The three spines that give this fish its name are located in front of the dorsal fin. The third spine (the one closest to the dorsal fin) is much shorter than the other two. This stickleback varies in colour from olive to silvery green, with a silvery belly. During the breeding season, the eyes of the male become blue and the belly turns bright red. The three-spined stickleback has a lifespan of up to five years. It measures between 4 and 6 centimetres long and weighs up to 9 grams.
The three-spined stickleback is found throughout the Northern Hemisphere. It usually lives in coastal waters but can also live in freshwater bodies connected to coasts, or in brackish water.
This stickleback eats a variety of foods, including plankton, insect larvae, worms and crustaceans. It also feeds on the eggs and young of small fish, including its own. The male builds a nest at the bottom of the water body, which it fills with plant material. The female deposits around 100 eggs in the nest, which are then fertilised by the male. The male then chases the female away. For around two weeks, while the eggs develop, the male chases away other males and takes care of the eggs by fanning them so as to bring fresh, oxygenated water to the nest. The three-spined stickleback is preyed on by leeches, frogs, snakes, tortoises, birds and mammals.