BIOLOGY Volume 2 - A Guide to General Biology - 2004

18. THE ANIMAL SKELETAL AND MUSCULAR SYSTEM

18.6. Locomotion in Vertebrates

18.6.1. Swimming in Fish

The density of Water, especially sea water, is very high and exceeds that of air by many hundreds of times. Water is a relatively viscous medium, which hinders movement within it, yet its density provides structural support for the fish's body and acts as a substrate against which the fish can push when swimming.

In any successful Organism, we find a multitude of adaptations to its environment, and fish are no exception. The bodies of most fish are streamlined and tapered at both ends. This allows water to flow smoothly around the body, keeping hydrodynamic drag to a minimum. Fish have no protruding body parts other than fins, and generally, the faster a swimmer a fish is, the more perfect its streamlined form. The scales of cartilaginous and bony fish are lubricated with mucous secretions from Skin glands, which further reduces friction between the body and the water. Fins serve as another common adaptation for aquatic locomotion. Unpaired fins, situated along the body's midline, maintain stability and control orientation, whereas paired fins (pectoral and pelvic) act as rudders for depth and steering; the caudal fin, working in conjunction with the paired fins, propels the fish forward through the water Column. The mechanics of fin operation will be discussed in detail in subsequent sections.