MEPS 239:169-179 (2002)  -  doi:10.3354/meps239169

ABSTRACT: Several studies have shown consistent macroinfauna community patterns in sandy beaches that could be mainly explained by variations in physical factors (i.e. grain size, beach slope, swash processes). However, the macroecology of population dynamics and biogeographic patterns in life history traits have not been adequately assessed in sandy beach ecology. Here, we examine the latitudinal variation of population dynamics and life history traits of the sandy intertidal mole crab Emerita brasiliensis in sandy beaches along the entire range of the species distribution of some 2700 km of the Atlantic coast of South America. Population structure by sex and size, individual growth, natural mortality, and the length-fecundity and length-weight relationships were compared. Most of the life history and population dynamics features of Emerita brasiliensis show clear geographical patterns: (1) a shift from continuous to seasonal reproductive and recruitment events from subtropical to temperate sandy beaches; (2) an increase in the individual size of the smallest ovigerous female and in the fecundity at size (length-fecundity relationship) from subtropical to temperate beaches; (3) a higher predominance of females towards temperate beaches; (4) an increase in male sizes towards subtropical beaches, following a direct relationship with mean water temperature of the surface zone; (5) a significant correlation between female growth parameters (L_∞ = inverse and K and φ¹ = direct) and surface water temperature; (6) an increase in the individual weight at size (length-weight relationship) from subtropical to temperate beaches, both for males and females; and (7) a linear decrease in life span and an asymptotic increase in natural mortality (both sexes) from temperate to subtropical beaches. The additional effect of morphodynamics at a regional scale was also detected, and in some cases masked or ameliorated clear latitudinal trends. Populations on dissipative beaches had a more extended reproductive season than in reflective beaches, as well as higher growth performance, fecundity and somatic weight at size. The decreasing occurrence of females in subtropical beaches might explain the unexpected but consistently increasing male crab sizes towards lower latitudes. This supports the hypothesis of asymmetric intraspecific competition between sexes, which may be considered a critical regulating process in sandy beach populations.

KEY WORDS: Mole crab · Emerita brasiliensis · Macroecology · Population dynamics · Sandy beaches · South America