stickleback

Any fish of the family Gasterosteidae, found in marine and fresh waters of the northern hemisphere. It has a long body that can grow to 18 cm/7 in. The spines along a stickleback's back take the place of the first dorsal fin, and can be raised to make the fish difficult to eat for predators. After the eggs have been laid the female takes no part in rearing the young: the male builds a nest for the eggs, which he then guards and rears for the first two weeks.

The common three-spined stickleback Gasterosteus aculeatus, up to 10 cm/4 in, is found in freshwater habitats and also in brackish estuaries.