Class Pycnogonida: Sea Spiders

Class Pycnogonida: Sea Spiders

About 1000 species of sea spiders occupy marine habitats ranging from shallow, coastal waters to deep-ocean basins. Some sea spiders are only a few millimeters long, but others are much larger with legspans up to nearly 0.75 m. They have small, thin bodies and usually four pairs of long, thin walking legs. In addition, they have a feature unique among arthropods: segments are duplicated in some groups, so that they possess five or six pairs of legs instead of the four pairs normally characteristic of chelicerates. Males of many species bear a subsidiary pair of legs (ovigers) on which they carry developing eggs, and ovigers are often absent in females. Many species also are equipped with chelicerae and palps. Chelicerae are sometimes called chelifores in this group.

The small head (cephalon) has a raised projection with two pairs of simple eyes. The mouth is at the tip of a long proboscis, which sucks juices from cnidarians and soft-bodied animals. Their circulatory system is limited to a simple dorsal heart, and excretory and respiratory systems are absent. The long, thin body and legs provide a large surface area, in proportion to body volume, that is evidently sufficient for diffusion of gases and wastes. Because of the small size of the body, the digestive system and gonads have branches that extend into the legs.

Sea spiders occur in all oceans, but they are most abundant in polar waters. Pycnogonum  is a common intertidal genus on both Atlantic and Pacific coasts of the United States; it has relatively short, heavy legs. Nymphon  Figuris the largest genus of pycnogonids, with over 200 species. It occurs from subtidal depths to 6800 m in all oceans except the Black and Baltic seas.

Some research suggests that pycnogonids belonged to an early diverging arthropod lineage outside any of the subphyla but morphological and molecular evidence strongly supports the placement of pycnogonids in the Chelicerata (see Phylogeny section).

Useful External Links

• Sea spiders (Arthropoda, Pycnogonida) from ten recent research expeditions to the Antarctic Peninsula, Scotia Arc and Weddell Sea – data by Jamie Maxwell
• The Pycnogonids (Arthropoda, Pycnogonida) of Torrevaldaliga (Tyrrhenian Sea), Italy, with data on Endeis biseriata, new record for the Mediterranean Sea by L.Galli
• Pycnogonida by Phil Myers