Paragordius tricuspidatus from southern France
|Vejdovský , 1886
The string worms (Nematomorpha) are an animal strain of the molting animals (Ecdysozoa). Its scientific name is composed of the ancient Greek words νήμα nēma "thread" and μορφή morphē "shape". The more than 320 species of this group live mainly in fresh water, but some are also found in the sea. The pale white to grayish black, sometimes also brownish to reddish colored worms are usually very long and extremely thin; extreme lengths of up to two meters can be achieved as with Gordius fulgur ; however, the animals only have a maximum diameter of three millimeters.
The juvenile forms of string worms are parasitic . They have a drilling apparatus with which they can drill into the host (usually an insect ). The adult animals only leave the host to lay their eggs and can be found at this time as a clump of worms, especially on streams.
The string worms belong to the groups of animals that have so far been largely ignored in research. Accordingly, little is known about this group of animals in comparison to other taxa . A string worm was first clearly mentioned in the Historia Animalium (1551–1587) by Conrad Gessner , where the string worms are called water calf or in Latin Vitulus aquaticus according to the popular name . At this point in time, the typical two-part naming scheme, with the well-known 10th edition of the Systema Naturae by Carl von Linné in 1758 , did not yet exist. Within the worms , Linné classifies an animal with the name Gordius aquaticus , named after the Gordian knot . He was referring to the comparison made at the beginning of the 17th century by Aldrovandus of a ball of worms with the famous knot in Greek mythology .
In the following years, both free- living string worms and parasitic forms of insects were assigned to the genus Gordius ; the latter was split off from the free- living Gordius species as Filaria in 1788 . However, it was only through a series of new observations that it could be clarified that the intestinal filaments were identical to the free-living worms. For example, F. Dujardin was able to observe in 1842 how a string worm passed from an insect into the open water. He named this newly discovered animal Mermis and explained that at least this worm has a parasitic and a free- living development phase . At that time it was still unknown that the Mermis did not belong to the family of the Gordius species ; it was not classified as a nematode until 1886 (today in the Mermithidae family). In 1849 and 1851 E. Grube and J. Leidy were also able to discover the morphologically different larvae of the string worms. Around the same time, the first articles on the internal anatomy of animals were published (F. Dujardin 1842, AA Berthold 1843, G. Meissner 1856).
In 1847 Friedrich Heinrich Creplin described a second genus of string worms, which he called Chordodes ; Addison Emery Verrill (1879) was the first to describe a sea-living string worm, which he named Nectonema agile . These sea worms were combined in 1887 by F. Vejdovsky with the "Gordiacea" to form the Nematomorpha. At the end of the 19th century, a number of new species were added, which were found mainly through expeditions to different parts of the world. Camerano, one of the group's most successful editors, introduced two new genera, Parachordodes and Paragordius , in 1897 ; all other generic names were added in the course of the 20th century.
More precise knowledge of the anatomy of animals was only gained in the second half of the 20th century, when various string worms were examined histologically and with the help of transmission electron microscopy (TEM). The scanning electron microscopy (SEM) was an indispensable tool in particular for the identification and determination of the animals. Among the most famous Nematomorphenforschern today include the at the Zoological Museum of the University of Hamburg working Andreas Schmidt-Rhaesa who wants to educate especially the evolutionary relationships within the string worms, and Ben Hanelt (Lincoln, USA), Fred Thomas (Montpellier, France) and Cristina de Villalobos (La Plata, Argentina).
Anatomy of the string worms
Adult worms anatomy
The string worms are very similar in structure to the roundworms , whose sister group they represent. The body is elongated, swiveling and unsegmented. In extreme cases, the body length is up to two meters; in most species, however, it is an average of five to ten centimeters. The diameter is as low as 0.5 to 3 millimeters.
- The body length of the females exceeds that of the males in almost all species.
- In the females of the Nectonema species, the rear end is rounded, in the males the rear end narrows and is bent downwards. In both sexes, the genital opening is at the terminal end.
- In the females of the freshwater species, too, the genital opening is always at the rear end, but in the males it is on the belly side. In about half of the species, the rear end of the males is also divided into two lobed runners called tail lobes. In addition to the structures of the outer skin layer, the cuticle, the differences between these praises are one of the most important features for differentiating between species.
The front end of most string worms is rounded, but it can also be "cut off" or pointed. Many species are characterized by a white front tip and a dark ring following it, behind which the actual body color begins.
- The cuticle, which only contains the polysaccharide chitin in larvae , is only very thin in the parasitic juveniles, but quite thick in the free-living adults. The change occurs via a molt shortly before leaving the host animal. The fibers of the cuticle, which are made up of collagen , run in a criss-cross helical manner and thus compensate for the lack of circular muscles. The angle of these fibers to the longitudinal axis of the body is about 55 °. A total of about 35 layers of these cuticular fibers lie on top of one another. The cuticula itself is occupied by different structures (areoles, superareoles, crown areoles, megaareoles), which are an important feature for differentiating between species.
- The epidermis is single-layered and protrudes through small cell extensions ( microvilli ) into the cuticle. Lateral epidermal ridges, as they occur in roundworms, are missing here, instead there is a back (dorsal) and an abdominal (ventral) ridge in the marine Nectonema species, which are covered with bluish-gray "floating bristles". The species living in freshwater have only one epidermal ridge on the abdomen.
- The longitudinal muscles are also single-layered and consist of narrow muscle cells. The contractile part of these cells is on the outside, the cytoplasm with the cell nucleus on the inside (towards the inside of the body).
A hard skeleton does not exist in string worms; its function is taken over by a so-called hydrostatic skeleton , which is formed by high fluid pressure in the body cavity, the pseudocoel. The cuticle can thus act as an opponent ( antagonist ) of the longitudinal muscles and in this way allows the winding movements with which the worms move.
The nervous system of the string worms consists of a nerve ring around the intestine, which forms the brain of the animals, located at the head end of the animals, an abdominal cord from which nerve fibers extend to the muscle cells and a ganglion in the area of the anal opening (cloacal ganglion ). Especially at the front and rear of the animals there are simply built sense organs , the sensilla . The perception of light is made possible by dimples lined with a black pigment layer below the cuticle, which is transparent at this point.
The intestine is largely regressed in all species and in all life stages and is not used for food intake; The mouth and anus are mostly non-existent. It is not yet clear whether the intestine still has an important function; It is probably used for excretion in some types , and nutrients are probably also stored there. The larvae and young worms are fed by the epidermis, adult worms do not eat. Special excretory organs do not exist in the string worms.
Between the muscle layer on the outside and the intestine and its hanging straps, there is a cavity filled with fluid and a matrix made up of collagen fibers, which is known as the body cavity. Since this is not covered by a single layer (commonly referred to as a coelom ), this form of the body cavity is called a pseudocoel . In freshwater species, the body cavity is divided into “pockets” one behind the other by lateral tissue flaps, in which germ tissue is located. The animals are of separate sexes and, with the exception of the Nectonema species, have paired ovaries and testes . In both sexes, the sex ducts open into the cloaca , i.e. a joint body opening with the intestine.
Anatomy of the larvae
The larvae of the string worms have a completely different structure than the adult worms. They are usually between 50 and 150 micrometers long and consist of a front section with an evertable mouth cone and an abdomen, which contains a large salivary gland in addition to a remnant of the intestine. These two body sections are separated from each other by a membrane (septum).
The salivary gland is connected to the trunk-like cone of the mouth via a long passage. This can be everted out by means of hydrostatic pressure, which is generated by the muscles in the front body, and has a drilling device at the front end as well as lateral spikes that allow an anchoring in the tissue of the host that is necessary for penetration.
distribution and habitat
String worms have been detected on all continents with the exception of Antarctica . The species known to date probably only represent a part of the species that actually exist, and further discoveries are expected, especially from the tropics and subtropics. About 100 species are known in Europe, with individual representatives also being found on the associated islands such as Madeira , Tenerife or the Shetland Islands . Most of the African species come from the Republic of the Congo , large parts of the continent have not yet been collected on these animals. Accordingly, the approximately 70 known species probably only form part of the African spectrum. Asia is also largely uncollected, with around 100 known species. So far seven species have been found in Australia and six in New Zealand, around 70 species are known from South and Central America and only 16. There are also four marine species of the genus Nectonema , which have been found on different coasts, in the Mediterranean Sea and the Northwest Pacific , on the coasts of New Zealand and in the South Atlantic.
The freshwater species live in small puddles, ponds, brooks, rivers and even lakes as well as in moist soils. It can be assumed that their distribution in these waters depends less on the animals' preferences than on the waters into which they are brought by their hosts. Desiccating small bodies of water such as puddles could represent a dead end for the string worms; it has not yet been clarified whether successful mating and infection of hosts is possible here.
The marine species live free swimming (pelagic) mostly in the intertidal zone.
Way of life, reproduction and development
Adult string worms only leave their hosts to reproduce and do not eat any food at this adult stage. The very active males then seek out the females, who usually hardly move from their place of exit. When two partners have found each other, the male wraps itself in tight loops around the rear end of the female, creating real mating balls. The transmission of uncultivated sperm or sperm packets (spermatophores) takes place in the string worms by copulation ; this is made possible by a foldable bundle of bristles ( cirrus ) on the cloaca. The sperm are usually still stored in the female before the internal fertilization takes place. While the males usually die immediately after copulation, the females of the string worms lay several 10,000 eggs, each with a diameter of 40 to 50 micrometers, in long spawning cords that are wrapped around plants or other underwater objects. The females then lay around these spawning lines and remain in this position. The marine species lay their eggs one by one in the open water.
The hatching larvae either passively enter the host when they eat, or they dig into the host at joint membranes or other soft spots. If the larvae cannot find a suitable host, they form a permanent stage (cyst) and can thus survive dehydration or other unfavorable environmental conditions for over a month. As a cyst, they can only enter a host passively, i.e. when they eat.
In a suitable host, the larva is likely to change through molting, in which the hard parts of the front body are completely shed. The animal then grows up and mainly absorbs nutrients from its host's fat body through its skin . In the case of an unsuitable host (such as a snail ), cyst formation can occur again until the animal is eaten by a predatory insect such as a praying mantis . As a result, string worms can also parasitize animals that do not come into contact with water. The Nectonema species living in the sea apparently only parasitize in decapods , such as those found in the shrimp Pandalus montagui , the hermit crab Anapagurus hyndmanni and the crab Cancer irrogatus . The freshwater species prefer ground beetles (Carabidae), catching horrors (Mantoptera) and long- feeler terrors (Ensifera) as hosts , but they have also been found in dragonflies (Odonata), spiders (Araneae), harvestmen (Opiliones), millipedes (Myriapoda) and leeches (Hirudinea) found. The infestation of hymenoptera (Hymenoptera) was observed in the 19th century on various plant wasps ( Symphyta), an ant species (Formicidae) and a wasp species (Vespidae) and should be checked. So far, no string worms are known from butterflies (Lepidoptera).
Shortly before the end of youth development, the string worm influences its host in such a way that it compulsively seeks out water. Hormonal influences are also taken into account, as well as extensive dehydration by the parasite. In 2005, a group led by researcher David Biron found out more about the mechanism of action during a closer examination of the proteins formed in the locust brain. So stringworms form certain neurotransmitter- like substances and molecules that trigger programmed cell death ( apoptosis ) in nerve cells. The parasite also releases certain growth factors that directly affect the development of the host's brain . However, it appears that the host animal itself also increasingly produces certain proteins that are supposed to act as a defense against the parasite, since they are produced to a much lesser extent in non-infected animals. This discovery not only provides insights into parasite-host relationships, but also shows that string worms can gain direct access to the central nervous system of their hosts.
In the water, the sexually mature stringworms leave the host through the anus or the joint membranes after a last moult in order to find a sexual partner in the open water. The hosts mostly die after the worms leave, but some live on.
For many species, the life cycle is a little more complex and runs through an intermediate host different from the ultimate host. While the definitive host always belongs to the arthropods, vertebrates such as fish or the juvenile stages of amphibians can also be used as intermediate hosts .
Depending on the beginning of sexual maturity, the duration of a life cycle is between two months and more than a year.
String worms and humans
Systematics of the string worms
Systematic position of the string worms
The string worms represent the sister group of the roundworms (Nematoda). With these they share a number of features, including the structure of the cuticle and the absence of circular muscles and epidermal cells carrying cilia . The life cycle of the string worms is also identical to that of the most primitive group of roundworms, the Mermithidae , and can be seen as a common characteristic of the string worms and roundworms.
The priapulida, the corset animal (Loricifera) and the hookweed (Kinorhyncha) are classified in the further relationship of the roundworms and string worms . All these groups, which (with the occasional inclusion of the abdominal curls (Gastrotricha)) are summarized as Cycloneuralia, have a cuticle that is regularly replaced by a moult. The hormone responsible for this molting is ecdysone , which also induces molting in arthropods . For this reason, the latter are now often combined with the groups mentioned to form the molting animals (Ecdysozoa). This summary is also based on molecular data, but is still highly controversial. The next sister group of molting animals (with or without arthropods) are the belly curls.
Alternatively, in addition to the classification as molting animals, there is also the group of round worms (Nemathelminthes), which in addition to the above-mentioned groups (excluding the arthropods) also includes the rotifers (Rotatoria) and the scraper (Acanthocephala).
Internally, the string worms are classified into two taxa.
- The sea worms (Nectonematoida) contain only the four known species of the genus Nectonema , which are characterized by their abdominal and reverse floating bristles and a liquid-filled pseudocoel and all of which have a body length of about twenty centimeters. They only attack decapods.
- The horse hair worms (Gordioida) include all other groups and live in fresh water or in moist soil. In contrast to sea worms, they only have one epidermal ridge on the abdomen; their pseudocoel also contains mainly a matrix of connective tissue .
The following figure shows a variant of the string worm system, but the monophyly of many of these groups (especially the genera) is very controversial, so a phylogenetic representation is largely dispensed with (representation according to Schmidt-Rhaesa 2002).
Very little is known about the evolution of the string worms as well as of the other animal groups in close relatives of these animals. Fossil specimens do not exist with a few exceptions. The oldest known roundworms were discovered in amber, which is around 120 million years old; the oldest string worm comes from the lignite of the Eocene no more than 60 million years ago. Based on these findings, it can be assumed that the string worms already existed at least at the beginning of the Tertiary , but the actual origin must be much further back.
The way of life of the first string worms can be reconstructed very well above all by comparing it with the other animal groups within the molting animals (without arthropods). All closely related groups, with the exception of the roundworms, consist primarily of marine animals and microscopic animals, so it is reasonable to assume that the ancestor of the stringworms and the roundworms also lived in this way. The decisive evolutionary step was obviously the transition to parasitism, which above all led to an enlargement of the body.
- DJ Biron: Behavioral manipulation in a grasshopper harboring hairworm: a proteomics approach. in: Proceedings of the Royal Society. Serie B. London 2005. doi : 10.1098 / rspb.2005.3213
- J. Bresciani: Nematomorpha. in: Frederik W. Harrison, EE Ruppert: Microscopic Anatomy of Invertebrates . Wiley-Liss, New York 1991. ISBN 0-471-56842-2
- S. Lorenzen: Nematomorpha, string worms. in: W. Westheide, R. Rieger: Special Zoology. Part 1. Protozoa and invertebrates. Gustav Fischer, Stuttgart 1996, Spektrum, Heidelberg 2004. ISBN 3-8274-1482-2
- EE Ruppert, SF Richard, RD Barnes: Invertebrate Zoology, A Functional Evolutionary Approach. Chapter 22. Brooks / Cole, Pacific Grove 7 2004, p. 770. ISBN 0-03-025982-7
- A. Schmidt-Rhaesa: To the morphology, biology and phylogeny of the Nematomorpha. Cuillier, Göttingen 1996. ISBN 3-89588-434-0
- A. Schmidt-Rhaesa: Nematomorpha. Freshwater fauna of Central Europe. Vol. 4/4. Gustav Fischer, Stuttgart 1997. ISBN 3-437-25428-6
- A. Schmidt-Rhaesa: The string worms. Westarp Sciences, Hohenwarsleben 2002. ISBN 3-89432-902-5
- A. Schmidt-Rhaesa: Are the genera of Nematomorpha monophyletic taxa? in: Zoologica Scripta. Oxford 31.2002, 185-200.
- AA Berthold: About the construction of the water calf ("Gordius aquaticus"). in: Treatises of the Royal Society of Sciences in Göttingen. Berlin 1843, 1-18.
- Lorenzo Camerano: Monografia dei Gordii. Memoria. in: Memorie della Reale Accademia delle Scienze di Torino. Series 2. Turin 47.1897, 339-419.
- F. Dujardin: Mémoire sur la structure anatomique de "Gordius" et d'un autre Helminthe, le "Mermis" ', qu'on a confondu avec eux. in: Annales des Sciences Naturelles. Zoology. Vol. 2. Paris 1842, 129–151.
- Conrad Gessner (1551–1587): Historia Animalium. Frankfurt 1551, 1621 (Latin), 1977 (Italian).
- E. Grube: About some Anguillulen and the development of "Gordius aquaticus". in: Archive for Natural History Leipzig 15.1849, 359–375.
- J. Leidy: On the Gordiaceae. in: Proceedings of the Academy of Natural Sciences. Philadelphia 1851, 383-384.
- C. Linnaeus: Systema Naturae. 1. Vol. Trustees of the British Museum, Natural History. London 1758 (10 ed.).
- G. Meissner: Contributions to the anatomy and physiology of the Gordiaceen. in: Journal of Scientific Zoology. Leipzig 7.1856, 1–144.
- AE Verrill: Notice of recent additions to the marine invertebrates of the Northern coast of America. in: Proceedings of the US Natural Museum 2.1879, 165-204.
- Siebold, CTE of 1858: About the nematode worms of the insects (fifth addendum). Entomologische Zeitung, Stettin 19: 325-344.