OSMOTIC BALANCE IN THE EGGS OF THE TURTLE CHELODINA-RUGOSA DURING DEVELOPMENTAL ARREST UNDER WATER

The tropical Australian turtle Chelodina rugosa normally lays its hard -shelled eggs in mud, under shallow freshwater, during the monsoon sea son. The eggs undergo developmental arrest until the water recedes and oxygen is able to diffuse into the embryo. This period of arrest can exceed 12 wk without embryonic mortality. To understand how the eggs a void osmotic absorption of water leading to shell rupture and embryoni c death, this study investigates the solute concentrations and volumes of the albumen and yolk compartments during submergence in distilled water. The albumen loses considerable sodium through the shell, partic ularly during the first week, and its osmotic concentration drops from 234 mmol/kg at laying to about 23 mmol/kg. Meanwhile, water from the albumen slowly moves through the vitelline membrane into the yolk comp artment, which enlarges at a constant rate until it approaches the ins ide of the shell at about 22 wk. Osmotic uptake dilutes yolk solutes, decreasing the osmotic concentration from 281 mmol/kg at laying to 132 mmol/kg at 157 d. Loss of embryonic viability is associated with cont act of the vitelline membrane with the inside of the shell. The princi pal adaptation of this species for protracted developmental arrest und er water is a vitelline membrane of such low permeability to water tha t the expansion of the yolk compartment occurs about 10 times more slo wly than in other chelonians.