Sea spiders (phylum Arthropoda, class Pycnogonida) are a group of arthropods that take their common name from their superficial resemblance to the true spiders. Although rarely seen, these are widespread animals occurring in every ocean, with a preference for cooler waters. Sea spiders occupy a wide range of habitats: some species have been recorded from a depth of 19,685 ft (6,000 m), but the majority live in shallow coastal waters. Some 600 species have so far been identified.
Most sea spiders are small animals, measuring from 0.04-0.4 in (1-10 mm) in length, but some deep sea species may reach a length of almost 2.4 in (6 cm). The body itself is usually quite small, the main mass of the spider being accounted for by its extremely long legs. The legs are attached to the anterior portion of the body (the prosoma) and are usually eight in number, although some species may have 10 or even 12 pairs. The body is segmented with the head bearing a proboscis for feeding, a pair of pincher-like claws known as chelicera, and a pair of segmented palps that are sensory and probably assist with detecting prey. Most sea spiders are either a white color or the color of their background; there is no evidence that they can change their body coloration to match different backgrounds. Many deep sea species are a reddish-orange color.
The majority of sea spiders crawl along the substrate in search of food and mates. They are often found attached to sea anemones, bryozoans, or hydra, on which they feed. They are all carnivorous species and feed by either grasping small prey with the chelicera, tearing off tiny polyps from corals or sponges, or by directly sucking up body fluids through the mouth, which is positioned at the extreme tip of the proboscis.
An unusual behavioral feature displayed by sea spiders is the male's habit of looking after the eggs once they have been laid by the female. As the female lays her eggs, they are fertilized by the male who then transfers them to his own body. Here they are grouped onto a special pair of legs known as ovigerous legs (which are greatly enlarged in males). Large masses of eggs may be collected—often as many as 1,000 on each leg. The male carries these egg clusters for several weeks until they hatch into tiny larvae that are known as a protonymphon. Even at this stage, some species continue to care for their offspring until they have further developed-a strategy designed to protect the vulnerable offspring from the wide range of potential predators that exist in these waters.