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Effects of Simulated Microgravity on Swimbladder Development in the Zebrafish (Danio rerio).  R.P. Croll¹, B.W. Lindsey², T.C. Dumbarton¹, G.N. Robertson², and F.M. Smith^{2 .  1}Department of Physiology & Biophysics, ²Department of Anatomy & Neurobiology, Dalhousie University, Halifax, N.S., Canada

   Teleost fishes have evolved gas-filled swimbladders to counteract normal gravitational forces, thus making these animals neutrally buoyant in their natural environments. In previous studies we examined the normal morphology of the swimbladder system together with its associated musculature, vasculature and autonomic innervation in the zebrafish. We have also examined the normal development of the swimbladder and its innervation from pre-inflation, through the initial single-chambered stage and finally to the double-chambered stage of the adult.  Here we test whether swimbladder development is dependent upon normal gravitational input by examining the effects of exposing eggs and larvae to simulated microgravity (SMG) in a NASA-designed rotating wall vessel (“bioreactor”) for the initial 96 hours after fertilization. Only 14% of SMG-exposed larvae had inflated swimbladders, compared with 62% of age-matched controls when first removed from the bioreactor. Moreover, in those animals with inflated swimbladders, the mean bladder volume was significantly reduced in experimental vs. control animals. A delay in the swim-up stage was also observed in SMG-exposed animals, but subsequent testing revealed little difference in swimming behaviour of animals at later developmental stages. There were also no obvious changes in the anatomical patterns of musculature, vasculature, or innervation between these two groups. Collectively, our findings suggest that despite initial differences in inflation rates and swimming behaviour, animals exposed to SMG early in life appear subsequently to develop normal swimbladders and swimming behaviours.

(Supported by Canadian Space Agency Contract 9F007-046016.)