Case Study: Gill Plasticity in Larval Fishes

Abstract

Adult teleosts possess an impressive degree of plasticity with respect to gas exchange and ion regulation. The gill is central to this plasticity, serving as the dominant site for both of these processes. Indeed, reversible changes to gill morphology can modify the oxygen and ion transport cascades such that flux is maintained despite changes to supply and demand. Gill plasticity thus provides clear selective benefits, allowing teleosts to thrive in habitats with fluctuating levels of oxygen and ions, and likely contributing to their broad distribution. However, this differs markedly in early life stages. Here, we use rainbow trout as a case study to review gill plasticity during larval development with respect to gas exchange and ion regulation. Surprisingly, the larval gill exhibits little plasticity relative to the adult form. Gill morphology is mostly fixed early on, and plasticity appears limited to changes in convection and protein function (hemoglobin isoform, ion affinity, Na+, K+ ATPase activity, tight junction permeability). Larvae thus have a reduced capacity to maintain trans-epithelial gas and ion flux in hypoxic and ion-poor water, respectively. Interestingly, larval development is slowed by hypoxia, but not by reduced ion uptake in ion-poor water. We propose that this limited gill plasticity is associated with selection for rapid development and minimal endogenous reserves when larvae rear in a stable environment on a fixed energy budget. As a consequence, this limited gill plasticity may contribute to the survival bottleneck often associated with larval development. However, these ideas are untested and based on few studies. We therefore conclude this review with several research directions aimed to provide a more comprehensive understanding of gill plasticity in larval rainbow trout specifically, and teleosts generally.