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.
Evolution
The three-spined stickleback appears to be a rather old species that has remained morphologically unchanged for more than 10 million years. The oldest record of the species is from the Alta Mira Shale of the Monterey Formation of California, which preserves an articulated skeleton that appears essentially identical to the modern G. aculeatus complex. A slightly younger specimen is known from diatomite deposits from Lompoc that also belong to the Monterey Formation. Both specimens are known from marine deposits, suggesting a marine or anadromous lifestyle. The presence of the three-spined stickleback in the Miocene suggests that the three-spined stickleback complex must have diverged from the blackspotted stickleback prior to this point.
The iconic evolutionary traits of the three-spined stickleback, including rapid evolution in isolated environments, reduction of armor and pelvis, and ecological divisions into different niches, appear to have been a longstanding tendency of the Gasterosteus lineage. These traits are all present in a fossil stickleback, Gasterosteus doryssus, known from Miocene-aged freshwater deposits of the Truckee Formation in Nevada, US, which saw rapid evolution and morphological change over a period of several thousand years.
Description
This species can occasionally reach lengths of , but lengths of 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 eleven rays. 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.
Habitat and distribution
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. Locally present in the Volga River basin. 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.
Variation in morphology and distribution
Three subspecies are recognized by the IUCN: G. a. 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 isolated reports have been made 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. Hybrids between some of these morphs show foraging disadvantages, a form of reinforcement in the course of speciation. This is evidence for speciation by reinforcement.
Overall these morphs 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 lifecycle in fresh water, 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 none and 12 lateral armor plates, and shorter dorsal and pelvic spines. However, also large morphological differences occur 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 upturned jaws. 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 relatively horizontal jaws and small eyes. These populations are referred to as the benthic form.
Since each watershed was probably colonised separately by anadromous sticklebacks, morphologically similar populations in different watersheds or on different continents are widely believed to have evolved independently. A unique population is found in the meromictic Pink Lake in Gatineau Park, Quebec. Populations have been observed rapidly adapting to different conditions, such as in Lake Union, where sticklebacks have lost and regained armor plates in response to pollution from human activity around the watershed.
One 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), 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. The lakes themselves only contain three-spined sticklebacks and cutthroat trout, and all are on islands. However, 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.Canada – Species At Risk Act. dfo-mpo.gc.ca
Other species pairs which consist of a well-armored marine form and a smaller, unarmored freshwater form are being studied in ponds and lakes in south-central 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 fresh water when conditions shift rapidly from salt to fresh water. 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 further legal protection may be warranted.
Diet 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.
Life history
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.
Reproduction
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.