Influence of Water Exchanges by Flexible-Shelled Eggs of Painted Turtles Chrysemys picta on Metabolism and Growth of Embryos

Abstract

Flexible-shelled eggs of painted turtles (Chrysemys picta) were incubated under controlled conditions eliciting different patterns of net water exchange between eggs and the environment. The temporal patterns of decline in dry mass of yolks and of increase in dry mass of embryos did not vary among eggs incubated in different hydric environments, indicating that rates of metabolism and growth of embryos were largely unaffected by variation in the amount of water available inside eggs to support embryogenesis. Nevertheless, embryos in wet environments assimilated more water during incubation than did embryos in dry conditions, and this extra water apparently enabled them to develop longer before hatching than was possible for embryos in the drier settings. Because of their longer incubation, hatchlings emerging from eggs in wet environments were larger (both in mass and in carapace length) and contained less residual yolk than turtles coming from eggs incubated in drier surroundings. Embryos accumulated three times more excretory nitrogen in the form of urea than in the form of ammonia, but the patterns of nitrogen accumulation did not vary among embryos exposed to different hydric conditions. Water potential of the yolk increased during the first 10 days of incubation, as water flowed from the albumen into the vitelline sac, and decreased linearly thereafter, as water was transferred from the yolk to the developing embryo. The predicted water potential of the yolk at the time of hatching was the same for eggs incubated in wet and dry environments, thereby raising the possibility that water potential of some compartment inside eggs provides the cue for hatching. Water exchanges by eggs of painted turtles incubating in natural nests probably affect survival of embryos to hatching as well as body size and level of tissue hydration in young turtles.