Coping with Life on Land: Physiological, Biochemical, and Structural Mechanisms to Enhance Function in Amphibious Fishes

The invasion of land by fishes was one of the most dramatic transitions in the evolutionary history of vertebrates. In this thesis, I investigated how amphibious fishes cope with increased effective gravity and the inability to feed while out of water. In response to increased body weight on land (7 d), the gill skeleton of Kryptolebias marmoratus became stiffer, and I found increased abundance of many proteins typically associated with bone and cartilage growth in mammals. Conversely, there was no change in gill stiffness in the primitive ray-finned fish Polypterus senegalus after one week out of water, but after eight months the arches were significantly shorter and smaller. A similar pattern of gill reduction occurred during the tetrapod invasion of land, and my results suggest that genetic assimilation of gill plasticity could be an underlying mechanism. I also found proliferation of a gill inter-lamellar cell mass in P. senegalus out of water (7 d) that resembled gill remodelling in several other fishes, suggesting this may be an ancestral actinopterygian trait. Next, I tested the function of a calcified sheath that I discovered surrounding the gill filaments of >100 species of killifishes and some other percomorphs. I found no evidence that this calcification evolved to provide support in amphibious fish out of water. Instead, my experimental data suggests that the calcified sheath maintains the position of gill filaments during aquatic ventilation. The role of gill mechanics has largely been neglected in previous studies of respiratory function, but my data suggests that filament stiffness may be critically important. Finally, I tested the hypothesis that prolonged survival out of water is enabled by slow metabolism and conservation of energy stores in K. marmoratus, which cannot feed in air. I found that low metabolism prolonged survival out of water by almost two weeks and this phenotype had increased fecundity in microcosms that were intermittently dry for half of a year. There was no obvious trade-off in fully aquatic environments. Overall, this thesis integrates physiological, ecological, and evolutionary perspectives to provide new insight into how amphibious fishes survive out of water.