Sea spider is an apt name for the pycnogonids (Fig. 23.4K, L), for they superficially resemble true spiders (Araneae), albeit with a more slender body and sometimes more than four pairs of legs (up to six pairs in some Antarctic species). The body of pycnogonids is small and slender. The head and thorax is fused to form the prosoma, and the abdomen (or opisthosoma) is small and unsegmented. Pycnogonids are also unusual in having the mouth placed at the end of a proboscis that may be as long as half the body. The pycnogonids are an ancient group dating back to the Devonian, probably representing the nearest relatives of the Che-licerata (spiders, mites, scorpions), or possibly the nearest relatives of the remaining Arthropoda. Around 1500 species of pycnogonid are known worldwide with about 30 species in 17 genera known from the GBR and adjacent Queensland coast. Some deep-sea pycnogo-nids have a leg span of 750 mm, but species on the GBR are much smaller, at less than 20 mm across. All are cryptic and slow moving, so detection can be difficult. Pycnogonids prey on bryozoans, polychaetes, hy-droids, xoanthids and even scleractinian corals.
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