The threespine stickleback (Gasterosteus aculeatus) is a species of fish in the family Gasterosteidae, which includes five genera and 15 other species. The threespine stickelback is small, between 4-6 cm, and is found in a circumpolar distribution that includes northern Europe, northern Asia and North America. It has also been introduced to central and southern Europe. Threespine sticklebacks are primarily and historically anadromous marine fish that feed on plankton for most of their adult lives in coastal waters and return to freshwater to mate and lay eggs. Threespine sticklebacks do not have scales, but have bony armor plates along their sides. In addition to the marine forms, threespine sticklebacks also exist in freshwater forms found in landlocked lakes. These many freshwater populations are thought to have derived from migratory anadromous G. aculeatus in many independent events, such as marine individuals permanently moving into a freshwater area or getting caught in land-locked lakes after the ice age, and subsequently adapting to an entirely freshwater existence. Freshwater threespine sticklebacks have notably reduced numbers of lateral armor plates as compared to marine populations and also show huge morphological diversity among different populations, so much that some populations living in the same lake do not interbreed. Gasterosteus aculeatus has contributed much to the study of species formation and are a research organism for evolutionary biologists and geneticists studying adaptation to new environments. Currently the IUCN (International Union for Conservation of Nature) recognizes three subspecies of threespine stickleback: Gasterosteus aculeatus aculeatus; G.a. williomsoni, the unarmored threespine stickeback; and G. a. santaeannae, the Santa Ana stickleback, but some taxonomists would classify the sticklebacks inhabiting isolated lakes into many more subspecies. Although the species itself is abundant and in no threat of extinction, various populations that represent specific diversity are in danger of local extirpation.

Froese and Pauly 2010; Hammerson et al 2012; Natureserve 2011; US Fish and Wildlife Service 2012; Wikipedia 4 January 2012; Wikipedia 6 February 2012)