KEMPF, S.C.; Auburn University: Changes in sensory morphology of the apical ganglion during larval development

Gastropod larvae, as well as those of many other invertebrate species, possess an apical ganglion that contains two or more types of sensory neurons, as well as interneurons that presumably mediate sensory function. Work by S.C. Kempf, L.R. Page and A. Pires. (1997. J. Comp. Neurology. 386: pp. 507-528) has provided a detailed description of the structure and sensory innervation of this ganglion in larval opisthobranchs. They suggested that the serotonergic sensory neurons function as a compensatory system regulating velar orientation during swimming and feeding, Serotonergic innervation of the velar lobes originating in the apical ganglion suggests that this is accomplished by modulation of velar muscular contractility. If this is the case, one might expect that as larvae feed and increase in size, the separation of the lateral, serotonergic, sensory dendrites would increase as a means of improving their ability to detect left/right differences in orientation of the velar lobes. In planktotrophic larvae of Melibe leonina, separation of these dendrites increased from14.5 + 1.3 um at hatching (0 d) to 65.6 + 7.2 um at competence (32 d), supporting this hypothesis. An increase in dendrite separation was also seen in larvae of Tritonia diomedea; however, separation in competent larvae was only about 1/3 that seen in competent larvae of M. leonina. In lab culture, older larvae of M. leonina are active swimmers usually present in the water column of culture vessels, while older larvae of T. diomedea are less active and usually demersal, remaining on the bottom of culture vessels. If this behavior mimics that in the field, the smaller increase in separation of the lateral sensory dendrites in less active, demersal larvae of T. diomedea may reflect a decreased need to sense differences in velar orientation than is the case in actively swimming, planktonic larvae of M. leonina.