High diversity of tropical intertidal zone sponges in temperature, salinity and current extremes

The wide intertidal zone of the Quirimba Archipelago (Mozambique, East Africa) encompasses many different habitats, which all experience wide environmental variation. Large daily fluctuations in temperature occur when standing water is heated up at low tide to >10 degrees C above ambient. Salinity may be high in the dry season and periodically almost fresh in the wet season. Very high current velocities (approximate to 3 ms(-1)) may occur and the direction of water flow is often complex. Sponges were a major component of the fauna land dominate the sessile forms) in most of the eight habitats studied: exposed reef, sheltered reef, sand-rock, cave, reef boulders, boulders on sand, mangrove swamp and seagrass meadows. The highest number of sponge species occurred in the two reef habitats: from mean values of between approximate to 2.5 and approximate to 0.5 species/m(2) (upper and lower shore, respectively). The diversity of species present yielded a high Shannon index value of H = 2.95 for the total of 33 species. A rank-abundance plot of the data showed a high level of equitability (J = 0.84). Community similarity measurement using Bray-Curtis index showed two clusters; the exposed habitats of the reefs and sand-rock and the refugia of cave, reef and sand boulders.