Three-spined sticklebacks (Gasterosteus aculeatus) are a type of fish native to most inland coastal waters north of 30°N. They have long been a subject of scientific study for many reasons. They show wide morphological variation throughout its range, ideal for questions about evolution and population genetics. Most populations are anadromous (they live in seawater but breed in fresh or brackish water, like salmon) and are very tolerant of changes in salinity. They display elaborate breeding behavior (defending a territory, building a nest, taking care of the eggs and fry) and they can be social (living in shoals outside the breeding season) making the species easy to find in nature and easy to keep in aquariums.
Coloration varies, but tends towards a drab olive or a silvery green, sometimes with brown mottling. The flanks and belly are silvery. In males during the breeding season, the eyes become blue and the lower head, throat, and anterior belly turn bright red. The throat and belly of breeding females can turn slightly pink.
the three spines that give the fish its name (though some individuals may have only two or four) are joined to the body by a thin membrane. The third spine (the one closest to the dorsal fin) is much shorter than the other two. All the stickleback’s spines can be locked in an erect position, making the fish extremely hard to swallow by a predator.