Gasterosteus aculeatus; Print; The three-spined stickleback (Gasterosteus aculeatus) is a fish native to most inland coastal waters north of 30癗. It has long been a subject of scientific study for many reasons. It shows great morphological variation throughout its range; ideal for questions about evolution and population genetics. Many populations are anadromous (they live in seawater but breed in fresh or brackish water) and very tolerant of changes in salinity; a subject of interest to physiologists. It displays elaborate breeding behavior (defending a territory; building a nest; taking care of the eggs and fry) and it can be social (living in shoals outside the breeding season) making it a popular subject of enquiry in fish ethology and behavioral ecology. Its antipredator adaptations; host-parasite interactions; sensory physiology; reproductive physiology; and endocrinology have also been much studied. Facilitating these studies is the fact that the three-spined stickleback is easy to find in nature and easy to keep in aquaria.