Evolution of the Cardiorespiratory System in Air-Breathing Fishes

Abstract

Fishes have evolved a wide variety of air-breathing organs independently along different lineages. Of these air-breathing fishes, only some (e.g., mudskippers) venture onto land but the vast majority of them remain in water and use air as an oxygen source to different degrees. With the development of air-breathing capacity, the circulatory system of fishes has often been modified in various ways to accommodate blood to and from the newly developed air-breathing surface. However, most air-breathing fishes, except snakeheads and lungfishes, seem to lack the ability of separating O2-rich effluent blood of the airbreathing organ from O2-poor systemic venous blood during passage through the central cardiovascular system, although this has been inferred usually only from anatomical studies. Mudskippers attest to the fact that transition from aquatic to amphibious life is possible without restructuring the gross anatomy of the cardiorespiratory system, at least to some extent. Why then have some fish and ancestral vertebrates evolved the capacity of blood separation? The purpose of this paper is to review the current knowledge about the form and function of the cardiorespiratory system of selected species of air-breathing fishes (eel gobies, mudskippers, swamp eels, snakeheads and lungfishes, arranged in the order of higher specialization of the cardiorespiratory system) and discuss important issues relating to the topic.