While sponges are well-known to be suspension feeders, consumption of dissolved organic carbon (DOC) has recently been highlighted as a mechanism whereby sponges may avoid food limitation. Further, the sponge-loop hypothesis proposes that sponges consume DOC and then release shed cellular detritus back to the reef benthos. We examined the carbon flux mediated by the giant barrel sponge, Xestospongia testudinaria, on reefs in the Red Sea across an inshore-offshore gradient that had previously been proposed to affect sponge nutrition in other parts of the tropics. Seawater samples were collected from the incurrent and excurrent flow of 35 sponges. Concentrations of total organic carbon and its components, DOC, live particulate organic carbon (LPOC), and detritus, were all significantly higher in incurrent seawater on inshore than offshore reefs. The diet of X. testudinaria was comprised primarily of DOC and detritus, with mean values across all reef sites of 61.5% DOC, 34.6% detritus, and 3.9% LPOC. Across the inshore-offshore gradient, there was evidence (1) of a threshold concentration of DOC (≈ 79 μmol C L seawater −1 ) below which sponges ceased to be net consumers of DOC, and (2) that sponges on offshore reefs were food limited, with a mean carbon deficit relative to sponges on inshore reef sites. Sponges on offshore reef sites exhibited higher pumping rates, perhaps indicating optimal foraging for POC. As previously demonstrated for Xestospongia muta, and contrary to the sponge-loop hypothesis, there was no evidence that X. testudinaria returned DOC to the benthos in the form of detritus.

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Recent studies have demonstrated that sponge-eating fishes alter the community of sponges on coral reefs across the Caribbean. Sponge species that lack chemical defenses but grow or reproduce faster than defended species are more abundant on reefs where sponge-eating fishes have been removed by overfishing. Does predatorremoval have an effect on the distribution of sponges at smaller spatial scales? We conducted transect surveys of sponge species that are palatable to sponge predators in proximity to refuge organisms that are chemically or physically defended (fire coral, gorgonians, hard corals) on the heavily overfished reefs of Bocas del Toro, Panama, and a reef in the Florida Keys where sponge-eating fishes are abundant. In Panama, palatable sponge species were not distributed in close association with refuge organisms, while in the Florida Keys, palatable sponge species were strongly associated with refuge organisms. The presence of fish predators alters the meter-scale pattern of sponge distribution, and defense by association enhances biodiversity by allowing palatable sponges to persist on reefs where sponge-eating fishes are abundant. K E Y W O R D S associational defense, coral reefs, indirect effects, overfishing, predation, sponges

Sponges act as important microhabitats in the marine environment and promote biodiversity by harboring a wide variety of macrofauna, but little is known about the magnitude and patterns of diversity of sponge-associated communities. This study uses DNA barcoding to examine the macrofaunal communities associated with Stylissa carteri in the central Saudi Arabian Red Sea, an understudied ecosystem with high biodiversity and endemism. In total, 146 operational taxonomic units (OTUs) were distinguished from 938 successfully-sequenced macrofauna individuals from 99 sponges. A significant difference was found in the macrofaunal community composition of S. carteri along a cross-shelf gradient using OTU abundance (Bray-Curtis dissimilarity index), with more amphipods associated with offshore sponges and more brittle stars and fishes associated with inshore sponges. The abundance of S. carteri also showed a gradient, increasing with proximity to shore. However, no significant differences in macrofaunal community composition or total macrofauna abundance were observed between exposed and sheltered sides of the reefs and there was no significant change in total macrofauna abundance along the inshore-offshore gradient. As climate change and ocean acidification continue to impact coral reef ecosystems, understanding the ecology of sponges and their role as microhabitats may become more important for understanding their full ramifications for biodiversity.

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