Life history of the sandhopper Pseudorchestoidea brasiliensis (Amphipoda) in sandy beaches with contrasting morphodynamics

Abstract

Exposed marine beaches are physically rigorous habitats in which macrofauna community patterns are related to physical factors such as sedimentary parameters and wave/swash processes. In this context, a consistent increase in species richness, abundance and biomass from reflective to dissipative conditions has been widely reported, and proposed as a paradigm of sandy beach ecology. Here, we examine this hypothesis on the demography and life history characteristics of the sandhopper Pseudorchestoidea brasiliensis of Uruguay. Abundance, population structure by sex and size, individual growth, natural mortality, fecundity, female maturity and size at maturity, and the length-weight relationship were compared between populations of 2 microtidal exposed sandy beaches that differed widely in physical characteristics (i.e. grain size, slope, penetrability and water content), during 20 consecutive months. Contrasting with the predictions of 'the sandy beach ecological paradigm', the population of P. brasiliensis at the reflective beach presented (1) higher abundance both for males and females; (2) higher egg production potential and recruitment levels; (3) lower natural mortality; and (4) no major differences in individual growth and estimated life span 10 those the dissipative beach population. On the contrary, growth in weight, individual fecundity and average size at maturity were higher for the dissipative beach population. We conclude that population level responses to variation in sandy beach morphodynamics may markedly differ from community level responses, and thus macroscale, world-wide community patterns could not necessarily characterise life history and demographic variations of individual species in a similar manner. We suggest that the paradigm of the forces generating patterns in sandy beach communities has underestimated the importance of population regulation mechanisms in these communities.