Chaetognatha
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Chaetognatha is a phylum of predatory marine worms that are a major component of plankton worldwide. About 20% of the known species are benthic and can attach to algae or rocks. They are found in all marine waters from surface tropical waters and shallow tide pools to the deep sea and polar regions. Most chaetognaths are transparent and are torpedo shaped. Some deep-sea species are orange. They range in size from 3 mm to 12 cm. The common term for the phylum is Arrow Worms. There are more that 120 modern species assigned to over 20 genera. Despite the limited diversity of species, the number of individuals is staggering.[1]
Chaetognaths are transparent or translucent and are covered by a cuticle. They have fins and a pair of hooked, chitinous, grasping spines on each side of their heads that are used in hunting. The spines are covered with a hood when swimming. They have a distinct head, trunk and tail. All species are hermaphroditic, carrying both eggs and sperm. Some species are known to use the neurotoxin tetrodotoxin to subdue prey.[2]
They have some developmental similarities to nematodes. Although they have a mouth with one or two rows of tiny teeth, compound eyes, and a nervous system, they have no respiratory, circulatory, or excremental systems. Materials are moved about the body cavity by cilia. Waste materials are simply excreted through the skin amd anus.
Chaetognaths swim using their tail fin for propulsion and the body fins for stabilization and steering. At least one species of chaetognath, Caecosagitta macrocephala, has bioluminescent organs on its lateral fins.[3]
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[edit] Classification
Chaetognaths are traditionally classed as deuterostomes by embryologists. Lynn Margulis and K. V. Schwartz place chaetognaths in the deuterostomes in their Five Kingdom classification.[4] Molecular phylogenists, however, consider them to be protostomes. Thomas Cavalier-Smith places them in the protostomes in his Six Kingdom classification.[5] The similarities between chaetognaths and nematodes mentioned above may support the protostome thesis - in fact, chaetognaths are sometimes regarded as a basal ecdysozoan.[6] Chaetognatha appears close to the base of the protostome tree in most studies of their molecular phylogeny.[7] This may explain their deuterostome embryonic characters. If chaetognaths branched off from the protostomes before they evolved their distinctive protostome embryonic characters, they may have retained deuterostome characters inherited from early bilaterian ancestors. Thus chaetognaths may be a useful model for the ancestral bilaterian.[8]
[edit] Fossil record
Chaetognaths fossilize poorly, but are thought to have originated in the Cambrian Period. Chaetognath grasping spines are found occasionally as fossils from the late Paleozoic onward. Complete body fossils that have not been formally described are reported from the Kicking Horse Shale member of the Middle Cambrian Burgess Shale of British Columbia and the Lower Cambrian Maotianshan shales of Yunnan, China. Chaetognaths are thought possibly to be related to some of the animals grouped with the conodonts. The conodonts themselves, however, are thought to be related to the vertebrates. It is now thought that the protoconodonts, which are known only from their teeth, might be chaetognaths rather than conodonts.
[edit] References
- ^ Bone, Q. et al. (1991). The biology of chaetognaths. Oxford University Press.
- ^ Thuesen, E.V. (1991), "The tetrodotoxin venom of chaetognaths", in Bone, Q. et al., The biology of chaetognaths, London: Oxford University Press
- ^ Haddock, S.H.D. & J.F. Case (2004). "A bioluminescent chaetognath". Nature 367: 225-226.
- ^ Systema Naturae 2000 Taxon: Phylum Chaetognatha per Margulis and Schwartz (select Margulis & Schwartz in 'Classification by') - last retrieved November 25, 2006
- ^ Systema Naturae 2000 Taxon: Phylum Chaetognatha per Cavalier-Smith (select Cavalier-Smith in 'Classification by') - last retrieved November 25, 2006
- ^ Matus, D.Q. et al. (2006). "Broad taxon and gene sampling indicate that chaetognaths are protostomes". Current Biology 16: R575-R576.
- ^ Marletaz, F. et al. (2006). "Chaetognath phylogenomics: a protostome with deuterostome-like development". Current Biology 16: R577-R578.
- ^ Papillon, Daniel et al. (2004). "Identification of chaetognaths as protostomes is supported by the analysis of their mitochondrial genome". Molecular Biology & Evolution 21 (11): 2122-2129.
