Existence of an extra-retinal and extra-pineal photoreceptive organ that regulates photoperiodism in gonadal development of an Osmerid teleost, ayu (Plecoglossus altivelis)

Ayu (Plecoglossus altivelis) is an Osmerid teleost whose gonadal development exhibits clear photoperiodism: it is stimulated and prevented under short and long photoperiod, respectively. However, the photoreceptor organ involved in this process remains to be identified. In the present study, we examined whether gonads of ophthalmectomized (Ex) and pinealectomized (Px) ayu respond to short photoperiod to test whether photoreceptors other than lateral eyes and pineal complex are involved in the photoperiodic response of gonadal development. Gonadosomatic index (GSI) and plasma levels of sex steroids (testosterone and 11-ketotestosterone for males and testosterone and estradiol 17-β for females) were significantly increased in the Ex+Px ayu kept under short photoperiod in both males and females as compared with the initial control. On the other hand, there were no significant increases in GSI and sex steroids in the Ex+Px ayu kept under long photoperiod. Histological observation of gonads in the Ex+Px ayu revealed that oocytes undergoing final maturation in females and proliferation of germ cells in males were observed under short photoperiod but not under long photoperiod. These results indicate that extra-retinal and extra-pineal photoreceptive organ regulates photoperiodic gonadal development in this species.