Bivalve death assemblages from shallow to abyssal depths in the Southwestern Gulf of Mexico: A broad-scale comparison of taxonomic and functional diversity

Bivalvia constitutes one of the most diverse taxa in the marine realm and its diversity in the southern Gulf of Mexico (GoM) is poorly known. We studied the bivalve death assemblages collected during five oceanographic cruises at 76 sites in the southwestern GoM. Sites encompassed a depth gradient from 14 to 3740 m and were grouped into four depth classes: shallow (< 60 m), lower shelf-upper slope (100–1000 m), lower slope (1001–2500 m), and abyssal plain (> 2500 m). We compared the diversity of bivalve death assemblages among depth classes, explored the relationships between some biological traits and depth, and updated the geographic distribution of species. We identified 1321 bivalves belonging to 14 orders, 39 families, 71 genera, and 110 species. Species richness (γ-diversity) declined with increasing depth likely due to reductions in both productivity and habitat heterogeneity. The β-diversity was influenced by both replacement and richness difference partitions suggesting a complex interaction of productivity, spatial heterogeneity and dispersal. The disturbance regime and habitat heterogeneity likely explain the lowest β-diversity in the abyssal plain and the highest β-diversity in the lower shelf-upper slope. Functional composition showed species adaptations to the environment, with shallow assemblages composed mostly of suspension feeder, actively mobile or sedentary, and unattached, byssally attached or cemented species, whereas abyssal assemblages were functionally less diverse. Based on death assemblages, we highlight the key role of bathymetry on diversity patterns, broaden the distribution records of bivalves across the southern GoM, and document 16 new bivalve species for this region.