Nobuyoshi Imada

Effect of 17β-estradiol on the reproduction of Japanese medaka (Oryzias latipes)

Abstract Estrogenic compounds such as 17β-estradiol (E2) and its analogs are present in the aquatic environment and can adversely affect the reproductive systems of aquatic organisms. Although E2 has been detected at a maximum concentration of 64 ng/l in effluents of sewage treatment works (STWs), few reports address the associated effects on reproduction in fish. Therefore, we exposed adult medaka (Oryzias latipes) to mean measured E2 concentrations of 29.3, 55.7, 116, 227, and 463 ng/l for 21 d and assessed the effects on the egg number and fertility of paired medaka during the exposure period. In addition, we determined the hepatic vitellogenin (Vtg) concentration and histologically assessed the gonads of these fish. The number of egg produced and fertility of the paired medaka exposed to 463 ng/l E2 were significantly less compared with those of the control fish. Males in all treatment groups had developed testis–ova. Males treated with E2 concentrations=55.7 ng/l contained relating great concentrations of hepatic Vtg. Therefore, although only the greatest E2 concentration tested in our study affected fecundity and fertility, effects of E2 were observed on induction of Vtg and testis–ova in male medaka exposed to environmentally relevant concentrations of E2.

Suppression of sexual behavior in male Japanese medaka (Oryzias latipes) exposed to 17β-estradiol

Mating pairs of Japanese medaka (Oryzias latipes) received 17beta-estradiol (E2; dose, 3 and 30 microg/g body weight daily) in their diet and E2-free diet (control) for two weeks. The effect on reproductive success was assessed in the mating pairs, and sexual and nonsexual behaviors in the male were quantified. The fecundities of the E2-treated groups were reduced significantly (p < 0.05) compared with those of control, and the sexual behaviors (following, dancing, floating, and crossing) of male fish in response to prostaglandin F2alpha (PG)-injected E2-unexposed females were suppressed dramatically. While the fertility of spawned eggs and the frequencies of the nonsexual behaviors of resting and swimming in a straight line performed by male were unchanged in E2-exposed groups. Therefore, sexual behavior in male may be linked with reproductive success in mating pair of madaka exposed to E2. This assay for sexual behavior using PG may provide a useful tool for assessing the effect of endocrine-disrupting chemicals.

Effects of 4‐nonylphenol on reproduction of japanese medaka, Oryzias latipes

We exposed mature Japanese medaka (Oryzias latipes) to 4-nonylphenol (4-NP) to elucidate its reproductive effects and estrogenic responses. For each treatment, eight breeding pairs were exposed for 3 weeks at measured 4-NP concentrations of 24.8, 50.9, 101, and 184 micrograms/L. Egg production of the pairs during the third week of exposure (15-21 d) was significantly decreased in the > or = 101-microgram/L treatment group, and fertility was significantly decreased in the 184-microgram/L group. Induction of testis-ova was observed in male fish at all 4-NP concentrations, whereas abnormality of spermatogenesis was found only in those receiving the 184-microgram/L treatment. The gonadosomatic index of the males in the 184-microgram/L group was also remarkably reduced. Hepatic vitellogenin (VTG) levels were significantly increased in both sexes receiving 4-NP treatments of > or = 50.9 micrograms/L. Overall, the lowest-observed effective concentrations (LOECs) of 4-NP for decreasing reproductive capacity (fecundity and fertility) and eliciting estrogenic responses (induction of testis-ova and VTG) were 101 and < or = 24.8 micrograms/L, respectively. The LOEC for reduced reproduction in medaka was at least four-fold greater than that required to elicit estogenic responses in male fish. Thus, we suggest that the estrogenic responses expressed in male fish are not responsible for the reduction in reproductive capacity.

Accumulation of tributyltin (TBT) in the blood of flounder and dab intraperitoneally administered with TBT

Abstract The present study was carried out to investigate the accumulation of TBT in the blood of cultured Japanese flounder, Paralichthys olivaceus, and dab, Limanda yokohamae. TBT-CI was intraperitoneally administered to fish at a dose of 2.5mg kg−1 b.w. At 2, 4, 6 and 8 days after the injection, the blood, liver, kidney, spleen and dorsal muscle were collected. The amounts of TBT within the tissues were determined by GC-FPD. High TBT concentrations were found in the serum of Japanese flounder, levels which corresponded to 22~37-fold those found in the muscle, and 7~12-fold those found in the liver. High TBT concentrations were also found in the serum of dab. The results of gel filtration of the serum showed that TBT was bound to an unknown substance (mol. wt ca 50000).

Genetic variation of the cytochrome b gene in the rosy bitterling, Rhodeus ocellatus (Cyprinidae) in Japan

Abstract According to conventional views, the rosy bitterling, Rhodeus ocellatus, comprises two subspecies, R. ocellatus kurumeus and R. ocellatus ocellatus, the former being native to Japan whereas the latter was introduced into Japan from China during World War II. To examine the genetic structure of Japanese R. ocellatus, part of the mitochondrial cytochrome b gene from 48 individuals collected from various locations in Japan was sequenced. Three major mitochondrial lineages were found. Based on historical evidence, two of these represent R. ocellatus ocellatus and the third R. ocellatus kurumeus. The existence of two distinct lineages of R. ocellatus ocellatus in Japan suggests at least two colonizations. Some local populations comprised purely R. ocellatus kurumeus, but those from Kashima and Ogori included both subspecies. Because the proportion of R. ocellatus ocellatus in Kashima increased from 1994 to 1995, invasion by R. ocellatus ocellatus into R. ocellatus kurumeus habitats is apparently in progress.