Ecology of North American Freshwater Fishes. Stephen T. Ross Ph. D.
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(Figure 7.3). Surge refers to movement forward or backward, slip refers to sideways movement, and heave refers to movement up or down. Rotational forces refer to movement around the center of mass and occur along three axes: yaw, pitch, and roll (Figure 7.3). Yaw describes the rotation about the center of mass from side to side, pitch is the rotation up or down, and roll is the rotation along the horizontal axis of the body. Some actions do not result in a change of rotational or translational state because they result in keeping the body in the same location (e.g., hovering) (Alexander 1967c; Webb 2006).
Body Shape, Fin Location, and Maneuverability
Control and maneuverability during hovering or active movement are related closely to fin placement relative to the center of mass, the control of fin rays and fin area by muscles, and swimming speed (Alexander 1967c; Webb 2006). Four zones are recognized relating to fin placement and function (Figure 7.4): (1) an anterior body zone of rudders and lift surfaces positioned anterior to the center of mass that are important in translational forces; (2) a zone of keels located at the center of mass that are particularly important in controlling roll; (3) a zone of stabilizers located immediately posterior to the center of mass and important in controlling yaw, pitch, and roll; and (4) a zone of locomotion and rudders located well posterior to the center of mass that is again important in translational forces (Aleev 1969; Gosline 1971). Anterior control surfaces (zone 1) can include pectoral fins, the head, or the anterior part of the spinous dorsal fin, with the head particularly important in turning motion in elongate body shapes (Webb 2006). A fin, such as the spinous dorsal in zone 1, acts to deflect the fish away from its forward course, but during rapid forward progress in a straight line, it is advantageous for it to be folded down, which also helps to reduce drag. Pectoral fins can also be furled during high swimming speeds (Webb 2006). A single dorsal fin located over the center of mass (zone 2) serves as keel but does not stabilize or deflect the forward course of the body. Many lower teleosts, such as herrings, minnows, suckers, catfish, and trout (groups in the Clupeomorpha, Ostariophysi, and Protacanthopterygii; Figure 7.5), have dorsal fins in this general position or in a position slightly posterior to the center of mass where the fin can also function as a stabilizer (rudder) or aid in propulsion (Figure 7.4B) (Aleev 1969; Gosline 1971). In higher teleosts, such as Moronidae, Centrarchidae, and Percidae (groups in the Acanthomorpha; Figure 7.4A), the dorsal fin consists of two parts, the more anterior spinous dorsal fin and the more posterior soft dorsal fin. The spines can be raised or lowered depending on need. It is important to remember, however, that fins can serve multiple purposes, including camouflage, communication, and in the case of spines, defense.
FIGURE 7.3. Terms used in describing translational (black font and arrows) and rotational (gray font and arrows) changes in state about the center of mass in fishes. Photograph of Colorado Pikeminnow (Ptychocheilus lucius) courtesy of Tom Kennedy. Based on Alexander (1967a) and Webb (2006).
FIGURE 7.4. Potential fin functions relative to the center of gravity in (A) higher teleosts illustrated by the Freckled Darter (Percina lenticula), and (B) lower teleosts illustrated by the Blacktail Shiner (Cyprinella venusta). Based on Aleev (1969) and Gosline (1971).
FIGURE 7.5. Major levels of fish evolution. Names at the base of the cladogram define inclusive groups (e.g., Osteoglossomorpha to Tetraodontiformes are included within the Teleostei). Names at the ends of branches refer to particular lineages. Black text identifies groups that have, or had, representation in North American freshwater habitats. The Sarcopterygii includes lobefin fishes as well as tetrapods. Based on Nelson (2006).
Many freshwater fishes achieve static lift (=buoyant lift) by having air bladders or low-density fatty inclusions within the body cavity so that the mass of water displaced approaches the mass of the fish (Gee 1983). However, because the vertebral column bounds the upper extent of the abdominal cavity, low-density inclusions result in the center of buoyancy being beneath the center of mass (Eidietis et al. 2003). The difference between the center of mass and the center of buoyancy is termed the metacentric height, and a negative value, typical of most fishes, results in a rolling torque (a reason why a recently dead or an incapacitated fish turns belly up). Fishes must use behavioral changes, such as resting on the bottom or leaning against structures, or fin movements, to compensate for this inherent instability. To a certain extent, this rolling torque likely was reduced by the location of the swimbladder dorsal to the gut in actinopterygians compared to the ventral position of the lung (the precursor to the swimbladder) in early bony fishes (the lobefin fishes within the Sarcopterygii) such as lungfishes (Lauder and Liem 1983; Webb 2002). Some actinopterygians also have a more anterior location of gas volume such that the pitching torque generated by the mass of the head skeleton is reduced (Webb 2006).
Types of Locomotion
Fish swimming modes can be divided into those involving the body and caudal fin (BCF) and those using various combinations of paired or median fins for locomotion (MPF) (Blake 2004). BCF locomotion is undulatory, involving alternate waves of contractions on either side of the body, because of sequential innervation of lateral body muscles (serial myomeres) that are three-dimensionally folded and divided into blocks by connective tissue (myosepta) (Danos et al. 2008). Furthermore, BCF swimming can be subdivided into steady, continuous swimming versus unsteady, transient (burst and coast) swimming (Blake 2004). Burst-and-coast propulsion occurs in many pelagic and nektonic fishes, and in fishes with streamlined bodies, it can provide considerable energy savings per distance traveled in contrast to steady swimming (Blake 1983b).
BCF swimming typically is categorized into three to five modes: anguilliform, subcarangiform, carangiform, thunniform, and ostraciiform (Breder 1926; Webb 1975; Lindsey 1978). The modes are named after exemplar species and characterized by increasing concentration of the propulsive force in the caudal fin, although they do not imply phylogenetic relationships (Webb 1975; Blake 2004). The ostraciiform mode has a complete, or nearly complete, absence of body undulation with all propulsive power generated by oscillation of the caudal fin. Because the ostraciiform swimming mode is exemplified by tropical marine box fishes, marine electric rays, and tropical African freshwater elephant fishes (Lindsey 1978; Helfman et al. 2009), and is not represented by any North American freshwater fish group, it will not be discussed further.
The remaining four modes were originally defined by perceived differences in swimming based on morphology and not on hydrodynamic analyses and, among other things, overlooked the three-dimensional geometry of the body during swimming. Recent research indicates that two-dimensional views of dorsal midline profiles of anguilliform, subcarangiform, carangiform, and thunniform modes are essentially indistinguishable, at least during certain swimming speeds. Because of this, the traditional modes of BCF swimming in fishes are not always representative of hydrodynamic differences and lack a functional basis (Blake 2004; Lauder and Tytell 2006). Current research suggests that thunniform and carangiform modes are quite similar in most, although not all, features. Because of the high similarity between the carangiform and thunniform modes (Blake 2004), and because I know of no North American freshwater fish using a thunniform swimming mode, it is not treated further. The remaining three BCF modes are not distinct in all attributes and are grouped differently based on different functional and morphological criteria, including propulsive wavelengths, wake patterns, tendon lengths, and red muscle activity (Table 7.1) (Lauder and Tytell 2006; Danos et al. 2008). Thus, although useful as general shorthand descriptors of BCF swimming, the taxon-named swimming modes are not totally distinct but share various features.
ANGUILLIFORM BCF LOCOMOTION In anguilliform swimming, which is ontogenetically and phylogenetically the basal mode of BCF swimming in ray-finned fishes, the Actinopterygii (Figure 7.5), the entire body is employed to generate thrust through a series of waves moving from head to tail (Gosline 1971). In contrast to early studies indicating that large amplitude undulations occurred all along the body over a range of swimming speeds, recent work indicates that body waves have increasing amplitude posteriorly, thus increasing water displacement