[67]

THE ROLE OF THE NERVOUS SYSTEM IN MODULATING  GRAVITY-DEPENDENT SWIMMING BEHAVIOUR IN THE LARVAE OF MYTILUS  EDULIS.  J. T. Plummer¹, D. L. Jackson² and R. P. Croll¹,  ¹Dalhousie University and ²Department of Fisheries and Oceans,  Halifax, N.S., Canada.

   Previous flight studies in which bivalve veliger larvae deviated from their normal helical swimming in a micro-gravity environment prompted our investigation into the possible role of the larval nervous system in controlling this behaviour.  Immunocytochemical techniques coupled with pharmacological studies have provided evidence for a direct control of swimming behaviour by the larval nervous system in in bivalve molluscs.  Both serotonin- and catecholamine-containing axons innervate the velum, which serves as tthe primary swimming organ in the larvae.  In addition, serotonin and its reuptake inhibitor, fluoxetine,  appeared to have a cilio-excitatory effect on larvae causing them to swim at a greater velocity.  Conversely, the serotonin antagonist, mianserin, had the opposite effect.   The catecholamines, dopamine and norepinephrine, appeared to directly decrease ciliary activity.  These results were confirmed by treatment with spiperone, an antagonist to catecholamines which increased swimming velocities. The cilio-inhibitory effect of  catecholamines caused the animals to not only decrease their swimming speeds, but also to deviate from their normal helical upward swimming pattern.  The compilation of these results suggest a specific role of the larval nervous system in mediating patterns in bivalve larval swimming, and are consistent with the hypothesis  that the alterations observed in micro-gravity may possibly result from underlying changes in  neuronal development.

(Funding provided by the Canadian Space Agency).

[