Anatomy of a bony fish.
Male and female anatomy.
A fish is a cold-blooded vertebrate that lives in water and uses gills for providing oxygen to the blood. Fish typically have fins, scales, a slender body, and a two-chambered heart. There are three main groups: the bony fish or Osteichthyes (e.g., goldfish, tuna); the cartilaginous fish or Chondrichthyes (e.g., sharks, rays); and the jawless fish or Agnatha (e.g., hagfish, lampreys). About 30,000 fish species are known of which approximately 2,500 are freshwater. The zoological study of fish, including their classification, structure, distribution, and ecology, is called ichthyology.
Bony fish (Osteichthyes)
Most fish have an internal skeleton mainly composed of bone. The bony fish include the teleosts, lungfish, and several surviving primitive forms, such as the bichir, gar-pike, and sturgeons (in the latter, bone is very much reduced and the skeleton is almost wholly made of cartilage).
In shape the typical teleost (e.g., salmon) is more streamlined than that of the cartilage fish (see below), being more flattened from side to side. The tail is usually symmetrical. The paired fins are small and have supporting rays. The scales are thin and flat, not tooth-like as in sharks. The mouth is larger and the lower jaw more mobile than that of sharks. The gills are covered by a flap, the operculum, and the skull has a complicated structure. The jaws are made up of several bones. The vertebral column is better developed than in sharks and bears more prominent ribs. Thin protective pads (the remains of the notochord) occur between each of the vertebra. A feature not found in sharks is the air bladder – an air-filled sac used as a floatation device. The brain is better developed than in sharks.
Cartilaginous fish (Chondrichthyes)
The sharks, rays, and skates have an internal skeleton of cartilage – bone is absent. They are the simplest of living vertebrates with jaws.
The skull is more developed than in the jawless fish (see below). In all except the king herrings the gill slits are visible and not covered by a gill flap or operculum as in bony fishes. The gills are respiratory.
The skin is covered with horny teeth-like scales and on the jaws these are modified to form teeth. The dorsal lobe of the tail is larger than the ventral lobe. Besides the dorsal fins (typically two in number) there are two pairs of paired fins. Each pair fin has a supporting structure at the base – pectoral (shoulder) girdle and pelvic (hip) girdle.
The notochord is reduced in comparison with that of the jawless fish and is largely replaced by vertebrae – short ribs are present in the front region. Movement is produced mainly by the serial contraction of the fibers of successive muscle blocks – arranged essentially as in simpler chordates. The gut is more elaborate and divided into more obvious regions.
Jawless fish (Agnatha)
The simplest of fish, and the simplest of all vertebrates, are the jawless fish, which include the lampreys and hagfish. These creatures have no jaws. The mouth is surrounded by a large, round sucker. The tail has a fin which extends forward halfway along the back and the muscle blocks are W-shaped. The skeleton is made up of the notochord and cartilage.