A world checklist of Pycnogonida, compiled by taxonomic experts and based on peer-reviewed literature. more
The Pycnogonida, or sea-spiders, is a distinct Class of the Arthropoda of unresolved affiliation with other arthropodan taxa. They are almost exclusively free-living, errant invertebrates, exclusively marine, and found from the poles to the tropics and from the littoral zone to the deepest sea. They range in size from littoral and interstitial species with leg spans of a few mm up to the large, deep-sea colossendeids, with leg spans up to 750 mm. There is one known parasitic species which lives in bivalves. Sexes are almost invariably separate and dimorphic, although gynandromorphs have been rarely recorded.
Their characteristic body form involves a cephalon and trunk comprising four body segments, each of which bears a pair of walking legs. In addition, the cephalon bears an anterior triradiate proboscis, and primitively a pair of chelifores, a pair of palps, and a pair of ovigerous legs (ovigers), these last being a feature exclusive to the Pycnogonida. Dorsally, the cephalon primitively bears an ocular tubercle with four eyes. The last trunk segment bears a small abdomen with a distal anus.
Variations on this theme include atrophy or loss of chelifores, of palps and of ovigers, and, particularly in deeper water forms, loss of eyes and even of the ocular tubercle. Further, there are six polymerous species known, four having five pairs of legs and two having six pairs.
Where reproduction is known, the male carries the eggs on its ovigerous legs, whence they hatch either as a protonymphon larva or at a later stage as a small post-larva, resembling the adult but with underdeveloped appendages. Pycnogonids have no active dispersal ability, but some taxa, for example species of Anoplodactylus and Bathypallenopsis, are passively dispersed by medusae, some of the latter thus achieving cosmopolitan distributions by drifting with bathypelagic scyphomedusae. Feeding is generally understudied, but appears to be restricted to sessile animals and, occasionally, algae: certain species from epizoic communities on rocks or algae are known to feed on bryozoans, cnidarians and filamentous algae. The diet of the many species collected from soft sediments is unknown. The biology of sea-spiders is markedly understudied. The most recent comprehensive review of their biology is by Arnaud & Bamber (1987). Extensive bibliographies are to be found in Fry & Stock (1978) and Nakamura & Child (1991).
The fossil record is sparse, with one known species from the Lower Silurian (~425 MYa), four from the Lower Devonian (~400 MYa) and three from the Jurassic (~150 MYa). Two of the Devonian species are not considered to be on the direct phylogenetic line to extant pycnogonids.
The classification is based on that of Bamber (2007) with subsequent amendments: notably, Nymphonella has been moved to the Ascorhynchidae, Pycnofragilia is added to the Ascorhynchidae, and Pycnosomia is added to the Phoxichilidiidae.
It is evident from recent surveys, both from shallow waters and particularly in the deep sea, that numerous species of pycnogonid await discovery. At the same time, molecular analyses are indicating the existence of cryptic species in what had been thought to be widespread taxa. This database must therefore be an ongoing process and it will certainly expand in the near future. By its existence will hopefully engender better interpretation of the world’s pycnogonids.