Chaetognatha

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Chaetognatha

Chaetognatha is a phylum of predatory marine worms that are a major component of plankton

Plankton

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Chaetognatha is a phylum of predatory marine worms that are a major component of plankton

Plankton

Plankton consist of any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. Plankton are defined by their ecological niche rather than their Phylogenetics or taxonomy classification....
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 2 mm to 12 cm. The common term for the phylum is Arrow Worms. There are more than 120 modern species assigned to over 20 genera. Despite the limited diversity of species, the number of individuals is staggering.

Chaetognaths are transparent or translucent and are covered by a cuticle

Cuticle

In biology, a cuticle or cuticula is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or part of an organism, that provide protection....
. They have fins and a pair of hooked, chitin

Chitin

Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world....
ous, 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

Ovum

An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization....
and sperm

Spermatozoon

A sperm, from the ancient Greek word sp???a and and more commonly known as a sperm cell, is the ploidy cell that is the male gamete. It Fertilization an ovum to form a zygote....
. Some species are known to use the neurotoxin

Neurotoxin

A neurotoxin is a toxin that acts specifically on nerve cells , usually by interacting with membrane proteins such as ion channels.Some sources are more general, and define the effect of neurotoxins as occurring at nerve tissue....
tetrodotoxin

Tetrodotoxin

Tetrodotoxin is a potent neurotoxin with no known antidote. Tetrodotoxin blocks action potentials in nerves by binding to the pores of the voltage-gated, fast sodium channels in neuron cell membrane....
to subdue prey.

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 and anus. Chaetognaths swim in short bursts using a dorso-ventral undulating body motion, where their tail fin assists with propulsion and the body fins for stabilization and steering. At least one species of chaetognath, Caecosagitta macrocephala, has bioluminescent organs on its lateral fins.

Classification

Chaetognaths are traditionally classed as deuterostomes by embryologists. Lynn Margulis

Lynn Margulis

Lynn Margulis is an United States biologist and University Professor in the Earth science at the University of Massachusetts Amherst. She is best known for her theory on the origin of eukaryote organelles, and her contributions to the endosymbiotic theory?which is now generally accepted for how certain Mitochondrion were formed....
and K. V. Schwartz place chaetognaths in the deuterostomes in their Five Kingdom classification. Molecular phylogenists, however, consider them to be protostomes. Thomas Cavalier-Smith

Thomas Cavalier-Smith

Professor Thomas Cavalier-Smith , Royal Society, Royal Society of Canada, Natural Environment Research Council Professorial Fellow, is a Professor of Evolutionary Biology in the Department of Zoology, at the University of Oxford....
places them in the protostomes in his Six Kingdom classification. The similarities between chaetognaths and nematodes mentioned above may support the protostome thesis - in fact, chaetognaths are sometimes regarded as a basal ecdysozoa

Ecdysozoa

The Ecdysozoa are a grouping of protostome animals, including the Arthropoda , roundworm, and several smaller phylum . They were first defined by Aguinaldo et al. in 1997, based mainly on trees constructed using 18S ribosomal RNA genes....
n. Chaetognatha appears close to the base of the protostome tree in most studies of their molecular phylogeny. 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 bilateria

Bilateria

The Bilateria are all animals having a symmetry #Bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside....
n ancestors. Thus chaetognaths may be a useful model for the ancestral bilaterian.

Fossil record

Due to their soft bodies, chaetognaths fossilize poorly. Even so, several fossil chaetognath species have been described. Chaetognaths appear to have originated in the Cambrian

Cambrian

The Cambrian is a geologic period that began about Mya at the end of the Proterozoic eon and ended about Ma with the beginning of the Ordovician period ....
Period. Complete body fossils have been formally described from the Lower Cambrian

Cambrian

The Cambrian is a geologic period that began about Mya at the end of the Proterozoic eon and ended about Ma with the beginning of the Ordovician period ....
Maotianshan shales

Maotianshan shales

The Maotianshan Shales area a series of lower Cambrian deposits, famous for their Lagerst?tten. They take their name from Maotianshan Hill in Chengjiang County, Yunnan Province, China....
of Yunnan, China (Eognathacantha ercainella Chen & Huang and Protosagitta spinosa Hu) and the Middle Cambrian Burgess Shale

Burgess Shale

The Burgess Shale Formation is one of the world's most celebrated fossil localities, and is famous for the exceptional preservation of the fossils found within it, in which the soft parts are preserved....
of British Columbia (Oesia disjuncta Walcott). A more recent chaetognath, Paucijaculum samamithion Schram, has been described from the Mazon Creek biota from the Pennsylvanian

Pennsylvanian

The Pennsylvanian is an epoch in the geologic timescale or a series in the stratigraphic column. It is a subdivision of the Carboniferous period lasting from roughly to Ma ....
of Illinois. Chaetognaths were thought possibly to be related to some of the animals grouped with the conodont

Conodont

Conodonts are extinct chordata resembling eels, classified in the class Conodonta. For many years, they were known only from tooth-like microfossils now called conodont elements, found in isolation....
s. The conodonts themselves, however, are thought to be related to the vertebrates. It is now thought that protoconodont elements (e.g., Protohertzina anabarica Missarzhevsky, 1973), are probably grasping spines of chaetognaths rather than teeth of conodonts. Previously chaetognaths in the Early Cambrian were only suspected from these protoconodont elements, but the more recent discoveries of body fossils have confirmed their presence then.

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- last retrieved December 13 2006