All bony fish share many physical characteristics, which are labeled in Figure below. One of their distinguishing features is scaly skin. Scales on fish overlap one another, much like shingles on a roof, protecting the skin from damage and slowing the movement of water into or out of the fish’s body.
Bony fish are outfitted with fins that facilitate maneuvering and positioning in the water. The fins, which are made of thin membranes supported by stiff pieces of cartilage, can be folded down or held upright.
Fins are named for their location: Dorsal fins are on the back, a caudal fin is at the tail, and an anal fin is on the ventral side. Two sets of lateral fins are located on the sides of the fish, the pectoral fins are toward the head, and the pelvic fins are near the tail. The caudal fin moves the fish forward in the water, and the others help change direction and maintain balance.
Although fish dine on a wide assortment of food, most species are predators whose mouths contain small teeth for grasping prey. Nutrients from digested food are distributed through the body by a system of closed blood vessels. The circulation of blood is powered by a muscular two-chambered heart. Blood entering the heart is depleted of oxygen and filled with carbon dioxide, a waste product of metabolism. Blood collects in the upper chamber, the atrium, before it is pushed into the ventricle. From the ventricle, it travels to the gills where it picks up oxygen and gets rid of its carbon dioxide. Water exits through a single gill slit on the side of the head. The gill slits of fish are covered with a protective flap, the operculum.
In many bony fish, some gases in the blood are channeled into another organ, the swim bladder. This organ is essentially a gas bag that helps the fish control its depth by adjusting its buoyancy. A fish can float higher in the water by increasing the volume of gas in the swim bladder. To sink, the fish reduces the amount of gas in the bladder.
Most bony fish reproduce externally. Females lay hundreds of eggs in the water, then males swim by and release milt, a fluid containing sperm, on the eggs. Fertilization occurs in the open water, and the parents swim away, leaving the eggs unprotected. Not all of the eggs are fertilized, and many that are fertilized will become victims of predators, so only a small percentage of eggs hatch.