Neuronal development in larval mussel Mytilus trossulus (Mollusca: Bivalvia)

Although our understanding of neuronal development in Trochozoa has progressed substantially in recent years, relatively little attention has been paid to the bivalve molluscs in this regard. In the present study, the development of FMRFamide-, serotonin- and catecholamine-containing cells in the mussel, Mytilus trossulus, was examined using immunocytochemical and histofluorescent techniques. Neurogenesis starts during the trochophore stage at the apical extreme with the appearance of one FMRFamide-like immunoreactive (lir) and one serotonin-lir sensory cell. Later, five FMRFamide-lir and five serotonin-lir apical sensory cells appear, and their basal fibres form an apical neuropil. Fibres of two lateral FMRFamide-lir apical cells grow posteriorly and at the time that they reach the developing foot, the first FMRFamide-lir neurons of the pedal ganglia also appear. Subsequently, FMRFamide-lir fibres grow further posteriorly and reach the caudal region where neurons of the developing visceral ganglia then begin to appear. In contrast, the five apical serotonin-lir neurons do not appear to project outside the apical neuropil until the late veliger stage. Catecholamine-containing cells are first detected in the veliger stage where they appear above the oesophagus, and subsequently in the velum, foot, and posterior regions. Though neural development in M. trossulus partly resembles that of polyplacophorans in the appearance of the early FMRFamidergic elements, and of scaphopods in the appearance of the early serotonergic elements, the scenario of neural development in M. trossulus differs considerably from that of other Trochozoa (bivalves, gastropods, polyplacophorans, scaphopods and polychaetes) studied to date.