Ecology of North American Freshwater Fishes. Stephen T. Ross Ph. D.
Trade-Offs in Form and Function
Does Morphology Predict Ecology?
Tests of the Ecomorphological Hypothesis
Studies Assuming Validity of the Ecomorphological Hypothesis
VERTEBRATE EVOLUTION BEGAN in an aquatic environment in the early Paleozoic (500+ mya), followed by the evolution of tetrapods and then the evolution of terrestriality in the middle Devonian (390 mya) (Clack 2002; Nelson 2006). The aquatic and terrestrial environments occupied by vertebrate organisms offer their own sets of challenges and opportunities. For instance, unlike air, water is incompressible for all practical purposes and has much greater viscosity (the resistance of a fluid to deformation because of internal friction). Viscosity becomes increasingly significant as body size decreases and so is an especially important issue for larval stages of fishes (Webb and Weihs 1986). Because the viscosity and density of water are much greater than in air, movement in water must overcome greater drag compared to terrestrial vertebrates moving over land or flying. As a consequence, aquatic organisms, other than those where speed is not an issue, have streamlined body shapes to reduce the energy requirements of locomotion. Also, volume for volume, oxygen content in water is about a thirtieth of that in air (Kramer 1987), and obtaining oxygen from water is additionally challenging by the need to move a viscous medium across respiratory surfaces. Compared to movement on land, the lack of a solid surface to push against reduces the resultant force, although water is a much more efficient medium to push against compared to air. In contrast to terrestrial vertebrates, because their density is close to that of water, aquatic vertebrates gain all or a majority of their bodily support from water rather than having to invest in a skeletal system that can carry the weight of the body. In addition, little energy is required to move vertically. In a now-classic study, Schmidt-Nielsen (1972) provided a way of comparing some of the costs and benefits of movement in water, air, and on land. He determined that the net energetic cost of powering 1 gram of vertebrate over 1 km relative to body size was lowest for swimming, intermediate for flying, and greatest for running. The disciplines of fish biomechanics and hydrodynamics are presently very active, due in part to new technologies allowing the precise quantification of water flow patterns around swimming fishes (Lauder and Tytell 2006). This chapter explores the interaction of morphological evolution in fishes with their success in various freshwater habitats.
BASICS OF FISH PROPULSION
The body of a fish essentially consists of a compression resistant notochord or vertebral column, surrounded by lateral musculature, and wrapped in a complex arrangement of connective tissue and skin (Danos et al. 2008). In contrast to terrestrial locomotion, where the limbs involved in locomotion must also support the body, fishes can use a variety of mechanisms for locomotion, both independently and in concert, and can employ a variety of control surfaces such as scales, body projections, and fins to affect their posture and position in the water column (Webb 1994, 2006).
Forces to Overcome
To achieve forward motion, the force generated by a swimming fish must equal (constant swimming speed) or exceed (acceleration) the resistance to movement caused by drag (Webb 1975; Blake 1983a). The two components of drag are friction drag and pressure drag, both of which can best be understood by boundary-layer theory. Water moving across the body of a fish, either by the fish moving through water or holding position in flowing water, has a gradient in relative velocity that increases from 0, where water molecules are in contact with the fish, to that of the free-stream velocity, the velocity of the undisturbed water at some distance from the fish (Blake 1983a). The region between the free-stream velocity and the velocity at the fish is referred to as the boundary layer (Figure 7.1). Flow in the boundary layer can be laminar, resulting in low friction drag, or turbulent, where the resultant eddies form a thicker boundary layer compared to laminar flow and overall friction drag is increased (Webb 1975; Blake 1983a). The change from laminar to turbulent flow is predicted by the Reynolds number, a hydrodynamic measure calculated as
where L = fish length; U = speed; and ν = the kinematic viscosity of water, which is approximately 0.01 cm2s−1 (Webb 1975; Purcell 1977).
Friction drag arises from the viscosity of water in the boundary layer. The greater the surface area of the body, the greater the friction drag. Friction drag also increases exponentially with swimming speed. For laminar flow, the exponent is 1.5, rising to 1.8 for turbulent flow in the boundary layer (Alexander 1967a). Pressure drag is caused by eddies generated along and behind the body by the separation of the boundary layer from the body of the fish (Figure 7.1A). The farther back that boundary separation occurs, the lower the underpressure and the size of the wake. Because turbulent boundary layers separate farther back than laminar boundary layers (Figure 7.1B), the pressure drag resulting from separation of a laminar boundary layer is higher than that for a turbulent one (Blake 1983a). Streamlining also reduces boundary layer separation and thus lowers pressure drag. Other things being equal, pressure drag increases at approximately the square of velocity (Alexander 1967c). Because of how friction and pressure drag are formed, a body shape that reduces friction drag has the opposite effect on pressure drag. Friction drag is related to surface area, so a body shape that minimizes the surface-to-volume ratio, such as a sphere, would have the lowest friction drag. Among freshwater fishes, a more globular shape, such as shown by some sunfishes, would have lower friction drag but a higher pressure drag in contrast to a more elongate, streamlined fish such as a trout, which would have higher friction drag but a lower pressure drag (Alexander 1967c).
FIGURE 7.1. Flow separation around a fish holding position in flowing water.
A. Flow lines, friction and pressure drag, and the boundary layer at a point tangential to the body. The relative thickness of the boundary layer is greatly exaggerated. The length of the arrows indicates the relative velocity of water, ranging from zero in contact with the body of the fish to the free-stream velocity indicated by the arrows of identical length on the right.
B. Changes in flow separation from the body in laminar (dashed lines, black arrows) and turbulent (dotted lines, white arrows) flow. Based on Webb (1975) and Blake (1983a).
Generated Forces
Water flowing over the body and fins of a fish can generate lift because the shapes are acting as hydrofoils—such lift is often referred to as dynamic lift. Bernoulli’s equation predicts that pressure will decrease as the velocity of fluid increases across a surface, so lift for a hydrofoil occurs when flow across the upper surface exceeds that of the lower, resulting in a pressure differential (Webb 1975). Such conditions occur when the angle incidence (α) of the hydrofoil increases from zero (Figure 7.2). The lift generated by a hydrofoil acts normal to the drag force and increases with the angle of incidence up to a point where flow lines begin to separate from the hydrofoil (usually about 15°), resulting in a sudden increase in pressure drag and a sudden decrease in lift so that a stall occurs. Because the amount of lift generated by turbulent flow is greater than for laminar flows, as a consequence of later separation of flow lines as described previously, higher values of lift occur at higher Reynolds numbers (Webb 1975; Blake 1983a).
FIGURE 7.2. Flow lines, lift, drag, and the resultant pressure force at three angles of incidence (α) of a hydrofoil. Drag is parallel to the axis of flow (or motion) while lift is normal to the axis of flow or motion. Based on Webb (1975) and Blake (1983a).
Freshwater fishes occupy a wide range of habitats with a correspondingly high range of current speeds and degrees of turbulence. To maintain hydrodynamic stability, change posture, initiate changes in course, or change location, fishes must control translational and rotational forces. Translational forces refer to movement of a body from one point in space to another without rotation and occur in three planes: surge, slip, and