Egg-mass size and cell size: Effects of temperature on oxygen distribution

Two processes strongly influence the distribution of oxygen within egg mass es and cells: the supply of oxygen by diffusion and the consumption of oxyg en hy embryos and mitochondria, These processes are differentially sensitiv e to temperature. The diffusion coefficient of oxygen depends only weakly o n temperature, having a Q(10) of approximately 1.4. In contrast, the consum ption of oxygen depends strongly on temperature, having a Q(10) between 1.5 and 4.0, Thus, at higher temperatures, the ratio of oxygen supply to deman d decreases. I show, by extending a model of oxygen distribution within met abolizing spheres, that maximal egg-mass sizes and cell sizes are predicted to be smaller at higher temperatures. For egg masses, definitive data are not get available. For ectothermic cells, this prediction appears to be sup ported; cells from a variety of ectothermic organisms, unicellular and mult icellular, are smaller when the cells are produced at warmer temperatures. Establishing a specific connection between this pattern and oxygen distribu tions requires demonstration of (1) oxygen concentration gradients within m etabolizing spheres and (2) central oxygen concentrations low enough to aff ect function. Egg masses from a variety of taxa show steep oxygen concentra tion gradients and often are severely hypoxic or anoxic in central location s, Severe hypoxia appears capable of retarding development or killing embry os, Similar kinds of data for ectothermic cells have not yet been collected , but the literature on oxygen gradients within mammalian cells suggests th at intracellular gradients may be important.