Meeting Abstract

50.4  Tuesday, Jan. 5  Fluid Dynamic Drag, Body Shape, and Endurance Swimming Performance among Coral Reef Fishes WALKER, J.A.*; ALFARO, M.E.; FULTON, C.J.; Univ. Southern Maine; Univ. California, Los Angeles; Aust. Nat. Univ. walker@maine.edu

The repeated evolution of streamlined body shapes among large, fast aquatic animals is a textbook example of evolutionary convergence. The oft-cited functional explanation is simple: streamlining reduces drag. A component of streamlining is fineness, which is the ratio of body length to diameter (a combination of depth and breadth). In a common but naïve model, endurance swimming performance is expected to increases monotonically if not linearly with fineness. A more hydrodynamically informed model recognizes that streamlined bodies have an optimum fineness ratio and that endurance performance should decrease as fineness moves away to either side of this optimum. Hoerner’s empirical drag data, however, show that the relationship between fineness and drag depends on how bodies are scaled. Drag decreases monotonically if bodies are scaled by length but there is a distinct minimum if bodies are scaled by cross-sectional area or volume. A problem with using these data to understand body shape variation is that Hoerner’s data were collected on rigid bodies while most fish undulate the body to power endurance swimming. While rigid bodies generate a distinct pressure drag (something that can be minimized), self-propelled undulating bodies generate a pressure distribution that ultimately contributes to thrust. Consequently, we explored the association between body shape (fineness) and swimming performance in 58 species from 5 families of coral reef fish that maintain a rigid body while powering endurance swimming using the pectoral fins. Regardless of how the fish body is scaled, swimming endurance increased with increasing fineness. This result is consistent with the naïve model but inconsistent with the more hydrodynamically informed models.