Pycnogonida are also called sea spiders because they somewhat resemble terrestrial spiders [Pycnogonum crassirostrum ]. However, pycnogonids have so many characteristic properties that they are presumed to be an early diverged branch of the ancestral spiders.
Most pycnogonids have an elongated and narrow body, divided in a number of segments, however, not as clearly as in other arthropods. The cephalon is bearing a cylindrical proboscis and paired chelifores and palps. Posterior to the cephalon is the four-segmented trunk (some species have a trunk with five or six segments). Each segment has a pair of laterally projecting processes, to which the legs are attached. The number of walking legs depends on the number of trunk segments. Each leg has nine segments, including a terminal "claw" or dactyl. There are no abdominal appendages, but the last segment has an anal process.
Size
Ranging from about 1 mm to 500 mm (span width of the legs).
Vertical distribution
Most are errant benthic species, some live on seaweeds or other invertebrates (for example, sea anemones, hydroids, ectoprocts and tunicates). Pycnogonids are typically not planktonic, but some species are able to swim, and incidentally get caught with pelagic sampling. Sea spiders occur from littoral to great depths, generally living on hydroids and bryozoans, their common food resources.
Distribution in the North Sea
Some 25 species of Pycnogonida are known from the North Sea; for example:
Pycnogonum littorale
Pseudopallene spinipes
Pycnogonum crassirostrum