Population of the porcellanid crab Petrolisthes armatus was studied on a rocky intertidal beach located at the Punta Morales peninsula in the mid upper Gulf of Nicoya estuary, Pacific coast of Costa Rica, from December 1997 to November 1998 (14 dates). Horizontal plankton tows (280 micron mesh net) were also made to verify the presence of P. armatus larvae. Crabs were collected every 3 m along three 18 m long transects, at two sites on the beach, by placing a bottomless bucket fringed with canvas to prevent the organisms from escaping under the rim. A total of 15 382 P. armatus were collected. Only 146 (0.95%) crabs had a carapace lenght longer than 10 mm, and 8 995 (58.5%) were in the size range of 2 to 4.5 mm. The remaining crabs 6 241 (40.5%) were in the size range of 4.6 to 10 mm. Male and female P. armatus were represented by 2 777 and 3 518 individuals respectively, with a sex ratio of 1:1.26. Ovigerous females were found at all dates and included 2 937 individuals (83 % of females). Plankton tows yielded only 73 larvae of P. armatus (Zoea 1), with a density of 1.2 larvae/m3. No statistically significant seasonal trends in the population of this species were detected. P. armatus appears to reproduce continuously the year around in Punta Morales, and some peaks of abundance were present during the dry and rainy seasons. These trends are similar to trends reported for other crustacean species in the Gulf of Nicoya.
Keywords: Crustacea, Porcellanidae, Petrolisthes, rocky intertidal, Gulf of Nicoya, Costa Rica