An alternative mechanism to reduce intracapsular hypoxia in ovicapsules of Fusitriton oregonensis (Gastropoda)

Oxygen supply inside egg masses has been reported as a constraint of embryo development. Many species that enclose their eggs in jelly masses or ovicapsules have strategies to avoid or reduce intracapsular hypoxia. In some amphibian species, a decrease in the wall thickness of the egg capsule over time produces an increase in oxygen conductance of the wall, reducing the problem of intracapsular oxygen limitation. Previous studies of gastropods have reported a decrease in the thickness of the capsules during development. However, there are no studies relating capsule thinning to the oxygen limitation problem in this group. This study links the thinning of egg capsules with oxygen diffusion as a possible mechanism to reduce or avoid hypoxia inside the capsules of the gastropod Fusitriton oregonensis. Capsule thickness, capsule area, oxygen partial pressure inside and outside the capsule, and oxygen consumption of the embryos at early and late developmental stages were measured. The conductance and the diffusion coefficients of the capsule were estimated using these measurements. Results showed that (1) capsule thickness decreased throughout development by about 50%, (2) oxygen consumption of embryos increased from early to late stages, (3) oxygen partial pressure inside the capsule did not change during development, (4) conductance coefficient increased with time, and (5) estimation of diffusion coefficient was lower than amphibian egg jelly, shark capsules, egg fishes, and eggs of giant cuttlefish. The reduction in the thickness of the capsule wall and the associated increase in its conductance during embryonic development may reduce oxygen constraints, especially at late developmental stages.