Sea spider

Anatomy of a pycnogonid: A: head; B: thorax; C: abdomen 1: proboscis; 2: chelifores; 3: palps; 4: ovigers; 5: egg sacs; 6a–6d: four pairs of legs

Sea spiders have long legs in contrast to a small body size. The number of walking legs is usually eight (four pairs), but the family Pycnogonidae have species with five pairs, and the families Colossendeidae and Nymphonidae have species with both five and six pairs.[9] Seven species distributed among four genera (Decolopoda, Pentacolossendeis, Pentapycnon, and Pentanymphon) have five pairs, and two species in two genera (Dodecolopoda and Sexanymphon) have six pairs.[10] Pycnogonids do not require a traditional respiratory system. Instead, gasses are absorbed by the legs and transferred through the body by diffusion. A proboscis allows them to suck nutrients from soft-bodied invertebrates, and their digestive tract has diverticula extending into the legs.

A pycnogonid grazing on a hydroid

Certain pycnogonids are so small that each of their very tiny muscles consists of a single cell, surrounded by connective tissue. The anterior region (cephalon) consists of the proboscis, which has fairly limited dorsoventral and lateral movement, the ocular tubercle with eyes, and up to four pairs of appendages. The first of these are the chelifores, followed by the palps , the ovigers, which are used for cleaning themselves and caring for eggs and young as well as courtship, and the first pair of walking legs. Nymphonidae is the only family where both the chelifores and the palps are functional. In the others the chelifores or palps, or both, are reduced or absent. In some families, also the ovigers can be reduced or missing in females, but are always present in males.[11] In those species that lack chelifores and palps, the proboscis is well developed and more mobile and flexible, often equipped with numerous sensory bristles and strong rasping ridges around the mouth. The last segment includes the anus and tubercle, which projects dorsally.

In total, pycnogonids have four to six pairs of legs for walking. A cephalothorax and a much smaller unsegmented abdomen make up the extremely reduced body of the pycnogonid, which has up to two pairs of dorsally located simple eyes on its non-calcareous exoskeleton, though sometimes the eyes can be missing, especially among species living in the deep oceans. The abdomen does not have any appendages, and in most species it is reduced and almost vestigial. The organs of this chelicerate extend throughout many appendages because its body is too small to accommodate all of them alone.

The morphology of the sea spider creates an efficient surface area-to-volume ratio for respiration to occur through direct diffusion. Oxygen is absorbed by the legs and is transported via the hemolymph to the rest of the body.[12] The most recent research seems to indicate that waste leaves the body through the digestive tract or is lost during a moult. The small, long, thin pycnogonid heart beats vigorously at 90 to 180 beats per minute, creating substantial blood pressure. The beating of the sea spider heart drives circulation in the trunk and in the part of the legs closest to the trunk, but is not important for the circulation in the rest of the legs.[12] Hemolymph circulation in the legs is mostly driven by the peristaltic movement in the part of the gut that extends into every leg, a process called gut peristalsis.[12][13] These creatures possess an open circulatory system as well as a nervous system consisting of a brain which is connected to two ventral nerve cords, which in turn connect to specific nerves.

A pycnogonid in its natural habitat

These animals live in many different parts of the world, from Australia, New Zealand, and the Pacific coast of the United States, to the Mediterranean Sea and the Caribbean Sea, to the north and south poles. They are most common in shallow waters, but can be found as deep as 7,000 metres (23,000 ft), and live in both marine and estuarine habitats. Pycnogonids are well camouflaged beneath the rocks and among the algae that are found along shorelines.

Sea spiders either walk along the bottom with their stilt-like legs or swim just above it using an umbrella pulsing motion.[15] Sea spiders are mostly carnivorous predators or scavengers that feed on cnidarians, sponges, polychaetes, and bryozoans. Although they can feed by inserting their proboscis into sea anemones, which are much larger, most sea anemones survive this ordeal, making the sea spider a parasite rather than a predator of anemones.

The class Pycnogonida comprises over 1,300 species, which are normally split into eighty-six genera. The correct taxonomy within the group is uncertain, and it appears that no agreed list of orders exists. All families are considered part of the single order Pantopoda.

Sea spiders have long been considered to belong to the Chelicerata, together with horseshoe crabs, and the Arachnida, which includes spiders, mites, ticks, scorpions, and harvestmen, among other, lesser-known orders.[16]

A competing theory proposes that pycnogonida belong to their own lineage, distinct from chelicerates, crustaceans, myriapods, or insects. This theory contends that the sea spider's chelifores, which are unique among extant arthropods, are not in any way homologous to the chelicerae in real chelicerates, as was previously supposed. Instead of developing from the deutocerebrum, they can be traced to the protocerebrum, the anterior part of the arthropod brain and found in the first head segment that in all other arthropods give rise to the eyes only. This is not found anywhere else among arthropods, except in some fossil forms like Anomalocaris, indicating that the Pycnogonida may be a sister group to all other living arthropods, the latter having evolved from some ancestor that had lost the protocerebral appendages. If this is confirmed, it would mean the sea spiders are the last surviving (and highly modified) members of an ancient stem group of arthropods that lived in Cambrian oceans.[17] However, a subsequent study using Hox gene expression patterns consistent with a developmental homology between chelicerates and chelifores, with chelifores innervated from a deuterocerebrum that has been rotated forwards; thus, the protocerebral Great Appendage clade does not include the Pycnogonida.[18][19]

Recent work places the Pycnogonida outside the Arachnomorpha as basal Euarthropoda, or inside Chelicerata (based on the chelifore-chelicera putative homology).[20]

According to the World Register of Marine Species, the order Pantopoda is subdivided as follows:[21]

• suborder Eupantopodida[22]
  • including the following superfamilies:
  • Ascorhynchoidea Pocock, 1904
  • Colossendeidoidea Hoek, 1881
  • Nymphonoidea Pocock, 1904
  • Phoxichilidoidea Sars, 1891
  • Pycnogonoidea Pocock, 1904
  • Rhynchothoracoidea Fry, 1978
• suborder Stiripasterida Fry, 1978[23] including the following family:
  • • Austrodecidae Stock, 1954
• suborder incertae sedis,[24] including the following extinct genera:
  • • • Flagellopantopus Poschmann & Dunlop, 2005 †
      • Palaeothea Bergstrom, Sturmer & Winter, 1980 †
• family Pantopoda incertae sedis, including the following genera:
  • • • Alcynous Costa, 1861 (nomen dubium)
      • Foxichilus Costa, 1836 (nomen dubium)
      • Oiceobathys Hesse, 1867 (nomen dubium)
      • Oomerus Hesse, 1874 (nomen dubium)
      • Paritoca Philippi, 1842 (nomen dubium)
      • Pephredro Goodsir, 1842 (nomen dubium)
      • Phanodemus Costa, 1836 (nomen dubium)
      • Platychelus Costa, 1861 (nomen dubium)

This taxonomic classification replaces the older version in which Pantopoda is subdivided into families as follows:

• Ammotheidae
• Austrodecidae
• Callipallenidae
• Colossendeidae
• Endeididae
• Nymphonidae
• Pallenopsidae
• Phoxichilidiidae
• Pycnogonidae
• Rhynchothoracidae

Although the fossil record of pycnogonids is scant, it is clear that they once possessed a coelom, but it was later lost, and that the group is very old.

The earliest fossils are known from the Cambrian 'Orsten' of Sweden, the Silurian Coalbrookdale Formation of England and the Devonian Hunsrück Slate of Germany. Some of these specimens are significant in that they possess a longer 'trunk' behind the abdomen and in two fossils the body ends in a tail; something never seen in living sea spiders.

In 2013, the first fossil pycnogonid found within an Ordovician deposit was reported from William Lake in Manitoba.[25]

In 2007, remarkably well preserved fossils were exposed in fossil beds at La Voulte-sur-Rhône, south of Lyon in south-eastern France. Researchers from the University of Lyon discovered about 70 fossils from three distinct species in the 160-million-year-old Jurassic La Voulte Lagerstätte. The find will help fill in an enormous gap in the history of these creatures.[26]

• Data related to Pycnogonida at Wikispecies
• Media related to Pycnogonida at Wikimedia Commons
• "Pycnogonida" . Encyclopædia Britannica (11th ed.). 1911.
• PycnoBase: World list of Pycnogonida
• Introduction to the Pycnogonida
• Images of Pycnogonida, and Pycnogonids in literature
• Bibliography (compiled by Franz Krapp)
• Macrobenthos of the North sea, Pycnogonida