External Anatomy   full-sized image

Shape Most fish have a streamlined body. The head is somewhat rounded at the front. Fish have no neck, and so the head blends smoothly into the trunk. The trunk, in turn, narrows into the tail. Aside from this basic similarity, fish have a variety of shapes. Tuna and many other fast swimmers have a torpedolike shape. Herring, freshwater sunfish, and some other species are flattened from side to side. Many bottom-dwelling fish, including most rays, are flattened from top to bottom. A number of species are shaped like things in their surroundings. For example, anglerfish and stonefish resemble rocks, and pipefish look like long, slender weeds. This camouflage, called protective resemblance, helps a fish escape the notice of its enemies and its prey.

Skin and colour Most fish have a fairly tough skin. It contains blood vessels, nerves, and connective tissue. It also contains certain special cells. Some of these cells produce slimy mucus. This mucus makes fish slippery. Other special cells, called chromatophores or pigment cells, give fish many of their colours. A chromatophore contains red, yellow, or brownish-black pigments. These colours may combine and produce other colours, such as orange and green. Some species have more chromatophores of a particular colour than other species have or have their chromatophores grouped differently. Such differences cause many variations in colouring among species. Besides chromatophores, many fish also have whitish or silvery pigments in their skin and scales. In sunlight, these pigments produce a variety of bright rainbow colours. The colour of most fish matches that of their surroundings. For example, most fish that live near the surface of the open ocean have a blue back, which matches the colour of the ocean surface. This type of camouflage is called protective coloration. But certain brightly coloured fish, including some that have poisonous spines, do not blend with their surroundings. Bright colours may protect a fish by confusing its enemies or by warning them that it has poisonous spines. Most fish can change their colour to match colour changes that are present in their surroundings. Flatfish and some other fish that have two or more colours can also change the pattern formed by their colours. A fish receives the impulse to make such changes through its eyes. Signals from a fish's nerves then rearrange the pigments in the chromatophores to make them darker or lighter. The darkening or lightening of the chromatophores produces the different colour patterns. Scales. Most jawed fish have a protective covering of scales. Teleost fish have thin, bony scales that are rounded at the edge. There are two main types of teleost scales - ctenoid and cycloid. Ctenoid scales have tiny points on their surface. Fish that feel rough to the touch, such as bass and perch, have ctenoid scales. Cycloid scales have a smooth surface. They are found on such fish as carp and salmon. Some primitive bony fish, including bichirs and gars, have thick, heavy ganoid scales. Sharks and most rays are covered with placoid scales, which resemble tiny, closely spaced teeth. Some fish, including certain kinds of eels and fresh-water catfish, are scaleless.

Fins Fins are movable structures that help a fish swim and keep its balance. A fish moves its fins by means of muscles. Except for a few finless species, all modern bony fish have rayed fins. Some primitive bony fish also have rayed fins. These fins consist of a web of skin supported by a skeleton of rods called rays. Some ray-finned fish have soft rays. Others have both soft rays and spiny rays, which are stiff and sharp to the touch. Some primitive bony fish have lobed fins, which consist of a fleshy base fringed with rays. Lobed fins are less flexible than rayed fins. Sharks, rays, and chimaeras have fleshy, skin-covered fins supported by numerous fine rays made of a tough material called keratin. Fish fins are classified according to their position on the body as well as according to their structure. Classified in this way, a fin is either median or paired. Median fins are vertical fins on a fish's back, underside, or tail. They include dorsal, anal, and caudal fins. The dorsal fin grows along the back and helps a fish keep upright. Almost all fish have at least one dorsal fin, and many have two or three. The anal fin grows on the underside near the tail. Like a dorsal fin, it helps a fish remain upright. Some fish have two anal fins. The caudal fin is at the end of the tail. A fish swings its caudal fin from side to side to propel itself through the water and to help in steering. Paired fins are two identical fins, one on each side of the body. Most fish have both pectoral and pelvic paired fins. The pectoral, or shoulder, fins of most fish grow on the sides, just back of the head. Most fish have their pelvic, or leg, fins just below and behind their pectoral fins. But some have their pelvic fins as far forward as the throat or nearly as far back as the anal fin. Pelvic fins are also called ventral fins. Most fish use their paired fins mainly to turn, stop, and make other manoeuvres.

Kinds of fish scales These drawings show examples of the four main types of fish scales. Most modern bony fish have ctenoid or cycloid scales. Some catfish and a few other species have no scales at all. Ctenoid scale  Cycloid scale  Ganoid scale Placoid scale