The Gasterosteiformes are an order of ray-finned fish that includes the sticklebacks and the seahorses.

They live in tropical and temperate waters and can often be found in the shallow waters on continental shelves, for example, in coral reefs, lagoons or tide pools.

Gasterosteiformes do not have scales. Instead, their bodies are partly or fully covered with large, bony armor plates.

They usually have long snouts and small mouths with no teeth.

Nine-spined sticklebackGasterosteiformes are carnivores. Small species mostly eat small invertebrates, such as worms and crustaceans. Larger species also eat other fish.

They usually eat by opening their mouths and then sucking in their prey as it passes by with the current.

Most species have one dorsal fin. Some have two.

Gasterosteiformes are often very colorful.

Some species change color. They may change color so that they are camouflaged against the background. They may also change color during breeding.

Gasterosteiformes are divided into two suborders: Gasterosteoidae and Sygnathoidei.

Gasterosteoidae include sticklebacks and tubesnouts.

Sygnathoidei include seahorses, pipefish, trumpetfish, cornetfish and shrimpfish.

Some taxonomists say that seahorses and their relatives should not be classed within the Gasterosteiformes - that they should be placed within their own order, the Sygnathiformes.


A stickleback has rows of spines on its back, in front of its single dorsal fin. It also has spines on its pelvic fins.

Male sticklebacks build nests out of vegetation. They use secretions from their kidneys to hold the nests together.

Once a male attracts a female, she lays her eggs in the nest and the male fertilizes them.

The male guards the eggs in the nest until the eggs hatch.

He will use his tail to fan water over the eggs, in order to make sure that they get enough oxygen.

Head of male three-spined stickleback, photo by Piet SpaansThree-Spined Stickleback

The three-spined stickleback (Gasterosteus aculeatus) lives in Europe, Asia and North America.

It is torpedo-shaped, usually ranges from two to four inches long, and has two to four sharp spines on its back, in front of its dorsal fin.

Some three-spined sticklebacks live in freshwater all their lives.

Others are anadromous – they usually live in the ocean, but breed in freshwater.

All three-spined sticklebacks have bony armor plates on their sides. Anadromous fish have more well-developed armor plates then freshwater fish.

Males are territorial.

In some varities, during breeding season, the males’ chins and abdomens turn red.

This red coloration comes from carotenoids that the fish get from their food.

Nine-Spined Stickleback

The nine-spined stickleback lives in North America, Europe and Asia.

It has from eight to twelve spines on its back.

Nine-spined sticklebacks are slimmer than three-spined sticklebacks.

They are usually dark brown, with males darker than females.

Males’ abdomens turn black during breeding season.

In 2009, scientists published the results of an experiment which showed that nine-spined sticklebacks can learn by observing the behavior of other nine-spined sticklebacks.

They placed a group of sticklebacks in a tank with two feeders - one of which contained much more food than the other.

The sticklebacks chose to eat from the feeder with the most food.

Then, the original group of nine-spined sticklebacks was removed from the tank, and the feeders were switched.

A second group of nine-spined sticklebacks entered the tank.

The first group watched the second group as the second group ate.

They then reentered the tank

Approximately three fourths of the first group of fish were able to figure out which feeder would now supply them with the most food, just by observing the behavior of the second group.


Seahorses make up the genus Hippocampus.

They have long snouts, which make them look like horses.

A seahorse has rings of bony plates around its body. Its skin is stretched over these plates.

Every seahorse has a crown-like structure on its head that is known as a coronet.

A seahorse can move each of its eyes independently.

Seahorses have prehensile tails (tails that can be used for grasing).

They use their tails to anchor themselves to corals and sea grasses, and they use their elongated snouts to suck up food as it passes by.

A seahorse's diet consists of animal plankton (zooplankton) and small crustaceans.

When it swims, a seahorse remains in an upright position.

It will flutter its dorsal fin in order to move forward.

Seahorses use their pectoral fins to steer.

They do not have caudal fins.

Seahorse Reproduction

When seahorses reproduce, it is the male who becomes pregnant.

Seahorse eggs are fertilized internally, within the male's body.

The male keeps the eggs inside his body until they hatch.

He has a pouch, which is known as a brood pouch, on the front, or ventral, side of his body.

The female deposits the eggs into the male's brood pouch using a tube called an ovipositor.

The male then fertilizes the eggs from within his body.

He keeps the eggs inside his pouch until they hatch.

Newly hatched seahorses look like tiny adult seahorses.