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McMurray SE, Stubler AD, Erwin PM, Finelli CM, Pawlik JR
A test of the sponge-loop hypothesis for emergent Caribbean reef sponges
The sponge-loop hypothesis proposes that sponges recycle dissolved organic carbon (DOC) from seawater and return it to coral reefs as shed cellular detritus, much like the famous microbial-loop. However, the sponge-loop was based on studies of encrusting sponges living in reef interstices. McMurray and co-authors tested the generality of the sponge-loop by quantifying carbon flux for 9 emergent sponge species that are common on Caribbean reefs. Uptake of DOC was found, particularly for sponges hosting abundant symbiotic microbes; however, none of the species produced significant quantities of detritus. The authors conclude that, instead of releasing assimilated DOC as detritus, emergent sponges retain carbon as they grow and pass carbon to the next trophic level through predation by spongivores.