A small torpedo-shaped fish with a slender tail and broad fan-like tail fin, usually 5-7 cm long but occasionally up to 10 cm in length. Two to four, usually three, sharp spines are born on its back in front of the dorsal fin. The first two spines are long and strong while the third is small. The pelvic fin is reduced to a single fin-ray and one sharp spine. The dorsal fin is longer than the anal fin. The sides bear a few bony plates, the number of plates increasing with increasing salinity. The body is greeny brown in colour, sometimes black dorsally and often bluish with silvery scales and belly in brackish waters.The three-spined stickleback breeds in early spring and summer. In the breeding season the males develop a bright orange to red colouration on the throat. The males build hollow nests from seaweeds and aquatic plants, into which they drive females to lay eggs that the males then guard until they hatch and the young leave the nest (see Wheeler, 1969; Dipper, 2001).
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