A review of the sponge increase hypothesis for Caribbean mesophotic reefs

Sponges are major components of benthic ecosystems, particularly on Caribbean reefs, where their importance in carbon cycling and ecosystem function is only beginning to be understood. There is a recurring statement in the literature, herein called the “sponge increase hypothesis,” asserting that the biomass and diversity of sponges increase with depth on Caribbean reefs through the mesophotic zone (to 150 m). We reviewed evidence for the sponge increase hypothesis, beginning with electronically searchable contributions to the literature, then working backward in time through the bibliographies of more recent citations. We found 17 studies that report one or more metrics associated with sponge abundance or diversity as a function of depth through all or part of the mesophotic zone. None of these studies reported data on either overall sponge biomass or diversity as a function of reef surface area. Among abundance metrics, including cover and density, patterns as a function of depth were disparate across sites and locations. We conclude that there is no evidence to support the sponge increase hypothesis for Caribbean mesophotic reefs and suggest that patterns of sponge abundance as a function of depth are likely to vary for a number of reasons, including substratum type, slope, and orientation. General theories of sponge abundance and diversity as a function of depth await more sophisticated survey studies that employ standardized methods for relating sponge biomass and diversity to reef surface area.