Class Osteichthyes: Bony Fish More than 95% of all fish belong to the class Osteichthyes. As implied by their name, these fish have a skeleton made up of bone. Being in the phylum Vertebrata, they also have a backbone made up of individual bones called vertebrae which surround and protect the spinal cord. Bony fishes can survive in most aquatic habitats including both freshwater and saltwater ecosystems.
Bony fish have adapted very well to life in the sea. They have a protective covering of scales, which are loosely attached to the skin and can even rub off in your hands. Bony fish fell slimy to the touch because their skin, which is living, secretes a protective mucus coating over their scales. The mucus serves two functions: it acts as a barrier against infection and it reduces friction so the fish can move easily through the water.
Scales serve another function: they can indicate the approximate age of fish. On the scales are growth rings called circuli. As the fish grows older, new circuli are produced. When the circuli are close together, they form bands. Although there is variation in the bands of each fish, a single band may represent about one year’s growth, mush like the bands on a clamshell.
Marine animals, such as fish, that have the ability to swim are referred to as nekton (as opposed to plankton, which means drifter). Bony fish have paired fins, the pectoral fins and pelvic fins. These fins are attached to body muscles so that when the muscles contract, the fins move and allow the fish to move all directions. Both the single dorsal fin and the single anal fin work to stabilize the fish which prevents its rolling from side to side. Factors that influence how fast a fish can swim include its body shape (fusiform, tapered at both ends, is streamlined), the use and shape of a caudal fin to propel for propulsion, and water temperature (cooler water slows fish down).
When a fish gets of swimming, it will want to rest. It does this by resting periodically by floating in place. The ability to float or rise in a liquid is called buoyancy. Bony fish are buoyant because they use a swim bladder which us an internal gas-filled organ. Muscles around the swim bladder cause the swim bladder to contract or enlarge causing the fish to sink or float respectively or even maintain a steady position in the water (neutral buoyancy).
Eating habits vary from fish to fish. A method seen in bony fish is called straining which is similar to filter feeding. When straining, a fish will swim with its mouth wide open and will filter out great quantities of plankton on its gill rakers. Once the water exits the gills, the plankton is swallowed. Some bottom dwelling fish are adapted for sucking their food into their rounded mouths equipped with small teeth. Other fish have beaklike mouths that are adapted for nibbling on chunks of algae-covered coral or strong teeth suitable for crushing the shells of invertebrates. Predators have large sharp teeth for catching fish and other fast-moving prey.
Fish are very sensitive to changes on their environment. They have a well-developed nervous system to carry out their responses. Fish have a good sense of hearing. Although they have no external ears, fish have an inner ear located behind their brain, and sound receptors located in tiny openings in the skin along each side of the body. Fish also have excellent sense of smell. In front of the eyes are nostrils or nares. Sensitive nerve endings inside the nares can detect tiny quantities of chemicals in the water. Vision is also good in fish. Fish that live in clear waters near the surface have the best color vision, whereas fish that live in deep, darker waters have a larger eyes that can take in more light.
(adapted from Marine Science, 2nd edition. Thomas F. Greene. Amsco School Publications Inc. Brooklyn, NY 2004. Pp. 290-301)