Studies on vestibular organs in the fry of mutant Medaka

We provide an overview of the features of vestibular organs, especially of otolith organs in mutant Medaka ha. The ha fish have a dysfunction in sensing gravity due to the absence of utricular otoliths either unilaterally or bilaterally. Especially in their early developmental stages, the posture control is predominantly depend on light rather than on gravity seen in the wild-type. The ha juveniles that were kept and tolerated in an environment with altered light directions indicated a strong light dependency on the posture maintenance, while the fish under an ordinary light environment gradually changed their posture control by sensing light to gravity around 30 days after hatching and beyond. Their behavior under microgravity agreed with such phenomena observed in ground laboratory.