IMIS

The Pycnogonida, or sea-spiders, is a distinct Class of the Arthropoda which phylogenetic position is still debated (a history of the discussion in Dunlop & Arango, 2005), with recent analyses suggesting sea spiders are sister group to all extant chelicerates (a review of Arthropoda phylogenetics in Giribet and Edgecombe, 2013). Pycnogonids are almost exclusively free-living, errant marine invertebrates, 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. Some species are parasitic, like Nymphonella tapetis which lives in bivalves. Sexes are almost invariably separate and dimorphic, although gynandromorphs have been recorded.
The sea spiders 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. Variations on this theme include atrophy or loss of chelifores, o palps and/or 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.
These relatively simple creatures with relatively small bodies and long legs, have gut diverticula extending into chelifores and along the legs, and the last trunk segment bears a small abdomen with a distal anus. Sea spiders lack respiratory organs or structures, and gas exchange occurs through the cuticle; recently Woods et al (2017) demonstrated how sea spiders use gut movements for internal transport of oxygen.
Where reproduction is known, the male carries the eggs attached to the ovigers, whence they hatch either as a protonymphon larva or as an advanced postlarva. A review of the different pathways of postembryonic development in Pycnogonida is found in Brenneis et al. (2017). Pycnogonids have no active dispersal ability, but some taxa, for example species of Anoplodactylus and Bathypallenopsis, are passively dispersed by medusa, achieving wide geographical distributions. The Antarctic species Nymphon australe is known as circumpolar, however, it is yet not clear what dispersal mechanisms are involved.
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 still sparse (only four pycnogonid fossil records where known by early 2000s) but new forms have described in recent years. So far, there is one known species from the Upper Ordovician (ca. 450 Ma), one from the Lower Silurian (ca. 425 Ma), five from the Lower Devonian Hunsrück Slate (Germany) (ca. 400 Ma), and three from the Jurassic (150 Ma).
PycnoBase is based on Bamber’s (2007) attempt of a holistic interpretation of the Pycnogonida classification, with subsequent amendments: notably, Nymphonella moved to the Ascorhynchidae, Pycnofragilia added to the Ascorhynchidae, and Pycnosomia added to the Phoxichilidiidae. However, these and other groupings are yet to be tested under a robust and comprehensive phylogenetic approach, in a follow up to the phylogeny in Arango & Wheeler (2007).
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 a working document and it will certainly continue expanding, hopefully contributing to a better understanding of the world’s pycnogonids.