High postural costs and anaerobic metabolism during swimming support the hypothesis of a U-shaped metabolism–speed curve in fishes

Significance Hydrodynamic theory predicts that the energetic costs required for fishes to swim should vary with speed according to a U-shaped curve, with an expected energetic minimum at intermediate cruising speeds. Empirical studies to date do not support this view. Here we report a complete dataset on a swimming batoid fish that shows a clear energetic minimum at intermediate swimming speeds. We also demonstrate that this species uses a combination of aerobic and anaerobic metabolism to fuel steady swimming at each speed, including the slowest speeds tested. This contradicts the widespread assumption that fish use only aerobic metabolism at low speeds. Kinematic data support this nonlinear relationship by also showing a U-shaped pattern to body angle during steady swimming.