Seahorses are bony fish (or teleosts) in the family Syngnathidae, which includes about 220 species in 52 genera, most of which are pipefishes. The “true” seahorses comprise some 25 species in the genera Hippocampus and Phyllopteryx, which make up the subfamily Hippocampinae.
Species of seahorses occur in warm-temperate and tropical waters of all of the world’s oceans. The usual habitat is near the shore in shallow-water places with seagrass, algae, or corals that provide numerous hiding places for these small, slow-moving fish. Seahorses may also occur in open-water situations, hiding in drifting mats of the floating alga known as sargassoweed or Sargassum. The lined seahorse (Hippocampus erectus ) is one of the more familiar species, occurring on the Atlantic coast of the Americas.
Seahorses have an extremely unusual and distinctive morphology. Their body is long, narrow, segmented, and encased in a series of ring like, bony plates. Seahorses have a long, tubular snout, tipped by a small, toothless mouth. They have relatively large eyes, and small, circular openings to the gill chamber. The head of seahorses is held at a right angle to the body, and it has a superficial resemblance to that of a horse; hence the common name of these small fish.
Seahorses swim in an erect stance, buoyed in this position by their swim bladder. Seahorses lack pectoral and dorsal fins, but use their anal fin to move in a slow and deliberate manner. Seahorses have a prehensile tail, which is used to anchor the animal to a solid structure to prevent it from drifting about.
Because they are so slow-moving, seahorses are highly vulnerable to predators. To help them deal with this danger, seahorses are cryptically marked and colored to match their surroundings, and they spend much of their time hiding in quiet places. Seahorses mostly feed on zooplankton and other small creatures, such as fish larvae. The size range of the prey of seahorses is restricted by the small mouth of these animals.
Seahorses take close care of their progeny. The female seahorse has a specialized, penis like structure that is used to deposit her several hundred eggs into a brood-pouch located on the belly of the male, known as a marsupium. The male secretes sperm into his marsupium, achieving external fertilization of the eggs. The male seahorse then broods the eggs within his pouch until they hatch. Soon afterwards, swimming, independent young are released to live in the external environment.
Because seahorses are such unusual creatures, they are often kept as pets in saltwater aquaria. They are also sometimes dried and sold as souvenirs to tourists. Seahorses for these purposes are captured in the wild. Seahorses are also prized in eastern Asian herbal medicine. Millions of seahorses are caught for these uses each year, particularly the medicinal uses. Unfortunately, the exploitation is much too intense, and is causing the populations of most species to decline precipitously. Consequently, all species of seahorses are considered threatened by conservation organizations. Regrettably, although their perilous situation is well known, seahorses are not yet being well protected from the over-exploitation.
Kuiter, Rudie H. Seahorses, Pipefishes and Their Relatives: A Comprehensive Guide to Syngnathiformes. Chorleywood, UK: TMC Publishing, 2000.
Nelson, Joseph S. Fishes of the World. 4th ed. New York: John Wiley & Sons, 2006.
Whiteman, Kate. World Encyclopedia of Fish and Shellfish. New York: Lorenz Books, 2000.