The three-spined stickleback, Gasterosteus aculeatus, is a fish native to most inland 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. Most populations are anadromous (they live in seawater but breed in freshwater 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 anti-predator 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.
The largest of all sticklebacks, it is usually 5 cm (2 in) long (but may reach, exceptionally, twice that length). 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 2, or 4). 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 8-11 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. There is only one ventral plate, 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. There are a few populations, however, where breeding males are all black or all white.
Habitat and distribution 
The threespine stickleback is found only in the Northern Hemisphere, where it usually inhabits coastal waters or freshwater bodies well connected (or once well connected) to the coasts. It can live in either fresh, brackish or salt water. It prefers slow-flowing water with areas of emerging vegetation. It can therefore 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°N 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 if it was 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.
Variation in morphology and distribution 
- Gasterosteus aculeatus aculeatus is found in most of the species range, and is the subspecies most strictly termed the three-spined stickleback; its common name in Britain is the tiddler, although "tittlebat" is also sometimes used.
- G. a. williamsoni, the unarmored threespine stickleback, is found only in North America; its recognised range is southern California, though there are isolated reports of it occurring in British Columbia and Mexico;
- G. a. santaeannae, the Santa Ana stickleback, is also restricted to North America.
These subspecies actually represent three examples from the enormous range of morphological variation present within three-spined sticklebacks. These fall into two rough categories, the anadromous and the freshwater forms.
The anadromous form spends most of its adult life eating plankton and fish in the sea, and returns to freshwater to breed. The adult fish are typically between 6 and 10 cm long, and have 30 to 40 lateral armour plates along their sides. They also have long dorsal and pelvic spines. The anadromous form is morphologically similar all around the Northern Hemisphere, such that anadromous fish from the Baltic, the Atlantic and the Pacific all resemble each other quite closely.
Three-spined stickleback populations are also found in freshwater lakes and streams. These populations were probably formed when anadromous fish started spending their entire life cycle in freshwater, and thus evolved to live there all year round. Freshwater populations are extremely morphologically diverse, to the extent that many observers (and some taxonomists) would describe a new subspecies of three-spined stickleback in almost every lake in the Northern Hemisphere. One consistent difference between freshwater populations and their anadromous ancestors is the amount of body armour, as the majority of freshwater fish only have between 0 and 12 lateral armour plates, and shorter dorsal and pelvic spines. However, there are also large morphological differences between lakes. One major axis of variation is between populations found in deep, steep sided lakes and those in small, shallow lakes. The fish in the deep lakes typically feed in the surface waters on plankton, and often have large eyes, with short, slim bodies and an upturned jaw. Some researchers refer to this as the limnetic form. Fish from shallow lakes feed mainly on the lake bed, and are often long and heavy bodied with a relatively horizontal jaw and a small eye. These populations are referred to as the benthic form.
Since each watershed was probably colonised separately by anadromous sticklebacks, it is widely believed that morphologically similar populations in different watersheds or on different continents evolved independently. There is a unique population in the meromictic Pink Lake in Gatineau Park, Quebec.
One fascinating aspect of this morphological variation is that a number of lakes contain both a limnetic and a benthic type, and these do not interbreed with each other. Evolutionary biologists often define species as populations that do not interbreed with each other (the Biological Species Concept), and thus the benthics and limnetics within each lake would constitute separate species. These species pairs are an excellent example of how adaptation to different environments (in this case feeding in the surface waters or on the lake bed) can generate new species. This process has come to be termed ecological speciation. This type of species pair is found in British Columbia in Western Canada. The lakes themselves only contain three-spined sticklebacks and cutthroat trout, and all are on islands. Tragically, the pair in Hadley Lake on Lasqueti Island was destroyed in the mid 1980s by the introduction of a predatory catfish, and the pair in Enos Lake on Vancouver Island has started to interbreed and are no longer two distinct species. The two remaining pairs are on Texada Island, in Paxton Lake and Priest Lake, and they are listed as Endangered in the Canadian Species At Risk Act.
Other species pairs which consist of a well-armored marine form and a smaller, unarmored fresh water form are being studied in ponds and lakes in Southcentral Alaska that were once marine habitats such as those uplifted during the 1964 Alaska earthquake. The evolutionary dynamics of these species pairs are providing a model for the processes of speciation which has taken place in less than 20 years in at least one lake. In 1982, a chemical eradication program intended to make room for trout and salmon at Loberg Lake, Alaska killed the resident freshwater populations of sticklebacks. Oceanic sticklebacks introduced through nearby Cook Inlet recolonized the lake. In just 12 years beginning in 1990, the frequency of the oceanic form dropped steadily, from 100% to 11%, while a variety with fewer plates increased to 75% of the population, with various intermediate forms making up another small fraction. This rapid evolution is thought to be possible through genetic variations that confer competitive advantages for survival in freshwater when conditions shift rapidly from salt to freshwater. However, the actual molecular basis of this evolution still remains unknown.
Although sticklebacks are found in many locations around the coasts of the Northern Hemisphere and are thus viewed by the IUCN as species of least concern, the unique evolutionary history encapsulated in many freshwater populations indicates that further legal protection may be warranted. The IUCN indicates that this evaluation may be out of date.
The three-spined stickleback is a bottom-feeder that feeds basically on benthic prey, normally the Chironomid larvae being the most abundant prey; however, this fish species can also consume terrestrial prey on the surface. It can cannibalize eggs and fry.
Life history 
Many populations take 2 years to mature and experience only one breeding season before dying and some can take up to 3 years to reach maturity. However, some freshwater populations and populations at extreme latitudes can reach maturity in only 1 year.
From late April until July, 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 comes 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. He approaches a female by swimming very short distances left and right, and then he 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, it is noteworthy that 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, Canada, a form of three-spined stickleback departs from the usual pattern of parental care. Unlike other sticklebacks who 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 stickleback". It is currently unknown whether they are a distinct species, or simply a morph of the common Atlantic stickleback.
Sensory biology 
Sticklebacks have four colour photoreceptor cells in their retina, making them potentially tetrachromatic. They are capable of perceiving ultraviolet wavelengths of light invisible to the human eye and use such wavelengths in their normal behavioural repertoire.
Three-spined sticklebacks have recently become a major research organism for evolutionary biologists trying to understand the genetic changes involved in adapting to new environments. The entire genome of a female fish from Bear Paw Lake in Alaska was recently sequenced by the Broad Institute and many other genetic resources are available. This population is under risk from the presence of introduced northern pike in a nearby lake.
In popular culture 
The three-spined stickledback is also known as a tittlebat, and was featured in The Pickwick Papers by Charles Dickens. Samuel Pickwick is said to have published a treatise on the subject of tittlebats, specifically the ones living in the ponds of Hampstead.
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|Wikimedia Commons has media related to: Gasterosteus aculeatus|
- "Gasterosteus aculeatus". Integrated Taxonomic Information System. Retrieved 19 March 2006.
- The section of the Canadian Species at Risk Act dealing with the species pairs
- View the stickleback genome in Ensembl