Features heightening cardiovascular performance in fishes, with special reference to tunas

Information on heart rate, cardiac mass, vascular resistance, and myocardial oxygen consumption are considered with a view to identifying special features that heighten cardiovascular performance in fish, and especially in tuna. Fish rarely have heart rates greater than 120 bpm. Skipjack tuna are an exception, with heart rates approaching those of similarly sized mammals. A large relative ventricular mass is a characteristic of, but not exclusive to, active fish. However, the pyramidal shape of this large ventricle in active fish is a distinguishing characteristic. This anatomical form may be important in the coupling of high cardiac stroke volumes (around 1 mL/kg) with high heart rates and blood pressures. Skipjack tuna excel in this regard. The characteristically high cardiac power output and large ventricular mass in active fish necessitates a coronary circulation. In most active fish, however, the coronary circulation is important under extreme conditions (e.g., swimming and hypoxia) rather than for routine performance. Skipjack tuna may be an exception, requiring their coronary circulation for routine performance. Neither total vascular resistance nor the relative O-2 cost of cardiac pumping stand out as being unusual in active fish. Even so, the unusually high resistance of the branchial circulation in skipjack tuna may represent an upper design limit for the structure of individual lamellae.