Form and function of the larval nervous system in molluscs

Recent studies have demonstrated that molluscs possess extensive larval nervous systems, which begin to develop by the early trochophore stage, and thus predate the first appearance of neurons within the ganglia of what will become the adult central nervous system. In addition, many of the larval neurons disappear after metamorphosis, as do the other larval structures (e.g., prototroch or velum, retractor muscles, etc.), which they innervate. Several studies have also demonstrated common features that are shared within the larval nervous systems of the various gastropods and bivalves examined to date. Specifically, the early larval nervous systems of such molluscs include an apical organ containing serotonergic neurons, various peripheral neurons associated with the velum, foot and mouth and often posterior peptidergic cells. Research is also beginning to elucidate the roles of some of these neurons in the control of larval behaviour. For instance, serotonin-, catecholamine- and FMRFamide-containing cells all appear to innervate the velum, thus suggesting roles in withdrawal behaviour and/or swimming and feeding. In later larval stages, all these neural elements must also operate in concert with neurons resident within the developing ganglia that will form the adult central nervous system. Together, such research illustrates a degree of complexity within molluscan larvae that was unappreciated until recently and suggests new foci for the study of the evolution of larval forms within this phylum.