Pycnogonids are a Class of Arthropods called sea spiders, because they resemble spiders and live in the oceans. They are slow-moving, crawling predators, usually white or pale yellow or orange in colour. Most have 8 jointed legs (although a few have 10 or 12 legs), an exoskeleton and two body parts: a cephalothorax with a large proboscis plus the leg-bearing segments followed by a very short, reduced abdomen.
There is much doubt about pycnogonid affinities. Are they actually related to the Araneae or are they modern descendants of the ancestors of the Arthropods? They have chelicerae that resemble those found in spiders but this may not indicate a close relationship as much as a similar solution for a common hunting technique. Like spiders, they are predators that inject their prey (mostly hydroids and polyzoa) with digestive juices and then they suck up the dissolved liquids. Or perhaps they are highly altered Crustaceans. The evidence for this is that their larval stages resemble the nauplius larvae of some crustaceans and both groups are marine. These characteristics could also have developed independently and do not necessarily indicate a close relationship. The most likely explanation is that the pycnogonids separated from the main line of Arthropods hundreds of millions of years ago and so deserve their separate class status.
Pycnogonid bodies are so reduced that their guts go into their legs and the main purpose of the body segments seems to be to connect the legs together. The cephalothorax starts with a proboscis, four simple eyes and a brain, then a heart and the reproductive organs (the sexes are separate). They are so small and their surface area to volume ratio so large that they do not need a respiratory system, but diffuse oxygen across their body surfaces. There is also no excretory system. To quote King (1973. p8): “The pycnogonids are unique in their method of intracellular digestion. Certain of the mucosal cells take up food material until they are gorged with food and they then strip away from the mucosa and float freely in the lumen of the midgut. Other mucosal cells absorb food from them until, with the food exhausted, the floating cells are eliminated from the anus.”
There are about 1300 species described and all are small, the largest being 75 centimeters across and the smallest a mere 2 mm. The larger animals are usually found in deep sea habitats. Most pycnogonids don’t swim, but crawl slowly around the bottom sediments or over rocks, looking for their prey. In most species, the larvae are planktonic and then settle on anenomes, molluscs or worms where they hide. Some are parasitic. In a number of species, the eggs and larvae are carried on the specialised ‘ovigerous’ legs of the males, a feature unique to the pycnogonids. Fertilisation is external and then the males attach the eggs to their ovigers.
With their hard exoskeletons, pycnogonids are often used as substrates for other organisms such as sponges, molluscs, anenomes and barnacles. This may give them some camouflage advantages. Like other arthropods, pycnogonids must moult in order to grow.
Pycnogonids may not be common or well known but they are widely distributed in the oceans. A few are found in tropical seas but the majority are found in temperate and polar seas. They can be found in the intertidal zones and down to depths of 6000 meters so pressure doesn’t bother them. They are never that common except in the Antarctic, where their prey also flourish. They are not something that most people will ever see but they are interesting little animals nonetheless.
References: http://marine-life.suite101.com/article.cfm/sea_spiders
King, P.E. 1973. Pycnogonids. Hutchinson of London
http://www.australasian-arachnology.org/arachnology/pycnogonida
