Dispersion in Populations of Tropiometra carinata (Crinoidea: Comatulida) in the São Sebastião Channel, São Paulo State, Brazil

To understand the patterns of spatial distribution in populations of Tropiometra carinata (Lamarck), bi-monthly samples were collected in two areas of shallow water (Ponta do Baleeiro; PB, and Cabelo Gordo Beach; CG), in the São Sebastião Channel, using a grid superimposed on the habitat. A pattern of clumped dispersion was observed, with values for Morisitaʹs Index significantly different from those of a random pattern. The mean densities at PB and CG, from December 1997 to October 1998, were respectively 2•20±0•21 (SD) and 2•32±0•28 (SD) individuals m−2. The length of the longest arm was used as an indicator of specimen size. The mean size at PB was 11•76±0•53 cm and at CG, 9•65±0•18 cm. The mean size of solitary individuals was greater than for clumped individuals in both areas. Small individuals tended to attach near clumps of greater individuals. The five principal microhabitats selected by these crinoids were lateral, upper and lower rock surfaces and lateral and upper rock crevices, with preferential attachment to lateral rock surfaces. The extent of substratum coverage by the zoanthidPalythoa caribaeorum varied between areas (74•1% at PB and 38•1% at CG), as did the association Tropiometra-Palythoa (81•4% at PB and 44•5% at CG). The clumped dispersion seen here optimizes filtration by enhancing the baffle effect.