A test of the sponge-loop hypothesis for emergent Caribbean reef sponges

The sponge-loop hypothesis proposes that coral reef sponges convert the dissolved organic matter (DOM) released by benthic primary producers into particulate detritus available to higher trophic levels. The hypothesis was developed and subsequently supported from studies of cryptic, encrusting sponges; however, the sponge-loop has yet to be considered for massive, emergent sponge species that dominate the surface of Caribbean reefs and represent a large proportion of sponge community biomass. We tested the generality of the sponge-loop for emergent sponge fauna by using direct In-Ex methods combined with acoustic Doppler velocimetry and sponge volume calculations to quantify carbon flux for 9 sponge species representing a variety of functional types. The diet of 5 species hosting abundant symbiotic microbes (HMA) primarily consisted of dissolved organic carbon (DOC), while 4 species with low microbial abundances (LMA) primarily consumed detritus and picoplankton. None of the sponge species studied were found to produce significant quantities of detritus. We conclude that, instead of releasing assimilated carbon in the form of detritus, as originally proposed by the sponge-loop for encrusting, cryptic sponges, emergent sponge species likely retain assimilated carbon as biomass. Given the high rates of DOC uptake observed for HMA species, we propose an additional pathway by which the sponge-loop fuels higher trophic levels via predation by fish, turtle, and invertebrate spongivores.