Coastal sponge communities of the West Indian Ocean: morphological richness and diversity

Sponges are a reasonably ubiquitous, abundant and highly morphologically plastic taxon. They are very unusual in showing considerable morphological plasticity, not only within higher taxa but within species and at macro and micro scales. In this study we determined the prevalence of sponge morphologies at four coastal study regions in the west Indian Ocean. We show that tropical and subtropical assemblages (in the present study) can be separated on the basis of morphological composition alone (by > 4% arborescent forms or presence of palmate forms in the former). Inter-tidal sponge assemblages can also be separated from those in the subtidal to a high degree of certainty, also on prevalence of morphologies (absence of tubular or branching forms in the former). The species diversity of many sponge assemblages has been quantified in various environments but, typically, only by specialists. Rarely have equivalent measures been made of morphological diversity. Values of morphological diversity (Shannon H') were similar, though more variable, to those measured in temperate waters. Substratum nature had a major influence on morphological diversity and evenness, in contrast to geography and bathymetry. As coral reefs were the most diverse and caves and boulders the least, we suggest that substratum heterogeneity and water-flow complexity are probably the chief determinant of sponge assemblage morphological diversity.