Vertical Distribution of Mollusks on the Rocky Intertidal of Easter Island

Abstract

Horizontal and vertical distribution of mollusks was studied in the rocky intertidal of Easter Island (27°09' S,109°26' W) in January and again in September 1986. Organisms are zoned from the upper to the lower intertidal. In the upper horizon dominant species are Nodilittorina pyramidalis pascua, Rehderella belyaevi, Nerita sp., and Planaxis akuana, together with crustaceans, decapods and echinoderms. In the middle horizon dominant species are Plaxiphora mercatoris, Dendropoma sp., Antisabia sp., and Pilosabia sp. Mid-horizon pools support algae; the only relatively abundant coral, Porites lobata; numerous gastropods such as Stomatella and Euplica; crustaceans; and echinoderms. In the lowest horizon Cypraea caputdraconis and Echinometra insularis are dominant. The latter species is a rock borer that builds "pots" forming microhabitats shared with other organisms. Some specimens of Dendropoma, Pocillopora, and Echinostrephus were also found in this horizon. No great differences in distribution of organisms among the south, northeast, and northwest sectors of the island were distinguished in an index of similarity. Distribution patterns at Easter Island are comparable with those in other tropical areas where the same families and genera are found. At Easter Island the species are different because of the high degree of endemism among the mollusks.