ISSN : 1225-3480
The gametogenic cycle and the spawning season in female and male Protothaca (Notochione) jedoensis were investigated by quantitative statistical analysis and sexual maturity was studied by qualitative histological analysis. In females, monthly changes in the percents of the ovarian tissue area to total tissue area, the percents of the follicle area to total tissue area, the percents of the follicles area to the ovarian tissue area, and the percents of the oocyte area to the ovarian tissue area increased in February and reached the maximum in April, and then gradually decreased from May to July, with the spawning peak between June and July. In males, monthly changes in the percents of the testicular tissue area to total tissue area, and the percents of the spermatogenic stage area to the testicular tissue area increased in February and reached the maximum in April and then showed a rapid decrease from May to July. From these data, it is apparent that the number of spawning seasons in female and male P. (N.) jedoensis occurred once a year, from May to July, with the spawning peak between June and July. Therefore, P. (N.) jedoensis in both sexes showed a unimodal gametogenic cycle during the year. Compared the gametogenic cycle by quantitative statistical analysis in 2007 with the previous qualitative results (Chung and Ryou, 2000) in female and male P. (N.) jedoensis, the results of the gametogenic cycle calculated by quantitative statistical analysis showed some differentiations between two spawning seasons in comparisons of results evaluated by the gonad index by qualitative histological analysis (through individual subjective assessment) including several errors. The spawning season evaluated by quantitative statistical analysis was from May to July, and that evaluated by histological gonad index was June to July. The interval of beginning of two spawning seasons showed one month between them. Therefore, the spawning season of this species should be checked in detail by quantitative analysis because this species has been noted as a target organism for aquaculture. The percentages of sexual maturities of female and male clams ranging from 30.1 to 35.0 mm in length by qualitative histological analysis of this species were 53.3% and 61.1%, respectively, and 100% for clams over 45.1 mm length. The biological minimum size (considering to 50% of group sexual maturity) in female and male clams by quantitative analysis of this species are 32.01 mm in shell length in females and 30.58 mm in males, respectively. According to the growth curves for the mean shell length fitted to von Bertalanffy’s equation by Kim et al.(2003b), individuals ranging from 25.1 to 30.0 mm in shell length were considered to be two years old because Kim et al.(2003) reported that 30.58 and 32.01 mm in shell length were considered to be two years old. Therefore, we assume that both sexes of this population begin reproduction from two years of age. In terms of natural resource management, the present study suggests that harvesting clams less than 30.6 mm in shell length (< 2 years old) can potentially lead to a drastic reduction in recruitment.
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