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Both female and male sea urchins accumulate the major yolk protein (MYP) in the nutritive phagocytes of immature gonads before gametogenesis. Additionally, the most abundant protein in the coelomic fluids of both sexes is MYP. In females, MYP in the coelomic fluid is taken up by the nutritive phagocytes and then transported into growing oocytes. In this study, quantitative changes of MYP in the coelomic fluid of both sexes were examined during the reproductive cycle of the wild sea urchin, Mesocentrotus nudus. In females but not males, positive correlation between MYP level and the gonadosomatic index was observed. These results indicate that MYP in the coelomic fluid is suitable as a biomarker of the onset of puberty and progression of maturity in female sea urchins.

We examined the reproductive cycle and transcription-level changes of the major yolk protein (MYP) in gonads of both sexes during the different reproductive stages of wild northern sea urchin, Mesocentrotus nudus, collected in two areas of southern Hokkaido, Japan. Histological evaluation revealed mature gonads in both April and August, suggesting two annual spawning seasons at Otoshibe, whereas one annual spawning season was observed in autumn (September) at Usujiri. The transcription levels of MYP, which is synthesized and stored in the gonads, increased from stage 1 (recovering) to stage 2 (growth) and thereafter decreased, in both female and male sea urchins. The current results indicate that two annual spawning seasons may occur in wild populations of the northern sea urchin in southern Hokkaido.