Beard worms
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| Caullery , 1914 |
The beard worms or bearded worms (Siboglinidae) are a family of tube-building annelids (Annelida) that live on the sea floor of almost all of the world's oceans, especially at a depth of 1,000 to 10,000 meters. Depending on the species, they are between 0.5 and 300 centimeters in size. The largest beard worm is Riftia pachyptila , which lives in the area of submarine hot springs ( Black Smoker ) . Due to their anatomical peculiarities - the animals have no intestines , for example - they were long considered a separate animal strain Pogonophora (pogonophores).
Beard worms can occur in large numbers. The beard worm Siboglinum veleronis was found off the California coast in an accumulation of up to 200 specimens per square meter. All beard worms are fed by symbiotic bacteria that convert sulfur compounds into organic nutrients.
The classic scientific name (Pogonophora) is derived from the Greek πώγων pṓgōn , "beard" and φέρω phérō , "I wear", so literally translated means "beard". The family's scientific name (Siboglinidae) recalls the first discovery of a beard worm species on the Siboga expedition (1899–1900) in Indonesian waters.
features
Beard worms are usually very thin with a diameter between 0.1 and three millimeters and a body length of five to 75 millimeters. Riftia pachyptila is the largest species and has a maximum length of 3 meters and a body diameter of four centimeters. All species live in tubes that are only slightly larger in diameter than the animals themselves, but can reach up to three times the length. Outside the tubes are often covered with yellow-brownish pigmented rings.
Beard worms have a three-part body structure: A short front section, which in turn divides into a front (prosoma or prostomium ) and a rear region (mesosoma or peristomium), is followed by a long, undivided trunk (metasoma), which is followed by a short rear section (Opisthosoma) is completed.
The front end is markedly different in the two subgroups within the beard worms:
- In the perviata , between one and 200 tentacles originate from the prosoma , the mesosoma is often ornamented by a sloping bar (frenulum).
- In the obturata, up to 200,000 sensory threads arise from the prosoma, which can sometimes be fused together to form ligaments and end in a structure known as the obturaculum. The mesosoma forms large wing-like folds that form an outer tube on the back, which is called the vestimentum.
In both groups, the tentacles can be pinnate, with numerous fine eyelashes and lateral outgrowths ( microvilli ). They are primarily used for gas exchange, in particular for the absorption of oxygen and sulfur-containing compounds.
The mesosoma usually separates the building material of the beardworms' living tubes, it merges into the trunk in a structure called a collar. In the perviata this is littered with numerous protuberances (papillae), annuli with hooks and cilia , but in the obturata it is evenly structured. Internal segmentation could not be demonstrated in either of the two groups.
The opisthosoma forms the end of the body and consists of between 5 and 82 narrow segments. Compared to the rest of the body, it is thickened and covered with bristles and serves to anchor the animals in the ground or their living tube, into which they can quickly retreat in this way.
Outer skin and tube structure
The material for the tubes of the beard worms is made and secreted by gland cells in the epidermis of the beard worms. These are β- chitin and hardened proteins , whereby the tubes of older animals are provided with thickenings and other structures. In the Perviata, the tubes are mostly embedded in the sediment or in other substrates and mostly protrude only a little, in Siboglinum poseidoni they are even completely buried. The tubes grow in them as new material is deposited at both ends. The tube is permeable for absorbing substances, so it lets dissolved gases , salts and amino acids through. The tubes of the obturata, on the other hand, are built on hard substrates - including fragments of wood, rubble and empty mollusc shells - and do not let any substances through from the outside, so the exchange of substances only takes place via the exposed parts of the body.
The body itself is surrounded by an outer covering, the cuticle . This consists of several layers of collagen fibers and varies in thickness depending on the region of the body. It is particularly thin in the area of the tentacles and in the torso areas that are important for the exchange of substances. The underlying epidermis is mainly made up of supporting cells, in which cells for absorbing substances (absorption cells) and numerous gland cells are embedded. Like all other annelids, the animals also have bristles made of chitin, which, apart from the pin-like bristles of the abdomen, are hook-shaped. They serve as anchoring in the tubes.
Within the epidermis there are initially individual thin circular muscles, which are followed by a thicker longitudinal muscle layer.
Trophosome
As adult worms, the animals have neither a mouth nor a continuous intestine or an anus. They feed on an organ developed from the intestine, the trophosome , which is created as a special tissue at an early larval stage. The obturata is a very extensive tissue, divided into many lobes, which, together with the gonads ( gonads ), fills the entire trunk. In the perviata, on the other hand, it is cylindrical and lies in the rear part of the trunk. The trophosome contains bacteria that are essential for the nutrition of animals .
Body cavity and blood circulation
Beard worms have a real body cavity, a coelom , which extends into all body regions. It is divided into two parts in the longitudinal direction by thin partitions, the mesenteries, and in the opisthosoma is segmented by transverse partitions (septa). The coelom is filled with a liquid that sometimes contains the respiratory pigment hemoglobin .
The vascular system is designed to be closed like all annelids. It consists of the back (dorsal) and abdominal (ventral) main vessels that run through the entire body; There are two blood vessels running in each tentacle that carry oxygen-poor blood in and oxygen-rich blood out. A muscular thickening in the head lobe or in the vestimentum, which is sometimes referred to as the heart, serves as the driving force. The blood contains a high concentration of free hemoglobin that binds oxygen and hydrogen sulfide .
Excretory and reproductive organs
Very little is known about the excretion of beard worms, they have paired nephridia in the pro or mesosoma , the exact nature of which as meta- or protonephridia is still unclear. With the exception of Siboglinum poseidoni , all known beard worms are segregated. Their paired cylindrical gonads ( gonads ) are located in the trunk and open out on the back at the front end of the animal, either on the mesosoma or in the upper area of the trunk.
Nervous system and sensory organs
An important characteristic of the relationship with the annelid worms is the ventrally located main nerve cord of the animals as the abdominal marrow . The brain lies just below the tentacles, further ganglia are only found in the perviata in the opisthosoma. Further nerve cords run from the brain into the tentacles and (in the case of the obturata) into the opturaculum; in the former they presumably take on a function as tactile receptors, that is, they enable the worm to react to touch. A network of nerves is embedded throughout the body below the epidermis. Except for a few unicellular, photoreceptor-like cells in the head lobe of Siboglinum fiordicum and Oligobrachia gracialis and receptors with an unknown function in the opisthosoma of various species, no sensory organs are known.
distribution and habitat
Most of the 150 known species live in the western Pacific , which has been the best studied to date. Also in the deep oceanic trenches, but also on the northeast coast of America, for example from Nova Scotia to Florida , in the Gulf of Mexico , in the Caribbean , off Brazil , near Greenland and in the northeast Atlantic between Norway and the Biscay , in the Indian Ocean and near New Zealand are home to a few species.
Beard worms have so far been detected at depths of 25 to over 10,000 meters, but mainly on the continental slopes and in the deep sea . Their habitat correlates closely with the cold water zones of the earth; in the Arctic and near the Antarctic, for example, shallow waters are populated, whereas in the middle latitudes it is more the deep-sea areas. The giant tube worms ( Riftia pachyptila ) in particular are characteristic creatures in the ecosystem of submarine hydrothermal springs, the so-called black smokers .
Way of life
Since the beard worms live in the deep sea, very little is known about their way of life. As fixed (sessile) organisms, they only move within their tubes, so they can leave them more or less far without crawling out of them. Several rows of adhesive papillae and rows of eyelashes make it easier for them to move, muscular annular bulges enable them to be stuck inside the tube.
nutrition
In contrast to other sessile organisms in the oceans, beard worms hardly feed on suspended matter or organisms in the surrounding water, even if they are capable of absorbing dissolved organic substances such as carbohydrates or amino acids . Your diet is almost completely adjusted to the symbiosis with chemoautotrophic bacteria . These live in the trophosome of the beard worms and oxidize absorbed inorganic sulfur compounds. They use the energy gained in this way to reduce carbon, i.e. to build up organic material and to synthesize high-energy substances such as adenosine triphosphate (ATP). These nutrients and the bacteria themselves serve as an energy source for the worm.
The bacteria are supplied with oxygen , carbon dioxide and hydrogen sulfide via a very finely structured blood vessel system. In order to transport the poisonous hydrogen sulphide, the hemoglobin molecules of the blood in the beard worms have a special structure and convert the hydrogen sulphide into non-toxic hydrogen sulphide ions. They also bind oxygen and hydrogen sulphide at two different binding sites at the same time in order to prevent oxidation . This particular form of hemoglobin is also called riftia hemoglobin. The required substances are absorbed on the outer skin of the entire body, but above all through the tentacles, which are colored red due to the strong blood circulation.
Riftia pachyptila has a particularly extensive trophosome - in this worm, the organ makes up about 50 percent of the total weight, up to 30 percent is pure bacterial mass. In contrast, the bacterial weight of the perviata is estimated to be less than one percent. The bacteria only populate the cells located in the center of the trophosome, the bacteriocytes. These are surrounded by bacteria-free cells (trophotheca) whose main function is probably the storage of glycogen and lipids .
The species of the genus Osedax feed on whale carcasses on which they live. They take in their food through their feet, as they have neither a stomach nor an opening for mouth.
Reproduction and development
So far, very little is known about the fertilization of the beard worms, as the animals largely evade observation due to their habitat. The sperm cells are present in the animals as sperm bundles or spermatophores . These should either be actively brought into the tubes of the females through the tentacles or passively drifted away. In the tubes fertilization of the eggs takes place, which, after they are fertilized, are expelled at Riftia pachyptila and sink onto the sediment. In the perviata, the eggs develop in the tube via a spiral furrow into ciliate larvae, which are free swimming for a short time and are very similar to the trochophora larva of other annelids. However, they obtain their metabolic energy exclusively from nutrients in the egg (lecithotrophy).
The earliest larval forms known to exist in the obturata are already living on the ground. In contrast to the corresponding forms of the perviata, they still have a continuous intestinal canal, which, however, quickly transforms into a trophosome. A ciliate stage with one or two tentacles first develops, later the body differentiates into the various body regions. The tube is already deposited during this development.
Research history
The first beard worm was caught on the Dutch Siboga expedition when it was caught in deep-sea nets and described by the French zoologist Maurice Caullery in 1914 . It was Siboglinum weberi from the Malay archipelago . In 1933 the second species, Lamellisabella zachsi , from the Sea of Okhotsk was described by KE Johansson and placed as a class with the annelid worms. It was not until 1955 that the similarities between the two species were recognized and the common trunk was formed. Difficulties in the taxonomic classification were mainly caused by the absence of the intestine and the presence of the trophosome. The specimens found were also mostly incomplete, since the opisthosoma was lost when it was removed from the tube. Through the training of the Coelom , AV Ivanov classified the pogonophores in several publications of the 1950s and 1960s in the relationship of the new mouth animals (deuterostomia), i.e. in the vicinity of the echinoderms and chordates . In these groups of animals the nervous system is dorsal ( spinal cord ); accordingly they got their name "beard worms" from the assumption that the tentacles are on the front instead of in the back area.
It was not until 1963 that Webb discovered the segmented opisthosoma and brought the animals into a relationship with the annelid worms. This hypothesis has been confirmed by many other characteristics that match those of annelids, including the special structure of the bristles and, more recently, also through molecular biological and embryological examinations that consolidate the currently favored classification within the annelid worms. As an alternative hypothesis, ML Jones arranged the two taxa within the beard worms as separate tribes on the stem line to the annelid worms in 1985.
Tribal history
The beard worms are a taxon whose systematic classification, as mentioned, was very controversial. While they were classically often placed as a separate strain in the closer relationship of the annelid worms and hedgehog worms , the classification in the annelid worms has now firmly established itself.
Due to the very similar structure of the hook bristles of beard worms are mainly living also in tubes as a sister group sabellidae accepted (Sabellida) into which they are now grouped mostly as a family; they then have the scientific name Siboglinidae. This classification is also supported by molecular genetic studies. However, some researchers prefer to continue to use the classic name Pogonophora without actually being of a different opinion about rank and lineage. The further relationship is formed by the largely sessile Spionida and Terebellida , with which both groups are combined in a taxon Canalipalpata, which in turn belongs to the polychaeta group.
The beard worms are very poorly known to be fossilized. There are no fossils with soft tissue preservation at all. Fossil tubes were found somewhat more numerous, the structure of which is similar to that of recent beard worms. On the basis of such tube fossils, the genus Hyolithellus from the lower Cambrian epoch , which was found in sites in Northern Europe, Greenland and North America, was assigned to the beard worms; but this is disputed by other researchers. Other fossil genera (e.g. the Silurian Yamankasia or the Devonian Tevidestus ) also encounter similar problems. Since, according to the methods of the molecular clock, a lower age of the beard worms seems more plausible today, the reliable assignment of paleozoic fossils is a particular problem here.
Systematics
More recent morphological as well as molecular studies have usually confirmed that the beard worms form a taxon that can be traced back to an original species of their own ( monophylum ). Within the beard worms, most scientists assume four monophyletic lines of development
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It is not entirely clear whether the large and predominantly rift-dwelling species grouped together in the Vestimentifera are really monophyletic. The discovery of a species that, like representatives of the genus Osedax , is bound to whale carcasses in the deep sea, suggests a possible path of descent of the rift-dwelling species from original saprophages.
supporting documents
- ↑ K. Fauchald (2012), in: G. Read, K. Fauchald, K. (2012): World Polychaeta database , World Register of Marine Species , Siboglinidae Caullery, 1914
- ↑ Shigeaki Kojima, Tetsuo Hashimoto, Masami Hasegawa, Shigenori Murata, Suguru Ohta, Humitake Seki and Norihiro Okada: Close phylogenetic relationship between vestimentifera (tube worms) and annelida revealed by the amino acid sequence of elongation factor-lα . In: Journal of Molecular Evolution . 37, No. 1, July 1993, pp. 66-70. doi : 10.1007 / BF00170463 . PMID 8360920 .
- ^ GW Rouse: A cladistic analysis of Siboglinidae Caullery, 1914 (Polychaeta, Annelida): formerly the phyla Pogonophora and Vestimentifera . In: Zoological Journal of the Linnean Society . 132, No. 1, 2001, pp. 55-80. doi : 10.1006 / zjls.2000.0263 .
- ↑ an overview in: Ana Hilário, Marıa Capa, Thomas G. Dahlgren, Kenneth M. Halanych, Crispin TS Little, Daniel J. Thornhill, Caroline Verna, Adrian G. Glover: New perspectives on the ecology and evolution of siboglinid tubeworms. In: PloS one. Volume 6, number 2, 2011, p. E16309, doi : 10.1371 / journal.pone.0016309 , PMID 21339826 , PMC 3038861 (free full text) (review).
- ↑ GW Rouse and K. Fauchald (1997): Cladistics and polychaetes. Zoologica Scripta 26 (2): 139-204.
- ^ Eve C. Southward, Anja Schulze, Stephen L. Gardiner (2005): Pogonophora (Annelida): form and function. Hydrobiologia 535/536: 227-251.
- ↑ Kenneth M. Halanych (2005): Molecular phylogeny of siboglinid annelids (aka pogonophorans): a review. Hydrobiologia 535/536: 297-307
- ↑ Feldman, RA, Shank, TM, Black, MB, Baco, AR, Smith, CR, Vrijenhoek, RC (1998). Vestimentiferan on a whale fall Biological Bulletin 194 (2): 116-119. open access
literature
- Richard C. Brusca, GJ Brusca: Invertebrates. Sinauer Associates, Sunderland Mass 2003 (2nd ed.), ISBN 0-87893-097-3
- Artemij V. Ivanov: Pogonophora. Academic Press, London 1963.
- KK Johansson: Pogonophora. in: Johann-Gerhard Helmcke, D. Starck, H. Wermuth: Handbuch der Zoologie. Vol. 3. De Gruyter, Berlin 1968. ISSN 1861-4388
- ML Jones: The Vestimentifera, their biology, systematic and evolutionary patterns. in: Oceanologica acta special issue. Gauthier-Villars, Montrouge 8.1988, 69-82. ISSN 0399-1784
- A. Nørrevang: The phylogeny and systematic position of Pogonophora. in: Journal for Zoological Systematics and Evolutionary Research. Special issue 1. Parey, Hamburg-Berlin 1975. ISSN 0044-3808
- G. Purschke: Pogonophora, beard worms. in: W. Westheide, R. Rieger: Special Zoology. Part 1. Protozoa and invertebrates. Spectrum Academic Publishing House, Heidelberg 1996. ISBN 3-437-20515-3
- GW Rouse, F. Pleijel: Siboglinidae Caullery, 1914. in: Polychaetes. University Press, Oxford 2001, 202-205. ISBN 0-19-850608-2
- GW Rouse: A cladistic analysis of Siboglinidae Caullerey, 1914 (Polychaeta, Annellida), formerly the Phyla Pogonophora and Vestimentifera. in: Zoological Journal of the Linnean Society. Blackwell, Oxford 132.2001, 55-80. ISSN 0024-4082
- EE Ruppert, RS Fox, RD Barnes: Invertebrate Zoology - a functional evolutionary approach. Chapter 13. Brooks / Cole, Pacific Grove Ca 2004, p. 455. ISBN 0-03-025982-7
- AJ Southward, EC Southward: Pogonophora. in: TJ Pandian, EJ Vernberg: Animal energetics. Vol 2. Protozoan through Insecta. Academic Press, London 1987. ISBN 0-12-544791-4
- M. Webb: Additional notes on "Sclerolinum brattstromi" (Pogonophora) and the establishment of a new family, Sclerolinidae. in: Sarsia. North atlantic marine science. Taylor & Francis, Basingstoke 16.1964, 47-58. ISSN 0036-4827
