Sticklebacks are small, slim members of the Gasterosteidae family that are rarely more than 3 inches long and are confined to the Northern Hemisphere, occurring most abundantly in North America. They are primarily freshwater fish, but some also occur in brackish or shallow inshore waters of seas.

The family contains seven genera, nine species, and several subspecies; they are of minimal forage value for predatory fish and are little used as bait, but they have a distinctive appearance and unusual courtship and spawning behaviors.

The stickleback gets its name from the short, stout spines in its first dorsal fin, the number of spines generally identifying the species. Each family member has from 3 to 26 well-developed isolated dorsal spines, preceding a normal dorsal fin having 6 to 14 rays. Almost every species also has a spine at the leading edge of the anal fin and each pelvic fin. The body lacks scales, but in most species it is armored along the sides with bony plates.

Several species of sticklebacks are kept in aquariums. They swim with short spurts of speed, then pause. This makes them interesting to watch, as does their spawning ritual, which people are unlikely to observe in the wild. At spawning time, the males adopt courtship colors, with the bellies bright red in some and velvety black in others.

Each male builds a nest among the stems of aquatic plants; the nest is hollow inside but completely covered on the top, the bottom, and the sides, with stems held together with a secretion of sticky threads. Once the nest has been built, the male searches for a female and drives her toward the nest, nipping at her fins and chasing after her if she turns the wrong way.

As soon as the female has laid her eggs, she leaves the nest, sometimes squirming out through the bottom. The male enters the nest immediately and fertilizes the eggs. Often he may go out again and get one or two other females to lay eggs in the nest. Some males build several nests at the same time. The eggs hatch in a week or less.

While the eggs are incubating, the males of most species aerate them by fanning currents of water through the nests (the male of one species builds a nest with two holes in the top and sucks water from one of the holes to cause circulation over the eggs). After the eggs hatch, the male tends the fry for several days, generally trying to keep them near the nest.

One of the common species in North America is the brook stickleback (Culaea inconstans), found in streams from southern Ohio westward to Montana and northward, and throughout southern Canada from Nova Scotia to eastern British Columbia.

It is generally less than 3.5 inches long. The five or six spines on its back are completely separate from one another, rather than joined by a membrane, and the caudal peduncle is especially slender. Like most sticklebacks, it is quarrelsome and guards its territory, particularly its nest, from intruders.

The threespine stickleback (Gasterosteus aculeatus) occurs in northern Eurasia and North America, living in both brackish water and freshwater. A number of subspecies are recognized. The ninespine stickleback (Pungitius pungitius), found in northern Europe, China, Japan, and northern North America, is dark brown, and the male becomes a rich black during the courtship and spawning periods.

The fifteenspine stickleback (Spinachia spinachia) is a European saltwater species restricted to northwestern Europe. The fourspine stickleback (Apeltes quadracus) is found only along the eastern coast of North America, from North Carolina to the Gulf of St. Lawrence. The blackspotted or twospine stickleback (G. wheatlandi) is another western Atlantic species.