Details
Gasterosteus aculeatus
A small fish with three dorsal spines. Marine, migratory, and freshwater forms make it a classic model for studying rapid evolution.
Map: Ecoregions 2017 © RESOLVE (CC BY 4.0) · Natural Earth (PD)
Widespread in Northern Hemisphere coasts, estuaries, lakes, and streams, especially cool shallow freshwater and brackish habitats.
The body is slender, with three sharp dorsal spines. Breeding males develop a red belly and bluish-green sheen.
Males build nests among vegetation or bottom material and defend territories during breeding. The fish is small but strongly territorial.
It eats small crustaceans, insect larvae, zooplankton, and fish eggs, picking prey quickly in shallow water.
A male attracts females to his nest, then guards and fans the eggs and larvae to keep fresh water flowing.
Freshwater populations often evolve changes in armor, spines, and body shape, making the species central to adaptation research.
