Belly hooks
| Belly hooks | ||||||||
|---|---|---|---|---|---|---|---|---|
A belly harling |
||||||||
| Systematics | ||||||||
|
||||||||
| Scientific name | ||||||||
| Gastrotricha | ||||||||
| Metschnikoff , 1864 | ||||||||
| without rank | ||||||||
|
The belly curls or bottle creatures (Gastrotricha, from ancient Greek γαστήρ, gaster - stomach and θρίξ, ( Gen. ) τριχός, thrix, trichós - hair) are 0.06 to 1.5 mm long multicellular inhabitants of fresh waters and seas.
features
outer appearance
In their appearance they resemble the ciliates ( ciliate animals such as the paramecium ). The body is generally elongated with a thickening that differs depending on the species, starting from the head, about halfway up the body, giving the animal its bottle-like shape. Characteristic is the pair of spike-shaped toe extensions at the rear end of the body, which with their adhesive glands serve to adhere to a surface. The flat belly side is covered with cilia with which the animals can glide over the ground. Also on the head there are usually four tufts of cilia which, in addition to their supporting function in eating, allow the belly to swim freely in the water for a short time. The body surface is formed by a syncytium , i.e. the fusion of many cells to form a multinucleated “large cell” and the cuticle separated from it . The cuticle, which is arranged in plates and partially provided with spines and other bulges, offers stability, but hardly restricts freedom of movement due to its joints and covers, unique in the animal kingdom - also the cilia.
Digestive and excretory organs
Similar to the nematodes , the abdominal curls ingest their food, consisting of algae , bacteria , small protozoa and detritus , by using their pharynx, which functions as a suction pump, to create a negative pressure that allows the prey to flow into the mouth opening. For this purpose, the pharynx, in addition to some cells that give off digestive secretions, is primarily made up of radial muscle fibers. This allows the lumen width to be changed, which leads to the generation of pressure differences. In addition to the digestive glands embedded in the wall, the ducts of the paired salivary glands also open into the pharynx . The intestine adjoining the pharynx runs in a straight line to the anus. In addition to the anus, the excretory organs are the protonephridia , which are one or more pairs, depending on the type, and the ducts of which end approximately in the middle of the body.
Musculoskeletal system
The active musculoskeletal system is made up exclusively of longitudinal muscles. The distribution is very constant across species. A ventral group, consisting of what is actually a ventral muscle train and a cord on each side of the body, extends over the entire length of the body. Opposite this is a dorsal group, which is somewhat shorter in length and which, depending on the type, consists of two to three pairs of muscle groups. Since the abdominal curbs do not have a skeleton or auxiliary muscles such as tendons, the only force that counteracts the active longitudinal shortening by the muscles is the pressure of the fluid inside the body.
Nervous system
The brain is at the level of the pharynx and rests on it. With its relatively large extension, it fills a significant portion of the head. A pair of nerve cord arises from it, which runs to the periphery on both sides.
Reproduction and life cycle
The reproduction of the abdominal curls takes place, depending on the species, either via parthenogenesis (virgin generation) or sexually. The Macrodasyida are always bisexual, while the Chaetonoidea are hermaphrodites , i.e. they have both male and female gonads and parthenogenesis is also common. The exact interpretation of the sexual organs is often difficult. Mating has only been observed in a few species, sperm are either injected into the partner or attached to the outside.
You can only provide a small number of mature eggs at the same time, as they are very large and rich in yolks. They are deposited individually via the fallopian tubes opening next to the anus. Embryonic development takes place very quickly, so that the young hatch about two days after they have laid their eggs. A larval stage does not exist, and the number of cells does not change from hatching, except when injuries are healed. However, the cell size increases as it develops into an adult animal. This process is completed in about two days and the animals are able to reproduce. The lifespan varies between 3 and 21 days depending on the species. A lifespan of up to 40 days has also been observed in the laboratory. In addition to the normal eggs (subitane eggs), the abdominal curls can also lay permanent eggs in order to ensure the continued existence of the population over longer periods of dry and cold weather.
distribution
The belly crooks are common worldwide. They live on the bottom of the water and are part of the meiofauna there . They are mostly found in the system of gaps between the sand and mud particles of the Benthals , both in fresh water and in the mesopsammon of the marine sediments . In some cases they also inhabit the surface of aquatic plants or other objects below the water level. Some species also live on land in damp foliage. They can survive dehydration in the egg, some species can also develop permanent stages in the form of cysts under unfavorable conditions .
In marine sediments, up to 364 ventral curls were found on ten square centimeters of soil. In addition to the roundworms (Nematoda) and copepods (Copepoda), they form one of the animal groups in this habitat that is rich in individuals. In fresh water they reach a density of up to 158 individuals per ten square centimeters. In various rivers in Germany, 7 to 23 different types of belly larvae were found, depending on the state of the water. An even greater variety of species can be found in stagnant waters, where up to 45 species such as in the Großenmoor in Hessen have been observed. In Canada , where there are large, natural wetlands , this number can be up to 64. Many species are still undescribed because of their small size and the fact that they often occur only for a short period of time.
Systematic classification
The phylogenetic classification of the belly curls is unclear. Due to their morphology, they seem to be related to the rotifers ( rotifers ) or nematodes (roundworms). Recent genetic studies, however, provide evidence of a relationship with the flatworms (platelets). These could be her sister group . The previously widespread assumption that the belly parsons had a body cavity ( coelom ) has now been refuted. These observations were dissection artifacts that arose in connection with the very loose connective tissue network inside the body. According to current knowledge, the Gastrotricha are acoelomats . About 830 individual species are known in two groups:
- The Macrodasyida live marine, are bisexual, have an inverted Y-shaped pharynx with a pair of pharyngeal pores and numerous adhesive tubes laterally on the body.
- The Chaetonotida can be found marine, in freshwater or terrestrial. They are bisexual as the Neodasys types, and the marine family of Xenotrichulidae or hermaphrodite. The terrestrial species reproduce purely parthenogenetically. Your pharynx is Y-shaped with no pores and only a pair of sticky tubes and sits on the back of the bottle-shaped body.
Individual evidence
- ↑ Sina M. Adl: The Ecology of Soil Decomposition. CABI Publishing, 2003, p. 52, ISBN 0-85199-661-2
- ↑ a b Gastrotricha Overview, Department of Biology, University of Modena , Italy, accessed on August 12, 2017
- ↑ Peter Schwank & Tobias Kånneby: Contribution to the freshwater gastrotrich fauna of wetland areas of southwestern Ontario (Canada) with redescriptions of seven species and a check-list for North America. Zootaxa, 3811, 4, pp. 463-490, June 2014
- ↑ Christopher E. Laumer, Nicolas Bekkouche, Alexandra Kerbl, Freya Goetz, Ricardo C. Neves, Martin V. Sørensen, Reinhardt M. Kristensen, Andreas Hejnol, Casey W. Dunn, Gonzalo Giribet, Katrine Worsaae (2015): Spiralian Phylogeny Informs the Evolution of Microscopic Lineages. Current Biology 25: 2000-2006. doi : 10.1016 / j.cub.2015.06.068
- ↑ Torsten H. Struck, Alexandra R. Wey-Fabrizius, Anja Golombek, Lars Hering, Anne Weigert, Christoph Bleidorn, Sabrina Klebow, Nataliia Iakovenko, Bernhard Hausdorf, Malte Petersen, Patrick Kück, Holger Herlyn, Thomas Hankel (2014): Platyzoan Paraphyly Based on Phylogenomic Data Supports a Noncoelomate Ancestry of Spiralia. Molecular Biology and Evolution 31 (7): 1833-1849. doi : 10.1093 / molbev / msu143
literature
- Wilfried Westheide and Reinhard Rieger (Eds.) Special Zoology - Part 1: Protozoa and Invertebrates , 2nd Edition, Elsevier, Munich, 2007, ISBN 3-8274-1575-6
