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  • Writer's pictureGrant Reed

About Fish

Updated: May 27, 2022

AGA Blog - About Fish


Description of a fish

A fish is a cold-blooded (endothermic) vertebrate animal, living in water, breathing by means of gills, and having fins for stability and movement. Fish are by far the most numerous and diverse group of vertebrates.

History and classification

Fish date back 500 million years ago and are the ancestors of amphibians, reptiles, mammals and birds. Fish themselves have evolved from jawless fish on to cartilaginous fish (sharks and rays) and later bony fish.

L1 Classification of fish is based on their ancestral line as well as their feeding and breeding behaviour. Jawless fish (Class Agnatha) include hagfish and lampreys (extinct armoured jawless fish are grouped together in the paraphyletic): they have mouths but no jaws and they have several gill openings, paired limbs are absent and there are no bones in the vertebrae; the skeleton is made of cartilage. They are elongate, scaleless, slimy parasites and scavengers.  Cartilaginous fish

Life cycle of fish

Life cycle of oviparous fish: the adult female lays unfertilized eggs; the male fertilizes the eggs; the larvae are born and become juvenile fish before becoming adults.

Annual killifish (family Cyprinodontidae) live in pans. As the name suggests, these fish live for barely a year: the eggs are laid, fertilized and buried at the bottom of a pan during the rainy season. When the pan dries up in winter, development of the egg continues until shortly before the hatching stage. All further growth is halted and the ‘egg’ remains in a state of diapause until the start of the summer rains. The embryo hatches and grows to adulthood in matter of a few weeks.

Respiration system

Fish breathe by means of gills, which are feather-like filaments arranged in rows along four or five pairs of gill arches at the back of the mouth. The gills are located under a bony plate, the operculum. The gills occupy the operular chamber and form the lateral wall of the pharyngeal cavity.

The fish continuously pumps water through its mouth and over the gill arches.

When the fish inhales, it enlarges its pharyngeal cavity, producing a negative pressure in the pharyngeal cavity and water enters through the mouth.

When the fish exhales, it closes its mouth and the oral valve behind the lips so that water is forced out between the gills. Muscular action produces the high positive pressure in the pharyngeal cavity that is necessary to force the water between the gills.

Blood circulates through the gill filaments as water passes amongst them. The lamellae contain capillaries that receive depleted of oxygen. Each gill filament has many even smaller extensions called secondary lamellae which further increase the surface area. The blood flows through the capillaries in the opposite direction to that of the water washing over the lamellae. As blood flows through the capillaries, it picks up oxygen from the water. In this counter-current exchange, the blood is charged with oxygen efficiently. The blood is circulated by the blood vessels.

The gills of a healthy fish are bright red in colour due to the rich supply of fine blood vessels.

Anatomy of fish

Body sections

The body of a fish consists of a head, trunk and tail. In fish it is not always easy to tell where one region ends and the next begin. The head includes the eyes, nostrils, mouth and gills and is measured from the tip of the snout to the end of the gill covers. The trunk comprises the body from behind the head to the end of the body cavity, usually indicated by the anus or the start of the base of the anal fin. The tail (Paul Skelton: Freshwater Fishes) is measured from the anus to the end of the tail fin.

i The caudal peduncle is measured from the end of the anal fin to the base of the tail fin.


A fish’s shape gives an indication of its biology and habitat requirements. For example, an eel-like shape is suited to living in holes or crevices or to moving through dense or tangled root stocks or vegetation; an eel swims by moving the body in a series of waves, the fins playing relatively little part.


Fins perform a variety of different functions, mainly in producing and controlling movement but also in defense, display and communication, spawning activities and for holding and touch. Their shape, size and position are important identifying characters. Fins are either paired (pectoral and pelvic fins) or lie in the midline (dorsal, caudal and anal fin). Fins are supported by spines and soft rays; spines are either true spines or are modified soft rays. Spines may be simple or serrated i.e. provided with barbs along one or both edges. The spines of many catfish can be held firmly erect by means of a special locking mechanism and form a very effective defense.

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