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Gasterosteus aculeatus williamsoni

(Unarmored Threespine Stickleback)


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Family : Sticklebacks and tubesnouts ;


Threat status

Common Names

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Common Names in English:

Unarmored Threespine Stickleback, Unarmoured Threespine Stickleback


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Physical Description

Species Gasterosteus aculeatus williamsoni

Threespine sticklebacks are laterally compressed , fusiform (torpedo) shaped fish with three sharp, erectile spines at the forward part of the dorsal fin. The pelvic fin usually consists of a spine on each side and one fin ray. Modified scales form a single row of armor-like plates along the length of the body. Although the unarmored threespine stickleback has some of the characteristics of threespine sticklebacks, it is noticeably different from other subspecies in several ways. The most important are that the dorsal and pelvic spines are substantially shorter and most specimens either completely lack or have very few scales or lateral plates, thus the name “unarmored.”

Sticklebacks have black to dark olive backs with silvery white sides and white to gold bellies. At spawning time males develop a red throat and belly, the usually clear fins take on a green hue, and the eyes become blue. Females develop a light red or pink throat and belly.

This species may reach a length of 58 mm (2.3 in), but most are less than 50 mm (2 in). Females tend to be slightly larger than males.


Most of these fish have about a one year life span. They do not normally survive to a second spawning season , but an occasional individual may survive for three years.


Although most species of stickleback can adapt to salt, brackish , or fresh water , unarmored threespine sticklebacks appear to be limited to fresh water. They require clear, flowing, well-oxygenated water with associated pools and eddies of quiet water and areas of dense vegetation or debris to provide adequate cover and food supply.

Sticklebacks are one of the most studied fishes by ichthyologists and behaviorists. Some researchers believe that the two lineages of marine and freshwater sticklebacks are an example of evolution in action since records indicate that some of their adaptations have occurred in only the past 10 years. This species is quite habitat specific and does not do well in other than their preferred surroundings.

Typically found at an altitude of 0 to 272 meters (0 to 892 feet).[1]

Biome: Marine .


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Sticklebacks are voracious omnivores and feed on a variety of food including some algae and invertebrates such as insects, snails, small crustaceans, and some types of small worms. Males sometimes feed on stickleback eggs that are not properly guarded. Because they are visual feeders , they require relatively clear water in order to see their food source.


A male first establishes a selected territory that he vigorously protects. He then excavates a small depression in the sand in which he builds a nest out of fragments of aquatic plants , strands of algae, twigs , and debris . After cementing the materials together with mucus threads spun from his kidneys , he burrows through it to make an exit and entrance and weights down the nest with pebbles . While building the nest he aggressively defends it against other males and not-as-yet welcome females. Courtship begins when the nest is ready.

Next he searches for a receptive female. When a female carrying eggs enters his territory the male swims near her in a zigzag dancing movement. If the female is attracted, she follows him to his nest where she deposits her eggs. The male immediately fertilizes them and drives the female away. Over the course of the breeding season , the male may entice several other females into his nest.

The male protects the eggs and the fry from predators . During the incubation period he fans the eggs with his pectoral fins to keep them clean and to provide a fresh supply of oxygenated water. Incubation time varies with temperature but is generally 6-8 days. After hatching the fry stay in the nest for a few days getting nutrition from their attached yolk sac . When they start to leave the nest area in search of food, the male continues to protect them, sometimes taking them into his mouth to return them to the nest. When dispersed from the nest they feed in the cover of dense vegetation.


Male sticklebacks are very territorial when breeding and actively protect their nests , eggs , and young, using their spines as weapons in offense and defense. Confronted by intruders or predators , they assume a threatening posture, and rush forward with raised dorsal spines and a wide, gaping mouth . If the intruder is still not intimidated, the stickleback becomes very still, points its head down , and jerks its body as if to bore a hole in the ground below. This display seems to be effective in driving intruders away. In off-spawning seasons they are much less territorial and may even show schooling tendencies.


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Gasterosteus williamsoni Girard


Name Status: Accepted Name . Synonym: Gasterosteus williamsoni. Latest taxonomic scrutiny: August 14, 1998.

Similar Species

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Members of the genus Gasterosteus

ZipcodeZoo has pages for 26 species and subspecies in this genus:

G. aculeatus (Three Spined Stickleback) · G. aculeatus aculeatus (Banstickle) · G. aculeatus santaeannae (Santa Ana Stickleback) · G. aculeatus williamsoni (Unarmored Threespine Stickleback) · G. canadus (Black King Fish) · G. carolinus (Carolina Pompano) · G. crenobiontus (Techirghiol Stickleback) · G. ductor (Pilot Fish) · G. gymnurus (Western Threespine Stickleback) · G. inconstans (Common Freshwater Stickleback) · G. islandicus (Iceland Stickleback) · G. japonicus (Japanese Pineconefish) · G. laevis (Smoothtail Ninespine Stickleback) · G. microcephalus (West Coast Threespine Stickleback) · G. ovatus (Short Dorsal Fin Pompano) · G. platygaster (Southern Nine-Spined Stickleback) · G. platygaster aralensis (Aral Stickleback) · G. pungitius (Nine-Spined Stickleback) · G. quadracus (Four-Spined Stickleback) · G. saltatrix (Santa Ana Stickleback) · G. sinensis (Amur Nine-Spined Stickleback) · G. spinachia (Fifteen-Spined Stickleback) · G. trachurus (Sakhalin Nine-Spined Stickleback) · G. volitans (Ornate Butterfly-Cod) · G. wheatlandi (Black-Spotted Stickleback) · G. williamsoni (Unarmoured Threespine Stickleback)

More Info

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Further Reading

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  1. Mean = 55.380 meters (181.693 feet), Standard Deviation = 211.350 based on 13 observations. Altitude information for each observation from British Oceanographic Data Centre. [back]
Last Revised: 2014-05-07