Fig. 4. The strength of the heartbeat depends on temperature. Symbols represent the estimated mean ratios (±95% confidence intervals) of contraction amplitudes measured at each temperature relative to those measured at 2°C. Statistical significance was determined from repeated-measures models using log scale. Compared to the values at 2°C, contraction amplitudes are significantly different at temperatures from 12 to 22°C (P<0.05). For temperatures of 2–16°C, N=9; 18 and 20°C, N=6; 22°C N=5.