Embryos at the Edge of Tolerance: Effects of Environment and Structure of Egg Masses on Supply of Oxygen to Embryos.

Oxygen concentrations in gelatinous egg masses of two species of opisthobranch gastropods were examined with microelectrodes. Embryos in central positions are near the limit of the oxygen supply required for development. This limit is approached despite a diffusion constant for oxygen in masses that is close to that in water. Closed-chamber respirometry shows that oxygen is consumed by masses in the dark but generated in the light. Internal oxygen concentrations were greater in bright than in dim light. Thus photosynthesis and respiration of microorganisms associated with the masses affects the supply of oxygen to embryos within the mass. This effect of light was confirmed for egg masses of a polychaete. These observations, together with other published observations on the effects of hypoxia on development, indicate that the developmental rates of embryos in egg masses may depend on algal photosynthesis and metabolism. Flow around the masses also affects delivery of oxygen to embryos, but masses in dim light are at the limit of adequate supply even in a strong flow with a very thin boundary layer. Because the central embryos are near the limit for adequate supply of oxygen by diffusion, their development rate thus depends on light, abundance of photosynthetic and heterotrophic microorganisms, flow, and oxygen concentration in the surrounding water.