Fish
Threespine Stickleback
Gasterosteus aculeatus · also called threespine sticklebacks, three-spined stickleback
In a coastal lake isolated from the sea generations ago, a threespine stickleback displays the reduced body armour and shortened spines typical of freshwater populations, the same predictable evolutionary change that has played out independently in lake after lake across the species’ entire range.
Overview
The threespine stickleback, Gasterosteus aculeatus, is a small fish native to coastal marine, estuarine, and freshwater habitat across a broad range of the Northern Hemisphere, including extensive coastal and inland waters of North America. Its role as one of the most important living examples of repeated, predictable evolution, elaborate male nest-building behaviour, and dual marine and freshwater lifestyles make it one of the most scientifically significant small fish species in the world.
A living laboratory for evolution
Each time a threespine stickleback population becomes isolated in a new freshwater lake, it tends to evolve the same reduced armour and spines as every other isolated population before it, turning an ordinary small fish into one of science’s most valuable natural experiments.
Status
Least Concern — widespread and abundant across suitable coastal marine, estuarine, and freshwater habitat throughout the Northern Hemisphere.
Threespine Stickleback: common questions
Why is the threespine stickleback important to evolutionary biology?
This species has become a major model for studying evolution because separate freshwater populations, each isolated after colonising a different lake from the sea, have repeatedly evolved reduced body armour and shorter spines in a highly predictable pattern, giving researchers a rare opportunity to study the same evolutionary changes happening independently in many different populations.
What are the three spines on a threespine stickleback used for?
This species has three sharp spines along its back, ahead of the dorsal fin, that it can lock upright when threatened, making the fish considerably harder and less appealing for a predator to swallow, a defensive adaptation that gives the species its common name.
How does a male threespine stickleback build a nest?
A breeding male of this species builds a nest from plant material glued together with a sticky secretion produced by his own body, then guards the resulting eggs closely and fans them with his fins to keep them supplied with oxygen until they hatch, a level of paternal care unusual among fish generally.
Does the threespine stickleback live in the ocean or in freshwater?
Both; this species includes marine populations that migrate into freshwater specifically to breed, as well as separate populations that have become permanently established in freshwater lakes and streams after becoming isolated from the sea, sometimes for thousands of generations.