Heterotardigrada
| Heterotardigrada | ||||||||||||
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Echiniscus sp., Illustration |
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| Heterotardigrada | ||||||||||||
| Marcus , 1929 | ||||||||||||
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As heterotardigrada refers to a class of tardigrades (Tardigrada), either by striking head attachments or hardened back armor plate (sclerites) are marked. It was described in 1929 by the German zoologist Ernst Marcus (1893–1968) and includes both sea-life (marine) and land and freshwater (limno-terrestrial) species. The marine forms in particular are characterized by a high level of biodiversity and great variability in body shape for tardigrade.
features
Tardigrades are a very homogeneous group of animals, whose body structure is only variable within narrow limits. In the following, therefore, only those features of the heterotardigrada are listed that are particularly characteristic of this class; For a more detailed introduction to anatomy, see the main tardigrade article. Most heterotardigrada reach a body length between 100 and 150 micrometers.
Cuticle
One of the most striking features of many heterotardigrada are hardening of the non-cellular outer skin, the cuticle , which can be found especially in the predominantly limno-terrestrial order Echiniscoidea , but to a lesser extent also in some species from the order Arthrotardigrada . This creates a division into individual back (dorsal) platelets, the sclerites , which can be viewed as armor of the body and often have thorny protrusions. The exact number and arrangement of the plates, some of which may have grown together, is an important feature for determining the species. Within the genus Testechiniscus , there are also ventral (ventral) armor plates, which are then referred to as sternites .
The microscopic fine structure of the cuticle is also typical of the heterotardigrada: In a layer called the epicuticle, characteristic "supporting pillars" surrounded by cavities are pronounced, so that the cross-section results in a honeycomb structure, which further stiffens the outer skin. The marine heterotardigrada are colorless, while land-living forms can often be colored by food or special pigments.
Head and body attachments
In addition to sclerites, many heterotardigradas, including in particular the species of arthrotardigrada, are also characterized by diverse head and body appendages. The so-called cirri on the head, which presumably serve to perceive mechanical contact stimuli, are particularly noticeable . A single cirrus usually consists of a base, the cirriphore , a short widened section, the collar or scapus , and the actual long and narrow sensory hair, which is called the flagellum . In addition to the Cirri, on the head of the Heterotardigrada one can also find structures with a likely chemosensory function, the Clavae, which are probably derived from Cirri, but are wider than the latter and are hollow inside. The exact shape, number and arrangement of the cirri and clavae is an important feature when determining the species within the heterotardigrada. Further characteristic features are long filaments that arise on the side of the body or at the rear end, which are known as alae , and large-scale outgrowths of the cuticula, which are found in many marine species and probably serve to spread the animals through ocean currents.
extremities
The legs have the same basic structure as all tardigrade; on the fourth pair of legs, however, there is often a ring of small cuticle teeth running around each leg, the function of which is unknown. The claws of the heterotardigrada are often not located directly at the end of the leg, but arise from separate "toes". They are sometimes provided with "spores", small protrusions, and in some species they are replaced by adhesive discs.
Body cavity and digestive system
There are also some peculiarities in the internal anatomy of the heterotardigrada. First of all, their body cavity, the haemocoelom , is smaller in comparison than in most of the Eutardigrada , the other large tardigrade class. The stilettos, with which the animals can pierce algae cells, for example, and suck them out, are usually thinner and almost always curved. The muscular throat is stabilized by continuous hardened longitudinal stripes, but not by individual placoids , rod-shaped structures that are found in the Eutardigrada. The pharyngeal muscles consist of 51 epithelial muscle cells of equal status; An earlier assumption that these are divided into 27 epithelial cells and 24 actual muscle cells, of which the former line the volume of the throat and the latter are responsible for the suction power, has proven to be incorrect according to electron microscopic examinations. The intestine usually has five or six protuberances, the diverticula .
Excretory and reproductive organs
Malpighian glands, which are used for excretion and osmoregulation , as found in the Eutardigrada, do not exist within the Heterotardigrada. Osmoregulation, i.e. regulation of the salt balance, is not necessary in the marine forms, since the body fluid is isotonic with sea water, so there is no difference in the salt content. In some terrestrial species, there are special organs on the abdomen between the attachment of the second and third pair of legs, which presumably have an excretory function. If present, they consist of one in the middle (medial) and two lateral (lateral) cells.
Characteristic of the heterotardigrada is the gonopore called opening point of the fallopian tube that with them always on the body outside, mostly before, often behind the anus, but never inside the rear intestine or rectum is. It is always surrounded by a rosette of up to six “leaves”, the petals . Sperm stores, in which the male can deposit his sperm, are duplicated, if available. In the males, the genital opening is oval or circular and protrudes slightly outwards.
distribution and habitat
Heterotardigrada can be found around the world, both in the sea, in freshwater and on land, although the population density of marine species is usually lower than that of land-dwelling species, which, however, have also been better studied. Some species, for example from the genus Batillipes , can be found worldwide and live as part of the sand gap fauna within the intertidal zone on almost all sandy beaches; others such as Moebjergarctus manganis seem to have specialized in South Pacific manganese nodules for reasons that are still unknown.
A number of species live as parasites or commensal , without damage to the host, to other animals, Echiniscoides Sigismundi approximately mussels (Bivalvia) and barnacles (Cirripedia) Pleocola limnoriae on isopods (Isopoda), Actinarctus doryphorus on sea urchins (Echinoidea) and Echiniscus molluscorum even in the terrestrial snail species Bulimulus exilis . Land-living parasites or commensals are otherwise rare, as tardigrade when active are dependent on moisture, which is not always guaranteed on land; Echiniscus molluscorum is therefore also endoparasitic or endoparasitic. A clearly ectoparasitic species, however, is the marine form Tetrakentron synaptae , which pricks and sucks out individual cells of sea cucumbers (Holothuroidea).
Way of life
Most aspects of animal life are not specific to the heterotardigrada and therefore are covered in the tardigrade articles in general .
Anhydrobiosis, the high resistance to dehydration through the formation of special survival structures, the barrels, is often considered a characteristic tardigrade trait, but only exists within the order Echiniscoidea. With one exception, all Arthrotardigrada live in the sea, which offers constant environmental conditions, so that these species never had to develop the ability to develop barrel formation.
Heterotardigrada can reproduce sexually as well as without sexuality. In the Echiniscoidea in particular, parthenogenesis is found in many species, i.e. asexual reproduction without males, in which the females lay unfertilized eggs, from which in turn only females emerge. This form of reproduction is coupled with anhydrobiosis and occurs particularly in habitats with strong fluctuations in environmental conditions. In the case of sexual reproduction, on the other hand, there are almost always separate sexes; hermaphroditic species that are able to fertilize themselves are only found in one genus within the Arthrotardigrada.
The eggs of the heterotardigrada are smooth and are either laid free by the females or in the old, shed skin, the exuvium . The development to the adult animal is considered to be direct, even if the presence of different molting stages is occasionally taken as an indication of an indirect development. The hatching young usually do not yet have an anus, so that waste materials can only be released together with the cuticle during the first moult, and they also lack the sexual opening (gonopore); the legs often have two fewer claws than in the adult animal. After the first moult, the anus comes to light and the full number of claws appears; the gonopore, on the other hand, only develops after the second molt. In Batillipes nourrevangi there is another molting stage; In the parasitic species Tetrakentron synaptae , on the other hand, the hatching young animals are equipped with all adult characteristics.
The lifespan of most Heterotardigrada species is unknown, but is thought to be between a few months and one to two years. cryptobiotic periods in which the animals do not age can increase the actual lifespan of terrestrial species of the order Echiniscoidea by years; for the mostly marine Arthrotardigrada this possibility does not exist.
Tribal history
Modern forms
The relationship to the other large tardigrade class, the Eutardigrada, is currently still unclear. It is often assumed that marine predecessors of the order Arthrotardigrada first developed the land and freshwater forms of the order Echiniscoidea and here in particular the family Echiniscidae, from which the Eutardigrada developed:
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This would mean that the Heterotardigrada do not form a natural group, since individual species, for example from the Echiniscidae family, would be more closely related to the Eutardigrada than to other Heterotardigrada. This hypothesis is supported by the fact that the heterotardigrada still have a relatively large number of primitive features, i.e. features that are regarded as characteristic of the ancestral form of all tardigrades.
While the above scenario seems very plausible, it could not be confirmed by preliminary molecular genetic studies; Instead, these suggest that the Heterotardigrada are a monophyletic taxon, so without exception include all descendants of the last common ancestor of the group. This hypothesis is shown in the following diagram:
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Fossil forms
It is very likely that the Heterotardigrada conquered the country very early, presumably in the geological era of the Paleozoic Era , even if there are no fossil finds from this period that can be clearly assigned to the Heterotardigrada .
A poorly preserved young animal has been preserved in Cretaceous amber , which can possibly be assigned to the Heterotardigrada, but otherwise does not provide any information on the tribal history of this taxon .
Systematics
The monophyly of the heterotardigrada is, as mentioned, controversial, that is, it is currently unclear whether all species taken together form a natural kin and not instead some forms are more closely related to Eutardigrada species.
Classically, one differentiates between two very different orders :
- The Arthrotardigrada come except Styraconyx hallasi exclusively in the sea before and are often seen as characteristic of the ancestors of all water bears. They are characterized by a particularly large number of primeval features and are characterized by numerous head structures such as cirri and clavae, which are also used to further subdivide the group into families. Their four to six claws often do not start directly at the end of the leg, but are located at the end of thin "toes". Most genera are monotypical , i.e. contain only one species , which is sometimes taken as an indication of their advanced age.
- The Echiniscoidea live mostly in fresh water or on land, rarely in the sea. The former forms are characterized by hardened rear and occasionally abdominal cuticle carapaces, sclerites and sternites. Their exact arrangement and structure is an important aid in the further classification of the Echiniscoidea. Head attachments are often present, but usually not particularly noticeable; the up to 13 claws always arise directly at the end of the leg.
In both orders, the fine structure of the claws and the structure of the pharyngeal muscles play a major role in further classification.
It is unclear whether the two orders are monophyletic groups; as already mentioned, it is often assumed that the Echiniscoidea arose from the Arthrotardigrada. In this case, the Arthrotardigrada family Stygarctidae is often cited, which could be particularly closely related to the Echiniscoidea:
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If this assumption is true, the Arthrotardigrada would be a paraphyletic group, that is, a subgroup would be more closely related to species outside the taxon than to other Arthrotardigrada. Cladist systematists would then no longer recognize them. However, the first molecular genetic studies currently speak against this view.
literature
- IM Kinchin: The biology of tardigrades. Portland Press, 1994.
- A. Jörgensen, R. Kristensen: Molecular Phylogeny of Tardigrada - investigation of the monophyly of Heterotardigrada. Molecular Phylogenetics and Evolution, 32 (2), 2004, p. 666.
Web links
- Species list for marine heterotardigrada (English)
