Sticklebacks, Seahorses, and Relatives: Gasterosteiformes

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Most sticklebacks, seahorses, and their relatives have a long snout. The mouth usually is very small and has no teeth. These fishes have enlarged protective plates on their bodies. Seahorses and their relatives have extreme camouflage coloring. Many species can change color at will. Most pipefishes and seahorses have no tail fin. Many have a grasping tail like that of a monkey.


Sticklebacks, seahorses, and their relatives live all over the world.


Sticklebacks, seahorses, and their relatives live in coral reefs, sea grass meadows, kelp forests, tide pools, bays, lagoons, and estuaries (EHS-chew-air-eez), areas where a river meets the sea. Many species hide among rocks and crevices in reefs or blend in with coral or sea grass. Others live over sandy or muddy bottoms. The young of many species live in open water and settle closer to the bottom as adults. Some species live in lakes, coastal rivers, creeks, marshes, and protected coastal inlets.


Most pipefishes and seahorses eat small invertebrates (in-VER-teh-brehts), or animals without a backbone, and the larvae (LAR-vee) of other fishes. Larvae are animals in an early stage that must change form before becoming adults.

The larger species eat other fishes. Most sticklebacks, seahorses, and their relatives suck in prey, or animals killed for food, by quickly opening their mouths to produce a strong inward current.


Sticklebacks, seahorses, and their relatives are active during the day. They live alone, in pairs, in small groups, or in groups as large as thousands. Many species change color according to their background, using this ability to sneak up on prey or to hide from predators (PREH-duh-ters), or animals that hunt and kill other animals for food. Many pipefishes and seahorses appear to hover in one location, controlling their position by coordinated movements of their fins.

Sticklebacks, seahorses, and their relatives are famous for their reproductive behaviors. In some species females carry fertilized (FUR-teh-lyzed) eggs, which are those that have been penetrated by sperm, on the outside of their bodies, protecting them with their fins. In other species the male carries the eggs. Many species use complex courtship dances.


Many sticklebacks, seahorses, and their relatives are sought out by recreational divers. Many are aquarium fishes and are raised in captivity.


Many male fishes tend their young, but male seahorses actually give birth to their young. The female transfers her eggs to a pouch on the male's belly, where they are fertilized by the male and stay until they hatch.


Seahorses eat constantly because they have no stomach in which to store food. They have no teeth, so they use their long jaws as a straw to suck up prey, which they swallow whole.


The World Conservation Union (IUCN) lists one species of sticklebacks, seahorses, and their relatives as Critically Endangered and one as Vulnerable. Critically Endangered means facing an extremely high risk of extinction in the wild. Vulnerable means facing a high risk of extinction in the wild. The U.S. Fish and Wildlife Service lists one species as Endangered, or in danger of extinction throughout all or a significant portion of its range.


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.

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. ∎


Physical characteristics: The back, belly, head, and tail of leafy seadragons are covered with spines that support flowing skin that looks like leaves. These fish are greenish brown or yellow with stripes along the trunk. The head has a slight mask and dark blotches on the "leaves." These fish are about 14 inches (35 centimeters) long. The body is long and slender and is encased in ringlike bony plates. The head is long and is almost at a right angle to the body. The snout is very long. The tail is slender and grasping, like that of a monkey. Leafy seadragons have no pelvic or tail fins, lateral line, or scales. Pelvic fins, the rear pair, correspond to the rear legs of four-footed animals. The lateral (LAT-uhr-uhl) line is a series of pores and tiny tubes along each side of a fish's body and is used for sensing vibrations.

Geographic range: Leafy seadragons live along the southern coast of Australia.

Habitat: Leafy seadragons live in shallow water and down to about 98 feet (30 meters), usually sheltered among seaweed and reefs but also over sandy areas.

Diet: Leafy seadragons eat shrimp and lobster.

Behavior and reproduction: Leafy seadragons appear to float aimlessly in kelp beds, protected by their elaborate camouflage. They may move rhythmically back and forth in a manner similar to seaweed being swept by currents. Adults may gather in shallow bays in late winter to pair and mate.

When a male leafy seadragon is ready to mate, his tail becomes swollen and turns bright yellow, and he releases sperm onto his belly. The female deposits her eggs onto the male's belly, pushing them into place. Egg pockets then form on the male to fasten the eggs securely in place under his tail. After about eight weeks, the eggs hatch and the male deposits the young over a wide area.

Leafy seadragons and people: Leafy seadragons are aquarium fishes that also attract recreational divers who want to see them up close.

Conservation status: Leafy seadragons are not threatened or endangered. ∎


Physical characteristics: The belly of lined seahorses faces forward rather than down. The head is at a right angle to the trunk and tail. The snout is long, and the mouth has no teeth. There are pairs of spines behind the eyes. There is one dorsal fin just in front of the tail. The pectoral and anal (AY-nuhl) fins are small, and there are no pelvic fins. The anal fin is the one along the midline of the belly. The grasping tail tapers into a slender stalk without a tail fin. The body is encased in ten to twelve bony rings, each with four spines. The tail has about thirty-five rings. The color can be light brown, black, gray, yellow, or red covered with small blotches, stripes, and spots. Small white

stripes extend from the eyes. Lined seahorses grow to a length of about 8 inches (20 centimeters).

Geographic range: Lined seahorses live in the western part of the Atlantic Ocean.

Habitat: Lined seahorses live in shallow waters and waters as deep as 240 feet (73 meters). They live in bays, near beaches, in salt marshes, in oyster beds, around piers, and in other environments with plants and shelter. They can withstand great variations in temperature and salt content.

Diet: Lined seahorses eat small crustaceans (krus-TAY-shuns) and larvae. Crustaceans are water-dwelling animals that have jointed legs and a hard shell but no backbone.

Behavior and reproduction: Lined seahorses swim slowly, belly forward, by making wavelike movements of the dorsal and pectoral fins. They use their tails to cling to plants and coral. These fish produce sounds to communicate with one another. Younger lined seahorses live in open water, sometimes swimming in groups.

After courting, male and female lined seahorses meet belly to belly and twine their tails together. The female then transfers her eggs to the male a few at a time until there are 250 to 400 eggs in his pouch. As eggs are being transferred, both seahorses rise in the water and may change color. The eggs develop in the pouch for twelve to fourteen days. The young seahorses look like tiny adults when they are born.

Lined seahorses and people: Lined seahorses are common in aquariums.

Conservation status: The IUCN lists lined seahorses as Vulnerable or facing a high risk of extinction in the wild. ∎



Burton, Jane. Coral Reef. New York: DK, 1992.

Byatt, Andrew, Alastair Fothergill, and Martha Holmes. The Blue Planet: Seas of Life. New York: DK, 2001.

Gilbert, Carter Rowell, and James D. Williams. National Audubon Society Field Guide to Fishes: North America. New York: Knopf, 2002.

Web sites:

"About Seahorses." Project Seahorse. (accessed on October 13, 2004).

"Leafy Seadragon." Shedd Aquarium. (accessed on October 15, 2004).

"Lined Seahorse." Shedd Aquarium. (accessed on October 15, 2004).