Contents of volume 128

Abstract

Observations were made to document habitat segregation and seasonal changes in density for the most common intertidal organisms at a study site on the Pacific coast of Costa Rica. The pulmonate limpets Siphonaria gigas (Sowerby) and Siphonaria maura (Sowerby) were more abundant in the mid-high- and high-intertidal zone, whereas the keyhole limpet Fissurella virescens (Sowerby) was more abundant in the mid zone. Coiled gastropods were found in the mid- and mid-high-intertidal zone but were absent or scarce in the high zone. Barnacles and coralline algae were found throughout the intertidal zone and foliose algae were mostly present in the splash zone. Abundance of all species changed over the 2-yr period of the study. S. gigas and S. maura and the barnacle Chthamalus fissus Darwin exhibited well-defined fluctuations in density related to recruitment. No recruitment was observed for F. virescens or Tetraclita panamensis Pilsbry and populations of both species declined. Changes also occurred for coiled gastropods and were probably related to movement of adults. Coralline and foliose algae were more abundant during the rainy season than during the dry months. Far from being constant, populations on these tropical shores were highly dynamic with temporal changes rivaling those in the temperate zone.