Testing the swash exclusion hypothesis in sandy beach populations: the mole crab Emerita brasiliensis in Uruguay

Abstract

Exposed marine beaches are physically rigorous habitats in which macrofauna commu- nity patterns have been well correlated with physical factors such as grain size, beach slope and wave/swash processes. In this context, the swash exclusion hypothesis (SEH) gained wide accept- ability in explaining the control of species abundance and diversity of the sandy beach macrofauna by swash climate, determined by wave height and beach face slope. This hypothesis predicts a con- sistent increase in species richness, abundance and biomass from reflective to dissipative conditions. However, predictions of the SEH have not been adequately tested at a population level. Here, demo- graphic and life history characteristics of the intertidal mole crab Emerita brasiliensis were compared between populations of 2 microtidal exposed sandy beaches with contrasting morphodynamics dur- ing 22 consecutive months. The major prediction of the paradigm does not hold for the mole crab: total abundance, as well as that of males, females, males with spermatophores and megalops did not differ between beaches. Moreover, the reflective beach population presented higher male growth rates in size and lower natural mortality. Other population processes and life history traits gave support for the SEH: the dissipative beach population presented (1) higher abundance of ovigerous females, female growth rates, fecundity, egg production potential, size at maturity, as well as weight at size; and (2) lower female mortality and burrowing rates (for both sexes). The SEH seems to be particularly useful in explaining female somatic growth and reproductive features, which could be related to a major amount of food availability in dissipative beaches. However, other competing, but not exclusive, hypotheses are needed to explain variations in population abundance and other pro- cesses such as male growth and survivorship. The empirical evidence from several recent studies on the Uruguayan coast showed that sandy beach populations that co-occur in contrasting environments are less sensitive to variations in beach morphodynamics, which should not be considered the primary factor affecting the abundance and some life history traits. The importance of alternative population regulation processes and mechanisms are stressed.