Spatial distribution of macrofauna within a sandy beach on the caribbean coast of Costa Rica

Abstract

Knowing the spatial pattern and densities of individuals in a population is basic to understand ecology such as their response to environmental gradients. The main goal of the present study is to describe the abundances and distribution of the benthic fauna in the lower intertidal level of a sandy beach at Pacuare Reserve, Caribbean Coast, Costa Rica. The count data of five taxa (in 29 cores) was analyzed to determine their spatial distribution. Two species of polychaetes were found: the first, Scolelepis (Scolelepis) squamata (O. F. Mueller, 1806) was common in the center of the beach in an aggregated pattern. The second, Pisionidens indica (Aiyar & Alikunhi, 1940) was present in low densities with a random distribution pattern. Moreover, two isopods were found: Excirolana braziliensis Richardson, 1912 was randomly distributed along the beach while Ancinus brasiliensis Lemos de Castro, 1959 appeared only in two stations. A meiofaunal nemertean attached to grains of sand was the most common organism in the intertidal zone, showing a clumped pattern. The patches of abundance within the beach could be due to a morphologic change that imposes the influence of the strong surf conditions. Moreover, food resource distribution, niche partitioning or competition within the same tide level could influence the different patterns of distribution found in the sediment among the species. In these unstable sediments, the fauna was limited to few opportunist taxa well adapted to withstand mechanical disturbances.