Bathymetric patterns of sponge distribution on the Bahamian slope

Using a manned submersible equipped with a high-resolution video camera, we documented vertical patterns of species abundance, substratum usage, color, and body form of sponges living on the Bahamian slope. Sponge abundance is not linearly related to depth, but has a bimodal distribution, with peaks located on the upper slope (100 m depth) and between 400 and 500 m. Sponge fauna decreases dramatically between 100 and 230 m and remains rather depauperated to 320 m. The deep-water mode of the vertical distribution is dominated by abundant lithistids and astrophorids, whereas the shallow mode includes a wide assortment of species belonging to most of the sponge orders. The observed bimodal pattern is not explained by the temperature/salinity profile of the water column. Sponges are much more abundant on vertical surfaces than horizontal ones, indicating that surface angle is a major source of patchiness on the slope. Diversity both in body form and color have bimodal bathymetric distributions similar to that of the species richness, mirroring an important faunal transition along the slope. Most of the morphs and colors occur at all depths, but erect whip-like forms as well as black and purple-pink colors are found only at the shallowest sites. Above 200 m, encrusting morphs and red-orange colors dominate sponge assemblages, whereas in the deepests stations cup-like morphs as well as yellow and white-grey colors are predominant. The shift from brightly colored sponges to white sponges with increasing depth may be explained by the fact that most of the organisms from which sponge pigments are derived, including both food and symbionts, are associated with the euphotic zone.