Catecholamine-Containing Cells in Larval and Postlarval Bivalve Molluscs.

Previous studies have suggested an involvement of catecholamines in the control of several larval behaviors, such as feeding, locomotion, and induction of settling and metamorphosis. In the present study we employed aldehyde-induced, blue-green fluorescence to indicate catecholamines in cells within representatives of two bivalve families, the Pectinidae (Placopecten magellanicus) and the Mytilidae (Mytilus edulis). Larvae were examined at different stages of development before and also shortly after settlement. The general distribution of fluorescent cells was similar in the two species. By midveliger stage, several fluorescent cells and fibers were located along the outer rim of each velar lobe, and a pair of flask-shaped cells was located lateral to the mouth. A single fiber from near the mouth projected to a region beneath the apical tuft. In the pediveliger, the cells by the mouth were joined by an additional two to four fluorescent cells. The developing foot also contained numerous such cells, some of which had processes that penetrated the epithelium on the "sole" and bore ciliated terminals. Fluorescent somata were also located around the edge of the mantle. Centrally projecting fibers appeared to terminate in the pedal and abdominal ganglia, which also contained a few fluorescent somata. After settlement, the velar lobes and resident fluorescent somata disappeared, but fluorescent cells in the foot persisted as this latter organ grew. Fluorescent cells within the developing gill were connected with the abdominal ganglia by means of fibers. Control preparations labeled with antibodies raised against serotonin indicated that the aldehyde-induced fluorescence was not due to the presence of indoleamines. The present study not only confirms previous chromatographic evidence suggesting the presence of catecholamines in the larvae of bivalve molluscs, but also identifies putative neuronal circuits that may control various larval behaviors.