Vasotocin expression is associated with social preference development of the medaka fish

•Social preference increased during development after hatching in young medaka fish.•VT expression increased during social behavior development in young medaka fish.•Social preference and VT expression decreased in the isolated larvae after hatching.•VT may play a key role in development of sociability in the medaka fish. The neurohypophysial peptide arginine vasotocin (VT) and its mammalian ortholog, arginine vasopressin, function in physiological and behavioral events. These functions have been identified in neuroendocrinological studies using adult animals; however, there is little information on whether VT is associated with social behavior development in fish. Here, we examined social preference in medaka fish of various ages and investigated how VT expression changes during development. The 1-, 2-, 4-, and 8-week post-hatching (wph) larvae, juveniles, and 5-month-old adults were individually introduced to the grouped fish of each age group, and the social preference index (SPI) was compared among ages based on the time spent in the interaction zone near the grouped fish in a test tank. The SPI was significantly higher in the 4-wph larvae, 8-wph juveniles, and adult fish than in the 1- and 2-wph larvae. VT expression increased with age from 1 to 4 wph. Similarly, the expression was high in 4-wph, 8-wph, and adult fish. Furthermore, it was also found that the SPI and the VT expression decreased in the socially isolated larva during the 4 weeks after hatching compared to the levels in the grouped 4-wph larvae. These findings suggest that social preference develops with age and that conspecifics are necessary for social development in medaka larvae. Furthermore, our results suggest that VT is associated with the development of social preferences in medaka.