Limits to Diffusive O(2) Transport: Flow, Form, and Function in Nudibranch Egg Masses from Temperate and Polar Regions

BACKGROUND: Many aquatic animals enclose embryos in gelatinous masses, and these embryos rely on diffusion to supply oxygen. Mass structure plays an important role in limiting or facilitating O(2) supply, but external factors such as temperature and photosynthesis can play important roles as well. Other external factors are less well understood. METHODOLOGY/PRINCIPAL FINDINGS: We first explored the effects of water flow on O(2) levels inside nudibranch embryo masses and compared the effects of flow on masses from temperate and polar regions. Water flow (still vs. vigorously bubbled) had a strong effect on central O(2) levels in all masses; in still water, masses were considerably more hypoxic than in bubbled water. This effect was stronger in temperate than in polar masses, likely due to the increased metabolic demand and O(2) consumption of temperate masses. Second, we made what are to our knowledge the first measurements of O(2) in invertebrate masses in the field. Consistent with laboratory experiments, O(2) in Antarctic masses was high in masses in situ, suggesting that boundary-layer effects do not substantially limit O(2) supply to polar embryos in the field. CONCLUSIONS/SIGNIFICANCE: All else being equal, boundary layers are more likely to depress O(2) in masses in temperate or tropical regions; thus, selection on parents to choose high-flow sites for mass deposition is likely greater in warm water. Because of the large number of variables affecting diffusive O(2) supply to embryos in their natural environment, field observations are necessary to test hypotheses generated from laboratory experiments and mathematical modeling.