Annual reproductive cycle of the helmet crab Telmessus cheiragonus on the sublittoral zones in Usujiri, southern Hokkaido, Japan

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Annual reproductive cycle of the helmet crab Telmessus cheiragonus was investigated based on the histological observations of the ovary and the gonadosomatic indices (GSI = ovarian weight x 100 / body weight). The ovarian maturity was divided into five phases (Phase I-V). Minimum mature size of female was 36.0 mm in carapace length. The annual reproductive cycle in the adult females (more than 36.0 mm carapace length) consisted of the incubating period (October-March) and the maturing period (April-September). During the incubating period, the ovarian maturity was Phase I-II (mean GSI±SD = 3.8±4.7 %) when most of the females were ovigerous. During the maturing period, the ovarian maturity increased to Phase V (GSI ≤ 21.6 %) when the females seemed to be frequently copulating, because the soft shell adult females, which could copulate, occurred with highest frequency. The spawning of most females seemed to occur at the end of the maturing period. These results suggest that the ovarian maturation cycle requires one year, and during the incubation period ovaries are immature.