Effects of environmental oxygen on development and respiration of Australian lungfish (Neoceratodus forsteri) embryos
The effects of oxygen partial pressure ($$ P_{{{\text{O}}_{2} }} $$) on development and respiration were investigated in the eggs of the Australian lungfish, Neoceratodus forsteri. At 20°C, embryonic survival and development was optimal at 15 and 20.9 kPa. Development was slowed at 5 and 10 kPa and embryos did not survive 2 kPa. At lower $$ P_{{{\text{O}}_{2} }} $$, the rate of oxygen consumption also decreased. Embryos responded to hypoxia by hatching at an earlier age and stage of development, and hatching wet and dry gut-free masses were reduced. The role of oxygen conductance ($$ G_{{{\text{O}}_{2} }} $$) in gas exchange was also examined under selected environmental $$ P_{{{\text{O}}_{2} }} $$ and temperatures. The breakdown of the vitelline membrane changed capsule geometry, allowed water to be absorbed into the perivitelline space and increased capsule $$ G_{{{\text{O}}_{2} }} $$. This occurred at embryonic stage 32 under all treatments and was largely independent of both $$ P_{{{\text{O}}_{2} }} $$ and temperature (15, 20 and 25°C), demonstrating that capsule $$ G_{{{\text{O}}_{2} }} $$ cannot adaptively respond to altered environmental conditions. The membrane breakdown increased capsule diffusive $$ G_{{{\text{O}}_{2} }} $$ and stabilised perivitelline $$ P_{{{\text{O}}_{2} }} $$, but reduced the convective $$ G_{{{\text{O}}_{2} }} $$ of the perivitelline fluid, as the large perivitelline volume and inadequate convective current resulted in a $$ P_{{{\text{O}}_{2} }} $$ gradient within the egg prior to hatch.