Abundance, tidal movement, population structure and burrowing rate of Emerita analoga (Anomura, Hippidae) at a dissipative and a reflective sandy beach in south central Chile

To evaluate the effects of beach morphodynamics upon the abundance, tidal movement, population structure and burrowing rate of the crab Emerita analoga (Stimpson) (Anomura, Hippidae) we sampled two beaches in south central Chile (ca. 42 degrees S), Mar Brava and Ahui with dissipative and reflective characteristics, respectively. The swash zone at the dissipative beach was 5-6 times wider than that of the reflective beach. At the dissipative beach, upwash speeds were higher and the number of effluent line crossings were lower by more than an order of magnitude. To examine the tidal movement of E. analoga, we collected crabs from 5 to 6 tidal levels of each beach every 2 h across 12 h of the tidal cycle. The :intertidal distribution of crabs differed between beaches; i.e., at the dissipative beach they were primarily located at the swash zone, while at the reflective beach they were mostly located at the lour tide level and shallow subtidal. The change in position of crabs was pronounced across the tidal cycle at the dissipative beach (Mar Brava), with most of the animals remaining in the active swash zone. Body size data were used to construct size frequency distributions for each population. Crabs from the dissipative beach reached larger sizes than those at the reflective beach. Sediments were coarser at the latter versus the former beach. Crabs burrowed at similar rates in the sand from both beaches, a result which supports the idea that E. analoga is a sediment generalist capable of burrowing successfully in a wide range of sediment types. This characteristic is likely a key to the broad success of this species on the full range of beach morphodynamic types along the coasts of South and North America.