The sex ratio (F:M) in the same population of oyster, Crassostrea gigas at the commencement of the study (2007) was 1:1.0, but changed to 1:2.8 by the end of the study (2008). The sex reversal rate in two-year-old oysters was 40.2%. Specifically, female to male sex reversal rate was 66.1%, which is higher than the male to female sex reversal rate of 21.1%. The sex reversal pattern of C. gigas appears to go from male⇒female⇒male, and as such is determined to be rhythmical hermaphroditism.

A well-established xenoandrogen, tributyltin (TBT) causes sexual interferences in various animal models including gastropods. In bivalves, the disrupting effects of TBT on reproduction are largely unevaluated. In this study, we carried out a test on reproduction of the bivalve (Gomphina veneriformis) affected by TBT (0.4, 0.6, and 0.8 μg L −1 ). The experimental period was 36 weeks, starting in October during the inactive stage of the clam's reproductive cycle. The sex ratio (F:M) was 1:1.00 in the control group but changed to 1:1.37 in all the exposed groups with 0.8 μg L −1 TBT, leading to significantly higher values in male (1:1.54; P = 0.009). Gonadal maturity (ripe stage frequency) of males in the exposure groups was higher than females. The gonad index (GI) of females significantly decreased when exposed to 0.6 and 0.8 μg L −1 TBT. The prevalence of intersex was significantly different between males and females in all exposure groups, which became significantly different (P < 0.001) after 20 weeks. A high prevalence of intersex (67.80%) was detected in 0.8 μg L −1 after 36 weeks. These results suggest that TBT, a xenoandrogen, may lead to skewed sex ratio toward males, inhibit sexual maturation of the females, and also induce intersex in G. veneriformis as an imposex of gastropods. Therefore, reproductive indicators (sex ratio, gonadal development, and intersexuality) of bivalves may be used as useful bioindicators in monitoring marine pollution by TBT compounds.

This study attempts to propose the possibility of the sex reversal in Sinonovacula constricta and Gomphina veneriformis by confirming the changes in the sex ratio with the shell length in the same population level. For analysis of sex ratio, 100 individuals of S. constricta (SL 26.5-95.0 mm) and 2385 individuals of G. veneriformis (SL 15.1-60.1 mm) were used. Sex was analyzed histologically. Both species displayed the tendency of increase in the female proportion with increase in shell length. In this study, changes in the sex ratio in accordance with the growth of S. constricta and G. veneriformis are determined to be indirect evidence that signifies their sex reversal.

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