Effect of Water Balance on Growth and Calcium Mobilization of Embryonic Painted Turtles (Chrysemys picta)

Abstract

Flexible-shelled eggs of painted turtles (Chrysemys picta) were incubated in wet (-150 kPa water potential) and dry (-750 kPa water potential) substrates to elicit different patterns of net water exchange between eggs and their environment. Embryos exposed to wet substrates consumed yolk more rapidly, grew faster, and incubated longer than did embryos exposed to dry substrates. As a result, hatchlings emerging in wet environments were larger and contained less residual yolk than hatchlings emerging in dry environments. Embryos exposed to wet and dry conditions relied on both yolk and eggshell to supply the calcium required for development. However, embryos exposed to wet substrates used the eggshell for a larger proportion of their calcium (56%) than did embryos exposed to dry substrates (40%). Residual yolk of hatchlings in both experimental groups contained less than 1 mg of calcium. Consequently, residual yolk cannot support growth of neonates at or near levels characteristic of late-term embryos. Thus, the larger energy reserve available to small hatchlings in the form of a large residual yolk probably can be used only to support maintenance metabolism, and young presumably must begin to forage soon after emerging from the nest if growth of both soft and hard tissues is to continue.