Role of urine in the spawning of female rose bitterling,Rhodeus ocellatus ocellatus

This study clarified the spawning mechanism of female rose bitterling, Rhodeus ocellatus ocellatus, with special emphasis on the physical role of urine. Ovulated females ingested a significantly greater quantity of water by drinking than non-ovulated fish. The body weight of ovulated females increased about 1.3% while it decreased 1.0% in non-ovulated females. Urine volume in the urinary bladder increased rapidly before spawning, synchronized with the spawning cycle. On the other hand, little urine remained in specimens which had completed spawning. Histological observation demonstrated that the oviduct and the ureter joined at the proximal part of the ovipositor. On the basis of these anatomical and physiological results, it is proposed that urine plays the physical role of pushing the ovulated eggs through the elongated ovipositor during spawning.