Spatio-temporal variation of the population structure and density of the shore crab Pachygrapsus gracilis (Grapsidae) in an estuary on the Brazilian Amazon coast

Abstract The present study investigates the spatio-temporal variation in the density of the adults and larvae of the shore crab Pachygrapsus gracilis and identifies the reproductive period and the population structure of the species in the Marapanim estuary, in northern Brazil, in order to describe the biological characteristics of this equatorial population on the Atlantic coast. Specimens were collected manually every month over the course of a year. Adults were collected at four rocky outcrops in the upper and lower mid-littoral. Larvae were sampled at six points using horizontal trawls of the surface water. The sampling points represent the inner and outer estuary, its two margins, and varying gradients of salinity. The density of the zoea I and adults were higher on the margins with the highest sediment deposition rates and salinity. Only the density of the adults correlated significantly with salinity. Although ovigerous females were only collected in the rainiest periods, the presence of juveniles throughout the year indicates that the species reproduces continuously. The population parameters indicate that the density of P. gracilis was related to salinity, and that part of the life cycle of these crabs is completed in the Marapanim estuary. This species reproduces in the estuary, exports zoea I to the coastal waters and then probably returns as megalopae, responding to local conditions through systematic shifts in its distribution and abundance over time and space. The population was relatively stable and able to adjust to the considerable variation in abiotic factors that are typical of this estuary.