Sponge erosion under acidification and warming scenarios: differential impacts on living and dead coral

Ocean acidification will disproportionately impact the growth of calcifying organisms in coral reef ecosystems. Simultaneously, sponge bioerosion rates have been shown to increase as seawater pH decreases. We conducted a 20‐week experiment that included a 4‐week acclimation period with a high number of replicate tanks and a fully orthogonal design with two levels of temperature (ambient and +1 °C), three levels of pH (8.1, 7.8, and 7.6), and two levels of boring sponge (Cliona varians, present and absent) to account for differences in sponge attachment and carbonate change for both living and dead coral substrate (Porites furcata). Net coral calcification, net dissolution/bioerosion, coral and sponge survival, sponge attachment, and sponge symbiont health were evaluated. Additionally, we used the empirical data from the experiment to develop a stochastic simulation of carbonate change for small coral clusters (i.e., simulated reefs). Our findings suggest differential impacts of temperature, pH and sponge presence for living and dead corals. Net coral calcification (mg CaCO₃ cm^?2 day^?1) was significantly reduced in treatments with increased temperature (+1 °C) and when sponges were present; acidification had no significant effect on coral calcification. Net dissolution of dead coral was primarily driven by pH, regardless of sponge presence or seawater temperature. A reevaluation of the current paradigm of coral carbonate change under future acidification and warming scenarios should include ecologically relevant timescales, species interactions, and community organization to more accurately predict ecosystem‐level response to future conditions.     

Population structure and dispersal of the coral‐excavating sponge Cliona delitrix

Some excavating sponges of the genus Cliona compete with live reef corals, often killing and bioeroding entire colonies. Important aspects affecting distribution of these species, such as dispersal capability and population structure, remain largely unknown. Thus, the aim of this study was to determine levels of genetic connectivity and dispersal of Cliona delitrix across the Greater Caribbean (Caribbean Sea, Bahamas and Florida), to understand current patterns and possible future trends in their distribution and effects on coral reefs. Using ten species‐specific microsatellite markers, we found high levels of genetic differentiation between six genetically distinct populations: one in the Atlantic (Florida‐Bahamas), one specific to Florida and four in the South Caribbean Sea. In Florida, two independent breeding populations are likely separated by depth. Gene flow and ecological dispersal occur among other populations in the Florida reef tract, and between some Florida locations and the Bahamas. Similarly, gene flow occurs between populations in the South Caribbean Sea, but appears restricted between the Caribbean Sea and the Atlantic (Florida‐Bahamas). Dispersal of C. delitrix was farther than expected for a marine sponge and favoured in areas where currents are strong enough to transport sponge eggs or larvae over longer distances. Our results support the influence of ocean current patterns on genetic connectivity, and constitute a baseline to monitor future C. delitrix trends under climate change.     

Reproductive cycle of the coral-excavating sponge Thoosa mismalolli (Clionaidae) from Mexican Pacific coral reefs

Abstract. Individuals of the recently described demosponge Thoosa mismalolli are common on Mexican Pacific coral reefs, excavating burrows in living corals and in other calcareous substrata. To better understand the propagative abilities of this sponge, we conducted a histological study over an 18-month period (May 2007–November 2008) to identify sexual and asexual reproductive structures. Members of the species are viviparous and hermaphroditic, with various developmental stages of oocytes, spermatic cysts, and embryos co-occurring in the mesohyl for most of the year. This nearly continuous reproductive activity intensified during the warm season. Fertilization was internal, and embryos developed inside the parental sponge to produce an unciliated hoplitomella larva, characterized by a peculiar siliceous skeleton. In addition to the sexually generated larvae, adults of T. mismalolli formed gemmules for asexual reproduction. Gemmules occurred within the mesohyl during all months of the year, but were most abundant in the coldest months. This combination of sexual and asexual processes enables individuals of T. mismalolli to reproduce almost continuously. This strategy may facilitate both long-term persistence within reefs and effective dispersal between distant reefs.     

Population genetics of the fissiparous holothurian Stichopus chloronotus (Aspidochirotida) on the Great Barrier Reef, Australia

 Population genetic structure was studied in one nearshore and two offshore populations of Stichopus chloronotus, a common holothurian species on Indo-Pacific coral reefs. Genetic variation at five polymorphic loci was examined using allozyme electrophoresis. The nearshore population consisted almost exclusively of male individuals, and more males than females were found in all populations studied. Deviations of heterozygosity from that predicted under Hardy-Weinberg equilibrium indicated that asexual reproduction occurred in all populations. Estimates of the level of asexual reproduction using the ratios of the number of sexually produced individuals to sample size, observed genotypic diversity to expected genotypic diversity, and number of genotypes to sample size confirmed that this reproductive mode was more important at the nearshore reef compared to the two offshore reefs. There were large differences in genotypic frequencies between males and females. F-statistics on clonal genotypic frequencies were not statistically significant between populations for neither females or males, suggesting high dispersal of larvae between reefs. A higher mortality of females during larval or early post-settlement stages, or reduced dispersal capability of female larvae are the most likely reasons for biased sex ratios. Accepted: 23 November 1998     

Microsatellite loci isolated from the Caribbean coral,Montastraea annularis

We report the isolation and characterization of seven microsatellite loci from the Caribbean reef‐building coral, Montastraea annularis. All loci are polymorphic with allele numbers ranging from five to 31 and observed heterozygosities from 0.17 to 0.89. These loci can be used in assessing gene flow patterns and diversity of this stony coral species both for local coral reef management purposes as well as for elucidating population connectivity within the greater Caribbean basin. These markers should also be applicable to other species of Montastraea and for resolving taxonomic relationships within the M. annularis species complex.     

Elucidating the sponge stress response; lipids and fatty acids can facilitate survival under future climate scenarios

Ocean warming (OW) and ocean acidification (OA) are threatening coral reef ecosystems, with a bleak future forecast for reef‐building corals, which are already experiencing global declines in abundance. In contrast, many coral reef sponge species are able to tolerate climate change conditions projected for 2100. To increase our understanding of the mechanisms underpinning this tolerance, we explored the lipid and fatty acid (FA) composition of four sponge species with differing sensitivities to climate change, experimentally exposed to OW and OA levels predicted for 2100, under two CO₂ Representative Concentration Pathways. Sponges with greater concentrations of storage lipid, phospholipids, sterols and elevated concentrations of n‐3 and n‐6 long‐chain polyunsaturated FA (LC PUFA), were more resistant to OW. Such biochemical constituents likely contribute to the ability of these sponges to maintain membrane function and cell homeostasis in the face of environmental change. Our results suggest that n‐3 and n‐6 LC PUFA are important components of the sponge stress response potentially via chain elongation and the eicosanoid stress‐signalling pathways. The capacity for sponges to compositionally alter their membrane lipids in response to stress was also explored using a number of specific homeoviscous adaptation (HVA) indicators. This revealed a potential mechanism via which additional CO₂ could facilitate the resistance of phototrophic sponges to thermal stress through an increased synthesis of membrane‐stabilizing sterols. Finally, OW induced an increase in FA unsaturation in phototrophic sponges but a decrease in heterotrophic species, providing support for a difference in the thermal response pathway between the sponge host and the associated photosymbionts. Here we have shown that sponge lipids and FA are likely to be an important component of the sponge stress response and may play a role in facilitating sponge survival under future climate conditions.     

Role of deep sponge grounds in the Mediterranean Sea: a case study in southern Italy

The Mediterranean spongofauna is relatively well-known for habitats shallower than 100 m, but, differently from oceanic basins, information upon diversity and functional role of sponge grounds inhabiting deep environments is much more fragmentary. Aims of this article are to characterize through ROV image analysis the population structure of the sponge assemblages found in two deep habitats of the Mediterranean Sea and to test their structuring role, mainly focusing on the demosponges Pachastrella monilifera Schmidt, 1868 and Poecillastra compressa (Bowerbank, 1866). In both study sites, the two target sponge species constitute a mixed assemblage. In the Amendolara Bank (Ionian Sea), where P. compressa is the most abundant species, sponges extend on a peculiar tabular bedrock between 120 and 180 m depth with an average total abundance of 7.3 ± 1.1 specimens m⁻² (approximately 230 gWW m⁻² of biomass). In contrast, the deeper assemblage of Bari Canyon (average total abundance 10.0 ± 0.7 specimens m⁻², approximately 315 gWW m⁻² of biomass), located in the southwestern Adriatic Sea between 380 and 500 m depth, is dominated by P. monilifera mixed with living colonies of the scleractinian Madrepora oculata Linnaeus, 1758, the latter showing a total biomass comparable to that of sponges (386 gWW m⁻²). Due to their erect growth habit, these sponges contribute to create complex three-dimensional habitats in otherwise homogenous environments exposed to high sedimentation rates and attract numerous species of mobile invertebrates (mainly echinoderms) and fish. Sponges themselves may represent a secondary substrate for a specialized associated fauna, such zoanthids. As demonstrated in oceanic environments sponge beds support also in the Mediterranean Sea locally rich biodiversity levels. Sponges emerge also as important elements of benthic–pelagic coupling in these deep habitats. In fact, while exploiting the suspended organic matter, about 20% of the Bari sponge assemblage is also severely affected by cidarid sea urchin grazing, responsible to cause visible damages to the sponge tissues (an average of 12.1 ± 1.8 gWW of individual biomass removed by grazing). Hence, in deep-sea ecosystems, not only the coral habitats, but also the grounds of massive sponges represent important biodiversity reservoirs and contribute to the trophic recycling of organic matter.     

Bacterial Diversity Associated with Cinachyra cavernosa and Haliclona pigmentifera,Cohabiting Sponges in the Coral Reef Ecosystem of Gulf of Mannar,Southeast Coast of India

Sponges are abundant, diverse and functionally important organisms of coral reef ecosystems. Sponge-associated microorganisms have been receiving greater attention because of their significant contribution to sponge biomass, biogeochemical cycles and biotechnological potentials. However, our understanding of the sponge microbiome is limited to a few species of sponges from restricted geographical locations. Here, we report for the first time the bacterial diversity of two cohabiting sponges, viz. Cinachyra cavernosa and Haliclona pigmentifera, as well as that in the ambient water from the coral reef ecosystems of the Gulf of Mannar, located along the southeast coast of India. Two hundred and fifty two clones in the 16S rRNA gene library of these sponges were grouped into eight distinct phyla, of which four belonged to the core group that are associated only with sponges. Phylogenetic analysis of the core bacteria showed close affinity to other sponge-associated bacteria from different geographical locations. γ-Proteobacteria, Chloroflexi, Planctomycetes and Deferribacter were the core groups in C. cavernosa while β and δ-Proteobacteria performed this role in H. pigmentifera. We observed greater OTU diversity for C. cavernosa (H^ǀ 2.07) compared to H. pigmentifera (H^ǀ 1.97). UniFrac analysis confirmed the difference in bacterial diversity of the two sponge species and also between the sponges and the reef water (p<0.001). The results of our study restate the existence of a host driven force in shaping the sponge microbiome.     

Genetic differentiation amongst populations of the coral Pocillopora damicornis off southwestern Australia

An electrophoretic study of genetic differentiation amongst local populations of the reef-coral Pocillopora damicornis was used to group coral heads into units defined as the area of effective dispersal of a clone, and termed colonies. For reefs off southwestern Australia, colonies were usually under a few hundred metres in extent. Although most new recruits within a colony were derived asexually, sexually produced propagules acted to connect populations outside the boundaries of a colony. Such connections were weak, and allelic frequencies varied considerably over a few kilometres. The primary agent of genetic differentiation was suggested to be the small effective population size resulting from the asexual proliferation of a few genotypes at any site. The effective number of genotypes per colony was approximately six. Asexual reproduction appears also to limit gene flow and accentuate selection in this species.     

Plectroninia celtica n. sp. (Calcarea, Minchinellidae), a new species of “pharetronid” sponge from bathyal depths in the northern Porcupine Seabight, NE Atlantic

Recent pharetronid sponges were regarded as relict species in tropical and subtropical waters, inhabiting cryptic habitats on coral reefs and in caves. More recent findings of a new species of the genus Plectroninia off northern Norway, with an inner fused skeleton have changed that view. Recent investigations on the sponge fauna of the “Propeller Mound”, northern Porcupine Seabight, focusing on sponges growing on the azooxanthellate cold-water coral Lophelia pertusa (Linné 1758) and Madrepora oculata Linné 1758, established the presence of a species of Plectroninia new to science. Its status as a common species within this deep-water coral habitat and the general status of the genus Plectroninia are discussed.     

Fragments or propagules? Reproductive tradeoffs among Callyspongia spp. from Florida coral reefs

Fragmentation and propagule formation are alternative reproductive strategies found in both plants and animals, with the latter generally providing greater dispersal capability. When both strategies occur, life history theory predicts that resources should be divided between the two. On coral reefs, both strategies are exhibited by branching corals and sponges, which are broken‐up after storm events and rapidly recolonize. In this study, we compared two congeneric Caribbean reef sponges, Callyspongia armigera, which is branched and easily fragmented, and C. vaginalis, which is not, to test whether there is a tradeoff in growth and propagule formation for C. armigera relative to C. vaginalis. Both species were equally abundant on coral reefs off Key Largo, Florida (10.1 ± 3.7 vs 11.9 ± 3.0 per 100 m², respectively), suggesting that they are equally successful relative to two other non‐fragmenting congeneric species (C. fallax and C. plicifera) that are much less common. The number of substratum attachment points per sponge was significantly higher for C. armigera compared to C. vaginalis (2.31 ± 1.47 vs 1.03 ± 0.18 sponge^?1), providing further evidence of the reliance of C. armigera on fragmentation, and of C. vaginalis on recruitment from larval settlement and subsequent growth. Growth rates in predator‐exclusion experiments were ～4‐fold higher for C. armigera compared to C. vaginalis (0.36 ± 0.31 vs 0.08 ± 0.11 % initial mass day^?1), but C. armigera produced ～13‐fold fewer propagules than C. vaginalis (0.04 ± 0.22 vs 0.53 ± 1.08 % tissue area). Our results support a tradeoff between growth and propagule output for C. armigera relative to C. vaginalis, suggesting that these closely related sponge species took different evolutionary trajectories in reconciling their resource constraints.     

Finding the relevant scale: clonality and genetic structure in a marine invertebrate (Crambe crambe, Porifera)

Important changes in genetic relatedness may occur at extremely small scales in benthic invertebrates, providing key information about structuring processes in populations of these organisms. We performed a small-scale study of the population structure of the sponge Crambe crambe, in which 177 individuals from the same rocky wall (interindividual distances from 0 to 7 m) were genotyped using six microsatellite markers. 101 sponges had unique genotypes and the remaining 76 individuals formed 24 groups of sponges sharing genotypes (clones). Mean intraclone distances were found to be c. 20 cm. Spatial autocorrelation analyses showed a drastic decrease in genetic relatedness over the first 100 cm of distance. If the contribution of clonality to this pattern was eliminated, the trend was attenuated, but remained a marked one and was still significant within the first distance classes (30-40 cm). Estimated mean dispersal distances per generation were c. 35 cm, and neighbourhood sizes were estimated at c. 33 sponges. Genetic similarities with sponges of the same locality, or from other Mediterranean localities, were within the same range as those found in sponges 2-7 m apart. It is concluded that asexual reproduction plays an important role in structuring populations in this species. However, over and above the effects of clonality, a strong fine-scale genetic structure was present at distances in the range of tens of centimetres, probably as a result of short dispersal of larvae. This fine-scale genetic structure may be common in invertebrates with lecitotrophic larvae.     

Pione lampa,a bioeroding sponge in a worm reef

A bioeroding sponge was found in a sabellariid worm reef in Florida, USA. The sponge was investigated with standard techniques for species identification (tissue digestion, microscopy techniques, examination of spicules) and was identified as the clionid Pione lampa (De Laubenfels, 1950). This species is extremely similar to Pione vastifica (Hancock, 1849). Morphological and ecological similarities and possible dissimilarities are discussed. Arguments for conspecifity were found to be weaker than evidence of P. lampa and P. vastifica being separate species. The sampled sponge contained asexual reproductive elements: superficial buds and internal gemmules. Buds were very delicate, consisting mainly of a radially-oriented mesh of collageneous threads. They further comprised abundant granular material and acanthose microrhabds. Numerous choanosomal gemmules were found in the sponge body attached to erosion chamber walls. They were subspherical to lenticular and had an aspicular, unstructured, smooth and rigid coat. It enclosed dense cell material and various spicule types. Whereas buds are interpreted to function as dispersal elements, gemmules will primarily ensure survival under adverse conditions such as smothering, exposure to air and high temperatures. Gemmules from this site might occasionally be freed and scattered, since the Florida reef can suffer heavy damage during periods of increased wave activity such as that created during hurricane season. Bioerosion activity of the sponge increases the chance to free gemmules, as the sponge not only etches into calcareous particles cemented into the matrix produced by the worms, but also into the matrix itself. This ability enables the sponge to utilise the reef as substrate. Within the Clionidae, Cliona annulifera and three species of the genus Pione are the only species known to produce typical gemmules. Possible reasons are: (1) reproduction of bioeroding sponges is understudied and asexual bodies may have been overlooked in other species, and (2) clionid gemmules are an adaptation to survive life in risky environments. Sponges of the genus Pioneare comparatively successful in environments in which they are close to their physical limits or in potentially unstable or mobile substrates.     

Reproductive patterns in three species of large coral reef sponges

The type and frequency of reproduction of three common sponge species was monitored histologically and by direct field observations of spawning events over 2 successive years at the reefs of Curaçao. Ircinia strobilina showed year round reproductive activity by the production of spermatic cysts in varying intensities in a major part of the population. Production of oocytes and larvae were only observed in the period from September through April, indicating an actual breeding season of 8 months a year. The sexes in this viviparous sponge seem to be separate, but protandry cannot be ruled out. Less than 10% of the sponge tissue was found to consist of reproductive elements. Neofibularia nolitangere is a gonochoristic oviparous sponge with a short annual breeding season of 2 months, in which two successive, highly predictable, short spawning periods occurred synchronously for the whole population. Up to half of the sponge tissue was transformed to reproductive elements. Agelas clathrodes showed low reproductive activity. Field observation suggests that this sponge is an oviparous hermaphrodite, probably annually releasing male and female gametes synchronously in a varying number of individuals of its population during a short breeding period.The variation in reproductive patterns in this small group of large massive coral reef sponges spans a range similar to that found in the morphologically highly variable group of stony corals and illustrates their ecological differentiation on the reef.     

Skeletal response of <Emphasis Type="Italic">Lophelia pertusa</Emphasis> (Scleractinia) to bioeroding sponge infestation visualised with micro-computed tomography

The skeleton morphology of the azooxanthellate cold-water coral Lophelia pertusa can be strongly influenced by invasive boring sponges that infest corallites in the still living part of the colony. Atypically swollen corallites of live Lophelia pertusa from the Galway Mound (Belgica Carbonate Mound Province, Porcupine Seabight, NE Atlantic), heavily excavated by boring organisms, have been examined with a wide range of non-destructive and destructive methods: micro-computed tomography, macro- and microscopic observations of the outer coral skeleton, longitudinal and transversal thin sections and SEM analyses of coral skeleton casts. As a result, three excavating sponge species have been distinguished within the coral skeleton: Alectona millari, Spiroxya heteroclita and Aka infesta. Furthermore, four main coral/sponge growth stages have been recognised: (1) cylindrical juvenile corallite/no sponge cavities; (2) flared juvenile corallite/linear sponge cavities (if present); (3) slightly swollen adult corallites/chambered oval sponge cavities; (4) very swollen adult corallites/widespread cavities. The inferred correlation between corallite morphology and boring sponge infestation has been detected in micro-computed tomography (micro-CT) images and confirmed in sponge trace casts and peculiar features of coral skeleton microstructure. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorised users.     

Specificity of larval settlement of the Caribbean Orange Icing Sponge,Mycale laevis

The Caribbean sponge Mycale laevis is often found growing in close proximity to living scleractinian corals. This commonly observed sponge–coral association has been considered a mutualism, with the coral providing substratum for the sponge, and the sponge protecting the coral skeleton from boring organisms. We examined the specificity of sponge recruitment to live corals, expecting a positive and specific settlement response if a mutualism exists. Benthic surveys conducted off Key Largo, Florida, and Bocas del Toro, Panama, revealed that individuals of M. laevis grew on substrata that included dead coral and other species of sponges. Selectivity analysis indicated that at three of the four survey sites, M. laevis was not randomly distributed, but associated with live corals more frequently than expected from proportional coral cover. However, settlement assays demonstrated that larvae of M. laevis did not preferentially respond to the presence of live coral. We have previously demonstrated that adults of M. laevis are chemically undefended and readily eaten by spongivorous fishes unless protected by adjacent substrata such as live corals. In overfished areas, where spongivore density is low, the sponge is not selectively distributed near corals. Initial results of settlement experiments with different substrata suggested that larvae of M. laevis responded positively to the presence of the chemically defended sponge Amphimedon compressa, perhaps indicating an associational defense. Further experiments revealed that larvae were reacting to artificially high concentrations of exudates from cut surfaces of Am. compressa; settlement was not enhanced in response to healed pieces of Am. compressa. In addition, the larvae of M. laevis did not selectively respond to live coral or to chemically defended heterospecifics. These results indicate that the commonly observed proximity of M. laevis to live corals is not driven by larval settlement behavior, but instead by post‐settlement mortality due to predation.     

Character release following extinction in a Caribbean reef coral species complex

Abstract The Pleistocene extinction of the widespread organ‐pipe Montastraea coral had measurable morphological and ecological effects on surviving lineages of the Montastraea“annularis” species complex. Extinction of the organ‐pipe Montastraea occurred after more than 500,000 years of dominance in the shallow‐water reef habitat of Barbados. Extinction resulted in a morphological shift of the columnar Montastraea lineage from thick to thin columns in modern reef environments. Pleistocene colonies of the columnar morphotype sympatric with organ‐pipe Montastraea showed greater column widths than those in allopatry. We subjected our data to a number of criteria for interpreting the morphological shift as character release following lifting of competitive pressure after extinction. The morphological differences do not appear to be due either to chance or to physical properties of the marine environment. Differential local extinction and recolonization of four members of the species complex did not occur on Barbados, so that the species coexisted and appear to have coevolved between more than 600,000 and 82,000 years ago. The morphological shift is related to coral growth form and growth rate, and thus reflects the acquisition of a primary resource in corals‐light. Character release occurred at the same oceanic Caribbean island (Barbados) where environments have fluctuated with similar variance throughout the period of coexistence. Not only has competition among living members of the Montastraea“annularis” species complex been convincingly demonstrated, but trends in relative abundance among fossil members of the species complex strongly suggest that a competitive hierarchy was operating during their Pleistocene coexistence on Barbados. We also observed an ecological analogue to character release on another Caribbean island, Curaçao. The distribution and abundance of living columnarM. annularis s.s. and massive M. faveolata from the leeward reef crest in Curaçao is greater now than in the Pleistocene, when organ‐pipe Montastraea dominated this shallow‐water reef habitat. Extinction of the faster growing, shallow‐water organ‐pipe Montastraea resulted in higher abundance of the columnar Montastraea lineage in shallow‐water habitats, where it shifted its morphology to one adapted to high light levels. The species extinction released surviving lineages from a competitive network that had resulted in lower rank abundance in the Pleistocene community and enhanced abundance of both columnar M. annularis s.s. and M. faveolata in modern communities. Full validation of our interpretation of character release must await experiments that demonstrate whether phenotypic differences between populations have a genetic basis. However, we believe the results of this study point to the important, yet heretofore neglected, role that biological interactions have played in the evolution of closely related reef coral species.     

Bitten down to size: Fish predation determines growth form of the Caribbean coral reef sponge Mycale laevis

Interactions between organisms add complexity to ecosystem function, particularly on coral reefs. The Caribbean orange icing sponge Mycale laevis is semi-cryptic, often growing under coral colonies or between coral branches. This association is reportedly a mutualism, with the sponge deterring boring sponges from invading the coral skeleton and the coral providing an expanding surface for sponge growth. But is there an alternative explanation for the proximity of sponge and coral? We examined the importance of fish predation on the growth of the sponge. While the semi-cryptic growth form of M. laevis predominates on reefs off the Florida Keys and the Bahamas Islands, M. laevis grows with a non-cryptic, erect morphology off Bocas del Toro, Panama. Surveys revealed that sponge-eating fishes were rare or absent at Bocas del Toro compared to sites in the Florida Keys. Because past studies were inconsistent about the palatability of M. laevis to fish predators, we conducted feeding experiments with sponges from all three sites. Crude organic extracts of M. laevis from all three sites were palatable to generalist fish predators in aquarium assays, and field feeding assays and caging experiments conducted in the Florida Keys confirmed that spongivorous fishes readily ate exposed fragments of M. laevis. Our results suggest that M. laevis is restricted to its semi-cryptic growth form by spongivorous predators, with corals providing a physical refuge from predation. This alternative explanation supports the broader hypothesis that Caribbean reef sponges can be categorized on the basis of chemical defense into defended, palatable, and preferred species, the last of which are restricted to refugia.     

Trophodynamics of the sclerosponge Ceratoporella nicholsoni along a shallow to mesophotic depth gradient

The sclerosponge Ceratoporella nicholsoni is a hyper-calcifying high microbial abundance sponge. This sponge has been observed at high densities throughout the Caribbean in the mesophotic zone (30–150 m), as well as cryptic environments in shallow (< 30 m) depths. Given the densities of this sponge, it could play an important role in the cycling of inorganic and organic sources of carbon and nitrogen at mesophotic depths. Additionally, there is broad interest in this sponge as a tool for paleobiology, paleoclimatology and paleoceanography. As a result, it is increasingly important to understand the ecology of these unique sponges in the underexplored Caribbean mesophotic zone. Here we show that this sponge increases in abundance from shallow depths into the mesophotic zone of Grand Cayman Island. We observed no significant differences in the stable isotope signatures of δ¹⁵N and δ¹³C of sponge tissue between depths. A predictive model of sponge diet with increasing depth shows that these sponges consume dissolved organic matter of algal and coral origin, as well as the consumption of particulate organic matter consistent with the interpretation of the stable isotope data. The taxonomic composition of the sclerosponge microbiome was invariant across the shallow to mesophotic depth range but did contain the Phylum Chloroflexi, known to degrade a variety of dissolved organic carbon sources. These data suggest that the depth distribution of this sponge may not be driven by changes in trophic strategy and is potentially regulated by other biotic or abiotic factors.