Morpho-biometric relationships and allometry of the “black clam” (Anadara mazatlanica) from two coastal lagoons in the southeastern Gulf of California

Background. Phenotypic plasticity in bivalve mollusks, such as Anadara mazatlanica (“black clam”), is influenced by both external environmental and internal biological factors, which causes changes in the metric variables of the shell and in the relationship between them. Goals. This study examined the association of environmental variables with the biometric indices and morphometric relationships of A. mazatlanica collected from two sites in the southeastern Gulf of California from March 2022 to February 2023. Methods. Monthly, environmental variables of the water were obtained (temperature, dissolved oxygen, salinity, pH, organic matter, inorganic matter, total suspended solids and chlorophyll a); also every month, samples of 120 clams were taken from the El Colorado (ECL) and Navachiste Lagoon (NL) systems in Sinaloa, Mexico, to obtain the height, length, and width of the shell (mm). Results. The clams from ECL were significantly larger in length, but exhibited lower shell compactness and cupping compared to those from NL (p < 0.05). At both locations, morphometric equations for A. mazatlanica displayed a linear positive relationship, with negative allometry (b < 1). The most representative growth ratios were shell width/shell height for ECL (R² = 0.80) and shell width/shell length for NL (R² = 0.78). Temperature and salinity were negatively related to shell dimensions in both locations. Conclusions. Significant monthly and annual variations were observed in all morphometric variables between the two populations, indicating that A. mazatlanica experiences environmental stress but also exhibits resilience to local conditions. This is the first morpho-biometric analysis of the “black clam” in this region, providing valuable insights for the species' management.