Fig. 2. Cardiac performance in bluefin tuna in situ. Values of cardiac parameters were recorded in spontaneously beating bluefin tuna hearts perfused in situ with oxygenated Ringer's solution at temperatures of 2–25°C. At each test temperature, values were recorded at standard conditions (open circles) of ambient input pressure and then input pressure was elevated to maximize stroke volume and cardiac output (A–C, filled circles). Input and output pressures were then elevated until power output was maximized (D, filled circles). (A) Heart rate, (B) stroke volume, (C) cardiac output, (D) power. Values are means ± S.E.M.; N=3 (2°), N=4 (10°, 20°, 25°), N=5 (5°), N=6 (15°). Significant differences between standard and maximal conditions (P<0.05) occur in A at 2°C only, in B at all temperatures, in C at all temperatures 5°C and in D at all temperatures 10°C. Significant differences between temperatures are indicated in Table 1. For details of standard and maximal conditions, see Materials and methods.