Bony Fish

The third group in the vertebrate fish class is the bony fish, or true fish, group. The fish in this group have jaws and paired fins. Most bony fish have scales that protect their skin. Some fish, such as the porcupinefish, have spines instead of scales.

Bony fish vary greatly in size. The goby fish can be as small as one and a half centimeters. The swordfish, one of the largest bony fish, may reach a length of over 11 meters. The vast majority of the world’s fishes belong to the class Osteichthyes. Osteichthyes means “bony fishes.” As the name indicates, these fishes have skeletons made of bone instead of cartilage. Like sharks, skates and rays, however, bony fishes also have jaws and scaly skin.

External structure

Mouth, nostrils and eyes without lids are located on the head of a typical bony fish. It has two sets of gills, one on either side of the body. A protective flap of tissue called an operculum covers the gills. The fins stabilize and maneuver the fish and propel it forward. The forward most pair are the pectoral fins. Midway along the sides of its body are the paired pelvic fins. Two dorsal fins extend from the trout’s back. An anal fin extends from its ventral surface, near its anal opening. A fish swims by moving its body from side to side while swimming its tail in the opposite direction. The paired fins help change course. Most fishes have skin covered by thin overlapping segments called scales. Fish are born with certain number of scales. Though the scales enlarge throughout the life of the fish, new ones are never grown.

Skeletal and digestive systems

The fish skeleton is composed almost entirely of bone. The anterior end of the vertebral column is the skull, which covers and protects the brain. Ribs project from the backbone. The spinal cord runs parallel to the backbone and nerves branch from it to various parts of the body. Fish feed on insects and fish eggs. The food moves from the throat cavity down the esophagus. In stomach the food is stored and digestion begins. Near the stomach is the liver, a large organ that secretes bile, a substance that breaks down the fats in the food.

Respiratory and circulatory systems

Like most fishes, bony fish obtains oxygen by means of gills. It has 4 gills, 2 in each gill chamber on either side of the head. Each gill consists of a bony gill arch fringed with thin-walled tissues called gill fragments. The gill filaments contain many small blood vessels. A two-chambered heart pumps blood through a series of vessels to all parts of the body. Arteries carry blood away from the heart; veins carry blood back to the heart. The small vessels form the connecting network between arteries and veins.

Fish reproduce by spawning

Fertilization in most fish is external and takes place in water. During the process of spawning, the female lays hundreds or thousands of eggs in the water. The male fish then spreads sperm – called milt – over the eggs. The fertilized eggs develop into embryos and then into young fish. In some fish, such as the seahorse, the female deposits her eggs into a pouch in the male. The eggs hatch in the pouch and young seahorses emerge into their watery environment.

Fish eggs do not have protective shells and would dry out if they were not in water. The yolk sac of a fish egg provides food for the fish as it develops.

Date: 2014-12-22; view: 968