11 - Mollusca: Bivalvia

Abstract

North American (NA) freshwater bivalve molluscs (class Bivalvia) fall in the subclasses Paleoheterodonta (Superfamily Unionoidea) and Heterodonta (Superfamilies Corbiculoidea and Dreissenoidea). They have enlarged gills with elongated, ciliated filaments for suspension feeding on plankton, algae, bacteria, and microdetritus. The mantle tissue underlying and secreting the shell forms a pair of lateral, dorsally connected lobes. Mantle and shell are both single entities. During development, the right and left mantle lobes extend ventrally from the dorsal visceral mass to enfold the body. Each lobe secrets a calcareous shell valve which remains connected by a mid-dorsal isthmus. Like all molluscs, the shell valves consist of outer proteinaceous and inner crystalline calcium carbonate elements. The lateral mantle lobes secrete shell material marked by a high proportion of crystalline calcium carbonate making them thick, strong and inflexible, while the mantle isthmus secretes primarily protein, forming a dorsal elastic hinge ligament uniting the calcareous valves. The hinge ligament is external in all freshwater bivalves. Its elasticity opens the valves while the anterior and posterior shell adductor muscles run between the valves and close them in opposition to the hinge ligament which opens them on adductor muscle relaxation. The key presented in this chapter, as well as many of the major keys to freshwater bivalves, relies on the integrated use of important anatomical structures and shell characters to identify unionoidean bivalves. This key is artificial, and it is divided into four sections corresponding to geographical provinces to facilitate identification.