Sea worms
| Sea worms | ||||||||||||
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| Systematics | ||||||||||||
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| Scientific name of the class | ||||||||||||
| Nectonematoida | ||||||||||||
| Rauther , 1930 | ||||||||||||
| Scientific name of the order | ||||||||||||
| Nectonematida | ||||||||||||
| Ward , 1892 | ||||||||||||
| Scientific name of the family | ||||||||||||
| Nectonematidae | ||||||||||||
| Ward , 1892 | ||||||||||||
| Scientific name of the genus | ||||||||||||
| Nectonema | ||||||||||||
| Verrill , 1879 |
The sea worms form the genus Nectonema and at the same time the family Nectonematidae and the class Nectonematoida with currently five known species of string worms that live in salt water in oceans worldwide and parasitize in decapods . Nectonema agile was described as the first and best known species today .
features
The sea worms have both a ventral and a dorsal median line, each with two rows of small, hair-like swimming bristles. The ventral and dorsal nerve cords run along these two median lines in the epidermis. This is where the Nectonematidae differ from all other string worms , comprising 300 species and grouped as Gordioida , which have only one ventral nerve cord and the associated median line and are completely bristle-free. The conical rear end of the male is bent over in the sea worms. The animals live in salt water . In contrast to the other string worms , the pseudocoel of the Nectonematidae is not filled with parenchyma. The females, however, have an extensive gonoparenchyma in the area of the ovaries , within which a cavity in the sexually mature female fills with eggs, while no epithelial female gonads are visible. The males, on the other hand, have an unpaired dorsal sperm sac. In both sexes, the sex opening is at the furthest end of the animal. The severely reduced intestine has neither a mouth nor an anus, so it ends blind, which is why there is no cloaca , but a pure sexual opening.
Spread and sample types
The Nectonematidae are found in seas around the world. Nectonema agile can be found in the Atlantic Ocean off the coast of North America , but also in the Mediterranean , where it parasitizes in crabs of the genera Periclimenes and Palaemonetes . Nectonema munidae is known as an endoparasite of the crab Munida tenuimana off the coast of Norway .
Development cycle
The larvae of the Nectonematidae have a proboscis with two hook rings and no stilettos (as known from the other string worms). The invasion of the larvae into the host crabs has not yet been observed, but it is considered likely that the larvae can be swallowed by the crabs. The hooked trunk apparently serves to pierce the host's intestinal wall so that the string worm can live in the body cavity, the host's mixocoel . Here the larva grows into a long worm, absorbing nutrients from the host through its skin. After all, the string worm takes up much of the interior of the host. When it is fully grown, it pierces the host's synovial membrane and ends up in the open sea. Depending on the size of the host, it will be killed, castrated or only damaged to a limited extent. The adult string worm no longer eats anything, but only looks for its sexual partner. New larvae emerge from the deposited eggs.
species
So far, five species of the genus Nectonema have been described:
- Nectonema agile Verrill, 1879 ( Atlantic Ocean , Mediterranean )
- Nectonema melanocephalum Nierstrasz, 1907 ( Indo-Pacific )
- Nectonema munidae Brinkmann, 1930 (off Norway )
- Nectonema svensksundi buck, 1908 ( Arctic )
- Nectonema zealandica Poinar & Brockerhoff, 2001 (off New Zealand , found in Hemigrapsus sp.)
literature
- Addison Emery Verrill (1879): Notice of recent additions to the marine fauna of the east coast of North America. American Journal of Science and Arts (3) 17, pp. 309-315.
- Carl Claus , Karl Grobben, Alfred Kühn: Textbook of Zoology: Special Part, 10th Edition. Springer Verlag, Berlin 1971 (1st edition by Julius Springer, Berlin / Vienna 1932). Pp. 524f., Nematomorpha .
- Peter Ax : The system of Metazoa III. A textbook on phylogenetic systematics. Spektrum Akademischer Verlag, Heidelberg 2001. Chapter Nematomorpha , pp. 21–26.
Web links
- Pierre Noël, Vincent Maran: Nectonema agile Verrill, 1879. Données d'Observations pour la Reconnaissance et l'Identification de la faune et la flore Subaquatiques (DORIS), 7 February 2018.