Hookweed
| Hookweed | ||||||||||||
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Zelinkaderes floridensis (Cyclorhagida) with everted and retracted introvert (after Lorenzen 1996) |
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| Scientific name | ||||||||||||
| Kinorhyncha | ||||||||||||
| Reinhard , 1887 | ||||||||||||
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The hookweed ( Kinorhyncha ) form an animal strain of worm-like organisms within the molting animals ( Ecdysozoa ). Your next evolutionary relatives are probably the priapulida ( Priapulida ) and loricifera ( Loricifera ), with whom they in a taxon scalidophora are summarized. Hook weevils were classified on the French Channel coast for the first time in 1841 and received their current scientific name in 1887 by the zoologist W. Reinhard. They have remained a rather obscure group to this day and largely unexplored; only one gene from a single species has so far been sequenced .
construction
The worm-shaped animals are almost always between 200 micrometers and one millimeter in size, often yellowish-brown in color and flattened on the abdomen. Your body has a triangular to circular cross-section and consists of three parts: the head or introvert , the neck, and the torso. The latter is divided into exactly eleven ring segments, the so-called zonites ; the segmentation here refers not only to the outer skin, the cuticle , but also to the muscles , glands and nervous system . In the past, the neck and head were also viewed as separate segments, but this assessment is controversial today.
Outer shape
The head is occupied by about fifty to ninety hooks or spikes, which are arranged in a maximum of seven concentric rings and from which the German name is derived. It can be drawn into the following neck and the first trunk segment. There are no spines on the former, but instead up to sixteen hardened plates formed by the outer skin, the placids , which close over the retracted introvert. The trunk segments are often covered by further spines and fine hairs, in particular the last segment often has end spines that are arranged laterally and pointing backwards. On the second segment there are sometimes fine tubes, which are believed to be used for attachment to a substrate.
Skin, muscles and body cavity
The skin consists of a chitin-containing , non- cellular and uncultivated outer layer, the cuticle , and an underlying single-layer cell layer, the epidermis .
The cuticle, in turn, is composed of an outer, thin epicuticle , a hard middle layer, the intracuticula , and an inner, fibrous procuticle . Per segment, one or two hardened plates are embedded in it on the back ( dorsal ) and on the abdomen ( ventral ) one or two, which are called tergites or sternites ; only the plates on the back have movable spines, the scalids . On the other hand, the cuticle is very thin between the segments and acts as a hinge, thus allowing a certain flexibility of the body shape. On the other hand, it cannot be stretched at will, so that the animals have to shed their skin at regular intervals in order to grow.
The cellular epidermis contains numerous mucous glands that open out through openings in the cuticle and secrete a layer of mucus that surrounds the animal.
The striated longitudinal muscles lie beneath the epidermis ; there are also dorsoventral muscles that run from the back to the abdomen, and a circular muscle that runs around the mouth cone. Two sets of introvert-retractor muscles allow the worm to retract its head. A peculiarity of the muscles of the hookweed is that, in contrast to most other animal groups, the individual muscle cells send off runners to the nerve cells and not vice versa.
The body cavity between the muscle layer and the digestive tract is probably formed by a hemocoel , so it is not surrounded by its own cell layer. Inside it is a colorless body fluid that contains numerous amoeboid cells. The Hemocoel takes on tasks as a hydrostatic skeleton , as well as in the distribution and absorption of nutrients and oxygen.
Digestive, excretory and reproductive organs
The digestive tract begins with the mouth, which is located at the front end of the body at the tip of an extendable cone surrounded by nine mouth stylets. The cuticle-lined oral cavity, which probably has a filter function, is followed by a muscular throat that is used to suck in food. It passes into the cylindrical intestine, with numerous invaginations, the microvilli , in which the nutrients are absorbed. It is surrounded by both longitudinal and circular muscles and ends in a rectum, again lined with a cuticle, which opens to the outside world at the eleventh segment on the abdomen in the anus.
Two paired protonephridia in the tenth segment serve to excrete fluid and regulate the salt balance . These are slender tubes that are closed on the inside by three terminal double-flagellated cells that protrude into the hemocoel and that open outwards in the eleventh segment through a nephridiopore .
Contact the gonads paired and are characterized by a Gonodukt said channel in an opening located between the tenth and eleventh segment gonopore connected opens to the outside world. Females usually also have a sperm store in which the male sperm can be stored, while males sometimes have special, penile spines that probably facilitate copulation.
Nervous system and sensory organs
The nervous system consists of a ring of nerves around the anterior pharynx, which consists of ten ganglia and can be viewed as the brain . Eight longitudinal nerve cords run back from it. Most noticeable are two ventral cords running in pairs, each with a ganglion per segment and connected to each other by two transverse nerve ligaments per segment.
The sensory organs of the hookweed consist of numerous bristles that are distributed over the entire trunk in species-dependent patterns and react to mechanical stimulation. The scalids of the head are also used for perception; in each of them there are up to ten simply flagellated sensory cells. Light sense organs, the ocelli , sometimes lie behind the mouth cone and are usually pigmented reddish.
distribution and habitat
Hookweed are marine animals and have so far been detected in all seas up to the 87th parallel north latitude. As part of the so-called sandgap fauna, they live in silt and mud ( interstitial ), i.e. between larger grains of sand on the seabed ( benthic ), on sandy beaches and in the brackish water of river mouths, as well as on sponges (Porifera), hydrozoa (Hydrozoa) and bog animals (Bryozoa) and algae mats from the sublittoral zone , the area that is barely covered at low water, to the abyssal zone in more than 5000 meters water depth. In muddy sediments they usually inhabit the upper millimeters, in sandy the upper centimeters of the sea floor. In some species, seasonal “migrations” of around 5 millimeters up and down can be detected.
The number of individuals depends on the habitat: while there are around 45,000 individual animals per square meter in shallow water, there are only around 1,000 to 10,000 individuals in the deep sea.
Diet and locomotion
Hookweed are of great importance for the food web in the sea. They themselves feed on detritus , organic waste and diatoms (Bacillariophyta), which are sucked in through the muscular throat and transferred into the intestine, but conversely they serve as food for numerous other inhabitants of the seabed such as various worm species. They are likely to act as hosts for the parasitic apicomplexa .
As living beings that live on the ground, they cannot swim, but only move forward by crawling or digging into the sediment. To do the latter, they pump fluid from the hemocoel into the head, which causes it to swell. As a result, the head hooks spread apart from this, move backwards and thereby pull the animal forward. In the end position, the head is firmly anchored in the sediment and the introvert retractor muscles now pull the trunk afterwards. This mode of locomotion has given them their scientific name, which comes from the Greek and derives from kinein , to move, and rhynchos , trunk.
Reproduction and development
Hookweed are separate-sex animals in which males and females can only be distinguished by the sometimes slightly differently arranged spines in the females and often not at all. It is believed that the eggs are fertilized internally; however, copulation has only been observed in one species, Pycnophyes kielensis . Here both partners turn their stomach side towards each other, but orient themselves with their heads in the opposite direction. The male now inserts his belly-side penile spines into the female's genital opening and thus ensures the mechanical cohesion of the partners. The sperm cells combined in sperm packets, the spermatophores , now presumably migrate to the female's sperm store and from there to the eggs released from the ovaries, which are thus internally fertilized.
The eggs develop externally without going through a larval stage, but the exact course of this ontogenesis is unknown. The hatching young usually already have nine segments, the missing two are built up by a growth zone at the rear of the animals; The number of head spines also increases in the course of development. After at least six moults, the animals have reached adult size; thereafter no more molting takes place.
Tribal history
In the county Nanjiang province Sichuan in China very well preserved were 535 million years old fossils of Hakenrüsslern the type Eokinorhynchus raru found.
The morphological comparison with other recent (currently living) animal groups reveals that its closest relative most likely the priapulida (Priapulida) and loricifera are (Loricifera) holding them to taxon are summarized shed carrier (scalidophora).
Which of the other two groups within the Scalidophora represents the immediate sister taxon of the hookweed is unclear, however, all three possible combinations have so far been represented and scientifically justified.
The presence of a corset formed by the cuticle, which is present in the former in the larval stage in the corset animals and also in the adult animals, suggests a closer relationship between priap worms and corset animals. The supporters of this thesis combine both groups in the taxon of the Vinctiplicata; accordingly, the hookweed would be the sister group of this taxon. On the other hand, the fact that the pharyngeal tissue does not consist of epithelial muscle cells but is derived from the embryonic mesoderm is cited for a close relationship between priap worms and hookweed . The third alternative, a sister taxon relationship between hookweed and corset animals, is justified by the protruding, but not everting, cone of the mouth that exists in both taxa.
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 a species that has been extinct for half a billion years than about 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 ( synapomorphy ) of both the priap worms and the corset animals and would thus underline their sister group relationship.
The Scalidophora themselves belong to a larger family group, which also includes roundworms (Nematoda) and string worms (Nematomorpha) and which are known as Cycloneuralia.
Similarities with the arthropods have long been viewed as characters created by convergent evolution , i.e. not attributed to a common predecessor characteristic. But they are increasingly interpreted as an indication of a closer ancestral relationship in the taxon of the molting animals (Ecdysozoa). The characters shared by both groups include the internal segmentation, the chitinous, skinned exoskeleton, the body cavity formed by a hemocoeloma, the presence of striated muscles, the anchoring of the longitudinal muscles to the cuticle by a structure of intermediate filaments and hemidesmosomes, and that Brain located around the throat, from which a pair of ventral nerve cords originate, each of which has a pair of ganglia per segment connected by nerve bands.
Systematics
To date, a little more than 140 species of adult hookweed and almost 40 larval stages have been scientifically described. Although one must assume that the "species" of the second group are often simply the early stages of development of the species of the first group, they have so far been recognized as independent by the zoological nomenclature ; however, the actual number of species discovered so far is estimated at only around 150. However, most of the deep-sea hookweed is probably still completely unknown.
The named species are divided into fifteen genera and two classes :
- The Cyclorhagida are characterized by fourteen to sixteen plazids in the neck region and have numerous spines and adhesive tubes arranged on the trunk. Your body cross-section is circular to triangular.
- The Homalorhagida grow up to a millimeter in size, have only six to eight placids in the neck region and also have hardly any spines on the trunk. Without exception, they have a triangular body cross-section and occur in muddy sediments below the intertidal zone.
literature
- RC Brusca, GJ Brusca 2003. Invertebrates. Sinauer Associates, Sunderland MA 2003, p. 348. ISBN 0-87893-097-3
- X.-P.Dong, PCJ Donoguhe, H. Cheng, JB Liu 2004. Fossil embryos from the Middle and Late Cambrian period of Hunan, south China. in: Nature . New York 427.2004, p. 237. ISSN 0028-0836
- RP Higgins 1971. A historical overview of kinorhynch research. in: NC Hulings (Ed.): Proceedings of the First International Conference on Meiofauna. Smithsonian Contributions to Zoology. Washington DC 76.1971, p.25. ISSN 0081-0282
- RM Kristensen, RP Higgins 1991. Kinorhyncha. In: FWHarrison, EE Rupert (Ed.): Microscopic Anatomy of Invertebrates. Vol. 4. Wiley-Liss, New York 1991, p. 377. ISBN 0-471-56103-7
- S. Lorenzen: Kinorhyncha. in: W. Westheide, R. Rieger 1996. Special zoology. T 1. Protozoa and invertebrates. Gustav Fischer, Stuttgart 1996. ISBN 3-437-20515-3
- B. Neuhaus, RP Higgins 2002. Ultrastructure, biology and phylogenetic relationships of Kinorhyncha. in: Integrative and Comparative Biology. Lawrence Kan 42.2002, p. 619. ISSN 1540-7063
- EE Ruppert, SF Richard, RD Barnes 2004. Invertebrate Zoology. A Functional Evolutionary Approach. Cape. 22. Brooks / Cole, Belmont Cal 2004, p. 778. ISBN 0-03-025982-7
Individual evidence
- ↑ Ellie Zolfagharifard: The spiky 'mud dragon' that lived half a Billion years ago. In: The Daily Mail Online. November 26, 2015, accessed November 26, 2015 .
Web links
- Picture of a hookweed species of the genus Antygomonas (about in the middle of the website)
- Extensive literature list on the hookweed
