The functional significance and evolution of the coronary circulation in sharks

The coronary circulation first appeared in the chordate lineage in cartilaginous fishes where it perfuses the entire myocardium, just like in birds and mammals but unlike in most teleost fishes. Yet, despite the pivotal position of elasmobranchs in the evolution of the coronary oxygen supply, the functional significance of their coronary circulation has never been investigated. Elasmobranchs are of special interest because of the morphological arrangement of their cardiomyocytes, which has resulted in the majority of the ventricular myocardium having access to oxygen from both a coronary supply and the venous blood returning to the heart. In order to determine the relative contribution of the coronary oxygen supply to cardiovascular function, I measured coronary blood flow (CBF) in the sandbar shark, Carcharhinus plumbeus, and leopard shark, Triakis semifasciata, while manipulating cardiovascular status using pharmacological approaches and in vivo temperature changes, respectively. By exploring inter- and intra-individual variation in cardiovascular variables I show that coronary blood flow is directly related to heart rate in both bradycardic (R²= 0.6, P <0.001) and trachycardic (R²= 0.8, P