Ecological, Evolutionary, and Physical Factors Influencing Aquatic Animal Respiration

Abstract

Compared to air-breathers, animals that respire aquatically have limited access to O2 and their habitats are more subject to hypoxia. Because O2 diffuses more slowly through water than air, animals in water experience greater diffusion boundary layer effects on respiratory gas diffusion. While ventilation and specialized exchange surfaces mitigate O2 diffusion limitations on respiration, most animal phyla, particularly those confined to aquatic habitats, lack these. Diffusion limitation influences the ontogeny of aquatic animals and may have also shaped Precambrian metazoans. In spite of a more limited O2 access, aquatic animals display a much greater spectrum of respiratory adaptation, ranging from the loss of Hb in icefishes to the independent evolution, invention, and acquisition of Hb in many invertebrates confined to hypoxic habitats. Three features of aquatic respiratory systems distinguishing them from aerial systems are the widespread occurrence of integumental respiration, the frequent presence of combined respiratory and feeding surfaces, and the profound effect of hypoxia on shaping respiratory adaptation, both in shallow water and in the deep sea.