Structure of the bivalve (Mollusca) assemblage of Mazatlan bay, Mexico, and its relationship to environmental variables

This work describes the structure of the assemblage of bivalve mollusks (i.e., composition, abundance, density, distribu- tion and species diversity) of four representative rocky beaches of Mazatlan bay, Mexico. Biological samples were taken in the upper intertidal, lower intertidal and shallow subtidal of the beaches together with environmental parameters: type of sediment, organic matter, water temperature, dissolved oxygen, wave exposure and concentration of chlorophyll a. A total of 19,848 individuals distributed in 77 species, 50 genera and 27 families were recorded. The permutational ANOVA and PERMANOVA indicated significant differences in the bivalve assemblage structure among environments across studied sites. However, the intertidal (IT) and SS environments had different bivalve assemblages. Six species had a broad distribution: Acar rostae, Arcopsis solida, Isognomon janus, Saccostrea palmula Chama buddiana and Carditamera affinis. Four species had the highest density in the IT: Brachidontes semilaevis (196.0 ind m 2 ), B. adamsianus (35.0 ind m 2 ), Arcopsis solida (18.0 ind m 2 ) and Lithophaga aristata (15.0 ind m 2 ), and in the SS were Lithophaga aristata (7.1 ind m 2 ), B. semilaevis and Acar rostae (3.2 ind m 2 ). This study shows that exposure to waves was determinant for the bivalve distribution that inhabit the intertidal zone. In contrast, chlorophyll a and organic material are the variables that best explain the distribution of the species in the SS. However, more detailed information is required regarding the importance of the interaction between the hydrodynamics and the topography of the rocky beaches of Mazatlan bay