Nearest neighbour analysis of spatial dispersion of intertidal prosobranch gastropods within two substrata.

The patterns of dispersion of Nerita atramentosa, Bembicium nanum and Cellana tramoserica within two major substrata, the encrusting alga Peyssonelia gunniana and bare rock were analysed from quadrat samples taken at low tide, over 1 year. Distances to the nearest members of each species from random points were analysed to determine spatial patterns of dispersion. The frequencies of nearest neighbours of each species to each animal in the sampled area were analysed to determine the degree of spatial segregation and symmetry in the three species. Nerita atramentosa and Bembicium nanum showed significant aggregated (clumped) dispersion in both substrata. Cellana tramoserica, however, was dispersed at random on Peyssonelia, and regularly dispersed on bare rock. There was no correlation between index of dispersion and density, or time of year, in any species. No useful tests for segregation and symmetry between Cellana and the other two species could be made because of the difference in dispersion of this species. Nerita and Bembicium were significantly segregated on both substrata, indicating that the microhabitat requirements of the two species are different. The two species were symmetrical on both substrata, indicating that there is no tendency for members of one species to be more isolated than members of the other species.Possible explanations of the differences in spatial pattern shown by the three species are discussed, and experimentally testable hypotheses are proposed to clarify the importance of physical and biological factors in the spatial patterns of distribution of the three species.