Developmental biology of medaka fish (Oryzias latipes) exposed to alkalinity stress

Alkalinity stress is common in cultured aquatic animals and considered to be one of the major stress factors for fishes when they are transferred to saline-alkali waters. To evaluate potential effects of alkalinity on the developmental biology of Oryzias latipes, fertilized eggs, larvae and breeding fish were exposed to different carbonate alkalinity concentrations of 1.5–64.5 meq l−1, for 9, 120, and 60 days, respectively. The mortality of embryos significantly increased when exposed to the high concentrations (16.5–64.5 meq l−1). Although more than 50% of survived embryos hatched in 16.5 and 31.4 meq l−1 concentrations of carbonate alkalinity, most were not able to swim up after hatching. Morphological abnormalities such as coagulated embryos, halted embryo development, and hatching failure were observed at stages 15, 29–33 and 38 in high concentrations (31.4, 64.5 meq l−1). Almost all larvae in 16.5 and 31.4 meq l−1 treatments died 70 d post-hatch. Growth of juveniles exposed to carbonate alkalinity of 5.3 and 8.8 meq l−1 was not significantly different at 70 d and 120 d post-hatch. The number of eggs released by breeders, the fertilization rate and the hatching rate of eggs were significantly lower in the 31.4 meq l−1 treatment than in other treatments. Although medaka are capable of surviving in high alkalinities (31.4, 64.5 meq l−1) for an extended period of time, these conditions are stressful to the fish, especially at the embryonic and reproductive stages.