Why polar gigantism and Palaeozoic gigantism are not equivalent: effects of oxygen and temperature on the body size of ectotherms

Abstract

Summary Organisms of gigantic proportions inhabited the world at a time of a hyperoxic prehistoric atmosphere (Palaeozoic gigantism). Extant giants are found in cold polar waters, with large quantities of dissolved oxygen (polar gigantism). Oxygen is usually deemed central to explain such gigantism. Examples of one category of gigantism are often cited in support of the other, but novel insights into the bioavailability of oxygen imply that they cannot be taken as equivalent manifestations of the effect of oxygen on body size. Recently, the availability of oxygen has been shown to be lower in cold waters, despite greater oxygen solubility. Consequently, gigantism in cold, oxygenated waters and gigantism in an oxygen‐pressurized world are fundamentally different: Palaeozoic gigantism likely arose because of greater oxygen availability, while polar gigantism arises in spite of lower oxygen availability. The traditional view of respiration focuses on meeting the challenge of extracting sufficient amounts of oxygen, which essentially is a toxic gas. We present a broader perspective, which specifically includes risks of oxygen poisoning. We discuss how challenges pertaining to balancing oxygen uptake capacity and risks of oxygen poisoning are very different for animals breathing either air or water. We propose a novel explanation for polar gigantism in aquatic ectotherms, arguing that their larger body size represents a respiratory advantage that helps to overcome the larger viscous forces in water. Being large helps organisms to balance the opposing risks of asphyxiation and poisoning, especially in colder, more viscous, water. This results in a selection for larger sizes, with polar gigantism as the extreme manifestation. Hence, a larger size provides respiratory benefits to water‐breathing ectotherms, but not terrestrial ectotherms. This can explain why clines in body size across temperature and latitude are stronger in aquatic ectotherms.