Ontogeny and Sexual Differences in Swimming Proximity to Conspecifics in Response to Visual Cues in Medaka Fish.

Adult medaka fish (Oryzias latipes) exhibit complex social behaviors that depend mainly on visual cues from conspecifics. The ontogeny of visually-mediated social behaviors from larval/juvenile to adult medaka fish, however, is unknown. In the present study, we established a simple behavioral paradigm to evaluate the swimming proximity to conspecifics based on visual cues in an inter-individual interaction of two medaka fish throughout life. When two fish were placed separately in a cylindrical tank with a concentric transparent wall, the two fish maintained close proximity to each other. A normal fish inside the tank maintained proximity to an optic nerve-cut fish outside of the tank, while the converse was not true. This behavioral paradigm enabled us to quantify visually-induced motivation of a single fish inside the tank. The proximity was detected from larval/juvenile to adult fish. Larval fish, however, maintained close proximity not only to conspecifics, but also to heterospecifics. As the growth stage increased, the degree of proximity to heterospecifics decreased, suggesting that shoaling preferences toward conspecifics and/or visual ability to recognize conspecifics is refined and established according to the growth stage. Furthermore, the proximity of adult female fish was affected by their reproductive status and social familiarity. Only before spawning, adult females maintained closer proximity to familiar males rather than to unfamiliar males, suggesting that proximity was affected by familiarity in a female-specific manner. This simple behavioral paradigm will contribute to our understanding of the neural basis of the development of visually-mediated social behavior using medaka fish.