Anguilliform swimming, at least as shown by eels, does differ from other swimming modes in several ways (Table 7.1). Red muscle activation tends to occur in short blocks ipsilaterally, in contrast to long blocks in the carangiform mode and intermediate blocks in the subcarangiform mode (Danos et al. 2008). One of the original descriptors of swimming modes, the propulsive wavelength adjusted for body length, is still useful, being short in anguilliform swimming, intermediate in subcarangiform modes, and high in carangiform modes (Tytell and Lauder 2004; Danos et al. 2008). Even though it tends to increase posteriorly, wave amplitude is also somewhat greater anteriorly in anguilliform swimming, in contrast to the other modes that are highly similar in this regard (Lauder and Tytell 2006). Wake form differs in anguilliform swimmers, with wakes having lateral momentum but not substantial downstream flow momentum (the momentum opposite the line of thrust of the body), in contrast to other swimming modes. The difference most likely is caused by the absence of a distinct caudal fin structure in eels in contrast to fishes having caudal fins that are distinct from the body (Lauder and Tytell 2006). In five other features, anguilliform and subcarangiform modes do not differ (Table 7.1). These include four features of the myosepta (the sheets of connective tissue separating blocks of myomeres and onto which muscle fibers insert) involving the lateral myoseptal tendon length, the presence of epineural (located on the dorsal surface of the vertebral centrum) and epipleural (located above the abdominal ribs) tendons, and the shape of the myosepta; the fifth similarity is in the firing duration of red muscle fibers (Danos et al. 2008). Red muscle fibers are oxidative and used in slow, prolonged swimming; as such, they are highly vascularized and contain abundant myoglobin, a red oxygen-binding pigment characteristic of muscle (Syme 2006).

      LARVAL FISHES AND ANGUILLIFORM LOCOMOTION During their larval period, the majority of all North American freshwater fishes use anguilliform locomotion in the sense of generating more than one complete propulsive wavelength within the length of the body (Webb and Weihs 1986). Anguilliform swimming in larvae occurs because the musculature and axial skeleton are not sufficiently developed to use lift-based subcarangiform or carangiform modes, both of which would place greater compressive force on the axial skeleton and require more muscular power. In addition, because of their small size and speed, larval fishes operate in an environment dominated by viscous rather than inertial forces so that any cessation of swimming movement stops forward progress—there is no coasting in the absence of inertial forces. The balance between viscous and inertial forces is determined by the Reynolds number (Re), the same equation described previously for prediction of laminar versus turbulent flow in a boundary layer. Re<1 indicates a totally viscous environment and Re >1,000 indicates a totally inertial environment; at intermediate values both forces are represented (Lauder and Tytell 2006), but for values of Re below 300–450, viscous forces predominate over inertial forces (Webb and Weihs 1986; Fuiman 2002). It is difficult for us to really imagine life at low Reynolds numbers. Purcell (1977), in discussing swimming in microorganisms and the impact of the primacy of viscous over inertial forces, said: “If you are at [sic] very low Reynolds number, what you are doing at the moment is entirely determined by the forces that are exerted on you at that moment, and by nothing in the past.”

      Once the yolk sac is absorbed, larval fishes generally swim at 1–3 body lengths per second (Fuiman 2002). Thus R_e of a 5-mm larval fish would be 25–75, at the lower end of the intermediate range, and subject to viscosity effects. In a viscosity-dominated environment, pushing against the water by an elongate body is more effective than using caudal fin propulsion (Webb and Weihs 1986), but because of the unimportance of inertial forces, larvae must swim continuously to move. (Recall that the law of inertia, or Newton’s first law, states that a particle at rest or moving in a straight line with a constant velocity will continue to do so, provided the particle is not subject to an unbalanced force.) As soon as the larvae stop actively swimming, they come to a halt (Purcell 1977; Blake 1983a). As fishes increase in size, the importance of inertial forces increase relative to viscous forces so that once Re reaches 300–450, they can employ an energy-saving burst-and-glide approach to locomotion (Fuiman 2002).

      NON-ANGUILLIFORM BCF LOCOMOTION Increased posterior localization of body undulation and power and the development of distinct caudal fins characterize the traditional modes of subcarangiform and carangiform locomotion (Table 7.1). Subcarangiform swimming occurs in the majority of nonlarval North American freshwater fishes, including salmonids, cyprinids, catostomids, centrarchids, and percids; however, specific studies on swimming are limited to relatively few species of salmonids, cyprinids, and centrarchids (Blake 1983a; Lauder and Tytell 2006). Subcarangiform fishes typically have fairly flexible but low-aspect-ratio caudal fins, such as the fins of many minnows, suckers, catfishes, sunfishes, and darters (Figure 7.4). Aspect ratio expresses the amount of lift generated by a hydrofoil, with lift increasing with aspect ratio, and is determined by the square of fin span divided by fin area. Examples of carangiform swimmers within North American freshwater fishes are less common but potentially include herring and shad (family Clupeidae). Carangiform swimming also is likely approached by two large cyprinid fishes endemic to the Colorado River system, Humpback Chub (Gila cypha) and Bonytail Chub (G. elegans); both have high-aspect-ratio caudal fins and narrow caudal peduncles, although there are no supporting biomechanical or hydrodynamic studies on these species. Consequently, in terms of BCF swimming, the vast majority of North American freshwater fishes occupy the anguilliform-subcarangiform range of swimming modes.