Mollusca: Bivalvia

AbstractWith approximately 15,000 described recent species, Bivalvia is the second largest molluscan class-level taxon. Compared to other conchiferan molluscs, the bivalve body plan has undergone several reductions, such as the lack of a distinct head including radula and buccal muscles, which is also reflected in the relatively simple nervous system exhibited by most bivalves. In autobranch bivalves, three pairs of ganglia are present: the cerebropleural ganglia (as a fusion product of the cerebral and pleural ganglia), the pedal ganglia, and the visceral ganglia. The pedal ganglia are often fused in the midline of the animal. Other ganglia such as, for example, buccal ganglia, are considered to be secondarily lost. The arrangement of the longitudinal nerve cords follows the molluscan-like tetraneural condition. The pedal ganglia are connected to the cerebropleural ganglia by two connectives. Bivalves have a remarkably high number of sensory organs, particularly at the mantle folds and in the oral region. The statocysts are situated within the foot and a paired osphradium, either innervated by an osphradial nerve from the visceral ganglion, or by a distinct osphradial ganglion, is often found. Photoreceptor cells or eyes may occur along the mantle margin, ranging from unicellular, cup-like ocelli to complex, lense-bearing eyes. Reliable data on bivalve neurogenesis and larval neuroanatomy are scarce. Larvae of Mytilus trossulus possess five serotonin-like and five FMRFamide-like cells in the apical organ. Taken together, the few data currently available permits no detailed comparison of neurogenesis in bivalves to that of other molluscs.