Ventilation in Cartilaginous Fishes

Lara Ferry-Graham and I have been investigating the mechanics of respiration in cartilaginous fishes. Aquatic vertebrates are faced with one of two options for respiration: physically moving their gills through the water ("ram"), or actively pumping water over their gills via "buccal pumping". The current model of the mechanics of respiration was described over 40 years ago, and has been incorporated into textbooks and physiological literature alike. It was suggested that alternating "suction" and "pressure" pumps allowed for the continuous and unidirectional flow of oxygenated water over the gills. This unidirectional flow of water is important for setting up and maintaining the counter-current oxygen exchange mechanism prevalent in fishes. This facilitates an extremely efficient transfer of oxygen to the blood, and allows more oxygen to be extracted from the water than could be if the flow were not continuous. We have used a variety of techniques to approach this question including sonomicrometry, pressure transduction, and direct visualization of the flow patters with an endoscope. Our results suggest that respiration is not as efficient as once presumed. The dominant paradigm does not account for the large amount of back-flow that we see in skates, and two species of shark. We have proposed a new, far less elegant, model of respiration that appears to better predict the flow patterns we observe.