The Stickleback FishAuthor: Alice YangPublisher: Dixon Biology Inc.
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Figure 1. Sticky the Stickleback Fish[1]
Figure 2[14] (BS)

Table of Contents

1. Classification/Diagnostic Characteristics
2. Relationship to Humans
3. Habitat and Niche
4. Predator Avoidance
5. Nutrient Acquisition
6. Reproduction and Life Cycle
7. Growth and Development
8. Integument
9. Movement
10. Sensing the Environment
11. Gas Exchange
12. Waste Removal
13. Environmental Physiology (Temperature, Water and Salt Regulation)
14. Internal Circulation
15. Chemical Control (Endocrine system)
16. Review Questions
17. References

Classification/Diagnostic Characteristics

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Class: Actinopterygii
Order: Gasterosteiformes
Family: Gasterosteidae
Genus: Gasterosteus
  • has a notochord
  • anterior skull with a large brain
  • rigid internal skeleton supported by vertebral column
  • well-developed circulatory system, powered by ventricle contractions[2]
  • most are 4 to 6 cm long, although some can grow to be 10 cm in length
  • It has a unique feature sharp spines, usually 2 to 4 of them, on the back in front of the dorsal fin.
  • the sides has large bony plates which is more developed in sea sticklebacks
  • the back can be dark grey, greyish or bluish-green, with the flanks being silvery[15] (SM)
  • have no scales but have bony hard plates on its surface[11] (NC)

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Figure 4. To the human eye, stickleback fish look like the conventional fish that the general population has in mind, unlike the blobfish pictured above [17]

Relationship to Humans

Researchers studying the stickleback population in Alaska (13) (DA)

Humans often use three-spined stickleback fish (Gasterosteus aculeatusmeaning “bony stomach with spines”) as the subjects of various experiments. One aspect of the stickleback fish that interest researchers is the ability for saltwater populations to survive in freshwater environments.[2]
Also, stickleback fish are known to be a pest in the Black sea region because of their high population. They eat young fish and compete with fish for food. Because of this, stickleback fish are sometimes turned into oil, animal feed, and fertilizer (LC). 3
Although the stickleback fish is not important to most Americans, Scandinavians boil the fish down for oil.[7] (MDS).

Stickleback fish compared to human hand. [Source:] (DM)
Stickleback fish compared to human hand. [Source:] (DM)

Habitat and Niche

Stickleback fish live in salt water (the Atlantic and PacificOceans) and freshwater (freshwater lakes) environments. Freshwater sticklebacks stay in freshwater waters their entire lives while marine sticklebacks (ones that live in salt waters) travel to freshwaters to breed.[2]

They tend to stick to the coasts and fresh waters of the Northern Hemisphere. In the Americas they range from Labrador Canada to Chesapeake bay, as well as a form of the stickleback that lives on the northwestern coast of North America. In Europe they spread from Norway to the Black Sea.[7] (MDS)

Predator Avoidance

Saltwater sticklebacks have strong pelvic bones with pelvic spines that can make it hard for a predator to swallow.[2]

The stickleback fish's predators include larger fish, kingfishers, herons, water shrews, and otters. The spines on the fish can be raised so that it is harder for predators to swallow the fish.[5] (JLau)

Nutrient Acquisition

Sticklebacks obtain oxygen through countercurrent exchange in the gills. They acquire other necessary nutrients from preying on other organisms.[2]

Even though sticklebacks are nocturnal they need sufficient light to hunt for prey, so best hunting occurs on well moonlit nights. Stickleback prey include worms, insect larvae, small snails, crustaceans, water fleas, young fish and fish eggs. Sticklebacks will even eat young fish and fish eggs of their own species. When a prey is spotted the stickleback chases it as necessary then its jaw quickly pushes open and forward to suck the prey into its mouth. Then with tiny sharp teeth the food is mashed and then swallowed.[9] (BH)

Reproduction and Life Cycle

Stickleback fishes reproduce in freshwater.

As a member of the animalia kingdom, sticklebacks reproduce sexually through the fertilization (fusion) of an egg from the female fish and a sperm by the male fish. After the female fish lays a batch of eggs, the male fish releases sperm to fertilize the eggs. Eggs and sperms are haploid cells (cells that have half the set of the number of chromosomes needed in an organism). The stickleback offspring hatch from the eggs after their embryonic growth is complete.

In this way, the offspring of the parent sticklebacks inherit half of their genetic information from their mother half of their genetic information from their father, allowing for a lot of genetic variation from generation to generation of stickleback fish.[2]

It has been found that a female stickleback fish selects her mate based on the color. For a male to have the best color possible, he eats food rich with carotenoids, similar to the way a flamingo eats shrimp with carotenoids to gain pink feathering. Stickleback fish can see the ultraviolet light, which the carotenoids help reflect in addition to the reds, oranges, and yellows. The female uses the amount of ultraviolet light reflected as a way to check the quality of a potential mate.[6] (WSS)

Stickleback fish live to about one to two years in the most favorable environments maybe three years. (HSC)

Male attacking pregnant female.[12] (PS)

Growth and Development

The fusion of the two cells creates a zygote (a diploid cell which contains the full number of chromosomes an organism needs). The zygote then cleavages, undergoing rapid cell division to increase cell numbers until the mass of cells forms different layers within the shape of a ball, the blastula, is formed. Major organs and the organ systems form through the next stage, organogenesis. In this stage neuralation, the creation of the central nervous system, occurs.

The stickleback fish develops throughout its entire life. While it stops growing when it hits its adult size, the organs still continuously repair and renew themselves until death.[2]


Marine stickleback fish have bony plates on their back that act like armor.[2]

They have a row of free dorsal spines in front of their dorsal fins. Stickleback fish unlike most fish lack scales. They use their bony plates in place of their scales.[8] (MDS)

In contrast to the saltwater form, freshwater populations are extremely morphologically diverse, with great variation between sticklebacks from different streams or lakes. They also have generally shorter dorsal and pelvic spines and substantially fewer armor plates (normally 12 or less) than their marine counterparts. In fact, many freshwater sticklebacks have no pelvic spines or armor plates at all. (10) (MC)


Skeletal muscle, bundles of muscle fibers, is responsible for both voluntary and involuntary movements (breathing, shivering). Groups of Actin and myosin (contractile proteins) are organized into filaments and slide past each other to contract muscles, which then pull on skeletal elements to generate force and cause movement. Contraction is stimulated by twitches, the unit of contraction. One action potential (impulse in the form of an electrical current through a cell) generates one twitch, and the sum of twitches fired in succession is proportional to the strength of the muscle contraction.

Stickleback fish have a nervous system that consists of neurons (cells that receive and send information throughout the body). The central nervous system, comprised of the brain and spinal cord, processes the information received from the environment and transmits the information to the peripheral nervous system. The peripheral nervous system is divided into two categories, the autonomic nervous system (ANS) and the Somatic Nervous System (SNS).

The somatic nervous system is responsible for voluntary motor movements such as swimming.

The ANS is responsible for the output of CNS information that calls for involuntary functions such as heart increasing or decreasing heart rate and sweating, that allow the stickleback fish to maintain homeostasis (an equilibrium of physiological processes in an organism) enhance rapid movement in reacting to the situation in fighting or fleeing from a predator.[2]

Sensing the Environment

Stickleback fish have five senses: sight, smell, touch, taste, and hearing. Sensory receptor cells convert stimuli (light, sound, etc) into action potentials in neurons that are carried into the central nervous system for processing and interpretation.

Sensory transduction, converting stimuli into membrane potential (electrical differences between the inside and outside of a cell membrane) occurs when ion channels react to a specific stimulus by allowing ions to flow across a membrane and into a cell, changing the charge of the cell. The amount of ions that flow inside the membrane is proportional to the intensity of the stimuli. Once the change in the membrane’s potential hits a certain point, either an action potential is released or neurotransmitters (chemical signals) are released to induce another neuron to release an action potential. The action potential travels from neuron to neuron until it reaches the central nervous system. Specific parts of the brain are responsible for each sense. For example, the olfactory bulb in the brain is the area in which smell is received and processed.

In the presence of danger, the ANS is stimulated and its sub division the sympathetic nervous system turns the body into the “fight-or-flight response” by allocating the fish’s energy and processes into fighting or fleeing from a predator, as mentioned in the Movement category.[2]

Gas Exchange

Stickleback fish exchange gases such as oxygen and carbon dioxide with their environment by intaking oxygen and removing carbon dioxide through diffusion, where concentration gradients (areas with differences in concentration of a substance) cause the substance to flow from a highly concentrated area to a less concentrated area.

Sticklebacks utilize countercurrent exchange in their gills to increase their blood oxygen supply. Water from the environment flows continuously in one direction: into the fish’s mouth, over the gills and out of the opercular flaps (openings behind the fish’s eyes) back to the environment. The gills are comprised of many gill filaments and covered by lamellae (the surfaces where gas is exchanged). Blood flows through the lamellae in the opposite direction of the flow of water, maximizing the amount of oxygen that is diffused from the water to the blood. Figures 8 and 9 further illustrates the countercurrent gas exchange.[2]

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Figure 8. Diagram of Countercurrent Gas Exchange by Concentration[2]

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Figure 9. Diagram of physiological process of gas exchange [16]

Waste Removal

Stickleback fish are ammonotelic (they excrete their nitrogenous waste mostly in the form of ammonia). Ammonia forms from the break down of proteins and nucleic acids. Because ammonia is highly soluble in water and easily diffuses through membranes, stickleback fish can continuously excrete ammonia from their blood to the environment through their gills.[2]

Environmental Physiology (Temperature, Water and Salt Regulation)

Homeostasis, the process of maintaining stable conditions in the internal environment of an organism, of sticklebacks is facilitated by their cells, tissues and organs. Negative feedback is the primary method that keeps physiological systems stable by returning it back to a set point. When receptor cells sense that the incoming stimulus is different from the set point, the fish will work to return the body condition back to that set point.

Sticklebacks may be osmoregulators (actively regulating the concentration of salt in their extracellular fluid) or osmoconformers (letting extracellular fluid and seawater equilibrate). Freshwater populations are osmoregulators and have to excrete water to conserve salt while salt water populations are osmoconformers. It is unknown whether or not salt water populations become osmoregulators when they reach freshwaters to breed.[2]

Internal Circulation

As a vertebrate, stickleback fishes have closed circulatory systems where the blood (circulating fluid containing blood plasma and blood cells) travels within vessels so that the blood does not mix with the interstitial fluid (fluid around the cells). Blood is pumped through the body from a multichambered heart to arteries (large vessels), diverge once into arterioles (smaller vessels), and then diverge again into capillary beds – networks of capillaries (even smaller vessels) – where the blood and the interstitial fluid exchange nutrients. The blood returns back to the heart through venules (vessels similar to capillaries) which join to form veins which connect back to the heart.

Like most fish, stickleback fishes have a two-chambered heart; one chamber is the atrium and the other is the ventricle. They also have a single circuit system: blood flows from the body into the atrium, which pumps the blood into the ventricle, which then pumps it further to the respiratory organs (the gills where oxygen is added to the blood), entering through afferent arterioles, and leaves through efferent arterioles and into the aorta (a vein) to the rest of the body (where oxygen distributed by capillaries to the rest of the organs), and then back to the heart. There is a limit to the nutrient supply by the fish circulatory system because the most of the ventricle pressure that powers the system is lost when the blood passes narrow passage way in the gills, slowing down the movement of blood and consequently the delivery of nutrients to the body.[2]

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Figure 4. Circulatory System of a Fish[2]

Chemical Control (Endocrine system)

The endocrine system in the stickleback fish, like other multicellular organisms, is used to send signals within the organism and to communicate with other organisms. Endocrine cells (cells that are part of the system) secrete chemical signals induce responses in other cells. Groups of endocrine cells form an endocrine gland. The chemical signals secreted by these cells are called hormones. Hormones secreted from the cell travel through the bloodstream and bind to the target cells. The reactions induced by specific hormones may induce short term responses, such as the fight-or-flight response, or permanent changes, such as the production of reproductive organs.[2]

Review Questions

1. Why are Stickleback fish ammonotelic? Why are humans not ammonotelic? Describe the difference between ammonotelic and ureotelic organisms. (SJ)
2. Why are stickleback fish often turned into oil, animal feed, and fertilizer? (E.S.S.)
3. True or false: stickleback fish that live in freshwater are osmoconformers. (JLev)
4. Why is countercurrent exchange more efficient than other methods of respiration? (BB-V)
5. Describe how the stickleback fish's circulatory system transports nutrients to all the cells within its body. How is this different from our circulatory systems? Why is the stickleback fish's circulatory system better suited to an aquatic environment? (FZ)
6. Describe two things that would limit the stickleback fish's circulation efficiency (one of which has to do with pressure and the other with the type of heart the fish possesses). (AA)
7.How do stickleback female fish choose mates and what ability do they rely on using while choosing the best mate? (KG)
8. How can stickleback saltwater populations that survive in places like the Dead Sea survive in freshwater? (AWC)
9. How does the nervous system of a stickleback fish play a role in movement and why? (ES)


1. McKinnon, Jeffrey S. A Stream-resident Male Threespine Stickleback from Alaska, in the Laboratory of Jeff McKinnon. N.d. Photograph. University of Wisconsin-Whitewater.
2. Hillis, David M., David Sadava, H. Craig Heller, and Mary V. Price. Principles of Life High School Edition. Sunderland, MA: Sinauer Associates, 2012. Print.
5. "Stickleback (Three-spined)." :: Animal Facts. Peoples Trust, n.d. Web. 13 Dec. 2012. <>.