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DEVELOPMENT OF THE NERVOUS SYSTEM AND MUSCULATURE IN THE BIVALVE LARVAE, M. EDULIS. J.T.Plummer and R.P.Croll. Department of Physiology and Biophysics, Dalhousie University, Halifax, N.S., Canada

     Gravity normally acts as an orientation cue allowing bivalve veliger larvae to migrate through a water column.  In addition, larvae from a previous spaceflight mission exhibited abnormal swimming behaviour, thus prompting our current efforts to understand the physiological mechanisms underlying locomotion in these organisms. The helical swimming pattern that is typical of bivalve larval behaviour is thought to be modulated by neuronal and muscle control. However, to date little is known about the either the nervous system or the musculature in bivalve larvae. Given that bivalve veliger larvae from a previous spaceflight mission exhibited abnormal swimming behaviour, it was necessary to investigate the possible role of the nervous system and musculature in larval behaviour. Histological studies revealed an extensive nervous system  and musculature in the very earliest stages of development. Do you have enough space to add details about which transmitters are found?Furthermore neuronal innervation of the velum, the main swimming structure, appears to coincide both spatially and temporally with  muscle innervation.  These findings suggest a possible role of the nervous system in regulating the behavioural changes seen in microgravity. Consequently, pharmacological studies have provided evidence which suggests possible roles for these early neuronal elements. The neurotransmitters, serotonin and catecholamines have been found in locomotory organs and also appear to act as excitatory and inhibitory agents, respectively, which alter the normal swimming pattern of bivalve veliger larvae. Together, these data provide a more complete understanding of the neuronal elements, musculature and pharmacology which may act to control larval swimming behaviour. This study provides the foundation for future work on the role of the gravity in modulating both muscle and neuronal development in molluscan systems. 

     (Supported by: the Canadian Space Agency)