The three-spined stickleback (Gasterosteus aculeatus) is a fish native to most inland and coastal waters north of 30°N. 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 inquiry 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.
This species can occasionally reach lengths of 8 cm (3.1 in), but lengths of 3–4 centimetres (1.2–1.6 in) at maturity are more common. The body is laterally compressed. The base of the tail is slender. The caudal fin has 12 rays. The dorsal fin has 10–14 rays; in front of it are the three spines that give the fish its name (though some individuals may have only two or four). The third spine (the one closest to the dorsal fin) is much shorter than the other two. The back of each spine is joined to the body by a thin membrane. The anal fin has eight to 11 rays and is preceded by a short spine. The pelvic fins consist of just a spine and one ray. All spines can be locked in an erect position, making the fish extremely hard to swallow by a predator. The pectoral fins are large, with 10 rays. The body bears no scales, but is protected by bony plates on the back, flanks, and belly. Only one ventral plate is present, but the number of flank plates varies greatly across the distribution range and across habitat types (see below); it is normally higher in marine populations (some freshwater populations may in fact lack lateral plates altogether).
Dorsal 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. A few populations, however, have breeding males which are all black or all white.
The three-spined stickleback is found only in the Northern Hemisphere, where it usually inhabits coastal waters or freshwater bodies. It can live in either fresh, brackish, or salt water. It prefers slow-flowing water with areas of emerging vegetation. It can be found in ditches, ponds, lakes, backwaters, quiet rivers, sheltered bays, marshes, and harbours.
In North America, it ranges along the East Coast from Chesapeake Bay to the southern half of Baffin Island and the western shore of Hudson Bay, and along the West Coast from southern California to the western shore of Alaska and the Aleutian Islands. It can be found throughout Europe between 35 and 70°N. In Asia, the distribution stretches from Japan and the Korean peninsula to the Bering Straits.
Its distribution could be said to be circumpolar were it not for the fact that it is absent from the north coast of Siberia, the north coast of Alaska, and the Arctic islands of Canada.
Many populations take two years to mature and experience only one breeding season before dying, and some can take up to three years to reach maturity. However, some freshwater populations and populations at extreme latitudes can reach maturity in only one year.
In its different forms or stages of life, the three-spined stickleback can be a bottom-feeder (most commonly chironomid larvae and amphipods) or a planktonic feeder in lakes or in the ocean; it can also consume terrestrial prey fallen to the surface. It can cannibalize eggs and fry.
Sexual maturation depends on environmental temperature and photo-period. Longer days and warmer days stimulate brighter colouration in males and the development of eggs in females.
From late April, males and females move from deeper waters to shallow areas. There, each male defends a territory where he builds a nest on the bottom. He starts by digging a small pit. He then fills it with plant material (often filamentous algae), sand, and various debris which he glues together with spiggin, a proteinaceous substance secreted from the kidneys. The word spiggin is derived from spigg, the Swedish name for the three-spined stickleback. He then creates a tunnel through the more or less spherical nest by swimming vigorously through it. Nest building typically takes 5–6 hours though it may also be spread out over several days. After this, the male courts gravid females that pass by with a zigzag dance. (In some populations, the male leads the female to the nest, rather than doing the zigzag dance.) He approaches a female by swimming very short distances left and right, and then swims back to the nest in the same way. If the female follows, the male often pokes his head inside the nest, and may swim through the tunnel. The female then swims through the tunnel as well, where she deposits 40–300 eggs. The male follows to fertilize the eggs. The female is then chased away by the male. For the duration of the eggs' development, the male will chase away other males and non-gravid females. He may, however, court other gravid females (more than one batch of eggs can be deposited in the same nest).
The sequence of territorial courtship and mating behaviours was described in detail by Niko Tinbergen in a landmark early study in ethology. Tinbergen showed that the red colour on the throat of the territorial male acts as a simple sign stimulus, releasing aggression in other males and attracting females. The red colouration may also be used by females as a way to assess male quality. Red colouration is produced from carotenoids found in the diet of the fish. As carotenoids cannot be synthesised de novo, the degree of colouration gives an indication of male quality (ability to find food), with higher-quality males showing more intense colouration. Also, males that bear fewer parasites tend to exhibit brighter red colours. Many studies have shown that females prefer males with brighter red colouration. However, the response to red is not universal across the entire species, with black throated populations often found in peat-stained waters.
The male takes care of the developing eggs by fanning them. He lines himself up with the entrance of the nest tunnel and swims on the spot. The movement of his pectoral fins creates a current of water through the nest, bringing fresh (well-oxygenated) water to the eggs. He does this not only during the day, but throughout the night, as well. Fanning levels tend to increase until the eggs are about to hatch, which takes 7–8 days at 18–20 °C. Fanning levels also increase when the water is poorly oxygenated. Towards the end of the egg development phase, the male often makes holes in the roof and near the rim of the nest, presumably to improve ventilation of the nest during fanning at a time when the eggs are more metabolically active. Once the young hatch, the male attempts to keep them together for a few days, sucking up any wanderers into his mouth and spitting them back into the nest. Afterwards, the young disperse and the nest is either abandoned by the male, or repaired in preparation for another breeding cycle.
In Nova Scotia, a form of three-spined stickleback departs from the usual pattern of parental care. Unlike other sticklebacks that nest on the substrate, Nova Scotian male sticklebacks build nests in mats of filamentous algae. Surprisingly, almost immediately after fertilization, the males disperse the eggs from the nest and resume soliciting females for eggs. Hence, there appears to have been a loss of parental care in this population. Because these males have reduced dorsal pigmentation, resulting a pearlescent white appearance, they have been dubbed "white sticklebacks". It is currently unknown whether they are a distinct species, or simply a morph of the common Atlantic stickleback.
As the breeding cycle of the three-spined stickleback is light and temperature dependent, it is also possible to manipulate breeding in the lab. For example, it is possible to stimulate sticklebacks to breed twice in a calendar year, instead of once, under the right conditions. This can be useful for genetic and behavioural multi-generational studies.
Infection with the cestode parasite Schistocephalus solidus can cause a reduction in egg mass or complete absence of eggs in female three-spined sticklebacks.
