Sponges assemblages were sampled in four coastal study regions (Malindi, Kenya; Quirimba Archipelago, northern Mozambique; Inhaca Island, Southern Mozambique and Anakao, Madagascar) in the west Indian Ocean. Sponge species were counted inmultiple 0.5m2 quadrats at depths of between 0 and 20m at a number of sites within localities within each region. Despite the relatively small areas sampled, sponge samples comprised a total of 130 species and 70 genera of the classes Demospongiae and Calcarea. Sponges are clearly a major taxon in these regions in terms of numbers of species, percentage cover or biomass, although their ecology in the west Indian Ocean is virtually unknown. Nearly half of the genera, e.g. Iotrochota, found were species with a so-called Tethyan distribution. Most of the other genera were cosmopolitan, e.g. Clathria, but some were coldwater (Coelosphaera), Indo-Australian (Ianthella) or circum-African (Crambe). Many of the species encountered in the present study occurred in at least two study regions, many in more and could occupy large areas of substratum. Some of these, e.g. Xestospongia exigua, are commonly found throughout the Indo-west Paci¢c region where they also occupy much space. The endemicity of the shallow water sponge faunas in East Africa (20^25%) seem to be high within the Indo-Pacific realm but are lower than northern Papua New Guinea. The tropical regions (Kenya and Northern Mozambique) were more speciose than subtropical regions (southern Mozambique and Madagascar) but not signi¢cantly more diverse (Shannon H0). Although latitude was not a major in£uence on sponge community patterns, hard substratum assemblages did form a cline from the tropics to Southern Mozambique,
linked by Madagascar. Substratum nature (habitat) was most important in in£uencing the suite and number of species present. Sponge assemblages of soft substrata were much more dissimilar, both within and between habitats, than those on hard substrata. There was a predictable variability in species richness between hard substratum
habitats: coral reefs being speciose and caves being less so. Our ¢ndings showed that both patterns and in£uences on species richness may be decoupled from those in£uencing diversity. In our data species richness, but not diversity, showed striking regional and bathymetric trends. In addition, sponge species richness mainly split at coral reef vs. non-reef habitats, whilst diversity divided principally into assemblages on hard and soft substrata.We consider this dichotomy of findings between species richness and diversity values to be important, as these are two principal measures used for the interpretation of biodiversity.
Indian Ocean, Western
Spacial and ecological distribution, Zonation, Microdistribution