Fish

I

Introduction

Fish, aquatic vertebrate animal, usually possessing gills in the adult stage and having limbs, when present, in the forms of fins. Unlike other commonly recognized groups of animals, the fishes are a diverse assemblage of groups that cannot be recognized by any defining trait (as mammals, for example, can be recognized by the presence of mammary glands or hair). Consequently, there are exceptions to the definition given here. Fishes are usually understood to include the jawless vertebrates such as the lamprey and hagfish; and the shark, ray, chimaera, lungfish, and bony fishes. The latter group includes the forms most commonly known as fish.

The word fish is commonly used to refer to an individual animal or to a species, or to fish in general. When referring to two or more kinds or species, the usual convention is to employ the plural fishes.

II

Age and Diversity

The earliest known vertebrates were jawless fishes that left fossilized remains in rock strata laid down during the Cambrian Period of the Palaeozoic Era, some 530 million years ago. Early forms were of small size, seldom more than a few centimetres in length, and possessed gills in a series of pouches. The first fishes with jaws evolved during the Devonian period (408 million to 362 million years ago), the so-called Age of Fishes, and became the dominant form of vertebrate life in marine and freshwater habitats. The major lines of fishes, including sharks, the coelacanth, and bony fishes, appeared in the latter part of the period.

Fishes constitute more than half the total number of known modern vertebrates. Scientists recognize an estimated 24,000 living species, compared with 21,500 extant (surviving) amphibians, reptiles, birds, and mammals. Unlike other vertebrates, new species of fishes continue to be discovered at a high rate and it is expected that the eventual number of recognized living species may exceed 28,000.

III

Anatomy

In general, fishes are somewhat torpedo-shaped, with bodies that are moderately compressed at the sides and taper more markedly at the tail than at the head. The fundamental feature is the serially repeated set of vertebrae and segmented muscles, which enable the fish to propel itself by moving its body from side to side. Typically, the body carries a number of fins, which are membranes supported by rays or spines, that function in propulsion or orientation. One or more dorsal fins may be located along the centreline of the back. A caudal fin lies at the end of the tail and is the primary organ for generating thrust in most species. One or more anal fins are situated on the ventral midline between the vent and the tail. The body has two pairs of lateral fins: the pectoral fins, which are usually located on the sides behind the gill openings, and the pelvic fins, which lie on the belly between the head and the vent. Great diversity exists in the shape and anatomical details of fishes, ranging from the snake-like eel to the globe-shaped puffer or greatly flattened fluke. Fins are often much modified or absent in accord with the diverse modes of life. Certain species of swamp eels lack most of the features by which fishes are commonly recognized, including gills, fins, and scales, and may even be primarily terrestrial. Some species of bony fishes lack eyes.

The various species of fishes also show extreme variation in size. A recently discovered goby from the Indian Ocean reaches only 10 mm (about 0.5 in) in length, whereas the whale shark may reach 15 m (50 ft) and an estimated weight of 68,000 kg (150,000 lb).

Fishes vary greatly in colouring as well, displaying a wide range of hues and patterns. In general the coloration is lighter on the belly than on the back, but in water the fish appears to be of a uniform shade because the only light comes from above. A number of tropical fishes, notably the butterfly fish, are brilliantly coloured and patterned. Their coloration may serve as a means of species recognition or, in the case of venomous species, as a warning to would-be predators. Many fishes have the ability to alter their colours markedly to blend with their surroundings. The flatfish has been observed changing colour to match a chequerboard pattern on the bottom of an aquarium.

Many species have specialized shapes and organs that aid them in hunting and feeding. Among such fishes are the lantern fish and other deep-water species, which have luminous organs to reveal or attract their prey. The angler fish lies on the ocean bottom and presents a small, worm-like knob of flesh on the end of a long spine as a bait to other fishes.

A

Scales

The bodies of most fishes are covered with a layer of scales, which are bony or horny plates arranged in overlapping rows, with the free rear end of one scale overlapping the attached front end of the scale behind it. A thin epidermis usually covers the scales. In some species, such as the eel, the scales are minute. In others, such as the catfish, they are almost absent.

Fishes have sometimes been classified according to the shape and characteristics of their scales. The most important types are ganoid scales, which are rhombic in shape and covered with an enamel-like layer; cycloid scales, which are almost round with smooth edges; and ctenoid scales, which are also round but have serrated, or comb-like, exposed edges. The epidermal layer of the body contains the pigment cells that give the fish its colour. Also in the epidermal layer are cells secreting a slippery mucus that covers the entire body.