Impact of Micro- and Hypergravity on Neurovestibular Issues of Fish

For decades, research in altered gravitational environments has been undertaken to elucidate the impact of gravity on a broad variety of biosystems from unicellular organisms to vertebrate animals. In the preparation of scarce and costly orbital missions, different short-term flight opportunities (drop-tower flights, parabolic aircraft flights, sounding rocket flights) as well as ground-based facilities like centrifuges and microgravity simulators are being used. Here, we present an overview of studies carried out under short-term and long-term altered gravity on fish, with a focus on vestibular issues of cichlid fish (Oreochromis mossambicus) larvae and juvenile swordtails (Xiphophorus helleri). These experiments were focused on their behaviour, analyses of neuronal tissues, epithelia of utricle and saccule and as well on inner ear stones, the otoliths. Kinetoses (motion sickness) were frequently observed in altered, especially diminished, gravity, and evidence could be provided that asymmetric otoliths are a major factor in kinetosis susceptibility. Furthermore, we could show that the biomineralization of otoliths is adjusted towards gravity by means of a neuronally guided feedback loop