funebris was observed every monitoring (Figure 11) while predation of fish was not observed. Bite marks on a number of colonies were also observed during monitoring. After 6 months, 70% of the explants survived and were healthy. Mean growth rate of healthy colonies collected after 6 months was 17.5 g per month (SD ± 6.4).11FIGUREJorunna funebris feeding on a blue sponge explant using the type 2 method.Biofouling on the PVC frames and on the surface of some explants were observed immediately a week from deployment (Figure S4b and c). However, a major observation was an increased level of biofouling intensity in March that lasted up to the 2nd week of June. Because of that, weekly cleanup activities became necessary. Minimal biofouling was observed again in the 3rd week of July and in the last week of August. During this time, cleanup effort became less. Another major observation was the difference in the biofouling intensity among the mariculture setups established in areas with different adjacent substrate types and abundance of fishes, which were observed to feed directly on fouling algae (Figure S5). This prompted us to measure adjacent coral cover and fish abundance, which results are presented in the next section.Adjacent coral cover and fish abundance on biofouling intensitySetup B, which was established in a sandy area with the lowest adjacent coral cover (Figure 12), always had the highest biofouling intensity every monitoring (Figure S5a). Setups A and C, which had higher adjacent coral covers, always had less biofouling (Figure S5b). In terms of fish abundance, setup A and C had significantly higher fish abundance than setup B (ANOVA, Tukey's test, p < 0.05) (Figure 13). Among the fishes observed, damselfishes (Pomacentridae) differed significantly among setups (ANOVA, p < 0.05) with setup B having a significantly lower damselfish abundance (Tukey's test, p < 0.05). Damselfishes were observed to frequent the setups and feed on algae attached to the PVC frames and explants, and were obviously residents of the adjacent coral colonies (Figure S6).12FIGUREPercentage adjacent (within 2 m from each setup) coral cover of the three mariculture setups.13FIGUREMean abundance (±SD) of the different fish families observed in the mariculture setups computed using four replicates.Phase 3 mariculture trials: floating methodAll explants in the floating method survived after 4 months. However, explants that were deployed near the surface (1 m depth) were severely fouled by algae days after transplantation (Figure 14a). Unlike fouling on the explants transplanted in the improved string method, which can be taken off via fanning (Figure S7), algae that fouled on the floating method attached better and were hard to remove without scraping the surface of the explants. However, explants that were deployed at 5 m depth were all healthy and were not severely biofouled as compared with those deployed at 1 m depth (Figure 14b). In the 4‐month monitoring done using the floating method, predation by J. funebris was not observed.14FIGUREDifference between the biofouling intensity of explants transplanted (a) near the surface and (b) at 5‐m depth.Growth (wild vs. mariculture)After two months, one of the three wild colonies tagged for branch growth measurement was lost probably due to predation or dislodgement. Three of the remaining six tagged branches were obviously preyed by J. funebris. All the tagged explants in the mariculture setups were still alive with all tagged branches still present. Branch growth was higher for the explants (2.05 cm ± SD 0.23/2 months) compared with wild colonies (0.25 cm ± SD 0.21/2 months) (Figure 15). ANOVA revealed significant difference (ANOVA, p < 0.05).15FIGUREMean tip growth (±SD) of colonies in the wild and explants in the mariculture setup, generated using tip growth of three mariculture and two wild colonies with three tips per colony. The Kruskal–Wallis test showed significant difference between the treatments (p < 0.05).DISCUSSIONSize–frequency distribution and abundanceThe variation in the size–frequency distribution and abundance may indicate variation in population health. Blue sponge colonies were more abundant in the embayed area and were generally larger compared with colonies outside that bay. In other colonial invertebrates (e.g., corals), size–frequency distributions are often used to assess the health of a population in an area (e.g., Adjeroud et al., 2015; Lalas et al., 2021). Variations in size–frequency distribution of populations may indicate variations in historical events in different areas that have implications on juvenile input and colony longevity (Bak and Meesters, 1998). Lower abundance and dominance of small colonies outside the bay may indicate that colonies were not able to grow and survive under the given environmental conditions. Inside the bay, the slight negative skewness of the size–frequency distribution suggests that the population is generally represented by large colonies, but with small colonies still having representatives. This indicates that colonies produce juveniles with higher colony longevity. Thus, this can also indicate a healthier blue sponge population in stations inside the bay.Differences in the abundance and size–frequency distribution of the blue sponge colonies among stations can be attributed to a mix of different biotic and abiotic factors (Duckworth and Wolff, 2007). In the study of Duckworth et al. (2009), abundance of the sponge, Coscinoderma matthewsi, differed at different spatial scales, including sites that are 200 m apart, which was less than the distance between our survey stations. Since the nineties, studies have highlighted pollution inside Puerto Galera Bay due to domestic wastes and restricted circulation causing low flushing rates (Gomez et al., 1994; San Diego‐McGlone et al., 1995). The study by San Diego et al. (1995) showed the formation of high nutrient pool in the area because of low flushing rates and high nutrient input from freshwater runoff and wastewater discharge from land‐based sources. In 2009, Iizuka and colleagues highlighted contaminated coastal waters due to poorly constructed sanitation and household facilities and runoff from the hillsides associated with the tourism development. Many sponges have been shown to benefit from polluted waters (Rose and Risk, 1985) and the blue sponge may be one of them. Furthermore, sponge microbiomes are said to be stable under high nutrient conditions, which means that they are able to withstand eutrophication pressure (Baquiran and Conaco, 2018). Iizuka et al.’s (2009) study also highlighted weak tidal currents and associated flow in the bay area. As with many filter‐feeding invertebrates, the blue sponge may benefit from these conditions making them more abundant and grow faster (see Gokalp et al., 2021). The blue sponge may also benefit from the relatively low turbulence inside the bay. High exposure of reefs outside the bay to waves and typhoons passing the area (Licuanan, 1991) may prevent colonies from growing tall, similar to tall branching formed colonies inside the bay. In Panama, Wulff (1995) highlighted the loss of sponge biomass and an increase in the abundance of small colonies after a storm. The dominance of small‐sized colonies of tetillid sponges in the northwestern Philippines was also hypothesized to be caused by strong wave action, which can limit growth or cause tissue loss and mortality (Baquiran et al., 2020). The weaker water movement inside the bay may also give more chance to dislodged colonies or detached fragments to attach to an available substrate, which is a common dispersal strategy of sessile organisms (Crisp, 1955; Rodriguez et al., 1993). High water flow can also damage sponges, remove tissue and decrease size (Trautman et al., 2000). The greater available space, such as rocks and dead corals, may also provide a space for settlement that has lesser chance of competition with other sessile organisms (i.e., corals). For the in situ mariculture trials that aim to increase biomass production of any organism, the most practical choice is to establish mariculture setups in areas where members of a population grow larger and more abundant.Observations of wild coloniesMajor factors observed that affected the survival and mortality of the blue sponge were predation, dislodgement and an unknown cause of partial mortality that was disease‐like in appearance (Figure 8b). The blue sponge was observed to have a lone predator, the nudibranch, J. funebris (Figure 8a). It was also observed to be the main predator of other blue sponges in other regions, such as in Hainan, China (Wu et al., 2021) and India (Fontana et al., 2000). Dislodgement was also observed in the study site. Whole colonies or fragments that were unattached to the substrate, which were obviously dislodged, were observed. This can be attributed to the relatively fragile morphology of the blue sponge colony, which can easily break when there is high water movement, such as caused by waves and underwater currents. This might also explain why blue sponges are not abundant or even absent in stations outside the bay, which are more exposed to waves, especially when there are typhoons passing the area. Another factor that contributed to whole and partial colony mortality was the occurrence of white colouration on parts of the colonies that eventually disintegrated. One of the possible causes of this is disease, which stresses out colonies and eventually kills a part or the whole colony. A similar discolouration was also reported in the Caribbean sponge, Amphimedon compressa (Angermeier et al., 2012). The discolouration, called ‘sponge white patch’, was characterized by distinctive white patches found irregularly throughout the diseased sponges and showed severe degradation of tissues. Meanwhile, the barrel sponge Xestospongia muta, a congener of the target blue sponge, was observed to be affected by the sponge orange band disease, characterized by a reddish‐brown colouration, which causes a destruction of the pinacoderm (Angermeier, et al., 2010). However, the cause of these sponge diseases remains unidentified. The white colouration that was observed was the same with the appearance when fragments are stressed due to suffocation when not aerated properly or when they were removed underwater and exposed to air for a long time. Although the factors listed were major factors that contributed to blue sponge survival and growth, information on the different aspects of its life history is still unknown (e.g., reproduction, eco‐physiology). Thus, more studies can be conducted to further understand blue sponge biology and ecology.Development of an open sea mariculture protocolAn ideal mariculture setup should be easy to construct, deploy, maintain, where explant mortality is reduced, growth is increased, and can easily be retrieved. From a general perspective, the string method had the highest number of explant survivors among the methods used. However, there was no clear pattern on the variations in the wet weight increase among treatments. This can be attributed to the fact that the effect of other factors (e.g., predation, biofouling and competition) were not minimized due to the lack of maintenance activities for 11 months because of travel restrictions caused by the COVID‐19 pandemic. However, descriptive observations on the status of the setups after 11 months gave us important information on blue sponge ecology, as well as ideas for improvement on the mariculture method. The bag method, which was also used in previous mariculture trials of the blue sponge (Santiago et al., 2019), may be protected from predators. However, the greater surface area of the bags where algae can attach to resulted to heavy biofouling, thus requiring more maintenance effort. The explants inside the bags also had the tendency to be overgrown by other organisms (e.g., tunicates, bivalves) that might have entered the bags through settling during their larval phase. The tube method performed better, but the limited substrate available may have limited explant growth. Also, the explants had a tendency to grow beyond the tubes they were attached to and encrust on the polyethylene screen. Encrusting blue sponges in the polyethylene screen were challenging to collect. Aside from the challenges in collection, the use of polyethylene plastic screen and additional PVC pipes to be used as sponge substrate would require additional expenses. The simplest method among the three methods tested was the string method. However, an explant with no substrate to grow on might limit its growth, since blue sponge colonies are observed to grow by encrusting substrate. However, unintentionally, some explants that were transplanted using the string method had strings that were long enough, which allowed them to attach to the PVC frames that resulted to having an artificial substrate, resulting to healthier explants with apparent high growth rates. Thus, it was decided to develop an improved string method, which was the ‘improved string method’ used in the 2nd phase of the mariculture trials. Even though the farming methods tested in the 1st phase were rejected, the initial trials were very essential to the improvement of the later versions used.The number of oscula during transplantation may also be a factor in the survival of the explants. Results show that number of survivors were higher on explants transplanted with three or more oscula compared with explants transplanted with less than three oscula, regardless of farming method used (Figure 9). The osculum is the most obvious feature in sponges, which is a chimney‐like structure where water is pumped out after being filtered by the sponge body (Leys et al., 2011). The osculum is also a very important structure in a sponge because it was shown to induce development of colony's aquiferous system (Windsor and Leys, 2010). Ludeman et al. (2014) suggested that non‐motile cilia in the osculum serve as sensory system to detect changes in flow and control the responses of the sponge colony as a whole. Thus, a blue sponge fragment collected with no osculum may have a lower growth rate and even a lesser chance of survival.The ‘improved string method’ was suitable for blue sponge farming. One criterion for usable transplantation methods is how fast explants can attach to an artificial substrate. In our trials, explants were observed to be attached to the PVC substrate 7 days from transplantation and have remained attached during the last monitoring activity. This indicates that the PVC pipes used are suitable as artificial substrates. Factors that were observed to be a potential source of stress of explants were fouling organisms and organisms that can attach on the artificial substrate and may compete for space. However, biofouling and attachment of space competitors were minimised by conducting more frequent monitoring and maintenance works. One of the challenges when using the improved string method was predation by J. funebris. Even though survival was high, there was still a need to improve the design of the farming medium.The ‘floating method’ was an improvised version of the improved string method. Since J. funebris can only go to one spot to another through crawling, it cannot reach the hanging artificial substrate. Thus, this design prevented predation, which was the main challenge in the improved string method. An advantage of the floating setup is also that deployment, maintenance, and collection can be done without SCUBA diving. Thus, may also have an advantage through lower costs. However, just like other methods used in sponge farming (Duckworth, 2009), the floating method has its disadvantage, which is mainly its higher exposure to winds and waves that can destroy the floating setup. Thus, it is important to consider different factors (i.e., ecological, logistical, and financial) when selecting what method to use in sponge farming. Also, it might be worth maintaining setups using multiple methods so there are backups when unfavourable situations occur.Biofouling intensity of algae varied temporally and spatially. One of the major observations during the monitoring is the increased intensity of biofouling in March and decreased after the 2nd week of June. During this time, more frequent cleanups were needed. This increase in biofouling intensity was in line with the increased in temperature in March to June (Figure S8). Thus, biofouling intensity, which affects maintenance effort needed, was affected by seasonal variation in temperature, as also observed in studies that have shown seasonal algal blooms (Lüning and Dieck, 1989). This should be factored‐in in maintenance planning since biofouling on the surface of the sponges may be a cause for colony stress that can lead to mortality. Some parts of the explants that were observed to be heavily fouled were observed to eventually disintegrate. Another major observation is the variation in the biofouling intensity among setups established in different areas with different benthic assemblages. Generally, the setup established in the area with the lowest adjacent coral cover always had the highest biofouling intensity. It was observed that browsing fishes, which resided in adjacent coral colonies, can aid in minimising biofouling by algae. Browsing fishes have also been utilized by other studies to minimize biofouling (Bannister et al., 2019). The condition of the mariculture setup may also be affected by depth. In the floating method, the explants and artificial substrate that were hung near the water surface showed very severe biofouling by algae as compared with those hanged in deeper water. This can be indirectly attributed to differences in depth, which may vary in light intensity (Mundy and Bobcock, 1998) and the browser abundance (Ferrari et al., 2018). Higher light intensity promotes algal growth as it enhances algal photosynthesis (Lüning and Dieck, 1989). The shallow part of the water column was also far from fish browsers. Fish browsers observed in the area were all residential and were observed not to move far from their territories. In the monitored wild colonies at deeper depth, heavy biofouling by algae was never observed, providing support for the influence of depth to the level of biofouling. This highlights proper depth selection when using hanging setups for the cultivation of sponges. High light levels on the shallower depths may be ideal for algae fouling than low‐light conditions at deeper water (Lewbel et al., 1987). Thus, careful planning on site selection for a mariculture area should be done and depth where explants should be suspended to increase chances of producing healthier and fast‐growing explants.Growth comparison (mariculture vs. wild)Transplanted fragments grew faster than colonies in the wild. This is one of the positive attributes of collecting fragments from the wild. The result was also similar to other species, where transplanted fragments had faster growth rates and often doubled their sizes after few months from transplantation (Duckworth, 2009). The difference of the growth rates might also be caused by the difference in the growth rates of sponges at different size classes. A number of studies highlighted decreasing growth rates as sponge age or as size increases (Cardone et al., 2010; Singh and Thakur, 2015). This needs to be further investigated. However, the results in the present study shows the advantage of sponge mariculture through transplantation.General criteria for the mariculture establishment site selectionBased on the observations, we came up with a protocol in site selection to increase the chances of success in the open sea farming. In selecting a site for mariculture establishment, different factors should be considered. (1) A mariculture site should be established in an area or adjacent to an area where a natural population of the selected species are abundant. This is to ensure that the environmental condition of an area is ideal for the species, which will increase the chances of survival and growth. It is also important to note that although renieramycin content was not measured here, previous work (Santiago et al., 2019) has shown that renieramycin content does not vary among setups, as long as mariculture is done in the same area. (2) A mariculture area should be easily accessed by maintenance workers. This is to minimise time of travel and maintenance cost. (3) A mariculture site should be established in an area with minimal exposure to wind and waves, such as in embayed areas and lagoons. This is to lessen chances of destruction, especially during typhoon season. However, this will depend on the environmental preferences of a species. (4) A mariculture setup should also be established at a depth similar to the depth range where natural populations of the selected species are abundant to ensure ideal environmental conditions. Ideally, setups should be established at the shallowest depth suitable for the species for easier maintenance. A greater depth would mean faster air consumption of SCUBA divers. Thus, working at a shallower depth would mean relatively more time for SCUBA divers to do maintenance work underwater. Last, (5) a mariculture setup should be established in a specific area where damage to benthic communities (e.g., corals) will be avoided (e.g., sandy, rocky). However, it is also important to establish setups in an area with adjacent coral colonies. Coral colonies usually have resident browsers (e.g., fishes) or can attract browsers that can help minimize biofouling by algae on the setups.ConclusionsThis study has shown insights on the distribution, ecology and the mariculture potential of the Philippine blue sponge, Xestospongia sp. We conclude that blue sponge colonies were more abundant and had bigger sizes in an area characterized by high nutrient levels and low exposure, indicating ideal environmental conditions for blue sponge growth. Higher growth rate of transplanted explants also justified the benefits of sponge transplantation. In general, this study has shown the feasibility of the use of PVC pipes as an artificial substrate for blue sponge transplantation, as long as proper site selection and frequent monitoring and maintenance works are done.AUTHOR CONTRIBUTIONJue Alef Lalas: Conceptualization; Data curation; Formal analysis; Investigation; Methodology; Writing – original draft; Writing – review & editing. Geminne Manzano: Conceptualization; Data curation; Investigation; Methodology; Writing – original draft; Writing – review & editing. Lee Arraby Desabelle: Data curation; Investigation; Methodology; Writing – original draft; Writing – review & editing. Lilibeth Salvador‐Reyes: Conceptualization; Funding acquisition; Project administration; Resources; Supervision; Validation; Writing – original draft; Writing – review & editing. Porfirio Aliño: Conceptualization; Methodology; Project administration; Supervision. Maria Vanessa Rodriguez: Conceptualization; Methodology; Project administration; Resources; Supervision; Validation; Writing – original draft; Writing – review & editing.ACKNOWLEDGEMENTSThis study was funded by the Philippine Council for Health Research and Development of the Department of Science and Technology (DOST‐PCRHD) through the ‘Anti‐infective and Anticancer Drug Candidates from Marine Microorganisms and Sponges: Discovery and Development’ project of the ‘Discovery and Development of Health Products’ program, which was implemented by the Marine Science Institute of the University of the Philippines. Samples were collected with the permission of the local government of the Municipality of Puerto Galera and the Bureau of Fisheries and Aquatic Resources of the Department of Agriculture of the Philippines through Gratuitous Permit no. 0196‐20. We thank the researchers and volunteers that helped establish the initial setups, namely, Joey Cabasan, Daryll Valino, Maxine Prado, Romina Lim, Marc Punzalan and Czarmayne Escoro. We also thank the people of the Municipality of Puerto Galera for their continuous support for research projects in the area.CONFLICT OF INTERESTThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.FUNDING INFORMATIONPhilippine Council for Health Research and Development of the Department of Science and Technology (DOST‐PCRHD) through the ‘Anti‐infective and Anticancer Drug Candidates from Marine Microorganisms and Sponges: Discovery and Development’ project of the ‘Discovery and Development of Health Products’ program.DATA AVAILABILITY STATEMENTData inquiries can be directed to the authors.ETHICS STATEMENTThe activities and specimen collections for this study were approved through the Prior Informed Consent Certificate issued through Municipal Resolution No. 2019 by the Office of the Municipal Mayor of Puerto Galera, and the Gratuitous Permit (No. 0196‐20) issued by the Department of Agriculture of the Republic of the Philippines.PEER REVIEWThe peer review history for this article is available at: https://publons.com/publon/10.1002/aff2.98REFERENCESAdjeroud, M., Mauguit, Q., Penin, L. 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Journal

Aquaculture Fish and Fisheries – Wiley

Published: Apr 1, 2023

Keywords: mariculture; open‐sea farming; Philippines; Porifera; sponge

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