Anatomy and Physiology

Skeletal System

The skeleton of bony fishes is made of bone and cartilage. The vertebral column, cranium, jaw, ribs, and intramuscular bones make up a bony fish's skeleton.

The skeleton of a bony fish gives structure, provides protection, assists in leverage, and (along with the spleen and the kidney) is a site of red blood cell production.


Muscular System

The muscles of the tail and trunk consist of a series of muscle blocks called myotomes. The myotomes usually resemble a sideways letter "W". A connective tissue called myosepta separates the myotomes. A horizontal septum separates the myotomes into dorsal (top) myotomes and ventral (bottom) myotomes.

Jaw muscles usually consist of adductor muscles that close the jaw and abductor muscles that open the jaw.
Fin muscles consist of abductor and adductor muscles that move the fins away from and close to the body, and erector muscles that provide stability and flexibility in the fins.


Nervous System

The nervous system of fishes is poorly developed compared to that of other vertebrates.

A bony fish's brain is divided into three sections: the forebrain, the midbrain, and the hindbrain.
  • The forebrain is responsible for the bony fish's ability to smell. Bony fishes that have an especially good sense of smell, such as eels, have an enlarged forebrain.
  • The midbrain processes vision, learning, and motor responses. Blind bony fishes, such as blind cavefishes in the family Amblyopsidae, have a reduced midbrain.
  • The hindbrain (medulla oblongata and cerebellum) coordinates movement, muscle tone, and balance. Fast-swimming bony fishes usually have an enlarged hindbrain.

The spinal cord and a matrix of nerves serve the rest of the body.


Cardiovascular System

A bony fish's heart has two chambers: an atrium and a ventricle. The venous side of the heart is preceded by an enlarged chamber called the sinus venosus. The arterial side of the heart is followed by a thickened muscular cavity called the bulbus arteriosus.


Blood flow
  • The sinus venosus receives oxygen-depleted blood from the body. A valve at the end of the sinus venosus opens into the atrium.
  • The atrium has thick, muscular walls. The atrium receives oxygen-depleted blood and pumps it into the ventricle.
  • The ventricle is the largest and most muscular chamber of the heart. When filled with blood, it constricts, forcing the blood through the bulbus arteriosus.
  • Blood flows through the bulbus arteriosus into the ventral aorta. A valve or series of valves in the bulbus arteriosus controls blood flow into the ventral aorta.
  • From the ventral aorta, blood flows to the gill filaments, where it is oxygenated.
  • Oxygenated blood flows from the gill filaments to the organs of the head and body. A complex system of arteries, veins, and capillaries circulates blood through the body and returns the blood to the sinus venosus.



  • Nearly all fish species are ectothermic, or cold-blooded. They are not capable of regulating their own body temperature and depend on their surrounding environment or behavior for heat.
  • The opah (Lampris guttatus) is the one species of bony fish considered to be truly warm-blooded. Heat is produced by a constant “flapping” of the opah’s large, winglike pectoral fins. Countercurrent heat exchange in the gills allows the opah to maintain an elevated body temperature — about 5C (41F) above the surrounding water at depths of 45 to 305 m (150—1,000 ft.). A higher body temperature allows the opah to be a more active and quick competitor in its cold water habitat.
  • Some tunas (family Scombridae, subfamily Thunninae) maintain a body temperature several degrees higher than that of the surrounding water. This heat is due to the modified circulatory system associated with the red muscle.
    • As red muscle functions, it generates heat. Muscle-generated heat warms the blood circulating through the red muscle, which then travels back to the heart through veins. Thus, blood returning to the heart from the muscle is warmer than blood traveling from the heart to the muscle.
    • Due to the nearness of arteries and veins, heat passes from warmer veins to cooler arteries within the fish's body, rather than dissipating to the cooler environment. This modified circulatory system retains heat in the red muscle.
    • A higher body temperature is an adaptive advantage for high-speed swimming.A similar modified circulatory system warms the brain and eye of some species of tunas and billfishes (family Istiophoridae).

Digestive System

The esophagus in bony fishes is short and expandable so that large objects can be swallowed. The esophagus walls are layered with muscle.

Most species of bony fishes have a stomach. Usually the stomach is a bent muscular tube in a "U" or "V" shape. Gastric glands release substances that break down food to prepare it for digestion.

At the end of the stomach, many bony fishes have blind sacs called pyloric caeca. The pyloric caeca are an adaptation for increasing the gut area; they digest food.
The pancreas secretes enzymes into the intestine for digestion.
Most food absorption takes place in the intestine. The length of the intestine in bony fishes varies greatly. Plant-eating bony fishes generally have long, coiled intestines. Carnivorous bony fishes have shorter intestines.
The digestive system terminates at the anus.

Respiratory System

Water enters the gill chamber through a fish's mouth and exits through gill openings under the operculum. Blood flowing through the gill filaments absorbs oxygen from the water.

Some fish have adaptations for getting oxygen from air. Lungfish must return to the surface to breathe air. A lungfish swallows air to fill up an air sac or "lung". This lung is surrounded by veins that bring blood to be oxygenated. Its gills alone can't keep a lungfish supplied with enough oxygen to live. Other species such as tarpon (family Elopidae) can gulp air at the surface to supplement their oxygen demand.
Some species of bony fishes can absorb considerable amounts of oxygen through their skin.

Swim Bladder

Many species of bony fishes have a gas-filled bladder called a swim bladder.

Apparently the swim bladder originally developed in fish as an organ of respiration, as evidenced by the "lung" of the lungfishes.

In modern bony fishes that possess a swim bladder, the organ serves principally in maintaining neutral buoyancy.

In some fishes the swim bladder has adapted to function as a sound amplifier.


Both marine and freshwater fishes regulate the movement of water across their body surfaces.

The tissues of marine fishes are less salty than the surrounding water, so water continually leaves the body of a marine fish through its skin and gills. To keep from becoming dehydrated, a marine fish drinks large amounts of water and produces a small amount of concentrated urine. In addition, its gills are adapted to secrete salt.
The tissues of a freshwater fish are saltier than its surrounding environment, so water is continually entering the body of a freshwater fish through its skin and gills. Freshwater fishes do not drink water, and they produce large amounts of dilute urine.