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Sticklebacks are small, bony fish in the family Gasterosteidae that rarely exceed 3 in (8 cm) in body length. Instead of scales, these fish have bony plates covering their body. Sticklebacks are found in North America and northern Eurasia. The name stickleback is derived from the sharp, thick spines arising in the first dorsal fin. The number of these spines forms part of the basis for the identification of the different species of sticklebacks.

Sticklebacks provide a good example of male dominance in mating and nesting behavior. At the beginning of the breeding season, a male stickleback selects a suitable spot in quiet water, where he builds a nest of plant parts stuck together by a sticky fluid produced by his kidneys. The fish shapes the nest by his body movements. A male threespined stickleback is normally bluegreen, but it develops a bright red color for the breeding season. The male tenspined stickleback becomes brown, while the 15spined stickleback changes to a blue color. Breeding male sticklebacks are aggressive during the breeding season and will readily fight with other males. The male performs a courtship dance to entice a female into the nest, but if this is not successful he attempts to chase the female into his nest, where she deposits her eggs. The male then expels sperm (milt) over the eggs to fertilize them. The male will then search for another female, repeating the process until his nest is filled with eggs. The male aerates the eggs by using his pectoral fins to set up a water current. He guards the eggs until they hatch, and then continues to guard the brood afterwards, maintaining the young in the nest until they are able to obtain their own food.

Some sticklebacks appear to be capable of rapid evolution, including the development of apparently separate species in different habitats within the same lake. For example, in Paxton Lake, British Columbia, morphologically and behaviorally distinct species of Gasterosteus sticklebacks utilize benthic and midwater habitats, and these have evolved in only a few thousand years since deglaciation.

The brook stickleback (Culaea inconstans ) iscommon in brooks in the United States from Pennsylvania to Kansas. It is a small fish less than 3 in (8 cm) in length, with five to six spines on its back. The 15spine stickleback (Spinachia spinachia ) is found in saltwater in the British Isles and around the North Sea. The ninespine stickleback (Pungitius pungitius ) is found on both sides of the Atlantic in northern latitudes. Its coloring is dark brown, but the male turns black during courtship and spawning.