A review of evidence for food limitation of sponges on Caribbean reefs

The diversity and biomass of sponges on Caribbean reefs exceeds that of all other benthic organisms. Wilkinson & Cheshire (1990; Mar Ecol Prog Ser 67:285�294) summarized evi- dence that sponge communities on Caribbean reefs were different from those on the Great Barrier Reef, Australia, reflecting food limitation of the latter, but not of the former. Since then, we have learned much about the ecology of sponges, and the concept of food limitation has recently re- emerged, seemingly without substantive reference to past inter-oceanic comparisons or pioneer- ing studies of sponge physiology. Here, we review the literature in an attempt to correct this digression. Based on current knowledge of sponge nutrition and bottom-up resource gradients (particulate food, dissolved organic carbon (DOC), light, turbulence), we predict 4 patterns of sponge abundance that would suggest food limitation on Caribbean reefs. After a critical review of survey data, correlative studies and manipulative experiments, we can find no evidence for food limitation. Although there is good evidence for higher availability of picoplankton at greater depths, sponge abundance does not mirror this gradient, suggesting that other sources of nutrition are also important, and particulate food is not a limiting factor. Recent studies have renewed inter- est in the uptake of DOC by both high and low microbial abundance sponge species, suggesting that the absence of bottom-up effects for sponges on Caribbean reefs may be attributable to this mysterious, and likely ubiquitous, food source. The recent unambiguous demonstration of top- down effects of predation on sponge community composition across the Caribbean may owe its clarity to the relative lack of confounding influences of abiotic and bottom-up effects in this study system.