seahorse, common name for certain small fishes of the family Syngnathidae, inhabiting warm waters but sometimes found as far north as Cape Cod. The elongated head and snout of a seahorse, flexed at right angles to the body, suggest those of a horse.
Members of different species range in size from .6 to 8 in. (1.6–20 cm); all feed on minute organisms. Protected by thin bony plates that are derivatives of the scales found in most fishes, the seahorse swims weakly in an upright position by means of rapid, hummingbirdlike beats of its fins; at rest it curls its thin, prehensile tail around seaweed. Some seahorses have deceptive leaflike appendages, and others are poisonous.
While linked in the mating embrace (during which the seahorses utter musical sounds) the female forces the eggs into a pouch on the underside of the male, where they are fertilized and where they remain, feeding on nutrients provided by the vascular lining of the pouch, until they are expelled as miniature versions of the adult.
The pipefishes, belonging to the same family, comprise about 50 species, whose members range in size from 4 to 12 in. (10–30 cm) long. They are slightly more fishlike in appearance and are able to change color, but have the long snout and unusual breeding habits of the seahorse.
Seahorses are classified in the phylum Chordata, subphylum Vertebrata, class Osteichthyes, order Gasterosteiformes, family Syngnathidae.
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