Mutant Medaka fish having a deficiency in otolith-vestibular system and their behavior in microgravity.

Vestibular and visual senses are two major factors fish use for controlling their posture under 1 G conditions. From various studies in the past (parabolic flights and some ground-laboratory experiments), I came to have a hypothesis that "heavily eye-dependent" fish are tolerant to microgravity (micro-G). Theoretically, for "heavily eye-dependent" fish, the following three types may exist. (a) Fish having good eyesight, and having an ordinary sensitivity to gravity, (b) Fish having an ordinary eyesight, and having less sensitivity to gravity, (c) Fish having good eyesight, and having less sensitivity to gravity. In Medaka fish (Oryzias latipes), these three types have successfully been obtained. In this paper, the behavior of such different types of Medaka fish under micro-G will be studied by use of parabolic flights of an airplane. The ground-laboratory experiments are also reported, which focus on the contribution of both vestibular and visual senses to the posture control for each of three types of Medaka fish.