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Stickleback
Stickleback
Stickleback
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For other uses, see Stickleback (disambiguation).

Sticklebacks
Four marine species of stickleback from the Atlantic Ocean coast of North America
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Perciformes
Suborder: Gasterosteoidei
Family: Gasterosteidae
Bonaparte, 1831[1]
Genera

see text

The sticklebacks are a family of ray-finned fishes, the Gasterosteidae which have a Holarctic distribution in fresh, brackish and marine waters. They were thought to be related to the pipefish and seahorses but are now thought to be more closely related to the eelpouts and sculpins.

Taxonomy

[edit]

The stickleback family, Gasterosteidae, was first proposed as a family by the French zoologist Charles Lucien Bonaparte in 1831.[1] It was long thought that the sticklebacks and their relatives comprise a suborder, the Gasterosteoidei, of the order Gasterostiformes with the sea horses and pipefishes making up the suborder Syngnathoidei. More recent phylogenetic work has shown that the Gaterosteoidei are more closely related to the Zoarcoidei and the Cottoidei, which would mean that this taxon would belong in the order Scorpaeniformes.[2] However, in other phylogenetic classifications it is treated as the infraorder Gasterosteales within the suborder Cottoidei or as a sister clade to the Zoarcales in the order Zoarciformes.[3]

FishBase recognises 16 species in the family, grouped in five genera.[4] However, several of the species have a number of recognised subspecies, and the taxonomy of the family is thought to be in need of revision.

Genera

[edit]

The family Gasterosteidae includes the following genera:[4]

Description

[edit]
1994 Faroe Islands postage stamp with three-spined sticklebacks

Sticklebacks are endemic to the temperate zone[5] and are most commonly found in the ocean, but some can be found in fresh water. The freshwater taxa were trapped in Europe, Asia, and North America after the Ice Age 10,000–20,000 years ago, and have evolved features different from those of the marine species.[6][7]

Sticklebacks are carnivorous, feeding on small animals such as insects, crustaceans and fish larvae.[8][9]

Sticklebacks are characterised by the presence of strong and clearly isolated spines in their dorsal fins.[10] An unusual feature of sticklebacks is that they have no scales, although some species have bony armour plates.

Sizes

[edit]

The maximum size of the best-known species, the three-spined stickleback (Gasterosteus aculeatus), is about 4 inches, but few of them are more than 3 inches long. They mature sexually at a length of about 2 inches.[11] Most other stickleback species are roughly similar in size or somewhat smaller. The only exception is the far larger fifteen-spined stickleback (Spinachia spinachia), which can reach 22 cm (approx. 8.8 inches).[12] Body form varies with habitat: sticklebacks in shallow lakes have developed a deep body specialized to enable feeding on benthic invertebrates, whilst those in deep oligotrophic lakes have adapted to feed on plankton and have slimmer bodies.[5]

Personality

[edit]

Research has shown that individual sticklebacks display distinct personality traits, specifically in the area of taking a risk, and, can be considered bold or shy. These personality traits were determined to directly influence if they would lead, and if discouraged, attempt to lead again.[13]

Mating

[edit]

All stickleback species show similar, unusual mating behaviour. Freshwater males develop a red colouration, and although this may be seen in oceanic and benthic species these tend to remain dull-coloured. The male then constructs a nest from weeds held together by spiggin,[5] a kidney secretion, then attract females to the nest. Females signal their readiness to mate with solitary rather than shoaling behaviour, a head-up posture; their bellies are also obviously distended with eggs.[5] Courtship typically involves a zig-zag 'dance' where the male approaches the female in an erratic side-to-side pattern, and dorsal pricking of the female's abdomen.[5] A female lays her eggs inside the nest, where the male fertilises them. The male then guards the eggs until they hatch 7–14 days later (depending on temperature),[5][9] and may continue to guard the fry after they hatch. This large investment in both the nesting site and guarding of the eggs limits the number of females a male can mate with however males spawn multiple times.[5] This introduces the ability for selection to favor male mate choice.[14] Some males die following spawning.[11]

Mating choice

[edit]

Typically, the sex with the greatest parental investment has the strongest mate preferences.[15] Stickleback species exhibit mutual mate choice in which both the male and female have strong mate preferences. This is due in part to the strong parental investment on behalf of the male in guarding the eggs.[16]

Female mate choice

[edit]

Female sticklebacks show a strong preference to male stickleback with bright red coloration under their throats. Females mate both more often with males with brighter red coloration and give on average, larger eggs to be fertilized by these males. This preference has led to brighter red coloring.[17][18] This association is possible because the red coloration can only be produced by males that are free of parasites. This is referred to in the Hamilton-Zuk hypothesis.[19]

However, there is also evidence that attractive male red coloration may be a faulty signal of male quality. Male sticklebacks that are more attractive to females due to carotenoid colorants may under-allocate carotenoids to their germline cells.[20] Since carotinoids are beneficial antioxidants, their under-allocation to germline cells can lead to increased oxidative DNA damage to these cells.[20] Therefore, female sticklebacks may risk fertility and the viability of their offspring by choosing redder, but more deteriorated partners with reduced sperm quality.

Female mate choice has also been seen to be condition dependent. Females are almost always the more choosy sex in most species. Female sticklebacks though, have been found to be less choosy of mates when in poor physical condition and inversely, more choosy in good condition.[21]

Male mate choice

[edit]

In some species, such as the three-spined stickleback, the large investment in both nesting site and guarding of eggs by males limits the number of females a male can mate with.[22] This introduces the ability for selection to favor male mate choice. Male mate choice is rarely studied or observed in many species but multiple studies have confirmed male mate choice within stickleback species. Males show a choosiness similar to females as to what female they are willing to court and mate. Male sticklebacks have been observed to show preference towards female sticklebacks that are larger and longer. This is believed to be because larger females on average produce larger eggs, which leads to a greater offspring survival and fitness.[16] In addition, male sticklebacks have also been observed to prefer females with more distended or bloated stomachs. The benefits of this is also due to larger eggs and thus offspring survival and fitness[23]

Inbreeding avoidance

[edit]

Female three-spined sticklebacks adjust their courting behaviour to the risk of inbreeding.[24] When gravid females are given the choice between a courting unfamiliar non-sibling and a familiar brother, they prefer to mate with the non-sibling and thus avoid the disadvantages that accompany incest.[24] Eggs from inbred matings compared to eggs from outbred matings have a lower rate of fertilization and hatching, and fewer progeny survive to reproductive age.[24]

Use in science

[edit]

Niko Tinbergen's studies of the behaviour of this fish were important in the early development of ethology as an example of a fixed action pattern. More recently, the fish have become a favourite system for studying the molecular genetics of evolutionary change in wild populations[25] and a powerful "supermodel" for combining evolutionary studies at molecular, developmental, population genetic, and ecological levels.[26] The nearly complete genome sequence of a reference freshwater stickleback was described in 2012, along with set of genetic variants commonly found in 21 marine and freshwater populations around the world. Some variants, and several chromosome inversions, consistently distinguish marine and freshwater populations, helping identify a genome-wide set of changes contributing to repeated adaptation of sticklebacks to marine and freshwater environments.[27] The adaptations seen in oceanic threespine sticklebacks make them an ideal organism for the study of parallel evolution.[28]

In culture

[edit]
Monument to the siege stickleback

There is a sculpture in Kronstadt dedicated to stickleback, which saved thousands of city residents from starvation during the Leningrad Siege of World War II.[29]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Stickleback

View on Grokipedia
from Grokipedia
Sticklebacks are small, carnivorous ray-finned fishes comprising the family Gasterosteidae in the order Gasterosteiformes, distinguished by their elongate bodies, reduced or absent scales replaced by bony scutes, and prominent series of dorsal spines numbering from 3 to 16. They are native to the , inhabiting a wide array of marine, brackish, and freshwater environments, from coastal and estuaries to lakes, rivers, and streams. The family includes five genera—Apeltes, Culaea, Gasterosteus, Pungitius, and Spinachia—and approximately 20 species, with the (Gasterosteus aculeatus) being the most widespread and ecologically versatile. Physically, sticklebacks measure 3–10 cm in length on average, though some reach up to 18 cm, and feature a streamlined form with a small , single preceded by isolated spines, and pelvic fins modified into sharp spines for defense against predators. Their coloration varies by and season, often greenish or brownish dorsally with silvery sides, while breeding males develop bright red bellies and blue eyes in species like the . Diet consists mainly of invertebrates such as , crustaceans, and larvae, supplemented by small in larger individuals. Reproduction is seasonal, typically spring to summer, with males exhibiting complex courtship displays, constructing tubular nests from vegetation bound by kidney-secreted glue, and providing sole by fanning eggs for oxygenation and guarding fry until independence. Sticklebacks hold significant ecological roles as intermediate links in aquatic food webs, serving as prey for birds, larger , and mammals while controlling populations. Their extraordinary capacity for rapid —exemplified by repeated post-glacial invasions of freshwater habitats leading to parallel morphological in traits like lateral plate armor and body shape—has established them, particularly the , as premier model organisms in evolutionary . Genomic studies reveal that adaptations often arise from standing at loci such as Eda for armor reduction, enabling diversification within decades and providing insights into and complex trait .

Taxonomy

Classification

Sticklebacks comprise the family Gasterosteidae, a group of ray-finned fishes (class ) classified within the order Gasterosteiformes and suborder Gasterosteoidei. This placement reflects their morphological affinities, including reduced pelvic fins and characteristic body armor in many species, distinguishing them from other percomorph fishes. The family encompasses five extant genera and 20 species, primarily adapted to temperate and waters. The evolutionary origins of Gasterosteidae trace to ancestral marine populations in the Holarctic region, spanning the northern portions of , , and . These marine forms underwent significant divergence following the retreat of Pleistocene glaciers at the end of the last around 10,000–12,000 years ago, leading to repeated colonizations of postglacial freshwater habitats such as lakes and rivers. This rapid adaptation from saltwater to freshwater environments exemplifies , with marine ancestors giving rise to diverse ecotypes across isolated drainages. Phylogenetically, Gasterosteidae shares close ties with the (encompassing pipefishes and ) and related families within the traditional Gasterosteiformes, forming part of a syngnathoid characterized by elongated snouts and specialized reproductive behaviors. Molecular and morphological analyses support this relationship, highlighting shared traits like male , though recent phylogenomic studies suggest the order may be polyphyletic, with sticklebacks positioned near the base of percomorphs alongside syngnathids. The fossil record of Gasterosteidae provides insight into their ancient lineage, with the earliest known fossils of the family dating to the epoch (approximately 23–5 million years ago), including well-preserved assemblages of like Gasterosteus doryssus from lacustrine sites in . Earlier Eocene fossils (48–50 million years ago) from sites such as Monte Bolca, , represent syngnathoid-like forms ancestral to the family, documenting early diversification and variability in traits like spine reduction, mirroring patterns seen in modern populations. These fossils underscore the family's long history of transitions and morphological evolution.

Genera and species

The family Gasterosteidae comprises five recognized genera: Apeltes, Culaea, Gasterosteus, Pungitius, and Spinachia. These genera encompass 20 (as of 2025), primarily small, elongate fishes adapted to freshwater, brackish, and marine environments, with significant diversity in spine morphology and geographic distribution. The genus Gasterosteus includes six species, with Gasterosteus aculeatus, the threespine stickleback, being the most widespread and extensively studied, occurring across the in marine, estuarine, and freshwater habitats. Other species in this genus, such as Gasterosteus wheatlandi (Blackspotted stickleback) from the western Atlantic, Gasterosteus nipponicus (Japanese stickleback) endemic to , Gasterosteus crenobiontus from , Gasterosteus islandicus from , and Gasterosteus microcephalus from , exhibit regional adaptations including variations in lateral plate armor. The genus Pungitius, known for ninespine sticklebacks due to their higher number of dorsal spines (typically 8–12), contains 11 species, including the widespread Pungitius pungitius across and , endemics like Pungitius hellenicus in and Pungitius tymensis in , and more recent additions such as Pungitius modestus from (described 2021). Culaea features a single species, Culaea inconstans (brook stickleback), confined to freshwater streams and lakes in central and eastern , distinguished by its lack of pelvic spines in some populations. Apeltes is monotypic with Apeltes quadracus (fourspine stickleback), found in coastal brackish waters of eastern , while Spinachia includes one species, Spinachia spinachia (fifteen-spine stickleback), marine and confined to European coasts. Spine count variations across genera—ranging from three in Gasterosteus to nine or more in Pungitius—reflect evolutionary adaptations for defense against predators. Hybridization occurs between closely related species pairs, particularly in post-glacial lakes where ancestral lineages admix, as documented in Gasterosteus aculeatus populations showing genetic exchange with ancient divergent forms. Such events contribute to contemporary ecological but are limited by environment-dependent incompatibilities.
GenusNumber of SpeciesNotable Species and Distribution
Apeltes1Apeltes quadracus: Eastern , brackish waters
Culaea1Culaea inconstans: , freshwater
Gasterosteus6Gasterosteus aculeatus: Northern Hemisphere, widespread; Gasterosteus nipponicus: ; Gasterosteus wheatlandi: Western Atlantic; Gasterosteus crenobiontus:
Pungitius11Pungitius pungitius: and ; Pungitius hellenicus: ; Pungitius modestus: (2021); Pungitius tymensis:
Spinachia1Spinachia spinachia: European coasts, marine

Physical characteristics

Morphology

Sticklebacks exhibit an elongated, body shape adapted for agile swimming in varied aquatic environments. The body is covered in lateral plates, which are modified scales forming a protective armor, particularly prominent in marine forms where they number 29-35 along the sides, providing structural support and defense. In freshwater ecotypes, these plates are often reduced or absent, reflecting adaptations to lower predation pressures and energetic costs in postglacial lakes. A key morphological feature is the presence of 3-16 sharp, erectile dorsal spines, typically three in the common three-spined stickleback (Gasterosteus aculeatus), which are buttressed by the lateral plates and pelvic girdle for enhanced rigidity. The pelvic girdle, comprising paired spines and reduced fin rays, shows significant variation; marine populations retain robust structures, while many freshwater populations exhibit pelvic reduction, often linked to regulatory mutations in genes like Pitx1, minimizing drag in low-predation habitats. The head features a small, terminal mouth suited for capturing small prey, with eyes that are relatively large in open-water forms; pectoral fins are fan-like and broad, aiding in precise maneuvering, while the caudal fin is truncate to slightly indented. Many freshwater sticklebacks lack true cycloid scales, relying instead on the variable bony plates for protection. Coloration serves cryptic and reproductive functions, with non-breeding individuals displaying mottled brown or greenish patterns above and pale undersides for against aquatic backgrounds. During breeding, males undergo striking changes, developing brilliant bodies with red or orange bellies and blue eyes, enhancing visibility in mate attraction and defense; marine forms may appear more silvery overall. These traits vary by , with freshwater populations often showing more subdued, mottled hues adapted to vegetated or benthic habitats.

Size and variations

Sticklebacks exhibit a range of adult body sizes, typically measuring 3 to 10 cm in total length across most , with the threespine stickleback (Gasterosteus aculeatus) commonly reaching 3 to 8 cm and the ninespine stickleback (Pungitius pungitius) averaging 6.5 to 9 cm. Marine forms of the threespine stickleback can attain larger sizes, up to 11 cm, compared to their freshwater counterparts, which are generally limited to a maximum of 8 cm. Sexual dimorphism is pronounced in sticklebacks, particularly during the breeding season, when males develop brighter red coloration on their throats and bellies to attract females and signal fitness. Males also tend to exhibit larger body depths, heads, and overall robust builds relative to females, enhancing their ability to construct nests and defend territories. Ecotypic variations further influence size and morphology, with low-plated freshwater populations of the threespine stickleback being smaller and possessing reduced armor plating compared to the high-plated, fully armored marine ancestral forms. These differences arise from parallel evolutionary adaptations to freshwater environments, where reduced predation pressure favors less armored, more streamlined bodies. Growth rates in sticklebacks are strongly influenced by environmental conditions, with individuals in nutrient-rich waters exhibiting faster growth due to increased availability of prey resources such as . In contrast, nutrient-poor or colder environments can slow development, resulting in smaller adult sizes.

Distribution and habitat

Geographic range

Sticklebacks, belonging to the family Gasterosteidae, exhibit a predominantly Holarctic distribution across the , spanning , , and temperate zones from approximately 30°N northward. This range encompasses coastal marine, brackish, and freshwater habitats in , , and , with the threespine stickleback (Gasterosteus aculeatus) being the most widespread species, found from to Baja California along the Pacific coast, Labrador to on the Atlantic coast, and across from the to the Mediterranean and basins. The ninespine stickleback (Pungitius pungitius) similarly occupies and Atlantic drainages in from to , as well as Pacific coastal areas, with extensions into the Great Lakes basin. Ancestrally marine forms of sticklebacks inhabit the northern Pacific and Atlantic Oceans, but numerous populations have independently colonized freshwater systems following the retreat of Pleistocene glaciers around 10,000–15,000 years ago. These post-glacial invasions have led to widespread establishment in deglaciated lakes and rivers, such as the in and fjords in , where anadromous marine ancestors transitioned to resident freshwater ecotypes. This repeated colonization pattern is evident in the threespine stickleback, with multiple independent freshwater entries documented across its range, driven by the availability of newly formed post-glacial habitats. Introduced populations of sticklebacks have expanded beyond native ranges in several regions, including parts of the . For instance, the threespine stickleback was introduced to the Mohave River drainage in between 1938 and 1940, likely via escaped or released baitfish, establishing non-native freshwater populations in the . Disjunct distributions occur in , where species like the ninespine stickleback extend eastward from to , though conspecificity of East Asian populations remains under study, highlighting isolated evolutionary lineages in remote freshwater systems.

Preferred environments

Sticklebacks, particularly the (Gasterosteus aculeatus), occupy diverse aquatic habitats ranging from shallow coastal marine environments to brackish estuaries and freshwater systems such as streams, ponds, and lakes often featuring abundant vegetation. These fish show a strong preference for vegetated areas, including weedy pools and backwaters, which provide essential cover from predators and suitable substrates for nesting. Such habitats are typically slow-moving or still waters that support the growth of submerged aquatic plants like and . Preferred water conditions for sticklebacks are temperate to cold, with temperatures generally between 4°C and 20°C, aligning with their optimal growth and activity ranges. Juveniles exhibit a final preferred temperature of approximately 15.2°C, independent of prior acclimation temperatures between 11°C and 20°C. Anadromous forms demonstrate tolerance to low in freshwater breeding grounds, while overall capabilities allow survival across salinities from near-zero to full seawater. Euryhaline species like G. aculeatus exhibit remarkable adaptations to fluctuations through osmoregulatory mechanisms, including upregulation of Na⁺/K⁺- isoforms in tissues during transitions between marine and freshwater environments. These physiological changes enable balance maintenance, with anadromous individuals showing heightened expression of specific ATPase subunits upon entry into low- waters. Seasonal migrations are a key aspect of habitat use in anadromous populations, which undertake annual journeys from marine habitats to freshwater and lakes for breeding, typically in spring when temperatures rise above 8–10°C. After spawning, adults and juveniles return to coastal marine areas, completing a life cycle that exploits both high-productivity estuarine zones and protected freshwater refugia. This migratory pattern underscores their adaptability to dynamic environmental gradients.

Behavior

Feeding and diet

Sticklebacks are primarily carnivorous, with a diet dominated by small such as , , and crustaceans. In coastal ecosystems like the western , their contents reveal a diverse array of prey, including chironomid larvae (midges), cladocerans (e.g., and Bosmina), and harpacticoid copepods, which occur in over 90% of samples analyzed via DNA metabarcoding. Larger individuals incorporate more benthic organisms, such as amphipods, gastropods, and isopods, reflecting opportunistic feeding adapted to available resources. Seasonal variations further influence composition, with copepods comprising up to 99.6% of the diet in spring and a mix including daphnids in autumn. Foraging in sticklebacks relies on visual cues to detect and select prey, often conducted in schools that enhance detection efficiency through collective vigilance. Once prey is identified, they employ feeding facilitated by highly protrusible , which rapidly extend toward the target to accelerate water flow and generate forces up to 35% greater than alone, effectively capturing attached or evasive items. This mechanism synchronizes jaw protrusion with opening, optimizing acceleration around prey regardless of size above 2 mm³, and allows for quick without mouthbrooding. Feeding activity peaks at dawn, afternoon, and dusk, with notable nocturnal capabilities that increase prey intake by 20% compared to diurnal competitors. Ontogenetic shifts in diet occur as sticklebacks grow, transitioning from pelagic to more benthic . Juveniles and smaller individuals (≤6.5 cm) predominantly consume microplankton like cladocerans in open water, aligning with their habitat use in vegetated shallows to avoid predation. Adults and larger (>6.5 cm) shift toward bottom-dwelling prey such as amphipods and isopods, broadening their niche and reflecting morphological adaptations like increased gape size. This progression supports growth and integrates them into varied microhabitats without rigid age-based changes, instead responding flexibly to prey abundance. As opportunistic mid-level predators, sticklebacks occupy a mesopredatory trophic position in aquatic food webs, exerting top-down pressure on and while serving as prey for larger . Their flexible diet enables proliferation in altered ecosystems, such as the , where reduced predator populations amplify their role in cascades affecting benthic communities and even macroalgae recruitment. This adaptability underscores their ecological versatility across freshwater and marine habitats.

Social interactions

Juvenile three-spined sticklebacks (Gasterosteus aculeatus) form tight schooling groups to enhance predator avoidance, maintaining close proximity and synchronized movement within two body lengths of conspecifics, a that is heritably stronger in marine populations compared to benthic ones. These cohesive schools provide antipredator benefits by diluting individual risk and improving detection of threats, with lab-reared marine juveniles consistently forming single large groups that persist longer than the transient pairings observed in benthic juveniles. In contrast, adults tend to disperse from such groups, particularly in benthic habitats, where they show reduced responsiveness to and prefer solitary shelter, reflecting a shift toward territoriality as they mature. This ontogenetic change in allows juveniles to leverage collective defense while adults prioritize resource monopolization. Territorial aggression is prominent among male three-spined sticklebacks, who vigorously defend nesting areas against intruders using a series of escalating displays, including the characteristic zigzag swimming pattern that signals threat and readiness to attack..pdf) During these confrontations, males erect their dorsal and pelvic spines as a defensive posture to deter rivals, enhancing their apparent size and making them harder to swallow by potential aggressors or predators. Such behaviors maintain exclusive access to breeding territories, with aggression levels peaking during the reproductive season and varying by population ecology, as marine-derived males often exhibit higher territoriality than freshwater residents. Dominance hierarchies emerge in groups of three-spined sticklebacks, particularly under resource competition, where larger individuals typically outrank smaller ones, gaining priority access to and shelter. These hierarchies stabilize over time in stable environments, reducing costly fights by establishing predictable rank orders that influence foraging success and shoal position, though disruptions like environmental stress can destabilize them and increase conflict. Size-based dominance is a key predictor, with body length correlating positively with aggressive success and resource acquisition in both lab and field settings. Social communication in three-spined sticklebacks relies exclusively on visual signals, as they produce no vocalizations, with interactions mediated through color patterns, body postures, and movements observable in habitats. Key signals include rapid flicks and spreading during mild threats, which convey or submission without physical contact, alongside UV-reflective patterns on flanks that modulate responses in conspecifics during encounters. These visual cues enable rapid assessment of rival intent or status, supporting and territorial boundaries in the absence of auditory or chemical dominance signals.

Reproduction and mating

In temperate zones, the breeding season of three-spined sticklebacks typically occurs in spring and early summer, with spawning often extending into late summer depending on environmental conditions. This timing is primarily triggered by increasing photoperiod, where longer day lengths stimulate gonadal maturation and reproductive behaviors, while temperature plays a supporting role by accelerating development at warmer levels above approximately 10–15°C. During this period, males undergo physiological changes, including the development of bright red nuptial coloration on their ventral surfaces to signal reproductive readiness. Nest construction is a key male behavior initiated early in the breeding season, where males select a site and assemble a tubular nest from plant fragments such as or . They secrete a glue-like protein called spiggin from hypertrophied s, which is induced by s like 11-ketotestosterone, binding the materials into a compact structure with entrances for deposition and protection. This process not only provides but also serves as an honest signal of male quality, as kidney size and spiggin production correlate with overall and androgen levels. Courtship begins once the nest is complete, with males performing a series of ritualized displays to attract gravid females. The prominent zigzag dance involves rapid side-to-side swimming toward the female, escalating in intensity to lead her to the nest, often combined with head-up postures to emphasize body size and coloration. Females exercise by inspecting the nest's quality, structure, and location, preferring those that indicate robust construction and male investment, which influences spawning decisions and patterns. Following spawning, males provide exclusive , fertilizing eggs immediately after deposition and then guarding the nest against intruders while fanning the to maintain oxygenation and remove debris. Fanning constitutes a significant portion of male activity, up to 40% of their time during peak periods, and is essential for embryonic development in low-oxygen environments. sizes typically range from 50 to 300 eggs per female, though males often incorporate multiple es into a single nest, with overall brood sizes varying by female body size and . After hatching in 7–10 days, males continue fanning and protecting the fry for several days until they become independent. To prevent inbreeding, female three-spined sticklebacks employ mechanisms during mate selection, preferentially courting unfamiliar non-siblings over familiar brothers in experimental setups. This avoidance is mediated primarily by olfactory cues, likely involving (MHC) alleles that produce distinct chemical signatures, allowing discrimination without visual input. Such preferences reduce the risk of genetic incompatibilities, as evidenced by females spending significantly more time near non-kin males.

Ecological role

Predators and defenses

Sticklebacks face predation from a variety of aquatic and semi-aquatic predators, including fish such as perch (Perca spp.), pike (Esox spp.), and trout, as well as birds like herons and kingfishers, and mammals such as river otters (Lontra canadensis) and mink (Neovison vison). Larger predatory fish often target adult sticklebacks through gape-limited ingestion, while birds strike from above the water surface. The eggs and early fry of sticklebacks are particularly susceptible to invertebrate predators, including dragonfly naiads (Odonata) and aquatic beetles (Coleoptera), which can infiltrate nests despite male parental guarding. To counter these threats, sticklebacks employ a suite of defensive mechanisms, including morphological and behavioral adaptations. The three dorsal and paired pelvic spines—key morphological features—can be erected and locked into position, increasing the fish's effective body diameter and deterring gape-limited predators like piscivorous by making difficult or painful. This spine-locking response often causes predators to release the stickleback after initial capture, as evidenced by high rates of spine fractures (up to 9.9% in sampled populations) among survivors, indicating frequent but unsuccessful attacks. In populations with complete armor, including intact pelvic spines, experimental predation trials demonstrate significantly higher rates against fish predators, with an approximately 11% increase in the probability of survival compared to spine-reduced individuals. Behavioral defenses further enhance stickleback survival. Schooling behavior, more pronounced in marine and high-predation populations, confuses predators by creating visual ambiguity and diluting individual risk during attacks. Rapid darting maneuvers allow sticklebacks to evade strikes, often in conjunction with schooling. Additionally, sticklebacks exhibit a C-start escape response, a fast-start reflex involving a rapid lateral bend of the body followed by a counter-bend to propel away from threats, similar to that observed in other fishes. This reflex enables quick acceleration, with double-bend C-starts being the predominant form in threespine sticklebacks during predator encounters.

Interactions with other species

Sticklebacks exert significant competitive pressure on native fish species, particularly by preying on their eggs and larvae, which disrupts recruitment and growth. In invaded systems, threespine sticklebacks (Gasterosteus aculeatus) consume the eggs and early life stages of salmonids such as ( nerka), leading to reduced juvenile survival and outcompetition for shared resources like . For instance, in Alaskan lakes like Karluk Lake, high densities of sticklebacks have been shown to limit the growth of age-0 through intense foraging overlap and predation. As an in certain freshwater ecosystems, threespine sticklebacks have caused notable disruptions since their introductions in the 20th century, including in streams and rivers. Released via baitfish escapes or stockings, such as in the drainage between 1938 and 1940, they have altered local food webs by voraciously consuming , which reduces availability for native planktivores and cascades through the community. In these systems, stickleback invasions have led to shifts in zooplankton composition, favoring smaller, less nutritious species and impairing the diets of endemic fishes. More recently, as of 2022, increasing stickleback densities in the have impaired the recruitment of piscivorous and altered coastal function. Similarly, in 2024, the rapid expansion of invasive pelagic three-spined sticklebacks in has led to ecosystem-wide effects on . Threespine sticklebacks serve as intermediate hosts for various parasites, including trematodes, contributing to symbiotic dynamics in aquatic ecosystems. They are commonly infected by trematode such as Diplostomum spp., which encyst in the fish's eyes and affect vision, and Schistocephalus solidus, a cestode that manipulates host behavior to facilitate transmission to avian predators. These interactions highlight sticklebacks' role in parasite cycles, where high infection rates can influence and serve as bioindicators of . Additionally, due to their sensitivity to pollutants, sticklebacks are employed in active to assess , with biomarkers like genotoxic damage in erythrocytes signaling contamination levels. In terms of biodiversity effects, stickleback grazing on can indirectly facilitate algal growth by relieving pressure on grazers. In eutrophic or invaded systems, such as shallow brackish lagoons, elevated stickleback populations reduce biomass, allowing blooms to proliferate and altering . This top-down control demonstrates sticklebacks' influence on structure, promoting conditions that support algal proliferation while potentially exacerbating impacts.

Scientific research

Model organism applications

The three-spined stickleback (Gasterosteus aculeatus) has emerged as a prominent in biological research due to its biological attributes that facilitate experimental manipulation and observation. It exhibits a short of approximately one year, enabling rapid multigenerational studies in settings. allows for straightforward artificial crosses, either through natural matings or controlled insemination, which supports genetic mapping and breeding experiments. Additionally, sticklebacks are robust, small in size (typically 3-10 cm), and easily maintained in captivity, requiring simple aquaria setups with controlled temperatures and photoperiods to induce breeding. Historically, the stickleback gained prominence in through the pioneering work of Tinbergen in the 1930s and 1950s, who utilized its conspicuous behaviors—such as the male's zigzag dance and nest-building—to dissect innate releasing mechanisms and fixed action patterns. Tinbergen's observations, often conducted in semi-natural setups, demonstrated how species-specific stimuli like the female's swollen abdomen trigger male responses, laying foundational principles for behavioral biology that remain influential. This early adoption highlighted the species' suitability for detailed behavioral assays, particularly in and , where mirror-image stimuli or live conspecifics elicit quantifiable territorial displays and sequences. Advancements in genetic tools have further solidified the stickleback's role as a model. The was fully sequenced in 2006 by the Broad Institute, providing a high-quality reference assembly that has enabled and identification of adaptive loci. More recently, CRISPR-Cas9 has been successfully applied to induce targeted mutations, such as deletions in loci controlling armor plate development or pigmentation, allowing functional validation of evolutionary traits with high efficiency in one-cell embryos. These tools support physiological studies, including responses to environmental stressors; for instance, exposure to oestrogenic pollutants like 17α-ethynylestradiol disrupts endocrine function, leading to altered vitellogenin production and reproductive behaviors in males. Such applications underscore the stickleback's value in assessing pollutant impacts on physiology and development.

Key evolutionary studies

One of the most striking examples of in sticklebacks involves the repeated loss of lateral armor plates in freshwater populations derived from marine ancestors following post-glacial . This , observed independently in numerous isolated populations worldwide, is primarily driven by mutations in the Ectodysplasin (EDA) gene, which regulates plate development. Studies have shown that low-plate phenotypes evolve rapidly through selection on standing at EDA loci, with freshwater sticklebacks exhibiting reduced plate numbers compared to their fully plated marine counterparts. This parallel pattern underscores how shared genetic mechanisms facilitate convergent adaptations to low-calcium freshwater environments, where armor is less beneficial due to reduced predation pressure from but increased costs from predators. Sticklebacks have undergone a remarkable from a marine approximately 10,000–15,000 years ago, coinciding with the retreat of Pleistocene glaciers that opened vast freshwater . Evidence from post-glacial populations in sites such as the Paxton Lake basin in reveals rapid morphological shifts, including changes in body shape and armor, occurring within decades to a few centuries after colonization, demonstrating the pace of post-glacial diversification. This radiation has produced diverse ecotypes adapted to varied niches, with genomic analyses confirming that much of the variation stems from ancient alleles recycled from the marine gene pool. Recent studies from subfossil stickleback bones dated 14.8–0.7 thousand years ago further confirm the chronology of , tracking the trajectory of adaptive alleles during transitions. Speciation in sticklebacks often occurs via sympatric divergence, as exemplified by benthic-limnetic species pairs in post-glacial lakes of . In these systems, such as Enos Lake and Paxton Lake, limnetic forms have evolved deeper bodies and more numerous gill rakers for open-water feeding, while benthic forms developed shallower bodies and stronger jaws for littoral foraging on macroinvertebrates. Disruptive on resource use drives through and hybrid inviability, with pairs forming independently in multiple lakes within the last 10,000–12,000 years. Ongoing is influencing stickleback , particularly through shifts in lateral plate morphology in response to warming waters. Experimental and observational studies indicate that elevated temperatures favor low-plate morphs in both freshwater and estuarine populations, as higher metabolic demands reduce the energetic burden of armor maintenance. For instance, in estuaries, populations exposed to warmer, more saline conditions due to reduced freshwater inflow are evolving fewer plates, mirroring ancient post-glacial patterns but accelerated by anthropogenic warming.

Cultural significance

Representations in media

Sticklebacks have appeared in literature as subjects of behavioral observation, notably in Niko Tinbergen's seminal 1951 book The Study of Instinct, where the (Gasterosteus aculeatus) serves as a key example for illustrating innate behaviors such as territorial aggression and courtship rituals triggered by visual cues like the red throat coloration of males. In this work, Tinbergen detailed experiments showing how male sticklebacks respond aggressively to red models, even non-fish objects, highlighting fixed action patterns that have influenced studies. Beyond scientific texts, sticklebacks feature symbolically in eco-fiction, representing adaptability and resilience in changing environments; for instance, in Lisette Auton's 2023 children's novel The Stickleback Catchers, the are woven into a of ecological wonder and transformation, with sticklebacks depicted as navigators of reversed rivers and shifting constellations to underscore themes of environmental flux. In media portrayals, sticklebacks often appear in educational documentaries focused on , such as the Howard Hughes Medical Institute's (HHMI) 2012 film The Making of the Fittest: Evolving Switches, Evolving Bodies, which uses footage of freshwater stickleback populations to demonstrate how genetic regulatory changes lead to anatomical adaptations like reduced pelvic spines in post-glacial lakes. They also play minor roles in animated nature programming, including Springwatch's 2020 segment featuring "Spineless Si," a real-life spineless nicknamed "Spineless Si" that educates viewers on and predation risks in wild populations. Sticklebacks hold a place in Scandinavian literary folklore, as seen in Selma Lagerlöf's 1906-1907 novel , where the Öland stickleback is praised as the finest variety, symbolizing the hardy inhabitants of Swedish waters in a tale blending adventure with natural observation. For educational purposes, sticklebacks are prominently featured in teaching resources on , including HHMI's Stickleback Evolution Virtual Lab (2011 onward), an interactive module used in classrooms to analyze morphological data from and modern specimens, helping students explore through hands-on simulations of trait inheritance. These materials, along with accompanying videos like Evolution of the Stickleback Fish (2020), emphasize the fish's role as a model for rapid evolutionary adaptation, appearing in curricula worldwide to illustrate concepts without requiring live specimens.

Economic and historical uses

Sticklebacks, particularly the three-spined species (Gasterosteus aculeatus), have been utilized incidentally as live bait in recreational angling for larger predatory fish, such as walleye, due to their abundance in shallow, vegetated waters. In regions like Ontario, Canada, their use is legal but limited to avoid unintended introductions, reflecting caution over their potential as vectors for ecological disruption. Historically, sticklebacks served as a low-value resource for and fertilizer, especially in the region where early 20th-century beach seine fisheries targeted them for extraction used in lamps and varnishes, as well as processing into feed for and birds. In , ichthyological interest in sticklebacks dates to the , with initial taxonomic descriptions and observations of morphological variation laying foundational work for later evolutionary studies. During the , European naturalists documented their distribution and adaptations in coastal and freshwater habitats, contributing to early understandings of systematics. Efforts to leverage sticklebacks for practical applications included introductions for biological , beginning in the early when ichthyologist Carl Hubbs advocated stocking them in water bodies to prey on larvae. However, these attempts often failed to achieve effective control, as studies showed limited predation on larvae compared to alternatives like , and sometimes resulted in unintended hybridization with native populations. In aquaculture, three-spined sticklebacks are bred experimentally on farms, primarily in , for the ornamental trade as native aquarium fish valued for their behavioral displays during and nesting. Today, their commercial value remains minor, with small-scale in the reporting average annual landings of 125 tons from 1991 to 2010, mainly for fishmeal production. However, invasive populations, such as in , impose economic costs on by preying on larvae and competing for resources, contributing to whitefish yield declines of over 50% in recent years, culminating in the closure of the commercial whitefish in 2024, and necessitating monitoring efforts.

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