Gasterosteus aculeatus, also known as three-spined stickleback, is a freshwater fish in the family Gasterosteidae. It is the only species in the genus Gasterosteus and is found in most of northern Europe, Asia and North America. The fish has a length of 8–10 cm (3.1–3.9 in) and a weight of 20–40 g (0.71–1.41 oz). It is characterized by its small size, streamlined body shape and conspicuous abdominal spines.
The three-spined stickleback is an opportunistic predator that feeds on a wide variety of small invertebrates such as crustaceans, insects and mollusks. The diet changes seasonally and depends on the availability of prey items. The fish breeds in springtime and the female build a nest out of vegetation where she lays her eggs. After hatching, the fry are looked after by the male until they reach adulthood.
The three-spined stickleback has been extensively studied for its interesting mating behaviour which includes territoriality, aggression and bright colouration during breeding season. The fish is also an important model organism in evolutionary biology due to its rapid rates of genetic change and adaptability to new environments.”