Underwater nesting by the Australian freshwater turtle Chelodina rugosa: Effect of prolonged immersion and eggshell thickness on incubation period, egg survivorship, and hatchling size

The northern long-necked turtle, Chelodina rugosa, occupies seasonally ephemeral freshwater wetlands in the wet-try tropics of northern Australia. The species has a highly unusual nesting strategy in that nesting takes place under water when wetland habitats are flooded. Embryonic development proceeds when floodwaters recede during the annual dry season and hatchling emergence coincides with the start of the following wet season. This study examined the influence of duration of immersion of eggs on hatchling size and hatching success under laboratory conditions. Prolonged immersion of eggs in water results in smaller hatchlings and shorter incubation times, but does not increase egg mortality during incubation. Leaching of calcium from the eggshell, resulting in reduced availability of calcium to the developing embryo, may explain the smaller hatchlings and shorter incubation period. Hatchlings from thin-shelled eggs, in which shell formation was incomplete, were smaller and hatched sooner, indicating that the eggshell plays a role in determining hatchling size and incubation period. Thin-shelled and normal eggs had equivalent egg survivorship, indicating that the eggshell is unimportant in egg survival during immersion.