Ontogeny and paleophysiology of the gill: New insights from larval and air-breathing fish

There are large changes in gill function during development associated with ionoregulation and gas exchange in both larval and air-breathing fish. Physiological studies of larvae indicate that, contrary to accepted dogma but consistent with morphology, the initial function of the gill is primarily ionoregulatory and only secondarily respiratory. In air-breathing fish, as the gill becomes progressively less important in terms of O2 uptake with expansion of the air-breathing organ, it retains its roles in CO2 excretion, ion exchange and acid–base balance. The observation that gill morphology and function is strongly influenced by ionoregulatory needs in both larval and air-breathing fish may have evolutionary implications. In particular, it suggests that the inability of the skin to maintain ion and acid–base balance as protovertebrates increased in size and became more active may have been more important in driving gill development than O2 insufficiency.

► We review the role of the gills for gas exchange and ionoregulation during development in fish.
► The initial function of the developing larval fish gill appears to be primarily ionoregulatory not respiratory.
► In developing air-breathing fish, gill morphology and function is also strongly influenced by ionoregulatory needs.
► Taken together these observations may have important evolutionary implications.
► We propose that ion regulation in proto-vertebrates may have been an important driving force in vertebrate gill evolution.