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Chaetognatha is a phylum of predatory marine worms that are a major component of plankton worldwide. Some other species are benthic and live in tidepools or attached to macroalgae. Chaetognaths are transparent and torpedo shaped, sometimes with arrowhead like opaque structures in their heads. They range in size from 3 mm to 12 cm. The common term for the phylum is Arrow Worms. There are over 120 modern species assigned to more than 20 genera. Despite the limited diversity of species, the number of individuals can be staggering. They can be especially abundant in midwater depths and in colder waters.
Chaetognaths are transparent or translucent and are covered by a cuticle. Some deep-sea species have pigmented guts. Some benthic species have patterns of pigmentation on their bodies. 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 lay their eggs into the water, and other species brood their young. Several species have been shown to use the neurotoxin tetrodotoxin to subdue prey. The toxin is produced by symbiotic bacteria. One species of chaetognath is bioluminescent.
Chaetognaths are traditionally classed as deuterostomes by embryologists. Lynn Margulis and K. V. Schwartz place chaetognaths in the deuterostomes in their Five Kingdom classification.[1] Molecular phylogenists, however, consider them to be protostomes. Thomas Cavalier-Smith places them in the protostomes in his Six Kingdom classification.[2] They have some developmental similarities to nematodes. Although they have a mouth with one or two rows of tiny teeth, 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.
Chaetognaths swim using their tail fin for propulsion and the body fins for stabilization and steering.
Chaetognaths fossilize poorly, but are thought to have originated in the Cambrian era. Chaetognath grasping spines are found occasionally as fossils from the late Paleozoic onward. Complete body fossils have been formally described from 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] External links
- ^ Systema Naturae 2000 Taxon: Phylum Chaetognatha per Margulis and Schwartz - retrieved February 3, 2006
- ^ Systema Naturae 2000 Taxon: Phylum Chaetognatha per Cavalier-Jones - retrieved February 3