The stickleback is a small, carnivorous fish found across the Northern Hemisphere. Inhabiting fresh, brackish, and marine waters, the common three-spined stickleback rarely exceeds 3 inches and preys on small insects, crustaceans, and fish larvae. The stickleback family includes about eight species. Their adaptability to diverse environments has made them a subject of scientific interest.
Distinctive Features and Environment
The name “stickleback” originates from the row of sharp, isolated spines on their backs. The number of spines varies by species. These fish lack traditional scales; instead, many species possess bony plates that form armor along their sides. The extent of this armor can vary significantly between populations.
Sticklebacks are remarkably adaptable. Some populations are anadromous, meaning they live in saltwater but migrate to freshwater to breed. The color of the fish can also differ with its environment; those in saltwater tend to be more silvery, while freshwater inhabitants are often a mottled, brownish-green.
This variation in physical traits and environments is a primary reason for their importance in research. Populations became isolated in freshwater lakes and streams across the Northern Hemisphere following the last Ice Age. Over thousands of years, these isolated groups have adapted to their specific local conditions.
Complex Mating and Parental Care
Stickleback reproductive behaviors are highly ritualized. During the spring breeding season, the male undergoes a transformation. In many species, his underside turns a vibrant red and his eyes may become blue, signaling his readiness to mate. This coloration serves to attract females and intimidate rival males.
The male’s first task is to build a nest. He selects a territory and constructs the nest using plant matter, which he glues together with a secretion produced by his kidneys. When a female, heavy with eggs, approaches his territory, the male performs a “zigzag dance” to entice her toward his creation.
Once he coaxes a female to the nest, she will enter and lay her eggs, and the male then follows to fertilize them. He may repeat this courtship with several other females, collecting multiple clutches of eggs in his single nest.
Following fertilization, the male assumes full parental responsibility, a notable trait among fish. He guards the nest against predators and fans the eggs with his fins to ensure they receive a constant supply of oxygenated water.
A Living Laboratory for Evolution
The stickleback is a model organism for studying evolution because of its rapid and observable adaptations. The fish display significant variation across their range, which are often direct responses to different environmental pressures, particularly predation.
A classic example is the evolution of their bony armor plates. Stickleback populations that live in the ocean, where they face a wide array of predatory fish, have extensive plating covering their bodies for protection. In contrast, many freshwater populations that became isolated after the Ice Age show a significant reduction in this armor. In these freshwater environments, the primary predators are often insects, which tend to grasp their prey rather than swallow it whole.
This difference in predation pressure has driven a rapid evolutionary change. In environments where heavy armor is less of an advantage, individuals with fewer plates have had a higher survival and reproduction rate. This has led to the prevalence of “low-armored” sticklebacks in many freshwater lakes and streams. Scientists can observe these changes, which have occurred over a few thousand years—a relatively short evolutionary timescale—providing a clear demonstration of natural selection at work.