Limits to diffusive O₂ 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 O₂ 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 O₂ 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 O₂ 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 O₂ consumption of temperate masses. Second, we made what are to our knowledge the first measurements of O₂ in invertebrate masses in the field. Consistent with laboratory experiments, O₂ in Antarctic masses was high in masses in situ, suggesting that boundary-layer effects do not substantially limit O₂ supply to polar embryos in the field. Conclusions/Significance: All else being equal, boundary layers are more likely to depress O₂ 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 O₂ supply to embryos in their natural environment, field observations are necessary to test hypotheses generated from laboratory experiments and mathematical modeling.