Patterns of sponge distribution in Cagarras Archipelago, Rio de Janeiro, Brazil

The structure and distribution of the sponge community in five sites and four habitats in Cagarras Archipelago, Rio de Janeiro, Brazil, are described. The archipelago has three major islands (Cagarra, Palmas, and Comprida), and four islets. Qualitative samples were taken by SCUBA diving in the three islands and in two islets, and quantitative samples were taken in Palmas Island only. Cluster analysis using Jaccard's coefficient on qualitative data grouped the two islets (Cagarra Bank and Cagarra Islet), which are more exposed to wave action, and the three islands formed a group of relatively sheltered sites. Cluster analysis using Bray-Curtis coefficient on quantitative data from different habitats in Palmas Island allowed distinction of three groups of samples: overhangs, shallow horizontal surfaces (6 m depth), and vertical walls plus deeper horizontal surfaces (17 m depth). Sponge abundance was greater in overhangs (61.2 ind m(-2)), which were dominated by Protosuberites sp. and Clathrina conifera. Abundance was reduced in vertical walls (31.1 ind m(-2)) and deep horizontal surfaces (16.8 nd m(-2)). Pachychalina sp. dominated the deep horizontal surfaces and Clathrina conifera was dominant in overhangs. Values of Shannon's diversity in overhangs. vertical walls (both with H' = 2.1 bits ind(-1)) and deep horizontal surfaces (H' = 1.7 bits ind(-1)) were moderate and similar, whereas in shallow horizontal surfaces the sponges were both less diverse (H' = 0.37) and less abundant (8.5 ind m(-2)). Shallow horizontal surfaces were dominated by Hymeniacidon heliophila. The reduced species richness of the sponge community in Cagarras Archipelago when compared with other sites in Brazil and elsewhere is probably due in part to the pollution from the city of Rio de Janeiro. Wave action appears to reduce the number of sponge species at a local scale, whereas substrate inclination affects more strongly the species composition and abundance than the diversity of sponges.