The Development of Form and Function in Fishes and the Question of Larval Adaptation

Gas Exchange, Ionoregulation, and the Functional Development of the Teleost Gill

Peter J. Rombough

doi: https://doi.org/10.47886/9781888569582.ch4

Abstract.—This review examines the transition from cutaneous to branchial respiration and ionoregulation in teleost larvae. Before gills develop, the skin is the only significant site of gas and ion exchange. The primary site of these two processes shifts from the skin to the gill as development proceeds. When the transition begins and the speed at which it occurs, both vary considerably depending on species. In general, ionoregulatory activity begins to shift to the gill before gas exchange. The transition to branchial ionoregulation also tends to proceed more rapidly than the transition to branchial respiration. The key morphological indicator of the shift in ionoregulatory activity to the gill is the appearance of large numbers of mitochondria-rich cells on gill arches. The corresponding indicator for gas exchange is the appearance of gill lamellae. For both activities, the transition occurs gradually, and even at the end of the larval period, the skin still plays an important role in both processes. This is particularly true for gas exchange. The ecological implications of the transitions and what they may mean in terms of gill evolution are discussed.