Is a Seahorse a Fish? How Seahorses Breathe and Swim

What makes a seahorse a fish?

Biologists decide whether an animal is a fish by its body plan and ancestry, not by its looks. A seahorse ticks every box:

• Backbone and skeleton: it is a vertebrate with a bony internal skeleton.
• Gills: it takes oxygen from water, not air.
• Fins: it has a dorsal fin, two pectoral fins and a tiny anal fin.
• Swim bladder: a gas-filled sac controls its buoyancy, as in most bony fish.
• Cold-blooded and aquatic: its body temperature follows the water around it, and it lives its whole life in the sea.

Seahorses belong to the ray-finned fishes, the class Actinopterygii, which includes most familiar fish from salmon to clownfish. Within that class they sit in the order Syngnathiformes and the family Syngnathidae. The lined seahorse of the western Atlantic is a typical example.

How do seahorses breathe?

Seahorses breathe like other fish, by passing water over their gills. Water is drawn in through the mouth at the end of the snout, flows over the gills inside the head and leaves through small gill openings behind the head. Blood in the gills picks up oxygen dissolved in the water and releases carbon dioxide.

Their gills are unusual, though. Instead of the comb-like gill filaments of most fish, seahorses and their relatives have small, tufted gills, like little bunches of grapes. The old name for the group, lophobranchs, means tuft-gilled. Their gill openings are also tiny, just small holes high on the sides of the head rather than the wide gill slits of a trout or a perch.

Because they rely on gills, seahorses cannot breathe air. They need clean, oxygen-rich water.

How do seahorses swim?

Seahorses swim upright, which is very rare among fish. Their main engine is the small dorsal fin on the back, which flutters dozens of times a second, so fast that it can look like a blur. Two pectoral fins behind the head steer and help with balance. Unlike most fish, seahorses have no tail fin and no pelvic fins.

To move up or down, a seahorse adjusts the amount of gas in its swim bladder, rising when it adds gas and sinking when it removes it. The result is slow, careful movement rather than speed. The tiny dwarf seahorse is often called the slowest-moving fish, at about 1.5 m (5 ft) per hour, and it features among the slowest animals.

Being a weak swimmer is less of a problem than it sounds. Seahorses spend much of their time holding on to seagrass, corals or ropes with their tails and ambushing prey that drifts within reach.

Why seahorses look so unlike other fish

Several features make seahorses look like anything but a fish:

• A bent neck: the head sits at a right angle to the body, giving the horse-like profile. This shape helps a seahorse strike at small prey from a distance.
• Armor instead of scales: the body is covered by skin over rings of bony plates rather than overlapping scales.
• A grasping tail: the tail can wrap around objects like a monkey’s. A 2015 study in the journal Science showed that its square cross-section, made of interlocking plates, resists crushing and grips better than a round tail would.
• A pipette snout: seahorses have no teeth. They suck in prey through the tube-shaped snout with a sudden upward flick of the head.
• No stomach: food passes quickly through a simple gut, so seahorses must feed often.
• Independent eyes: each eye can swivel on its own, like a chameleon’s.

And then there is reproduction: in seahorses, the male gets pregnant, as explained in do male seahorses give birth?

Seahorse relatives: pipefish and seadragons

Seahorses’ closest relatives make their fish identity much clearer. The family Syngnathidae holds roughly 300 species, most of them pipefishes: long, thin fish with tube snouts and armor rings that swim horizontally like ordinary fish. Genetic studies show that seahorses are highly modified pipefishes that began standing upright roughly 25 million years ago, probably in the western Pacific. The oldest seahorse fossils, about 13 million years old, come from Slovenia.

Seadragons are the family’s showpieces. The leafy and weedy seadragons of southern Australia drift among kelp, disguised by leaf-shaped skin flaps, and a third species, the ruby seadragon, was described only in 2015. Like seahorses, male seadragons carry the eggs, but their tails cannot grip.

Two seahorses live along the coasts of Europe: the long-snouted seahorse (Hippocampus guttulatus) and the short-snouted seahorse (Hippocampus hippocampus), both found from the British Isles to the Mediterranean.

Animals with fishy names that are not fish

English names can be misleading. Jellyfish, starfish, cuttlefish and crayfish are all invertebrates, not fish at all; the reasons are explained in are jellyfish fish? Whales and dolphins look like fish but are mammals that breathe air and feed their young on milk, as described in are whales fish?

The seahorse is the opposite case: an animal that looks nothing like a typical fish, yet is a fish through and through.