Molluscan communities of the rocky intertidal zone at two sites with different wave action on Isla La Roqueta, Acapulco, Guerrero, Mexico

Molluscan communities were studied on two shores, one exposed and the other sheltered from wave action by determining the diversity, richness, density, dominance, and similarity of the groups of species found in the upper part of middle intertidal zone. The sample area per location and date was 20 m super(2). The samples were taken quarterly during the year. Species richness was found to be 44 species for both sites combined. The exposed site had a greater richness (41 species) and lower density (64.2 organisms/m super(2)), whereas the sheltered site showed a higher density (88.08 organisms/m super(2)) and smaller richness (33 species). Thirty species (68.8%) were determined to be common. The exposed shore had more confined species. The Shannon-Wiener diversity index (H') was higher for the exposed site (3.165 bits/individual). A comparison of means by the Mann-Whitney tested indicated that the annual means of H' were equal between the two sites. Qualitative (Jaccard) and quantitative (Morisita-Horn) similarity scores were high at 0.682 and 0.622, respectively. The mean density of gastropods was the same at both locations, because bivalves showed significant differences with higher densities at the sheltered site. The malacologic community of the two sites was very similar. However, changes in richness and diversity over time were greater at the sheltered shore, while these changes were more gradual at the exposed shore, reflecting a greater stability of the malacologic community at the latter location.