|Latreille , 1810|
The woodlouse spiders (Pycnogonida, also Pantopoda) form a class within the jaw-claw carriers (Chelicerata). Despite their name, they are not counted among the arachnids . They are a purely marine group of animals with a distribution in all oceans and a distribution focus in the Southern Ocean . Their number is estimated to be over 1300 species.
The Pantopoda are particularly noticeable for their rod-shaped body, which is tiny in relation to the legs and which is often only a narrow connecting piece between the leg bases. The front body is divided into the undivided prosoma , which carries the first four pairs of extremities (including the first pair of legs), and a rear section, which is divided into several segments by transverse furrows, to which the other pairs of legs are attached. Usually there are four, in some species up to six pairs. The back of the body ( abdomen , opisthosoma) is extremely reduced and usually only a small bulge without attachments, which at the end supports the anus. Only in the fossil paleopantopods is it still sack-shaped and shows three to five segments. In addition to species with very long limbs, there are also more compact forms. The smallest woodlice spiders are 1 to 10 mm in size, the largest among those living in the deep sea can grow up to 900 mm. The length of the body is between 0.8 and 100 mm.
The extremities are similar to those of other Chelicerata, but you will look in vain for their endites. There are four different types of limbs. The first pair, the scissor-studded chelicerae (mostly called chelifora among the Pycnogonida), usually consist of three, rarely four members. They are important in nutrition. This is followed by button-like palps of varying lengths (up to ten limbs ), which, with their thick hair, serve to absorb stimuli .
The third pair of extremities is the so-called breeding legs. It arises ventrally , is also button-like and serves as an egg carrier (Oviger) in the male. The egg packets are formed by the male by stirring the egg mass deposited by the female with his brood legs and gluing them into lumps with the putty released by the legs. Depending on the size of the eggs, a package can contain 50 to 1000 eggs. With the help of a ring consisting of the last four links of the leg, which is covered with many bristles, the male ensures safe transport. The bristles serve the male z. B. for cleaning the eggs. In females, on the other hand, this pair of legs is often regressed or completely absent.
The next pairs of extremities, the walking legs, have a specialty. They can, like those of the harvestmen , be thrown off in case of danger. Throwing it has two advantages: on the one hand, enemies attack the long legs left to them while the woodlice spider takes flight; on the other hand, it is beneficial to throw off an injured leg instead of carrying it around and risking fluid loss. The fracture site closes very quickly and the leg grows back after the next moult. The long running legs, consisting of nine limbs, are usually 4, occasionally 5 (7 species, including Pentanymphon) or rarely even 6 pairs (2 species, genus Dodecalopoda). These multi-legged species are each closely related to eight-legged ones and are considered secondary deviations from the basic plan. The end phalanx, the praetarsus , is usually claw-shaped and often has a secondary claw and is used, among other things, to hold on to food. The three pairs of front extremities can be very variable or, just like the gills and antennae, completely absent.
These representatives of the jaw-claw bearers, just like other arthropods, have an exoskeleton with chitin deposits. As with the spiders, excreta is stored in the underlying skin, so that the animals are often colored. But stocks are also stored there. The exoskeleton is very impermeable and sometimes extremely thick. On the other hand, there is no accumulation of lime, which means that the Pantopoda's skin is leather-like or parchment-like.
Sense organs and nervous system
There are numerous glands in the skin, such as B. cement glands on the femora of the legs of males and spinnerets on the chelicerae of larvae. The sense organs are poorly developed. In the front part of the body there are four small lens eyes (median eyes). Only sensory bristles are known of skin sensory organs. Split senses have not yet been found in this class .
The nervous system is more primitive than that of other chelicerata because the abdominal ganglia remain largely separate. The sub- canal ganglion innervates the palpial and breeding bone segments. One or two abdominal ganglia still appear during development but fuse with the last trunk ganglion. The ganglia in the trunk are usually clearly visible. From these one can often see strong nerve cords pulling into the legs.
The mouth rests on an extensive proboscis , which protrudes ventrally or forwards. The trunk consists of three longitudinal parts (antimers), one dorsal and two ventrolateral. The triangular mouth at the tip of the trunk itself is covered with three bristle-covered plates (lips) and three movable chitin hooks . The part of the intestine in the trunk is called the pharynx . Its triangular lumen is widened by radial muscles that pull towards the wall of the trunk, and the rear area becomes a trap apparatus by chitin hooks protruding into the lumen. An esophagus leads into the midgut . Since the volume of the trunk is significantly smaller than that of the legs, there are also long extensions of the midgut (blind sacs) that extend into the legs, but in some species also into the chelicerae and trunk. This has the consequence that the absorbing and digesting surface is considerably enlarged. The esophagus, which is occupied by an apparatus made of rigid and flexible bristles, which finely chops the coarse absorbed food until only fragments of cells remain, is long and narrow. From here they enter the intestine and are absorbed by the intestinal cells in which the actual digestion takes place. The straight end part of the intestine opens with a terminal anus.
Vascular system and breathing
The blood vessel system consists only of one back vessel. It has two pairs of inflow openings (ostia) for the colorless blood and runs through the trunk from the rear end to the region of the ocular hillocks and is dorsal with a broad surface on the back wall, ventrally on the pericardial septum. Often there is also an unpaired ostium at the rear end. The pericardial septum runs horizontally through the trunk just above the intestines and also extends into the legs.
Because woodlouse spiders lack gills, breathing is taken over by another organ, probably the intestine or fine blood capillaries into which the oxygen diffuses.
Construction of the genitals
The gonads arise ventrally on the pericardial septum, but extend into the legs, which also contain most of the sexual organs. The genital openings are also located at the coxes of the legs, usually in the second member, so eggs ready for laying are never found in the trunk, but only in the legs. Interestingly, there are often multiple pairs of openings, often on all pairs of legs. In some genera, they are limited to certain pairs of legs, but most often to the last. The number and location of the genital openings can vary depending on gender.
The eggs laid, which reach a size of 0.02 to 0.7 mm depending on the species, are carried by the male as already described. The development shows some peculiarities and is also not homogeneous within the pantopods. The furrowing is initially total and can be equivalent or inequitable depending on the yolk content. Sooner or later, however, cells fuse to form syncytial masses. The formation of the cotyledons is difficult to understand. Dorsally, large cells are shifted to the interior, which form the endoderm (partly starting from a urentoderm cell) and the mesoderm . Later the longitudinal groove appears ventrally, which corresponds to the blastopore area of other arthropods. The mesoderm always seems to transform itself into muscles and connective tissue via a simple strip stage.
Embryonic development leads to a typical larva, the protonymphon larva. The three pairs of extremities that the larva initially possesses are chelicerae, palps, and brood legs. The chelicerae of the protonymphone larva have a spinneret gland opening into a tube at the side and some scissor glands. The other two extremities, however, are only three-part hooks, which are later more or less regressed, while the definitive palps and breeding bones arise through new formation. The larva lacks the heart and anus. The further development takes place through the gradual formation of the legs on the back of the body, the stages are separated by skins . The larval stages only rarely remain on the brood legs of the males (for example chaetonymphon ), they mostly leave the brood legs as protonymphons and live in the next phase as ecto- or endoparasites ( Phoxichilidium , Anoplodactylus ) on other animals, especially polyps .
Way of life and distribution
The representatives of the Pycnogonida are exclusively at home in the marine environment and live among all kinds of bottom vegetation. They are not tied to a specific depth, but are native to both the surface and the deep sea at depths of more than 4000 m. Some particularly small species live in the sand gap system (interstitial). They are only dependent on a certain salt content, which is around 3.5%. They also prefer cold water. They are therefore found in the Antarctic (around 250 of the 1000 known species, 100 of which are endemic to the Antarctic and around 60 in the sub-Antarctic waters), there also in large forms. In the warm seas, for example on the coasts of the Mediterranean , only small specimens with a maximum diameter of 30 mm have been found so far. All pantopods are consistently lazy animals, whereby the short-legged, clumsy species are characterized by a very special clumsiness. If you put them in a bowl, they sink to the floor and remain motionless. However, the slimmer forms can swim more or less gracefully and thus spend a long time in the open water. All woodlouse spiders are climbers who move slowly and deliberately and can cling to any suitable object. The speed of movement is quite slow (approx. 1 to 3 mm / s), but can be increased enormously in dangerous situations.
All representatives of the woodlouse spider feed predatory. Only soft-skinned animals are part of their diet, such as snails, moss animals and sponges , but above all hydroid polyps . The stinging cells of the polyps seem to have no effect on woodlouse spiders. The food, for example a polyp head, is grasped with scissors and sucked out with the trunk. This process can take up to 10 minutes. The ingestion of copepods and polystyrene was also observed. The copepods are grabbed with the help of the claws of the walking legs, brought to the mouth and sucked out. In addition, the eating of mussel meat was investigated in experiments. Here, too, the claws on the legs are used to hold the meat and then take the food through the proboscis . This process can take several hours.
A number of species live ectoparasitic (on coelenterates, sponges, molluscs and echinoderms).
Defense against predators
In the case of the knotty woodlice spider ( Pycnogonum litorale ), a method of chemical defense was found for the first time in a marine predator-prey relationship . It has been proven that the common beach crab ( Carcinus maenas ), which otherwise eats almost everything, avoids woodlice spiders because they have a very high content of 20-hydroxy- ecdysone in all stages . This substance is a hormone that triggers the molting (ecdysis) in insects and crustaceans . Frequent molting is disadvantageous for the beach crabs, not least because freshly molted animals still have very soft mouthparts, which make it impossible to eat for a certain time. The woodlouse spiders apparently control their moulting differently. A molting hormone for these animals is still unknown.
As with all animal and plant groups, several names have been preserved for the woodlouse spiders. So they were called Podosomata by the Englishman William Elford Leach around 1815 , which means something like "body (only) made of legs". In 1863, the German zoologist Carl Eduard Adolph Gerstäcker described them as Pantopoda , which can be freely translated as "the all-legged". The German name Asselspinnen expresses a certain degree of uncertainty . For a long time they were grouped with crabs, but since they also look spider-shaped and have some things in common, they were classified in the arachnid class. But because one wanted to emphasize their peculiarity, they were finally called woodlouse spiders .
Assumptions that have arisen in the meantime that the woodlouse spiders formed an independent trunk basal to all other Arthropoda are now considered disproved. Most taxonomists set the woodlouse spiders as the most basic group of the Chelicerata, as opposed to all other representatives as sister groups. This means that the horseshoe crabs and arachnids are more closely related to each other than any of these groups is to the woodpecker spiders.
The traditional system presented in this way (based primarily on morphological series with progressive reduction of individual limbs) is only partially supported by more modern molecular and cladistic-morphological studies. Most orders and some families were found to be paraphyletic .
Woodlouse spider fossils are rarely found. A number of problematic fossils with strongly deviating physique are no longer considered to be core group representatives, but have been assigned to other relatives. The remaining species can be assigned to modern orders on the basis of their blueprint, their formerly common summary as “Palaeopantopoda” is therefore an artificial division. The Lower Devonian roof slate of the Hunsrück is particularly rich in fossil species . The oldest forms are preserved larvae from the Upper Cambrian of Sweden (not only as an imprint) (the body wall was replaced by calcium phosphate and the animal was then embedded in limestone, so-called "Orsten" fossils). The adult forms of this are unknown.
- Claudia P. Arango, Ward C. Wheeler: Phylogeny of the sea spiders (Arthropoda, Pycnogonida) based on direct optimization of six loci and morphology. Cladistics 23, 2007, pp. 255-293, doi: 10.1111 / j.1096-0031.2007.00143.x .
- F. Arnaud, RN Bamber: The biology of Pycnogonida. In: JHS Blaxter, Alan J. Southward (Eds.): Advances in Marine Biology. 24: pp. 1-95. Academic Press Inc. 1988, ISBN 0-12-026124-3 .
- Volker Storch , Ulrich Welsch : Systematic Zoology. 6th edition. Spectrum publishing house. Munich 2004, ISBN 3-8274-1112-2 .
- Hans Eckard Gruner, Hans-Joachim Hannemann, Gerhard Hartwich: Invertebrates. (Urania Tierreich; Volume 2). Urania, Freiburg / B. 1994, ISBN 3-332-00502-2 .
- Joel W. Hedgpeth: Pycnogonid. In: Encyclopedia Britannica . 1977.
- Philip Ernst King: Pycnogonids. Hutchinson, London 1973, ISBN 0-09-116460-5 .
- Jan C. Loman: Biological observations on a pantopod. In: Tijdschrift van de Nederlandse Dierkundige Vereniging. Volume 2, 1907, pp. 255–284, plate V.
- T. Munilla, A. Membrives: Check-List of the pycnogonids from Antarctic and sub-Antarctic waters: zoogeographic implications. Antarctic Science 21, 2009, pp. 99-111, doi: 10.1017 / S095410200800151X .
- Pantopōs . In: Meyers Konversations-Lexikon . 4th edition. Volume 12, Verlag des Bibliographisches Institut, Leipzig / Vienna 1885–1892, pp. 659–660.
- Lotz G (1968): Feeding and catching prey in a pantopod, Anoplodactylus petiolatus Krøyer . Oecologia 1: 171-175. doi: 10.1007 / BF00383137
- Clémençon H (1961): Woodlouse spiders - inhabitants of the seashore. Part 1: Building and Living, Observation and Preparation. Part 2: Identification key. Microcosm 50: 262-270.
- Tomaschko KH (1994): Ecdysteroids from Pycnogonum littorale (Arthropoda, Pantopoda) act as chemical defense against Carcinus maenas (Crustacea, Decapoda) . Journal of Chemical Ecology 20: 1445-1455. doi: 10.1007 / BF02059872
- Arthur D. Chapman (2009): Numbers of Living Species in Australia and the World. 2nd Edition. Report for the Australian Biological Resources Study, Canberra, Australia, September 2009, ISBN 978-0-6425-6861-8 , download: http://www.environment.gov.au/biodiversity/abrs/publications/other/species- numbers / 2009 / index.html
- JW Hedgpeth (1978): A reappraisal of the Palaeopantopoda with description of a species from the Jurassic. Zoological Journal of the Linnean Society 63: 23-34. doi: 10.1111 / j.1096-3642.1978.tb02088.x
- Jump up D. Waloszek & JA Dunlop (2002): A larval sea spider (Arthropoda: Pycnogonida) from the upper Cambrian Orsten of Sweden, and the phylogenetic position of pycnogonids. Palaeontology, 45, 421-446. open access