PLUMMER, J T; CROLL, R P; Dalhousie University; Dalhousie University: The development and role of the larval nervous system in bivalve molluscs.

A comprehensive approach was used to investigate the morphology of the larval nervous system (LNS) in bivalves and how its neuronal subcomponents function to regulate behaviour. The development of the LNS in the model bivalve species, Mytilus edulis, was investigated by means of immunocytochemical and histofluorescent labelling. Neuronal development began with the establishment of cells and processes which predate gangliogenesis and lay outside the boundaries of the CNS. Neuronal components were added during larval development, such that by the time that the larva is competent for metamorphosis, the LNS is comprised of an apical organ, a rich network of peripheral cells and processes and the connectives and ganglia of the developing CNS. At metamorphosis certain components of the LNS were lost, while others appeared to become incorporated into the CNS and peripheral nervous system of the juvenile and adult mussel. Comparisons of the LNS involved larvae of six bivalve species which vary in their larval lifestyle. Although cell types and numbers vary, the general design of the bivalve LNS described above holds true for all species presently examined. Furthermore, interspecific similarities and differences elucidated the potential use of the arrangement of the LNS in further understanding the dynamics between phylogenetic relationships and functional life history constraints for the basis of its design. Finally, the role of identified monaminergic neurones examined through the use of synthetic analogues to serotonin and catecholamines. A series of behavioural tests suggested neuronal control in regulating larval swimming which involves a dual innervation of cilio-excitatory serotonergic and cilio-inhibitory catecholaminergic input to the cells of the main swimming structure, the velum.