Abstract

Field sampling and other experiments were carried out during February 2001 to determine whether different morphodynamic characteristics occurring within an intermediate sandy beach of southern Chile (ca. 39°S) convey differences in population abundance, tidal movement, burrowing ability and oxygen uptake of the anomuran crab Emerita analoga (Stimpson, 1857). Crabs were collected along transects extended between the lowest swash levels and the retention zone above the effluent line of the south and north end of the beach. Burrowing times of nearly 70 crabs collected at each study site were measured in saturated sands collected from the lowest swash level of each site. Oxygen uptake of crabs was measured in incubation glass bottles. The intertidal zone of the north end of the beach was wider (56 m) and flatter (1/14) than that of the south end (45 m and 1/9, respectively). In general, the swash zone of the north end was significantly wider than the south end throughout the sampling period. The frequency of swashes and number of swash crossings above the effluent line, plus up-wash speed, were usually higher at the steeper south end of the beach. The mean population abundance of E. analoga per linear metre of beach was significantly higher at the north end, whereas density per square metre was significantly higher at the south end. No differences were found in biomass figures. Although the highest abundance of crabs at the north end was usually observed at the lowest swash levels, similar population abundances occurred along all the tidal levels sampled at the south end. Burrowing times of crabs collected from both ends of the beach increased significantly with increasing carapace length and body mass. The mean burrowing time of crabs collected at the south end of the beach was smoooth negative slope. The differences between the metabolic cost of juveniles and larger stages are consistent with the fact that most of the migrating crabs at both ends of the beach were represented by those stages, which is a similar relationships to that found in other marine invertebrates (Brown, 1983). Those allometric relationships may explain why crab populations with similar biomass but different size class distribution across the beach have different energetic requirements. This assertion is supported by the across-shore distribution of crabs and oxygen consumption. Although biomass values in the lowest level of the swash were similar at both ends of the beach (3000 gm-2), oxygen consumption in the north end was 24% higher than in the south end: 0.300 and 0.228 mol O2, respectively (estimations of biomasses and oxygen consumptions from Figs. 4 and 9). This is probably because more small individuals inhabited the lowest swash level of the north end. © 2004 Blackwell Verlag, Berlin.