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DEVELOPMENT OF THE NERVOUS SYSTEM AND ITS CONTROL OF GRAVITY-DEPENDENT BEHAVIOURS IN LARVAE OF BIVALVE MOLLUSCS. J.T. Plummer, D.L. Jackson, and R.P. Croll. Dept. of Physiology and Biophysics, Dalhousie University, Halifax N.S. Canada.

     The effects of microgravity on the behaviour of larval mussels (Mytilus edulis) were studied in the Aquatic Research Facility (ARF) during the ten-day STS-77 mission. Locomotion patterns observed in microgravity differed significantly from those seen in normal gravity. Larvae in microgravity frequently altered their swimming directions and the dimensions of their helical swimming paths. Gravity was determined to be the primary orientation cue for these animals, as no directional orientation was exhibited by larvae in space. These findings suggest that microgravity either unmasks behaviours not expressed in normal gravity or causes certain behaviours to developed abnormally. In an effort to understand these effects on behaviour, we are currently examining the development of the larval nervous system and its innervation of peripheral target tissues such as the locomotory cilia of the velum. To date we have used immunocytochemistry and other histochemical techniques to provide a comparative study of the normal positions and morphologies of neurones which contain a range of transmitters including serotonin, catecholamines and various peptides such as FMRFamide, APGWamide and small cardiac peptide (SCP) in bivalve larvae. We are also developing techniques to permit detailed studies of the statocysts, the gravireceptive organs in these organisms. This work on normal larval development in M. edulis and other bivalves will form the foundation for studies of the effects of microgravity on neural development in a future ARF flight.

(Funding provided by the Canadian Space Agency.)