Dorsal and anal

In 1926, when C. M. Breder published his classic monograph on the locomotion of fishes (Breder, 1926), little was known about how fish coordinate movement among fins. Despite Breder’s wide-ranging functional and phylogenetic analysis of different modes of fish swimming, followed in 1930 by the pioneering physiological work of Eric von Holst (1973), who looked at general patterns of fish fin movement, our knowledge of coordinated fin function remains poor. Harris (1936, 1937), Breder and Edgerton (1942), Arreola and Westneat (1996), Gordon et al. (2000), Consi et al. (2001), Hove et al. (2001) and Liao (2002) have contributed important new data on multiple fin use during locomotion in fish, but in comparison to current understanding of body surface kinematics and hydrodynamics, quantitative information on how fish coordinate use among their fins is scant. Fin use is an important issue for understanding fish locomotor biomechanics because fish are statically unstable (Eidietis et al., 2003; Harris, 1936, 1937; Webb, 2002, 2004b; Weihs, 1993). The size and position of fins relative to an animal’s centre of mass (COM) will greatly affect how dynamic stability is achieved during locomotion. Perturbations in fish body position are caused by external environmental forces such as water currents, or by forces produced by their own fins, which may result in roll, pitch and yaw body movements (Fig.·1A). Fish use their fins to control body posture, and the relative position of fins to the centre of mass and centre of buoyancy (COB) is important in determining the forces and torques a fish experiences and produces during swimming. For example, Fig.·1B depicts dorsal and anal fin positions relative to the COM and COB of a typical fish. In this case, as is found in most fishes (Webb, 2004a,b), the COB is below the COM, causing rolling instability. The greater the distance of the fins from the midpoint between the COM and COB, the larger the moment arm for that fin. Assuming fins of similar size, one may predict that the anal fin in Fig.·1B would require less force to produce the same torque as the dorsal fin due to the length of its moment arm. Similarly, longitudinal and mediolateral position of fins on the fish’s body will also affect pitch and yaw instabilities around the COM. Webb (2002, 2004b) studied three species of fish with a diversity of fin morphologies and discovered that, for all species, postural disturbances that induced roll provoked the The Journal of Experimental Biology 208, 2753-2763 Published by The Company of Biologists 2005 doi:10.1242/jeb.01706