The number of dorsal spines can vary between 7 and 12 but most fish have 9 or 10 dorsal spines, 2 pelvic spines and a single anal spine. The dorsal and pelvic spines are protected from collapse by a pelvic girdle that connects them in a strong, crush-proof structure surrounding the body. Body is more elongated than that of the Three-spined Stickleback and is devoid of plates. The dorsal and anal fins are set well back along the body close to the long, thin caudal peduncle, and the pelvic fins are reduced to one spine only. Colour varies due to location and conditions. Both sexes can be olive green - sometimes nearly black - above, fading to bronze or silver below; or an overall bronze with grey mottling above.
Takes place March to August. Males attract as many females as possible to lay their eggs in a weed nest he has previously built. Each female lays from 10 to 100 eggs and the male guards these and the resulting fry. Hatching takes from 10 to 20 days, depending on water temperature.
Found in ponds, slow moving rivers, canals and sometimes in estuaries and the sea. A common habitat is in brackish ditches and dykes, draining into the lower reaches of estuaries. It favours dense reed reds standing in a metre or so of water.
Carnivorous, they eat worms, insect larvae and crustaceans.
Common throughout parts of Britain and Western Europe but easily overlooked.
The Aquarium Project