Stickleback Fish
Richard Shen
Brook Stickleback.JPG
(SL 13)

Classification/Diagnostic characteristics

The stickleback fish belongs to the kingdom Animalia and the phylum Chordata.

The stickleback fish is a chordate, specifically a vertebrate. Vertebrates are classified by four key features: an anterior skull containing the brain, an skeleton supported by the vertebral column, internal organs suspended in a coelom, and a well developed circulatory system.

Deuterostomes encompass chordates, echinoderms, and hemichordates.

Kingdom: Anamalia
Phylum: Chordata
Subphylum: Vertabrata
Class: Actinopterygii (ray-finned fishes)
Order: Gasterosteiformes
Family: Gasterosteidae (sticklebacks and tubesnouts)
Genus: Culea
(SL 14)

Relationship to humans

Both humans and the stickleback are of the kingdom Animalia and the phylum Chordata. The stickleback fish and humans are both deuterostomes, chordates, and vertebrates. A type of stickleback called the three-spined stickleback is considered a pest in the Black Sea region of southeastern Europe because it eats the fish that are caught for human consumption and competes with these fish for food.
However studied for behavior, physiology and evolution, Stickleback fish can be used to test for water pollution in areas such as Britain, where they are not treated as pests due to their habitats having been drained and polluted. (SP)

There is significant research in evolutionary studies concerning the diversity and sympatric pairs, two specialized populations living together, of the three-spined stickleback. Most of these studies are conducted in the marked morphological divergent and geographically isolated lakes of British Columbia (16). Some studies have shown that the same type of regulatory changes that appear particularly important during stickleback evolution have also contributed to both loss and gain traits in the human lineage. Further studies of sticklebacks may thus reveal general features of evolutionary change, with broad implications for the understanding of evolution in many other vertebrates, including humans. (17)

Habitat and niche

The marine stickleback fish inhabits the Atlantic and Pacific oceans and breeds in many freshwater lakes. The freshwater stickleback fish inhabits only freshwater lakes.

The freshwater stickleback fish is found in freshwater in vegetated sites that usually have sandy or muddy bottoms. Stickleback fish in saltwater are usually found in coastal areas. Younger stickleback fish are found around drifting patches of seaweed. They are also common in estuaries, partly enclosed coastal bodies of water with rivers or streams flowing into them. Some stickleback fish migrate between fresh and saltwater. They're only found in the Northern hemisphere and they prefer slow-moving waters. (5) (6) (LK)

Predator avoidance

Using its caudal fin, or tail fin, the stickleback fish can quickly dart in the water to avoid predators. Specific to marine stickleback fishes, well-developed pelvic bones with spines and bony plates help protect them from predators. Freshwater stickleback fishes have reduced body armor features. They also use camouflage to blend in with their environments (SL 14).

Parental care is given by the males to the young as they grow and develop in all stickleback species but the white stickleback fish. Once the eggs have hatched the father protects the young and teaches them self-defense from predators by chasing them. Stickleback fish, when reaching adulthood, can have an average of three spines on their back. These sharp spines and large bony side plates provide essential armor against an array of fish and bird predators. (12) (NU)

Nutrient acquisition

The stickleback fish acquires nutrients using its mouth, which are lined with teeth that aid in digestion.

Stickleback fish acquire nutrients by preying on other organisms such as: worms, insect larvae, small snails, crustaceans, and water fleas. Occasionally they will prey on other fish and fish eggs, including some of their own species. Stickleback fish are nocturnal predators, however, they need some light to spot the movement of their prey, and catch them in order to gain the necessary nutrients (7). (AG)

Reproduction and life cycle

Members of the oceanic stickleback fish population are born in freshwater and spend most of their lives in saltwater. They return to freshwater to breed. Members of the freshwater stickleback fish population never enter saltwater and spend all of their lives in lakes. The reproductive gland is the gonad.

The male fish is in charge of the reproductive cycle. The male begins by establishing a territory. In that area he builds a nest out of aquatic plant branches, and other organic materials including a mucus excreted from his kidney. Once done, he creates a burrow. Only after this process is completed does he allow females to come near. He entices the female by swimming toward her and she then proceeds to lay her eggs in the burrow. He will make her leave as soon as she is done laying and may have other females lay their eggs in the same spot. Once he fertilizes the eggs, the female is then no longer involved in the life process. The male protects the eggs for the duration of incubation (about 6-8 days). When they hatch, the fry (baby fish) feed on their yoke and then swim out into the world. Each fish only lives for about one spawning season (1). (RG)
All fertilization occurs externally. (SL 14)

Growth and development

As with all deuterosomes, the mouth of the stickleback fish forms opposite the blastospore, which later develops into the anus. A vertebral column, which distinguishes the vertebrates from other chordates, replaces the notochord early in development. The vertebral column becomes the structure which supports the stickleback fish. Eventually all chordates, including the stickleback fish, develop a dorsal hollow nerve cord and a post-anal tail. The marine stickleback has the Pituitary homeobox transcription factor 1 (Pitx1) gene which results in the development of a pelvic spine. The freshwater stickleback has a mutated and inactive version of this gene and thus the pelvic spine is absent. This is evidence of parallel evolution, as the populations of marine and freshwater sticklebacks are geographically distant and isolated.

(SL 15)

external image three-spined_stickleback1.gif

Figure 168: Egg
Figure 169: Larva, newly hatched, 4.3 mm
Figure 170: Larva, 6.3 mm

Three-spined stickleback larvae are usually spawn in May or June. The female lays about 100-150 eggs in the cavity of the nest (see Reproduction and Lifestyle), which the male then fertilizes. At birth, they are 4.25 to 4.5 mm, though they reach about 8 mm after a week, during which time the yoke sac is absorbed. The offspring reach adulthood at about 6 weeks, growing to about 16 mm. At about 2 years of age, they are 40-50 mm, and at 3 years, they are 50-55 mm long. (S.S.-22).


The marine stickleback fish has bony plates for protection.

Sticklebacks, like all fish, are covered in a thin layer of slime to assist smooth movement through water. While marine sticklebacks have long dorsal and pelvic spines and up to 36 plates covering their sides to protect against predators, freshwater sticklebacks don't need as much armor and have shorter dorsal spines and small pelvic spines if any at all. Spines are important defense mechanisms of marine sticklebacks as they can be extended toward a predator when the stickleback is in danger of being eaten. While some freshwater sticklebacks may have up to 12 protective plates, some lack plates altogether, and in some cases, heavy armor acts as a burden to the sticklebacks. Predation by dragonfly larvae was a factor that drove armor-less sticklebacks to evolve in freshwater environments, as sticklebacks with low body armor grew faster than sticklebacks with heavy armor and could more easily bypass the size range that the dragonfly larvae preyed on and move quicker through the water. (AC) (8)

Stickleback Evolution Video (AC) (9)


The stickleback fish uses a caudal fin, or tail fin, to propel itself in the water. The caudal fin also enables the stickleback fish to turn quickly. There are a pair of pelvic fins and pectoral fins, which are located on either side of the body for stabilization and sometimes propulsion. There are also median dorsal and anal fins which help further stabilize the fish.

The stickleback fish has a broad tail fin. The stickleback fish's fins help it with movement and stability. It has 2 to 4, usually 3, sharp spines on its back in front of its dorsal fin. The dorsal fin on a stickleback fish is the usually unpaired fin found in the back of its body. The pectoral fins are the primary propulsive organ for the locomotion of the stickleback fish. Male stickleback fish use the pectoral fins to also fan water through the nest. The pectoral fins usually have ten soft rays and the size, shape vary depending on the environment (19). The pectoral fins motion for propulsion and maneuvering (18). (SM)

Stickleback fish building nest:

(SM) (23)

Sensing the environment

The decreased pressure from predation in freshwater lakes most likely contributes to the lack of a Pitx1 gene in freshwater sticklebacks. The freshwater sticklebacks can use the energy elsewhere to achieve higher reproductive success, as the pelvic spines are not necessary.

Stickleback fish have neuromasts, which let them sense the movement of water currents. These neuromasts have similar sensory hair cells to those in our ears. They have large eyes which are capable of seeing in color. They use this color vision during mating season to tell them when a male with a red belly (which they only get in mating season) is approaching, which they then attack. Stickleback fish have the senses of touch, taste, sight, hearing, and smell. (JM 10, 11)

Stickleback fish can perceive chemical, visual, and acoustic signals. Males communicate through body language with other sticklebacks to communicate their territorial boundaries and sexual interest. (SL 14)

Gas exchange

The stickleback fish has a two-chambered heart, which uses pressure on the blood to circulate it. The gills in the fish oxygenate the blood, which then travels to the aorta, arteries, and then systematic capillaries. Then, the deoxygenated blood enters the atrium through veins, and the ventricle pumps the blood to the gills. There are two different arterioles on the gills: an afferent arteriole and an efferent arteriole. The blood flows into gill arches through the afferent arteriole then is carried out by the efferent arteriole into the aorta.

Gas exchange in the stickleback fish consists of an intake of oxygen and removal of carbon dioxide through the process of diffusion. Like in all fish, gas exchange in stickleback fish occurs in the gills, which have a large surface area made up of many fine, threadlike filaments composed of lamellae that contain capillaries, short, narrow blood vessels. Because the amount of dissolved oxygen in water is very low, it is necessary for gills to have a very large surface area so that enough oxygen can be taken in. The most important feature of gas exchange in fish is countercurrent exchange, which ensures that a constant concentration gradient is maintained between oxygen rich water and oxygen poor blood. Water comes in through the mouth of the fish, is forced over the gills, and passes out of the body though the operculum, which is a flap protecting the gills of the fish. The water flow in fish is a one way process that maximizes the amount of oxygen able to be taken in by the fish. With the countercurrent flow, the oxygen from the water is always able to flow to oxygen poor blood, and the carbon dioxide from the blood is able to leave the fish and return to the water. This function in fish takes advantage of diffusion of oxygen and carbon dioxide down their concentration gradients. (CM)

Process of countercurrent gas exchange in fish that maximizes the amount of oxygen that can be taken in by the fish due to a large concentration gradient. (3) (CM)

Waste removal

Stickleback fish release nitrogenous waste in the form of ammonia. The ammonia is excreted through the gills of the fish back into the water.

Environmental physiology (temperature, water and salt regulation)

Marine sticklebacks can survive in both freshwater and saltwater. They are born in and come back to breed in freshwater lakes, but they spend most of their lives in saltwater. Freshwater sticklebacks can only survive in freshwater.

The body temperature of the fish will always be the same temperature as the water surrounding it, indicating that it is an ectotherm. By producing more or fewer enzymes or enzymes with different optimal temperatures, the fish can stay active year round.

Internal circulation

The stickleback uses a two-chambered heart to oxygenate blood through gills and move it to capillaries and back to the heart where it is pumped back to the gills.

Blood vessels crisscross the entirety of the interior of the stickleback fish, transporting blood to each body part. Oxygenated blood flows from larger blood vessels to capillaries, which are only one cell wide, where oxygen diffuses from the blood cells to body cells. De-oxygenated blood flows over capillaries in the gills, where counter-current exchange ensures that it becomes re-oxygenated. The beating of the fish's heart ensures that enough pressure is maintained to constantly keep blood cycling through the circulatory system. Unlike mammals, stickleback fish do not have two circulatory circuits, so in some regions oxygenated and de-oxygenated blood are mixed, resulting in less efficient transport of oxygen in general. (YR) (20)

The circulatory system of a fish, which uptakes oxygen in the gills and is powered by a two-chambered heart. (YR) (21)

Chemical control (i.e. endocrine system)

The stickleback can stay active year-round by releasing more or less of certain enzymes. Because enzymes have certain optimal temperatures and temperatures change throughout the year, different enzymes are needed at different temperatures.

Review Questions

1. Why is it necessary for the gills on a stickleback fish to have a large surface area? (SM)
2. What behavior occurs following the fertilization of the egg? (SF)
3. Although the stickleback fish uses countercurrent exchange in gas exchange, it also has an internal circulatory system that does not separate oxygenated and deoxygenated blood. How are these two mechanisms contradictory in allowing the organism to maximize oxygen transfer to cells of the fish? (CM)
4. What mechanisms does the stickleback fish have for protection? How do these forms of protection help the stickleback fish?
5. How does the stickleback fish sense its environment? What are the benefits to this type of sensing?


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12. __
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