stickleback, common name for members of the family Gasterosteidae, small fishes, widely distributed in both fresh- and saltwaters of the Northern Hemisphere. Sticklebacks range from 11/2 to 4 in. (3.7–10 cm) in length and lack true scales; they are equipped with short, strong spines in front of the dorsal and on the ventral fins, the number varying with the species. These are used as offensive and defensive weapons, often against other sticklebacks during the breeding season, when the male is brightly colored and pugnacious. Each male constructs a roofed nest by gluing together bits of vegetation with a sticky secretion from glands near the kidneys. Under his persuasion, several females deposit eggs in the nest, which he guards jealously until well after the young hatch. Sticklebacks feed on smaller invertebrates and on the fry and eggs of other fish. Best known are the common stickleback, Eucalia inconstans, a coastal species, and the brook stickleback, a smaller freshwater variety. Sticklebacks are classified in the phylum Chordata, subphylum Vertebrata, class Osteichthyes, order Gasterosteiformes, family Gasterosteidae.