Population structure of the bivalve Anomalocardia brasiliana, (Gmelin, 1791) in the semi-arid estuarine region of northeastern Brazil.

For several decades, the bivalve mollusk Anomalocardia brasiliana has been extracted in estuarine areas in the semi-arid region of northeastern Brazil. However, information on both dynamics and abundance of their populations are missing. In this context, the present study, therefore, investigated several aspects of the population structure of A. brasiliana on beaches near the estuary of the Apodi River in Northeast Brazil. The aims were to determine the population density of A. brasiliana during different times of the year, to estimate population parameters, and to relate species density and distribution to the abiotic characteristics of the region. Sampling was performed from March/2007 and May/2008, we collected samples of sediments and mollusks along 180 m transects in the intertidal area on the beaches Barra and Pernambuquinho, Grossos - RN. At each sampling site, samples of sediments and mollusks were collected in the intertidal area along 180 m transects. The highest and lowest average densities on Pernambuquinho beach were 1148 (April/2007), and 100 individuals.m-2 (May/ 2008), respectively. On Barra beach, the highest and lowest densities were 1813 (April/2007) and 951 individuals.m-2, (November/2007), respectively. The densities of A. brasiliana on both beaches were significantly different only in the months of January, April and May/ 2008. Length of the individuals ranged from 1 to 28 mm, with most individuals measuring between 22 and 24 mm. Length growth rate parameters for A. brasiliana were L∞ = 28.68 mm and k = 0.61 year-1 on Barra beach, and L∞ = 29.87 mm and k = 0.48 year-1 on Pernambuquinho beach. The growth rate curves for A. brasiliana suggest the presence of three well defined cohorts, thus pointing to a continuous reproduction cycle with peak recruitment between October/2007 and March/2008. A. brasiliana density decreased predominantly during the heavy rains of 2008, probably due to an accumulation of sediments and decreasing salinity. This disturbance probably affected population of this species in the region.