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Chronic effects of methylmercury on the reproduction of the teleost fish, Oryzias latipes Chan, Kenneth Ka-Sing

Abstract

This study evaluates the toxicity, accumulation, chronic effects and mode of action of methylmercury on the reproduction of the teleost fish, Oryzias latipes. The median lethal concentration, 96h-LC50, for adult Oryzias was + 88+9.8 μg CH₃Hg⁺/1 as determined in a static system. Residue analysis by gas chromatography showed that fish exposed to 43 to 1000 μg CH₃Hg⁺/1 had tissue levels below 40 μg CH₃Hg⁺/g while fish exposed to more than 1000 μg CH₃Hg⁺/1 accumulated methylmercury steadily and reached levels as high as 408.1 μg CH₃Hg⁺/g. Death seems to occur when tissue level reaches 25 μg CH₃Hg⁺/g. Studies on long-term exposure to 0.0, 4,3, 10.7 and 21.5 μg of methylmercury in a flow-through system confirmed this observation. Four-hour exposure of 8.5 and 42.9 μg/1 of methylmercury on alternate days during the fish's normal oviposition period resulted in inhibition of oviposition. This observation occurred only oh days when fish were exposed to methylmercury but not on days when fish were returned to clean water. However, at a concentration of 85 μg CH₃Hg⁺/1, complete inhibition was observed even on days when fish were returned to clean water. High rates of accumulation with low rates of excretion of methylmercury were suggested explanations for these observations. Six-week exposure to 4.3, 10.7 and 21.5 μg/1 of methylmercury resulted in inhibition of spawning. This inhibition was directly related to the log of exposure concentrations. At the end of six weeks, both male and female gonads showed reduction in size; the females were more sensitive. However, hatchability of the spawned eggs was not affected by the exposure. Juvenile fish were very sensitive to methylmercury. After two weeks of exposure, one-week old juvenile exposed to 0.0, 4.3, 10.7 and 21.5 μg CH₃Hg⁺/1 had mortality rate of 2.2%, 54.3%, 64.9% and 99.4% respectively. Synthetic LH-RH, at concentrations of 100 and 1000 ng/g body weight, was effective in stimulating ovarian development in Oryzias. This shows that the LH-RH (synthesis based on structure of porcine LH-RH) has biological activity in Oryzias. When exposed to methylmercury, spawning activities were inhibited. LH injections were able to restore the spawning activities inhibited by the methylmercury treatment, but not LH-RH. However, histology of the pituitary gland showed stimulation of gonadotropic cells by LH-RH injection with no restoration of spawning activities. This suggests that methylmercury- may be blocking the release of gonadotropin. In vitro ovulation was affected by previous exposure to methylmercury. Addition of methylmercury directly to the incubation medium further reduced the percentage of in vitro ovulation in the previously treated fish. Using oocytes from untreated donor fish, the percent inhibition of in vitro ovulation by methylmercury was directly related to the log of doses used. A possible bioassay with in vitro ovulation was suggested. Among the various steroids used (progesterone, cortisone, estradiol and testosterone), cortisone was the only steroid effective in restoring in vitro ovulation blocked by the presence of methylmercury in the incubation medium. Ecological implications of these findings are discussed.

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