Maternal transfer and reproductive effects of Cr(VI) in Japanese medaka (Oryzias latipes) under acute and chronic exposures.

Maternal transfer of metals can be an important exposure route for animals. The maternal transfer of Cr and its effects on reproduction in fish are still largely unknown. In this study, Japanese medaka were exposed to a sublethal Cr(VI) concentration for 6 days (acute) and for 3 months (chronic). Chromium accumulation in the gonads, maternal transfer of Cr, and effects of Cr on the reproduction, histopathology and expressions of antioxidants in the gonads were evaluated. Both acute and chronic exposures resulted in significant Cr accumulation in gonads, eggs and larvae. In chronic Cr-exposed fish, approximately 61% of the Cr accumulated in the ovary was depurated by spawning during the first 3 days after exposure, suggesting that maternal transfer is a very important pathway for accumulation in offspring. The chronic exposure caused decreases in body weight, standard length, gonad weight, gonad-somatic index (GSI) and fecundity. The last of these was most severely affected: the total number of broods and eggs per female decreased by 57.1% and 75.9%, respectively. Moreover, egg weight and fertilization rate were also reduced (by approximately 20%) following chronic Cr(VI) exposure. Histopathological analyses showed that the Cr exposure resulted in the onset of follicular atresia and a reduction in the number of mature oocytes, along with a reduction in abundance of mature spermatozoa in testes. The GSH/GSSG ratio was greatly elevated after chronic Cr(VI) exposure, implying that GSH played a role in scavenging the reactive oxygen species generated by the reduction of Cr(VI) inside cells. This study provides evidence for the maternal transfer of Cr, highlights the importance of spawning in Cr depuration from the ovary, and demonstrates that chronic Cr(VI) exposure has serious impacts on reproduction in the Japanese medaka. Our results suggest that the issue of chronic Cr pollution deserves more attention than it has received to date.