Analysis of patterns of dispersion of intertidal prosobranch gastropods in relation to macroalgae and rock-pools.

The patterns of dispersion of Nerita atramentosa, Austrocochlea constricta, Bembicium nanum and Cellana tramoserica are analysed from quadrat samples taken between June 1972 and June 1973. In each quadrat, the proportion of animals in each of two substrata, the encrusting alga Peyssonelia gunniana and small pools, was recorded, with the proportional area of cover of each substratum, height on the shore, density of the species and the time of sampling. Multiple regression and covariance analysis showed that, in all cases, there was a linear relationship between proportion in a substratum and proportional area of substratum. No other variables were significant.Juvenile Nerita and Austrocochlea showed preferences for both substrata. Adult Nerita and all Bembicium were dispersed at random with respect to both substrata. Adult Austrocochlea were randomly dispersed with respect to Peyssonelia, but showed a preference for water. Cellana showed a preference for water and against Peyssonelia. These microhabitat preferences are discussed in relation to previously described effects of substrata on the densities of these four species.