- Norwegian: havedderkopper
The appearance of the long-legged sea spiders are very similar to the "true", terrestrial spiders. There are usually four, but some species have six pair of legs. The body is slim. A respiratory system is not necessary, due to the relatively large body surface and small volume. The required oxygen diffuses through the surface. At the back of the head there is one or two pair of eyes. One pair is usually directed forwards, the other backwards. Some deep water species may reach a length of 90 cm, but many species are in the range of 3 to 30 mm.
The sea spiders are marine arthropods found in all oceans. There are over 1300 registered species worldwide, 39 of them are known from the coast of Norway. The most common family Nymphonidae, is represented by 13 species in Norway. They are not "true" spiders, but are regarded as members of the same subphylum as the terrestrial spiders, in contrast to other arthropod groups, like the crustaceans.