Factors regulating the distribution and abundance of estuarine bivalves in a hypersaline ecosystem of the Brazilian semi-arid region

This study aimed to analyze factors that regulate the spatial-temporal distribution of Anomalocardia flexuosa (Linnaeus, 1767) in a hypersaline estuary on the coast of the Brazilian semiarid region. From November 2015 to October 2017, rainfall data were consulted and samples of biological material and sediment for granulometric analysis were collected, and abiotic parameters (salinity and temperature) were measured in 3 different areas (A, B, C) of the Apodi/Mossoró estuary. Salinity showed an inverted decreasing pattern within the estuary, with the highest values ​​recorded in A. The granulometric characterization differed between areas and showed a higher percentage of silt and clay in area B and a higher percentage of sand in area C. The density of A. flexuosa differed between areas, with A always showing the highest values, recording up to 4,728 ind/m2, and C the lowest, with an average of 370 ind/m2. Significant differences in density were observed between transects in areas A and B. The abiotic variables temperature and percentage of sand in the sediment showed negative correlations with the density of A. flexuosa, and salinity, silt+clay and precipitation showed positive correlations. Hypersalinity did not limit the distribution and abundance of the species, and rainfall regulated temporal variations. Competition for space with a meadow of Halodule wrightii, located in C, restricted the establishment and density of A. flexuosa. Intraspecific competition was also important for adjustments in the abundance of juveniles and adults. Therefore, the species presents broad tolerance to variations in several abiotic factors, however, biotic factors, such as competition, may be influencing the structure of this population.