Patterns of distribution of dominant porcelain crabs (Decapoda: Porcellamidae) under boulders in the intertidal of northern Chile

Porcelain crabs live in dense aggregations tinder boulders in the intertidal zone of northern Chile. A quantitative approach was performed to examine the distribution patterns of dominant species and to correlate It with emersion and temperature under the boulders. The study involved six tidal levels across the intertidal with six replicates each, totalling 36 samples of 0.09 m(2). From the seven species found, Petrolisthes granulosus, P. violaceus and Allopetrolisthes angulosus took in 49.06%, 25.20% and 25.51% respectively, of the total number of individuals collected (N=10,631). The distribution patterns showed P. granulosus dominating in the upper levels (up to 73% of emersion and 22 degrees C), P. granulosus and P. violaceus coexisting in the middle levels (48% of emersion and 21.6 degrees C), and A. angulosus dominating in the lower levels (below 20% of emersion and 17.8 degrees C). Size distribution for P. granulosus declined at its limit levels of distribution, the same tendency appeared at the upper limits of P. violaceus and A. angulosus. Temperature and emersion gradients strongly affected the species richness, abundance and mean size of the species across the intertidal, thus clearly influencing the occurrence and distribution patterns of the species studied. The ability of the upper middle shore species to tolerate high levels of temperature and emersion is suggested to explain their distribution patterns. Size distribution differences might have been caused by biological interactions as intra and interspecific competition. A foregut content analysis revealed trophic differences among the dominant species, a result that could be related to the distribution.