LONGSHORE DISTRIBUTION OF MESODESMA-DONACIUM (BIVALVIA, MESODESMATIDAE) ON A SANDY BEACH OF THE SOUTH OF CHILE

Monthly samples were taken from February 1989 to january 1990 to evalu te the longshore distribution and density of the bivalve Mesodesma don acium in a dissipative beach in southern Chile. The results showed tha t its distribution was patchy. Adult clams were confined to the surf z one, while the vast majority of juveniles occurred in the swash zone. The highest densities of adults were found in summer and autumn (up to 1.59 individuals per 0.25 m2 in February 1989), while the minimum occ urred during winter. juveniles had similar densities all year round (u p to 16-20 individuals per 0.25 m2). Most clams collected in the surf zone had similar shell lengths (70-75 mm); those collected in the swas h zone were smaller than 25 mm. No relationships were found between di stribution and abundances of clams and variability in textural charact eristics of the surf or swash zone. Due to the limited longshore varia bility in grain size and sorting of sands, it is suggested that the va riabilities in distribution and abundances of clams may be related to large-scale habitat characteristics rather than to small-scale textura l variability.