Effects of beach morphodynamic features on production and P/B ratio of the crab Emerita brasiliensis Schmitt, 1935 (Decapoda: Hippidae) in sandy beaches of South America

Abstract

The mole crab Emerita brasiliensis inhabits exposed sandy beaches, both reflective and dissipative, along the Atlantic coast of South America, where it usually attains high biomass and production. We assessed the effects of physical characteristics of beaches (mean grain size, beach width and slope) on production and turnover rate (P/B ratio) of E. brasiliensis. Six production and P/B ratio datasets, estimated with cohort-based and size-based methods, were gathered from studies carried out on subtropical (~25°S) and temperate beaches (~32°S). Based on the estimates available, general trends were identified. Production was positively related to the intermediate/dissipative conditions of wider beaches with finer sand grains and gentler slopes. P/B ratio was negatively related to grain size (mm) and positively related to beach width. These results suggest that P/B ratios also increase towards intermediate/dissipative features due to higher growth rate, as indicated by higher values of the curvature parameter of the von Bertalanffy growth function (K) in more benign conditions (wider beaches with finer grains). In addition, the recurrent pattern of size structure on intermediate/dissipative beaches, with high frequency of recruits, leads to higher P/B ratio estimates on these beaches than on reflective beaches. The relationship of production and P/B ratio with physical features, such as mean grain size, beach width and slope indicates that a ‘short-cut’ approach to estimate Emerita brasiliensis production is a promising prospect and, therefore, more data on the production of this species should be provided.