Three-spined stickleback (Gasterosteus aculeatus) is a fish of the stickleback family. Length 5-6 cm, sometimes up to 10 cm. In front of the dorsal fin are 3 barbs. On the sides of the body are bony plates. The pelvic fins are transformed into a strongly serrated barb. Three-spined stickleback is distributed in the northern parts of the Atlantic and Pacific Oceans, in Europe - from the Black Sea to Novaya Zemlya. Euryhaline gregarious fish. Marine, passable and freshwater forms. Life span 3-4 years. Sexual maturity in the first year of life. Spawning from April to July, portioned. Builds a nest on the ground, in which it lays eggs. Fecundity is not more than 350 eggs. It feeds on crustaceans, insect larvae, worms, fish eggs and larvae. It has no commercial importance.

The three-spined stickleback can live in both saltwater and freshwater. The body is somewhat elongated and compressed from the sides. The tail stalk is short. The fish is covered not with scales, but with bone plates in the number of more than two to three dozen. Subtail fin (7-10 branched rays) shorter than the dorsal fin (9-13 rays) and resembles it in shape. In front of the caudal fin there is a small sharp barb, three more are located in front of the dorsal fin. Fan-shaped pectoral fins consist of unbranched rays. The pelvic fins are shaped like jagged bony barbs. Dorsal and pelvic barbs in the raised position are closed with a special latch, representing a formidable weapon. The broad caudal fin is slightly rounded. The head is relatively large with a slightly pointed snout. The mouth is small, with the upper jaw shorter than the lower. Eyes are relatively large.

Colouration depends on age, physiological condition, habitat or time of year. If sticklebacks are silvery grey in winter, they become greenish and brownish with a silvery tinge in summer. Males and females do not differ in colouration except during the breeding season. During this time, the back of the male takes on a bluish tinge, while the underparts and head become red and the eyes bright blue. Females develop dark transverse stripes on the back and sides of the body just before spawning, and the underside of the body becomes pale yellow. This colouration disappears after spawning.

The three-spined stickleback is widespread in the northern basins of the Atlantic and Pacific Oceans. In Europe it is found from Novaya Zemlya, the White Sea, the Kola Peninsula and Iceland to the Mediterranean and Black Seas, and is present in the Baltic Sea. In America it lives in waters from Greenland to New York. Along the Pacific coast it is found from the Bering Strait to Korea, on the Kuril and Japanese islands, and along the American coast - from Alaska to Southern California. In Russian waters, it is common in the European part (except for the Caspian Sea basin) and in the waters of the Pacific basin.

There are three types of stickleback: marine, freshwater and migratory. The marine form lives permanently in coastal areas of the sea and breeds in shallow waters with salinity up to 20-25 ppm. This form is known from the Kandalaksha Bay of the White Sea. Sea sticklebacks tend to be larger and better armed than freshwater sticklebacks, as predation pressure is much higher in marine waters.

Passerine sticklebacks live in the sea but spawn in fresh water such as streams, rivers and lakes in spring and summer. After spawning, the adults die or return to the sea and spend the winter close to the shore or far away over deep water. Juveniles also return to the sea after some time. It has been shown that the marine form and the migratory form can merge. Apparently, these sticklebacks spawn in the sea because there was not enough space on the freshwater spawning grounds. Freshwater sticklebacks live and reproduce in freshwater without going to the sea, although the freshwater body is not isolated from the sea. Sticklebacks feed on a variety of small organisms: invertebrates of the upper water layers, diatom algae, insect larvae, worms, fish eggs and fry, molluscs, airborne insects. The food spectrum in each water body depends on the availability of food during the different seasons.

Spiny stickleback usually spawn from April to August, depending on the temperature and light regime of the water body. In Southern California, breeding individuals can be found throughout the year. A few days before spawning, the male will choose a spot on the bottom and dig a hole. He then takes small grasses or other plant material in his mouth and places them on the bottom of the hole, fixing them in place with mucus produced by the kidneys and excreted through the urogenital opening. The male then builds the side walls of the nest in the same way, followed by the roof. He then tidies up the nest, giving it a more regular, almost spherical shape, widening the entrance hole and smoothing the edges. At the same time, the male is busy chasing away all the insects and other fish from the nest. It will usually attack the stranger with a swift, direct lunge and, if it does not turn to flee, will bite it or grab its tail and drag it away from the protected territory. Barbs are not usually used in skirmishes with conspecifics.

Females lay eggs in portions in the nests of different males. One portion, depending on the size of the female, may contain from 20 to 400 eggs, and the total fecundity for the season is up to 1400 eggs. Males and females may participate in reproduction up to 10 times per season. In some populations, the bulk of individuals die after the first spawning. In general, sticklebacks live 1-5 years, but usually 2-3 years. Many of them, including juveniles, are preyed upon by various predatory fish and other animals, including insects and birds.

The fishery value of the teree-spined stickleback is low.