The existence of extra-retinal and extra-pineal photoreceptive organ regulating gonadal development of ayu, Plecoglossus altivelis

The photoreceptors regulating photoperiodic gonadal development of ayu, Plecoglossus altivelis, are not known. In this study, we examined whether the gonads of ophthalmectomized and pinealectomized (Ex+Px) ayu respond to short photoperiod. Gonadosomatic index (GSI) and plasma levels of sex steroids were significantly increased in Ex+Px ayu kept under short photoperiod in both males and females. On the other hand, there were no significant increase in GSI and sex steroids in Ex+Px ayu kept under long photoperiod. The histological observation of the gonads revealed that oocytes undergoing final maturation in females and proliferation of germ cells in males were observed only in Ex+Px ayu kept under short photoperiod. Altogether, our results strongly suggest unknown photoreceptive organ regulates photoperiodic gonadal development of ayu.