Carnivory is unusual among bivalve molluscs and is limited to a few families in the distantly related orders Pectinida, Mytilida and Anomalodesmata. Despite the significance of dietary shifts in the evolution of the bivalves, the anatomy of the alimentary system, and of the gastric chamber in particular, has been described in detail for only a few carnivorous species. Here we describe the anatomy of the gastric chamber in a pectinid, Propeamussium jeffreysii, and an anomalodesmatan, Bathyneaera demistriata, expanding the known morphological disparity of the alimentary system in both groups. We found the stomachs of both to be modified to varying degrees for a carnivorous habit, with thickened, muscular walls, extensive cuticular linings, and reduced sorting areas and gastric chamber compartments (i.e. the dorsal hood, the left pouch and the food-sorting caecum). Despite some superficial similarity, each retains distinct hallmarks of their ancestry among filter-feeding relatives, allowing precise homology assessment of individual characters to differentiate between them. In addition, we found that the gastric chamber of P. jeffreysii represents an intermediate morphology between previously described P. lucidum and filter-feeding pectinids. Consequently, variation in the anatomy of the gastric chamber in Pectinida parallels a previously identified trend towards greater specialization for carnivory in the Anomalodesmata. Our results indicate that the current classification scheme of stomach types does not reflect phylogenetic affinity across the Bivalvia and highlight the need for accurate homology assessment of individual characters of the gastric chamber for inferring evolutionary relationships.
