Gill Biomechanics and Functional Morphology in Elasmobranchs (Sharks, Skates, and Rays)

Ventilation—the biomechanical process which brings the respiratory medium into contact with the gas exchange surface—is a critically important process in vertebrates. In fishes, ventilation is described writ large as a four-phase process driven by two pumps: a “suction” pump which pulls water into the mouth and over the gills, and a “force” pump which pushes water over the gills and out of the body. However, there is a great deal of variation in ventilation both within and across species, shaped by ecological and evolutionary demands. In the first chapter of my dissertation, I investigated the contributions of individual gill chambers to the overall generation of ventilatory flow in the Pacific spiny dogfish, Squalus suckleyi, at rest. I found that pressure generation attenuates posteriorly, and bimodality of function in the posterior most chamber, gill 5. In the second chapter, I investigated the impact of increasing swimming speed on ventilatory mechanics in the same species. I found that frequency and amplitude of breaths increase in all chambers in response to increased swimming speed, but gill chamber 5 experienced the greatest relative increase in pressure, further supporting its functional modularity. In the third chapter, I investigated ecological correlates to the evolution of gill morphology in batoids, the most specious group of cartilaginous fishes. I found that gill morphology is significantly related to habitat depth and electrogenicity but does not vary with habitat salinity. Taken together, my dissertation investigates both the short- and long-term pressures shaping ventilation; understanding the organismal response to these pressures is key to understanding their capacity to adapt to a changing environment.