ABSTRACT
This chapter reviews the existing patterns of morpho-physiological divergence across oxygen gradients in fishes, and highlights the value of aquatic oxygen gradients as novel systems to study mechanisms generating and maintaining phenotypic diversity. It discusses the potential role of environmental effects on interdemic variation in gill traits across oxygen gradients, and explores performance trade-offs that may contribute to the maintenance of respiratory phenotypes in the field. The variation in gill morphometrics across oxygen gradients is accompanied by variations in other physiological and biochemical characters. Interdemic variation in respiratory traits could be genetically based, fixed by environmental pressures at a critical period of ontogeny, and/or simply be a phenotypic response that remains labile throughout an individual’s lifetime. An important step in understanding morphological divergence across aquatic oxygen gradients is to explain why alternative respiratory phenotypes persist in different environments. The significance of variation in dissolved oxygen as a driver of phenotypic divergence is a largely unexplored aspect of aquatic biodiversity.
