Sea Spider, a sea-dwelling animal that looks somewhat like a spider. There are about 500 species. Sea spiders are found in all oceans. Species found in coastal waters are usually small and have a leg span of about 1 inch (2.5 cm), those living at great depths, up to 24 inches (60 cm).

The sea spider has a small, narrow body. It usually has four pairs of long, thin legs attached to the abdomen. Attached to the head there are usually three other pairs of appendages—a pair called chelicerae, used for grasping food; a pair of sensory projections called palps; and a pair of egg-carrying legs (sometimes underdeveloped or absent in the female). The female lays round masses of eggs upon the egg-carrying legs of the male, which carries the eggs until they hatch.

On top of its head, the sea spider has a knobby projection bearing two, three, or four simple eyes. The head ends in a snout with a sucking mouth. Sea spiders feed by sucking the body juices of such marine animals as sea anemones, sponges, and sea squirts.

Sea spiders belong to the class Pycnogonida of the phylum Arthropoda.