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In-Depth Information
unsuspecting prey. Some fish have extending mouth parts that enable them
to sift food from silt, to reveal teeth, or to remove food items from a variety of
surfaces, including other fish.
Fish often engage in the behavior of leaping out of the water. For some this
could be a tactic to avoid predation, but for the salmon and migratory trout
it is a way of leaping obstacles such as waterfalls to access breading grounds.
Some fish may use leaping to get rid of parasites such as lice and ticks.
Swimming
As you would expect, animals have developed different ways of propelling
themselves on or through the water, and moving on the surface of water is a
very different matter from moving in it.
Slow-speed swimming, sustained swimming, and high-speed swimming
demand different approaches, and often the generation of thrust is created
by different processes (oscillation, rowing) and from different parts of the
anatomy (caudal fin, pectoral fins), though all forms of swimming must
overcome issues such as inertia and drag.
On the face of it, swimming and flying seem not too dissimilar. Neither flying
birds nor swimming fish are supported by the ground; both seem suspended
within their particular medium, air or water. However, the forces needed
for swimming vary considerably from those needed to fly. Animals that get
around by flying need to overcome strong vertical forces (gravity) to support
themselves in the air. This requires a great deal of effort on the part of the
animal in countering those forces with forces generated by their own actions
(lift). By comparison, animals moving in liquid are not subject to the same
degree of vertical forces. Animals that live in water are much closer in density
to that of their environment; as a result they need to exert far less effort in
creating vertical forces to counter those that impact on them. Fish and other
water-dwelling animals expend far less energy to maintain their position in
their medium than birds do in theirs.
FIG 4.81
The forces acting on animals moving through air and water differ in the effects they have on the ani-
mal. Gravitational effects are felt more strongly by birds than by fish. However, the force required for locomotion
through water is greater than that required for motion through air.
