Respiration of the eggs of the giant cuttlefish Sepia apama

On the roofs of subtidal crevices, the giant cuttlefish (Sepia apama) of southern Australia lays clutches of lemon-shaped eggs which hatch after 3 to 5 mo. Diffusion of oxygen through the capsule and chorion membrane to the perivitelline fluid and embryo was modelled using the equation Vo(2) = Go(2) (Po-2out - Po-2in), where where Vo(2) = rate of oxygen consumption, Go(2) = oxygen conductance of the capsule, and Po-2 values - oxygen partial pressures across the capsule. During development, Vo(2) rose exponentially as the embryo grew, reaching 5.5 mu l h(-1) at hatching. Throughout development, the capsule dimensions enlarged by absorption of water into the perivitelline space, increasing Go, by a combination of increasing surface area, and decreasing thickness of the capsule. These processes maintained Po-2in high enough to allow unrestricted Vo(2) until shortly before hatching. Diffusion limitation of respiration in hatching-stage embryos was demonstrated by (1) increased embryonic Vo(2) when Po-2out was experimentally raised, (2) greater Vo(2) of resting individuals immediately after hatching, and (3) reduced Vo(2) of hatchlings at experimental Po-2 levels higher than Po-2in before hatching. Thus, low Po-2in may be the stimulus to hatch. Potential problems of diffusive gas-exchange are mitigated by the relatively low incubation temperature (12 degrees C), which may be a factor limiting the distribution of the species to cool, southern waters.