Corset animals
| Corset animals | ||||||||||||
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Corset animal Pliciloricus enigmatus |
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| Loricifera | ||||||||||||
| Kristensen , 1983 | ||||||||||||
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A corset animal (Loricifera) is the name given to an animal strain that today is mostly attributed to the molting animals ( Ecdysozoa). The scientific name of the taxon is derived from the Latin lorica ("armor, corset") and ferre ("to wear"), so it can be translated as "corset bearer". Their closest relatives are probably the hookweed (Kinorhyncha) and priapworms (Priapulida), with which they are grouped together in a taxon Scalidophora .
Since the tiny animals attach themselves extremely tenaciously to grains of sand or other particles on which they live, to this day they can only be isolated by immersion in fresh water. The osmotic shock causes a separation from the substrate, but also kills the animals, so that with the exception of a single larva no animal has been examined alive so far. About physiology , nutrition, locomotion, behavior or development of animals, therefore, is virtually unknown. Molecular genetic data are also not yet available; at the beginning of the 21st century, for example, not a single gene was sequenced.
Due to the named circumstances and because of the relatively low population density, the corset animals were not scientifically described until 1983 , although they had been known from the French Atlantic coast since the 1970s ; the first species was named Nanaloricus mysticus . Since then, they have been continuously researched, and two other genera have been discovered. There are only a few biologists in the world who specialize in this group of animals.
construction
With a size of only 100 to just under 500 micrometers, the mirror-symmetrical corset animals are more similar to single-cell organisms , but they have up to 10,000 cells. Your body can be divided into a spiked head fachsprachlich Introvert is called, a short neck, a breast part of the thorax is called, and a hull that tapered plates pointed by a tank from the mouth end of the eponymous Lorica is surrounded, . The head, neck and part of the thorax can be drawn into it by sliding into one another.
On the head sits the stretchable, tapering mouth cone, followed by up to 400 spines called scalids , which are equipped with their own muscles and probably serve for sensory perception and locomotion. They are usually arranged in nine rings, of which the first with the clavoscalids are directed to the front, the other eight with the spinoscalids are directed backwards and are somewhat reminiscent of the rods of an umbrella.
Body wall and muscles
The body wall consists of a single layer of cells, the epidermis and an overlying cell-less outer skin, the cuticle , which in turn consists of three layers, which are called the epicuticle , intracuticle and procuticle . The epicuticle is hardened ( sclerotized ) in most places and thus forms either six or twenty-two longitudinally oriented plates, depending on the type, which wrap around the trunk as a corset; In contrast, the epicuticle is very flexible between the plates and functionally acts as a hinge there. On the Lorica plates there are mostly all sorts of pores and ridges, towards the end of the mouth they run out in some species into forward-facing thorns.
The musculature is striated across and consists exclusively of individual fibers, which can be oriented lengthways ( longitudinal ), from the stomach to the back ( dorsoventral ) or diagonally; ring fibers also occur; continuous muscle layers, however, do not exist. To retract the head and neck into the lorica, the animals use two groups of special introvert retractor muscles, while stretching is done by hydrostatic pressure.
Digestive, excretory and reproductive organs
Special respiratory or circulatory organs are not necessary in the corset animals because of the small size of the animals, nor are they available. The digestive tract begins with the mouth at the top of the cone of the mouth, followed by a buccal canal , which consists of a flexible tube and into which the salivary glands open. The adjoining throat is formed by epithelial muscle cells. Its interior or lumen has a three-beam cross-section and thus enables efficient suction. Connected by a short "duodenum" or esophagus , it is followed by the midgut , interspersed with numerous indentations and protuberances, the microvilli , in which nutrient absorption probably takes place; Waste materials are passed through a short rectum to the terminal anus at the rear end of the body.
The paired gonads are sack-like with the excretory organs, the protonephridia , which consist of simply flagellated cells, the solenocytes , and, like the priapses , are connected to the outside world by a common urogenital tract that opens next to or in the anus.
Nervous system
The nervous system of the corset animal consists of a relatively large brain located in the introvert, from which nerves extend into the scalids. A ring of ten ganglia is the starting point for a total of ten nerve fibers that run longitudinally backwards; two of them, which are located in the middle of the abdomen, are significantly larger than the others and also have ganglia at regular intervals. They supply nerves to the chest and probably also to the trunk. At the rear end of the animals there are sometimes sensory bristles arranged like a rosette on the rear side.
distribution and habitat
Corset animals live as part of the sand gap fauna ( mesopsammon ) in the sea, so they are found attached to small grains of sand or other particles in the silt and mud of the seabed. So far they have mostly been found around 350 to 400 meters below the surface of the water, but can also be detected in the deep sea; one species is known from a depth of more than 8000 meters. The late discovery and difficult research of the corset animals is probably not due to their rarity: today it is assumed that they represent one of the dominant groups of the meiofauna . The larvae of some species live in the marine plankton .
The animals are presumably distributed worldwide. Finds have been reported from the Danish coast, from Brittany , the Azores , from the North Atlantic off Florida and North Carolina , the Arctic Ocean and the Coral Sea of the South Pacific.
In addition, three new species from the genera Spinoloricus , Rugiloricus and Pliciloricus were discovered in 2010 , which live in oxygen-free , salty environments in sediments of the Mediterranean . These are the first known multicellular cells that can exist permanently without oxygen.
Way of life
There is practically no knowledge about the way of life of the corset animals. From the structure of the mouth cone and the structure of the throat as a suction throat, it is sometimes concluded that the animals live predatory or as ectoparasites and suck the nutrient-rich body fluid from their victims. One find is known in which an animal was found attached to a benthic copepod , further finds have not yet been proven. Nutrition by bacteria cannot be ruled out either.
Reproduction and development
Male and female gonads are always found separately on different individuals. While in some species the sexes can be distinguished from one another by the structure of the foremost row of spines, this is not possible in others. The meaning of the difference in the clavoscalids is unclear, but may be related to the use of these structures in the search for a partner.
In the female gonads, the ovaries, only a single, very large egg develops. Since a kind of sperm store was found in the females, it is assumed that fertilization takes place internally, but nothing is known about the further embryonic development.
Corset animals go through a larval stage known as Higgins larva. This has large, leaf-shaped "toes" at the rear end, which are either used together with two to three spikes on the belly side as paddles for locomotion in the plankton or they have adhesive glands with which the animals attach themselves to the sediment.
Even the larvae already have an introvert, but the cone of the mouth is not yet reinforced with rigid stilettos as in the adult animals. The adult stage is reached after several moults via a metamorphosis , which is sometimes preceded by the formation of a resistant cyst .
The phenomenon of neoteny can be found in some larvae, i.e. they become sexually mature while still in the larval state and reproduce parthenogenetically , i.e. without a fertilization process. To do this, they form a cyst in which they molt. In ovarian now more eggs from which four to twelve daughter larvae emerge that gestaltlich can not be distinguished from the resulting in sexual ways larvae develop. This then dies, whereupon the daughter larvae are released from the skinned shell of the animal. For their part, they are capable of single-sex reproduction, which is why one speaks of a parthenogenetic life cycle .
Tribal history
So far no unambiguous fossil finds can be assigned to the corset animals. Comparisons with modern taxa see their closest relatives quite clearly in the Hakenrüsslern (Kinorhyncha) and Priapswürmern (Priapulida), with which they form the taxon scalidophora. The three groups share numerous characteristics, such as the outer skin reinforced with chitin , the chitin-containing bristles or spines located on it, dimples (flosculi) serving for sensory perception and two groups of introvert retractor muscles.
Which of the two animal phyla represents the evolutionary sister group of the corset animals is, however, much more controversial; all three possible combinations have been proposed and justified by zoologists. The presence of a corset formed by the cuticle, which is present in the latter in the larval stage, suggests a closer relationship between corset animals and priapworms, whereas the cone of the mouth common to them, which can be extended but not everted, suggests a close relationship between corset animals and hookweed. The third alternative, a sister taxon ratio between hookweed and priap worms with the corset animals as the outer group, is justified by the pharyngeal tissue of the embryonic mesoderm occurring in the first two taxa . The thesis that the corset animals arose from priapworms through pedomorphosis , i.e. through retention of larval characteristics in the adult stage, was considered, but has so far not been accepted.
Embryo fossils of the species Markuelia hunanensis have been known from Hunan in southern China since 2004 . They come from the geological epoch of the middle to late Cambrian about 500 million years ago and are regarded by a cladistic analysis as representatives of the lineage of the Scalidophora, so they cannot be assigned to any of the three modern groups that make up this taxon. Due to the unique conservation conditions in fine-grain calcium phosphate , the embryonic development of Markelia hunanensis is quite well known, which results in the unusual situation that more is known about the early development of this species, which has been extinct for half a billion years, than about that of its modern relatives.
Markuelia hunanensis was possibly segmented - if this finding and at the same time the cladistic analysis were to be confirmed, the loss of segmentation would probably be a common derived feature, a synapomorphy , of the priapic worms and the corset animals and would thus underline their sister group relationship.
Threadworms (Nematoda) and string worms (Nematomorpha), with which the Scalidophora form the taxon Cycloneuralia, are also related to the corset animals . All of them are classified in the molting animals (Ecdysozoa), to which the Panarthropoda with the arthropods (Arthropoda) belong as the most important group.
Systematics
Today it is considered undisputed that corset animals form a natural family group, i.e. in the sense of cladistics are a monophyletic taxon and thus include all descendants of their common ancestor. Common derived characteristics of the group, synapomorphies, are, for example, the protonephridia sacked together with the gonads, the special spinoscalids on the head, which have their own muscles, and the "toes" of the Higgins larvae.
At the beginning of the 21st century, almost one hundred species are known, of which hardly more than ten have so far been scientifically described. They are divided into two families in the order Nanoloricida:
- The Pliciloricidae are characterized by twenty-two only slightly hardened ( sclerotized ) Lorica plates, on which there are no thorns. The anus and genital opening are on the stomach side. Male and female of the Pliciloricidae cannot be distinguished externally; their larvae have toes with glue glands. Two genera have been described so far , Pliciloricus and Rugiloricus .
- In the Nanaloricidae the strongly hardened Lorica consists of six plates, of which a total of fifteen hollow thorns point to the front. The rear position of the anus and genital opening, the different appearance of males and females and the paddle toes present in the larvae also set them apart from the Pliciloricidae. All Nanaloricidae belong to one genus Nanaloricus .
For the species not yet described, two new families should possibly be established.
literature
- Richard C. Brusca, GJ Brusca: Invertebrates. Sinauer Associates, Sunderland Ma 2003, p. 348. ISBN 0-87893-097-3
- S. Lorenzen: Loricifera. in: Wilfried Westheide, R. Rieger: Special Zoology. Part 1. Protozoa and invertebrates. Gustav Fischer, Stuttgart 1996. ISBN 3-437-20515-3
- Edward E. Ruppert, R. Fox s., RD Barnes: Invertebrate Zoology, A Functional Evolutionary Approach. Brooks / Cole, Pacific Grove 2004, p. 776. ISBN 0-03-025982-7
References
- ↑ Roberto Danovaro et al. (2010). The first metazoa living in permanently anoxic conditions. BMC Biology 8 (30): 30. doi: 10.1186 / 1741-7007-8-30
- ↑ Andy Coghaln: Zoologger: The mud creature did lives without oxygen on: NewScientist from 7 April 2010
Scientific literature
- XP. Dong, PCJ Donoguhe, H. Cheng, JB Liu: Fossil embryos from the Middle and Late Cambrian period of Hunan, south China. in: Nature . London 427.2004, p. 237. ISSN 0028-0836
- RP Higgins, RM Kristensen: Loricifera. in: Robert P. Higgins, H. Thiel (Ed.): Introduction to the Study of Meiofauna. Smithsonian Institution Press, Washington 1988, p. 319. ISBN 0-87474-488-1
- RM Kristensen: Loricifera, a new phylum with Aschelminthes characters from the meiobenthos. in: Journal for Zoological Systematics and Evolutionary Research. Parey, Hamburg 21.1983, p. 163. ISSN 0044-3808
- RM Kristensen: Loricifera. In: FW Harrison, EE Ruppert (Ed.): Aschelminthes. Microscopic Anatomy of Invertebrates. Volume 4. Wiley-Liss, New York 1991, ISBN 0-471-56103-7 , p. 351.
- RM Kristensen: An Introduction to Loricifera, Cycliophora, and Micrognathozoa. in: Integrative and Comparative Biology. Lawrence Kan 42.2002, p. 641. ISSN 1540-7063
Web links
- Extensive bibliography on the corset animals ( Memento from February 16, 2008 in the Internet Archive )
