Arrow worms
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Detail of the head region of an arrow worm |
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| Chaetognatha | ||||||||||||
| Leuckart , 1854 |
Arrowworms (Chaetognatha ( Gr .: bristle jaw )) are an animal tribe whose species are mainly found in the plankton of the warmer oceans . Their shape is streamlined with a length of 3 mm to 12 cm. Normally they float motionless in any orientation in the water before a dorsoventral wave of contraction of the longitudinal muscles ("dolphin strike") fires forward (when attacking) or backward (when escaping). Arrowworms are lurker hunters, their complex eyes enable visual orientation, the characteristic gripping hooks and teeth in the mouth area are used to catch prey. Due to their embryonic development, the arrowworms used to be counted among the new mouths (deuterostomia), according to the latest molecular genetic findings, an isolated basal position on the part of the original mouths (protostomia) is likely. An earlier molecular genetic classification of molting animals (Ecdysozoa) must therefore be discarded .
description
The German name is based on arrow-shaped structures on the head of some species. The animals are transparent and have a cuticle that is only partially developed . The body is torpedo-shaped and divided into head, trunk and tail. Arrowworms have one or two pairs of lateral fins to stabilize their position, as well as a caudal fin. The fins are stiffened by fin rays and are primarily understood as floating devices (enlargement of the buoyancy area), especially since some species can regulate the gelatinous mass in the fins up and down. On the head there are hook-shaped gripping bristles made of chitin , from which the name bristle jaw (Chaetognatha) is derived. These bristles are used for hunting and drawn in when swimming. There are teeth above the mouth.
All species are hermaphroditic . The nervous system consists of a brain , a ganglion on the abdomen and nerves lying on the body surface. Usually there are also eyes.
Respiratory, circulatory or excretory organs are missing. Most of the metabolism is carried out directly through the skin.
Some species have the paralyzing neurotoxin tetrodotoxin (TTX), which is produced in gland cells in the mouth area. These species include Flaccisagitta lyra , Parasagitta elegans , Eukrohnia hamata and Zonosagitta nagae . In an investigation of these species, the bacterium Vibrio alginolyticus , known as TTX-producing, was isolated. It is assumed that these bacteria are responsible for the formation of poison and that there is an association or symbiosis between these bacteria and the arrowworms. However, the proof of tetrodotoxin formation by bacteria was also called into question again. Protection through the formation of tetrodotoxin is widespread in the animal kingdom. For example, this protective mechanism can also be found in puffer fish and blue-ringed octopus .
Fossils
Since the arrowworms have hardly any hard parts, they only fossilize under special conditions. From the Chengjiang fauna community from the Lower Cambrian of Yunnan (China), a well-preserved, fossil arrowworm with soft tissue preservation was found that was almost identical to modern forms in numerous key anatomical features. Fossil bristles of arrowworms are known from the Upper Cambrian. In the past, the arrowworms were brought closer to the fossil conodonts because of their shape . However, recent studies have shown that the conodonts are to be counted among the chordates , which excludes a closer relationship to the arrow worms. The so-called proconodonts have now been identified with some certainty as fossil remains of the jaw spines of arrowworms or close relatives. This means that they have already been proven at the base of the Cambrian and were among the earliest fossil predators (predators) in the ocean.
Systematics
Embryologically, the arrow worms are new mouths (deuterostomia), but according to molecular genetic studies they are a basal branch on the part of the original mouths (protostomia). This contradiction led to two different ancestral interpretations. In any case, the stem of the arrowworms must have split off relatively early, in the Precambrian , when the basic lines of development of the Protostomia and Deuterostomia had just branched off. One thesis is that the deuterostomy developed again within the protostomia. The second thesis postulates the deuterostomy as the more original line of development. The protostomia evolved from the deuterostomia, and the arrowworms have consequently retained some features of the deuterostomy.
Around 100 species of arrowworms are assigned to 15 genera . These have recently been divided into two groups: Archisagittoidea and Sagittoidea .
See also: Systematics of the animal kingdom
swell
- ↑ Kevin G. Helfenbein, H. Matthew Fourcade, Rohit G. Vanjani, Jeffrey L. Boore (2004): The mitochondrial genome of Paraspadella gotoi is highly reduced and reveals that chaetognaths are a sister group to protostomes. Proceedings of the National Academy of Sciences 10 (29): 10639-10643. doi : 10.1073 / pnas.0400941101
- ↑ Kenneth M. Halanych (2004): The new view of animal phylogeny. Annual review of ecology, evolution, and systematics 35: 229-256. doi : 10.1146 / annurev.ecolsys.35.112202.130124
- ↑ Zrzavý, J., Hypša, V., Tietz, DF (2001): Myzostomida Are Not Annelids: Molecular and Morphological Support for a Clade of Animals with Anterior Sperm Flagella. Cladistics 17: 170-198. doi : 10.1111 / j.1096-0031.2001.tb00116.x
- ↑ Erik V. Thuesen, Kazuhiro Kogure: Bacterial Production of Tetrodotoxin in Four Species of Chaetognatha . In: Biological Bulletin . tape 176 , no. 2 , April 1989, pp. 191-194 , doi : 10.2307 / 1541587 (English).
- ↑ Kendo Matsumura: Reexamination of Tetrodotoxin Production by Bacteria In: Applied and Environmental Microbiology , September 1995, pp. 3468-3470 AEM Online
- ↑ K. Matsumura, D.-S. Kim, C.-H. Kim: No Ability To Produce Tetrodotoxin in Bacteria . In: Applied and Environmental Microbiology . tape 67 , no. 5 , May 1, 2001, pp. 2393-2394 , doi : 10.1128 / AEM.8.3.2393-2394.2001 (English).
- ↑ J. Vannier, M. Steiner, E. Renvoisé, S.-X. Hu, J.-P. Casanova Early Cambrian origin of modern food webs: evidence from predator arrow worms. Proceedings of the Royal Society Series B (2007) 274: 627-633. doi : 10.1098 / rspb.2006.3761
literature
- Storch, Volker; Welsch, Ulrich: Systematic Zoologie 6th edition. Spectrum Akademischer Verlag 2003. ISBN 3-8274-1112-2
