A stickleback is a type of spiny fish found in the northern hemisphere. Sticklebacks are part of the genus gasterosteus and encompass eight different species, all distinguished by a varying number of back spines. Sticklebacks are often studied by evolutionary biologists because of the speed with which they evolve.
Growing to about 7 inches (18 cm) at their largest, sticklebacks can have between two and 16 spines in front of their dorsal fin. Most species have large eyes, square tails, and, instead of scales, bony plates covering their bodies. Colors vary between species. During mating season, however, males change color, becoming shades of bright yellow to black. Sticklebacks usually live one to three years.
Breeding among sticklebacks is extremely ritualized. In the spring, males build nests using plants and excretions from their kidneys. They then swim in patterns near the nest to attract a mate.
Once attracted, the female will lay her eggs in the nest, then the male will fertilize the eggs. After fertilization, the male will remain at the nest guarding and aerating the eggs until they hatch and for a period after hatching. This behavior is seen in all species of stickleback except the white.
Nocturnal feeders, sticklebacks usually eat invertebrates, fish eggs, and sometimes other fish. Large fish, birds, and some mammals, such as otters, prey on sticklebacks. Although humans do not eat these fish, they may use them for oil, fertilizer, and animal food.
Sticklebacks live in both salt water and fresh water. Their ancestors originally all lived in the open ocean, but many species have since moved into lakes and rivers, adapting to fresh water environments. The three-spined stickleback is one of the most common freshwater fish found in Britain. It is also common in other areas of Europe, the United States, and Asia.
Evolutionary biologists study the stickleback because of its ability to evolve quickly in a new environment, sometimes making noticeable changes within ten years. Study of these fish is compared to Charles Darwin's study of finches on the Galapagos islands. Many freshwater sticklebacks have lost some or all of the bony plates and belly spines present on their marine ancestors. These plates and spines were necessary for protection in marine waters, but made feeding and maneuvering difficult in freshwater environments. The sticklebacks' basic coloring also lightened to help provide camouflage in the new freshwater environment.
There is evidence that sticklebacks share a few genes, such as skin color, with other animals including humans. Scientists believe that studying sticklebacks evolution may lead to information about the human evolutionary process. As a result, extensive research is still being performed on the evolution of the stickleback.