A study of the sex ratio and fin morphometry of the Thai medaka, Oryzias minutillus, inhabiting suburbs of Bangkok, Thailand

概要

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Thai medaka were collected from 10 localities in the suburbs of Bangkok, Thailand. The sex ratios (male to female) were almost 1:1 in localities 1, 2, 4, 5, 7, and 9 and seemed to be normal. In contrast, in localities 3, 6, 8, and 10, the sex ratios were unbalanced; they were 1:2.9, 1:3.4, 1:3.0, and 1:2.8, respectively. Furthermore, irrespective of the normal or abnormal sex ratios, many individuals whose morphological characteristics did not identify them as males or females were found. The indexes showing the secondary sex characters of the sex-undeterminable individuals, namely, the values (%) of dorsal fin height (HD) divided by standard length (SL) and anal fin height (HA) divided by SL were between the values of normal males and normal females. From the sediment of two ponds in which the sex ratio was unbalanced, a low level of DDT was detected. Although no hermaphrodite gonads were histologically found from those sex-undeterminable individuals, it may be noted that the number of individuals which had testes was almost twice that of those with ovaries. Taking these results into considerations, it is surmised that some feminizing stresses may be triggered in males of the Thai medaka, probably by artificial chemicals.