Seahorses are tiny fishes that are named for the shape of their head, which looks like the head of a tiny horse. There are at least 25 species of seahorses. You’ll find them in the world’s tropical and temperate coastal waters, swimming upright among seaweed and other plants. Seahorses use their dorsal fins (back fins) to propel slowly forward. To move up and down, seahorses adjust the volume of air in their swim bladders, which is an air pocket inside their bodies.