Seahorses Sticklebacks and Relatives: Gasterosteiformes

Physical characteristics: Threespine sticklebacks have three strong, widely spaced spines in front of the first dorsal (DOOR-suhl) fin, the one along the midline of the back. The first two spines are very tall. These fish reach a length of about 3½ inches (9 centimeters). The body is pointed at the ends, and the eyes are large. The fish are silvery on the sides, bluish black on the back, and orange on the belly. Males become more reddish when courtship begins and drab when it ends.

Geographic range: Threespine sticklebacks live in the Northern Hemisphere on both sides of the Atlantic and Pacific Oceans.

Habitat: Threespine sticklebacks live in tidal pools, coastal rivers and creeks, lakes, salt marshes, protected coastal inlets, and the open ocean. Adults live near plants such as eel grass.

Diet: Threespine sticklebacks eat small invertebrates and their larvae and sometimes the eggs of other sticklebacks.

Threespine sticklebacks swim by "rowing" with their pectoral fins and are strong enough to swim upriver. (Illustration by Joseph E. Trumpey. Reproduced by permission.)

Behavior and reproduction: Many populations of threespine sticklebacks live in the open sea but enter coastal habitats to spawn, or reproduce, and die. They swim by "rowing" with their pectoral (PECK-ter-uhl) fins and are strong enough to swim upriver. The pectoral fins are the front pair, corresponding to the front legs of four-footed animals. During spawning periods, males become strongly territorial.

Before spawning, male threespine sticklebacks establish a territory and build a nest. When an egg-filled female enters the territory, the male performs a zigzagging courtship dance. Once a female is impressed, the male leads her to the nest and points to it with his open mouth. The female enters the nest and releases her eggs. The male then fertilizes the eggs and forces the female out. After the eggs hatch, the male destroys the nest and guards the young.

Threespine sticklebacks and people: Threespine sticklebacks are kept in aquariums. They also are studied intently by scientists who specialize in fish behavior.

Conservation status: The U.S. Fish and Wildlife Service lists the unarmored threespine stickleback as Endangered, or in danger of extinction throughout all or a significant portion of its range. ∎