KEMPF, S.C.; Auburn University: SEROTONERGIC CIRCUITRY IN THE NERVOUS SYSTEM OF A BIVALVE VELIGER LARVA

While a considerable amount of research has focused on the larval shell, larval feeding, assays of neuroactive compounds, etc. of bivalve veligers, there is surprisingly little work on the structure of the larval nervous system and the circuitry responsible for behaviors such as swimming, feeding, settlement and metamorphosis. In this preliminary study, serotonergic innervation in the eyed, pre-pediveliger larva of Crassostrea virginica is examined. Serotonin specific antibodies label 3-5 neuron parikarya in each cerebral ganglion (CG) and 1-2 parikarya in the apical ganglion (AG). One serotonergic neuron in the AG extends a thick, presumably dendritic, process to the pre-trochal surface. There is also a single neuron positioned along both the left and right cerebral-visceral connectives about half-way between each CG and visceral ganglion. Neurons in the CG and AG project axons into the velar lobes, across the cerebral commissure, through the cerebral-visceral connectives, and across a visceral commissure. Axons projecting from the AG and/or CG into the velar lobes form a dense network that is in close association with the velar ciliated cells. Additional axons project into the larval foot and also the mantle tissues. While serotonergic innervation is not present in the developing gill rudiment or the digestive organs, a few putative serotonergic neuron parikarya and axons are found along the ventral side of the esophagus. In addition, serotonergic axons are present adjacent to the mouth and anus. The observed labeling suggests both sensory and modulatory functions for the serotonergic component of the nervous system. Serotonergic innervation of densely ciliated structures such as the velum and foot may indicate a role in modulation of ciliary activity as is known for serotonin in adult mollusks. (MASCG Project #R/SP-11-PD)