Temperature-oxygen interactions in Antarctic nudibranch egg masses

The Southern Ocean is one of the coldest, most stable marine environments on Earth and represents a unique environment for investigating metabolic consequences of low temperature. Here we test predictions of a new diffusion-reaction model of O₂ distributions in egg masses, using egg masses of the Antarctic nudibranch mollusk, Tritonia challengeriana. When warmed from -1.5° to +1.5°C, embryos of T. challengeriana showed large increases in O₂ consumption (Q₁₀ values of 9.6-30.0). Oxygen electrode measurements in intact masses showed, however, that O ₂ levels were high throughout and virtually unaffected by temperature. The model suggested that both effects stemmed from very low metabolic densities in egg masses. Detailed morphological measurements of egg masses of T. challengeriana and a temperate congener, T. diomedea, revealed large differences in structure that may be related to O₂ availability. Egg masses of T. challengeriana were approximately twice as thick. However, the most dramatic effects were observed in embryos: embryos of T. challengeriana were >32 times larger (by volume) than embryos of T. diomedea. Antarctic embryos also were contained singly in large egg capsules (∼500 μm diameter). Consequently, Antarctic embryos occurred at much lower densities, with very low metabolic densities.