Eutardigrada
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The Eutardigrada tardigrade Hypsibius dujardini |
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| Eutardigrada | ||||||||||||
| Marcus , 1929 | ||||||||||||
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As eutardigrade refers to a class of water bears (tardigrades), which in contrast to other water bears class, the heterotardigrada missing noticeable features such as head attachments or hardened carapace. For this they have special excretory organs, the Malpighian glands .
The taxon described for the first time in 1929 by the German zoologist Ernst Marcus comprises mostly freshwater and land (limno-terrestrial) forms; however, some species can also be found in salt water.
features
Tardigrade represent an overall very homogeneous group, the species of which differ anatomically only in details. In the following, therefore, only the peculiarities that are particularly characteristic of the Eutardigrada are described; a more detailed introduction to the body structure of animals can be found in the main article on tardigrade. As with all tardigrade animals, the average body size is around 100 to 150 micrometers, while a species from the genus Macrobiotus can grow up to 1.5 millimeters long.
Cuticle
The non-cellular outer skin (cuticle) of the Eutardigrada is very often completely smooth, occasionally granular, that is, of a grainy appearance, and sometimes also covered with small cusps, the tubercles . But it never has conspicuous hardening like the armor plates called sclerites , which are found on the back of the heterotardigrada; thorny cuticle processes are also by far not as distinct as in that group. Instead, the cuticle is very often colored by pigments.
Head and body attachments
The eutardigrada do not have long head appendages like the filamentous cirri of the heterotardigrada, but there are often knob-shaped elevations, the papillae or papules and lobed structures arranged around the mouth opening . The basic leg anatomy is that of all tardigrade; however, the claws are always located directly at the end of the leg and are never converted into adhesive discs. They always consist of a pair of double claws, with each double claw consisting of a common base, the claw base and two arms, the primary and the secondary arm, the exact shape and arrangement of which are important features in species identification. Around the base of the claws there is often a thickening consisting of a cuticle, which is called the lunulus .
Body cavity and digestive tract
The body cavity known technically as hemocoelom is usually somewhat more spacious in the Eutardigrada; like all tardigrade it is crossed by the digestive tract. This begins with the oral cavity, which in some species is divided into a rigid front and an elastic rear part. The border between the two regions lies approximately at the level of the stiletto holders, which fasten the two often powerful and sometimes curved stilettos located on the side of the oral cavity. Towards the front end of the oral cavity one can often find two ligaments with small "teeth", the mucrones , behind which elevations running transversely to the longitudinal axis of the body can occur on both the back and the abdomen, which are also identifying features when determining the species. Occasionally the oral cavity is reinforced by a ridge running in the middle of the abdomen.
Depending on the taxon, the muscular throat is either spherical, egg -shaped or pear-shaped and reinforced by placoids : These are short strips of hardened cuticula, which usually run in three pairs of longitudinal rows from the front end of the throat to the rear end. In contrast to the heterotardigrada, these longitudinal stripes are not continuous, but interrupted at regular intervals. The first three placoids in a row are usually larger than the following and are referred to as macroplacoids, followed by one or two smaller microplacoids. The intestine is always smooth and has no protrusions called diverticula .
Malpighian glands
A notable peculiarity of the Eutardigrada are the Malpighian glands, which are normally used for excretion. Most species have both a back (dorsal) and two laterally arranged (lateral) glands, which open at the junction of the midgut and hindgut. They consist of three to twelve cells and are usually divided into a distal (far from the bowel) and a proximal (near the bowel) segment. Both segments often have the same number of cells, with 3 + 3 and 6 + 6 being combinations that are often found. The task of the distal segment is to produce the primary urine; This probably happens through active material transport across the separating membrane, since there is no evidence of ultrafiltration of the body cavity fluid. The proximal segment then modifies the primary urine by selectively reabsorbing water or nutrients and releasing other pollutants or waste.
A specialty can be found in the marine genus Halobiotus , whose Malpighian glands are greatly enlarged. It has probably returned to the sea secondarily and probably uses its glands to regulate the salt balance ( osmoregulation ). Limno-terrestrial species are also dependent on actively controlling their water uptake, since their body fluids are anisotonic to fresh water, i.e. they do not have the same salt content. The widespread species Mil Magnesium tardigradum probably has another Malpighian gland, which is located on the ventral side and apparently plays a role in sexual reproduction; this is sometimes taken as an example of exaptation , the use of a structure originally created for another reason for a new function.
Reproductive organs
In contrast to the Heterotardigrada, the females of the Eutardigrada do not have a separate sexual opening. Instead, the fallopian tube opens into the hindgut, which can thus be called a cloaca . This opens to the outside world in a slit running transversely to the body axis (transversely) in front of the fourth pair of legs.
Some females have an unpaired sperm store in which the males' sperm is stored after copulation.
distribution and habitat
Eutardigrada are found worldwide on all continents and are also widespread in the sea, although they have probably only returned to this habitat in a secondary manner. But they are particularly common on land, especially in the leaf litter, the uppermost centimeters of the forest floor or in mosses (bryata). Different species prefer different habitats within a moss pillow; as a result, different species can often be found in different zones in a piece of moss.
Way of life
The way of life of the Eutardigrada does not differ too much from that of the Heterotardigrada, so that at this point reference is again made to the main article on the tardigrade for the basics. Parasitism is not widespread among the Eutardigrada, which is not surprising in view of their mainly terrestrial habitat: Eutardigrada, like all tardigrades, are dependent on a thin film of water when active, so that when drought occurs, either the connection with the host is lost or the individual in question becomes uncontrolled would dry out.
For this, the group includes cyclomorphosis, the appearance of different body shapes (morphs) in different seasons, cyst formation and cryptobiosis, the most extreme form of adaptation to adverse environmental conditions, in which the affected individuals pass into a near-death state without metabolic activity, the one after his Form is called a barrel . Since cryptobiotic times can also be found within the heterotardigrada, this special adaptation of the tardigrade is described in the main article.
The reproduction of the Eutardigrada can occur both asexually and sexually. In the first case, individual females reproduce parthenogenetically , i.e. without the participation of males. A distinction is made between two different forms, both of which occur within the Eutardigrada:
- In meiotic or automictic parthenogenesis, the meiosis called meiosis occurs , in which the number of chromosomes in the progenitor cells of the egg cells is halved; However, this is followed by a chromosome duplication before the normally subsequent second division, a mitosis , so that after the overall process consisting of meiosis, duplication and mitosis, the number of chromosomes has remained unchanged. The young animals then develop from the unfertilized eggs with a double set of chromosomes (diploidy).
- In ameiotic or apomictic parthenogenesis, females who reproduce sexually and those who reproduce asexually are distinguished by their number of chromosomes: The latter are often tri-, sometimes even tetraploid, that is, they have three or four sets of chromosomes instead of two and can match therefore do not reproduce sexually; their eggs are therefore created by simple mitotic division, in which the number of chromosomes remains unchanged.
In sexual reproduction, the sexes are almost always separated, although hermaphroditic species are also known in two genera, Isohypsibius and Amphibolus , which have an unpaired gonad that simultaneously functions as testicle and ovary. However, there is a very high probability that self-fertilization is a derived, not an original, characteristic of the group.
If, as is usually the case, the sexes are separate, they can be distinguished by the fact that the males have a separate sexual opening , the gonopore . In adult males, the claws of the first pair of legs are often modified compared to the same structure as the females and sexually immature young animals. Males are also often slightly smaller than females, although adult males and younger females cannot be distinguished according to this criterion.
The eggs of the Eutardigrada can either be smooth or ornamented by a variety of structures, all of which can be used to determine the species. Smooth eggs are usually deposited in the old outer skin (cuticula) that is shed during the molting process, which is now referred to as the exuvium . Females of the genera Pseudobiotus and Isohypsibius often drag this exuvium around with them for a while, which can be interpreted as a kind of primitive brood care. Eggs with a sculpted surface, on the other hand, are usually released without an additional protective cover. Some freshwater species from the genus Murrayon lay their eggs in the empty exoskeletons of water fleas (Cladocera), ostracods (Ostracoda) and insects (Insecta).
The development from the hatched young to the sexually mature individual is always direct, with no intervening larval stage. The normal lifespan is between several months and one to two years, but can be extended to several years, in isolated cases even decades, by cryptobiotic periods in which the animals do not age.
Tribal history
Modern forms
The exact phylogenetic relationships of the Eutardigrada to the other class of tardigrade, the Heterotardigrada , are still unclear. A frequently held view assumes that the Eutardigrada have their roots in an already terrestrial order of the Heterotardigrada, the Echiniscoidea , in particular the Echiniscidae family. This would result in a development from the marine Arthrotardigrada, the other heterotardigrada order, via the Echiniscoidea to the Eutardigrada:
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However, preliminary molecular genetic studies have not been able to confirm this hypothesis and instead speak in favor of an independent development of the Eutardigrada:
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It is very likely that the marine species of the Eutardigrada from the genus Halobiotus have returned to the sea as a secondary feature . This is supported by the greatly enlarged Malpighian glands of this genus, which probably only took on a prominent role in osmoregulation on a secondary level.
Fossil forms
A fossil species, Beorn leggi , which was found in Cretaceous amber from Canada , can already be classified in the Eutardigrada, as well as finds from the US state of New Jersey of about the same age .
Apart from the fact that the group has apparently remained extremely stable morphologically for more than 65 million years, none of the finds shed any light on the natural history of the Eutardigrada.
Systematics
The Eutardigrada are considered to be a monophyletic taxon , which means that it is assumed that all the added species go back to a common parent species and that all descendants of this parent species are also included in the Eutardigrada.
In the class there are two orders with a total of six families that are primarily defined by the fine structure of the bone claws and the pharyngeal muscles:
- The Parachela are the larger group and include both marine and limno-terrestrial forms. The larger primary claw does not arise directly at the base of the leg, but from the smaller secondary claw. The throat usually has a spherical or egg-shaped appearance and has stabilizing longitudinal stripes (placoids). According to the structure of the claws and the details of the morphology of the pharyngeal muscles, a distinction is made between five families, the ancestral relationships of which are still unclear:
- In the land and freshwater living Macrobiotidae , the leg claws are arranged symmetrically in relation to the median plane of the leg; the primary claws are both inside, the secondary claws both outside, the pattern is described accordingly by the formula 2-1-1-2.
- In the terrestrial Calophysibiidae the two small pairs of leg claws are shaped similarly, but arranged asymmetrically in relation to the leg median plane; the primary and secondary claw branches follow the sequence 2-1-2-1 from the outside in. In contrast to most other species, Calophysibiidae species do not have a claw base.
- In the marine Hypsibiidae , the two pairs of claws on each leg are not only arranged asymmetrically, but also differ in size and shape. The claw branches alternate (2-1-2-1).
- The limno-terrestrial Eohypsibiidae are characterized by a three-part claw morphology, in which the claw base, first and secondary claw are separated from each other by partitions (septa). Inner and outer pair of claws are always the same size, the sequence of primary and secondary rest is described by the formula 2-1-2-1; however, it can happen that the inner pair of claws is rotated 180 degrees, resulting in the symmetrical sequence 2-1-1-2.
- In the land-living Necopinatidae , the greatly reduced claws are only present on the first pair of legs.
- The Microhypsibiidae live on land and fresh water; their claw pairs alternate (2-1-2-1), but are similar in size and shape, the claw base is very narrow.
- One genus, Apodibius , could not be assigned to any of the aforementioned families.
- The Apochela form the second order and are found exclusively on land. All species are formally assigned to a single family Milnesiidae and are characterized by the fact that both the long, slender, primary and the short, stocky, secondary branch of each double claw arise directly at the end of the leg; the latter often has two additional hooks. The throat is pear-shaped in the Apochela and does not reveal any supporting structures called placoids.
Apochela and Parachela also differ in details of the sensilla morphology.
literature
- IM Kinchin: The biology of tardigrades. Portland Press, 1994
- DR Nelson, NJ Marley: The biology and ecology of lotic Tardigrada. In: Freshwater Biology. 44. 2000, p. 93
