Spatial and temporal variations in the species composition and abundance of benthic molluscs along 4 rocky shores of Karachi

A total of 97 species of macromolluscs were collected from 4 rocky beaches (Manora, Buleji, Nathiagali, and Cape Monze) along the Karachi coast from December 1993 to December 1995. More species were recorded from Buleji (68) and Manora (63) than from Nathiagali (55) and Cape Monze (57). Gastropods dominated at all 4 of the sites studied. Four species were commonly abundant at all 4 sites: Turbo coronatus, Nerita albicilla, Morula granulata, and Cerithium rubus. Th e average number of species decreased with an increase in the tidal heights at Manora, Buleji, and Nathiagali, whereas this was greater in the high tidal zone at Cape Monze’s rocky shore. Th e average number of individuals of molluscs was greater in the midtidal zone at the Manora and Buleji rocky shores, whereas it was greater in the low tidal zone at Nathiagali and Cape Monze. Two-way nested analysis of variance showed that the number of individual molluscs was signifi cantly diff erent among zones and among sites in most of the samples. Species richness, diversity, and evenness, however, were not found to be signifi cantly diff erent among zones and sites. Cluster analyses, derived from Bray-Curtis measures of similarity based on abundance data, showed the discrimination between carnivore gastropods and an increase in the abundance of herbivore gastropod molluscs. Diff erences in community structure amongst sites appeared to be due to habitat structure, substrate instability, human disturbances, and degree of exposure.