Any of about 12 species (family Gasterosteidae) of slender, scaleless fishes inhabiting temperate fresh and salt waters of the Northern Hemisphere. Sticklebacks grow to 6 in. (15 cm) long. They have a row of spines on the back, in front of a soft-rayed dorsal fin, and a sharp spine in each of the pelvic fins. They also have a slender tail base, a squared tail, and hard armour plates on their sides. The male builds a nest of plant materials and coaxes one or more females into it to lay eggs, fertilizes the eggs in the nest, and aggressively defends eggs and young.
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The Gasterosteidae are a family of fish including the sticklebacks. FishBase currently recognises sixteen species in the family, grouped in five genera. However several of the species have a number of recognised subspecies, and the taxonomy of the family is thought to be in need of revision. Although some authorities give the common name of the family as "sticklebacks and tube-snouts", the tube-snouts are currently classified in the related family Aulorhynchidae.
The family includes the three-spined stickleback Gasterosteus aculeatus aculeatus, common in Northern Temperate Climates around the world including Europe, Alaska, and Japan and colloquially known in United Kingdom as the "tiddler", or "sprick". In Southern Ireland they are commonly known as "pinkeens" due to the reddish colour of the male three spined stickleback's throat during breathing season. Niko Tinbergen's studies of the behaviour of this fish were important in the early development of ethology as an example of a fixed action pattern.
One unusual features of sticklebacks is that they have no scales, although some species have bony armour plates. They are closely related to pipefish and seahorses.