Polychaete

Taxobox
fossil_range = Cambrian (or earlier?) - present
name = Polychaetes

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image_width = 250px
image_caption = "A variety of marine worms": plate from "Das Meer" by M. J. Schleiden (1804–1881).
regnum = Animalia
phylum = Annelida
classis = Polychaeta
classis_authority = Grube, 1850
subdivision_ranks = Subclasses
subdivision = Palpata
Scolecida

The Polychaeta or polychaetes are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. Indeed the polychaetes are sometimes referred to as bristle worms. More than 10,000 species are described in this class. Common representatives include the lugworm ("Arenicola marina") and the sandworm or clam worm "Nereis".

Anatomy and physiology

The polychaetes' paddle-like and highly vascularized parapodia are used for movement and act as the annelid's primary respiratory surfaces (parapodia can be thought of as kinds of external gills that are also used for locomotion). Polychaeta also have well-developed heads compared to other annelids.

Ecology

s.

A few groups have evolved to live in terrestrial environments, like Namanereidinae with many terrestrial species, but are restricted to humid areas. Some have even evolved cutaneous invaginations for aerial gas exchange.

One notable polychaete, the Pompeii worm ("Alvinella pompejana") is endemic to the hydrothermal vents of the Pacific Ocean. Pompeii worms are thought to be the most heat-tolerant complex animals known.

A recently discovered genus "Osedax" includes the Bone-eating snot flower.

Another remarkable polychaete is "Hesiocaeca methanicola", which lives on methane clathrate deposits.

"Lamellibrachia luymesi" is a cold seep tube worm that reaches lengths of over 3 meters and may be the most long lived animal at over 250 years old.

Evolutionary history

The oldest crown group polychaetes fossils come from the Sirius Passet Lagerstatte, which is tentatively dated to the lower-middle Atdabanian (early Cambrian). [Conway Morris, S. and Peel, J.S. 2008. The earliest annelids: Lower Cambrian polychaetes from the Sirius PassetLagerstätte, Peary Land, North Greenland. Acta Palaeontologica Polonica 53 (1): 137–148.] Many of the more famous Burgess Shale organisms, such as "Canadia" and "Wiwaxia", may also have polychate affinites. An even older fossil, "Cloudina", dates to the terminal Ediacaran period; this has been interpreted as an early polychaete, although consensus is absent.citation
author = Miller, A.J.
year = 2004
title = A Revised Morphology of Cloudina with Ecological and Phylogenetic Implications
url = http://ajm.pioneeringprojects.org/files/CloudinaPaper_Final.pdf
accessdate = 2007-04-24 ]

Being soft bodied, the fossil record of polychaetes is dominated by their fossilized jaws, known as scolecodonts, and the mineralized tubes that some of them secrete.

Taxonomy and systematics

Taxonomically, the polychaetes are thought to be paraphyletic, meaning that as a group it contains its most recent common ancestor, but does not contain all the descendants of that ancestor. Groups that may be descended from the polychaetes include the earthworms, the leeches, sipunculans, and echiurans. The Pogonophora and Vestimentifera were once considered separate phyla, but are now classified in the polychaete family Siboglinidae.

Much of the classification below matches Rouse & Fauchald, 1998, although that paper does not apply ranks above family.

Older classifications recognize many more (sub)orders than the layout presented here. As comparatively few polychaete taxa have been subject to cladistic analysis, some groups which are usually considered invalid today may eventually be reinstated.

*Subclass Palpata
**Order Aciculata
***Basal or "incertae sedis"
****Family Aberrantidae
****Family Nerillidae
****Family Spintheridae
***Suborder Eunicida
****Family Amphinomidae
****Family Diurodrilidae
****Family Dorvilleidae
****Family Eunicidae
****Family Euphrosinidae
****Family Hartmaniellidae
****Family Histriobdellidae
****Family Lumbrineridae
****Family Oenonidae
****Family Onuphidae
***Suborder Phyllodocida
****Family Acoetidae
****Family Alciopidae
****Family Aphroditidae
****Family Chrysopetalidae
****Family Eulepethidae
****Family Glyceridae
****Family Goniadidae
****Family Hesionidae
****Family Ichthyotomidae
****Family Iospilidae
****Family Lacydoniidae
****Family Lopadorhynchidae
****Family Myzostomatidae
****Family Nautillienellidae
****Family Nephtyidae
****Family Nereididae
****Family Paralacydoniidae
****Family Pholoidae
****Family Phyllodocidae
****Family Pilargidae
****Family Pisionidae
****Family Polynoidae
****Family Pontodoridae
****Family Sigalionidae
****Family Sphaeodoridae
****Family Syllidae
****Family Typhloscolecidae
****Family Tomopteridae
**Order Canalipalpata
***Basal or "incertae sedis"
****Family Polygordiidae
****Family Protodrilidae
****Family Protodriloididae
****Family Saccocirridae
***Suborder Sabellida
****Family Oweniidae
****Family Siboglinidae (formerly the phyla Pogonophora & Vestimentifera)
****Family Serpulidae
****Family Sabellidae
****Family Sabellariidae
****Family Spirorbidae
***Suborder Spionida
****Family Apistobranchidae
****Family Chaetopteridae
****Family Longosomatidae
****Family Magelonidae
****Family Poecilochaetidae
****Family Spionidae
****Family Trochochaetidae
****Family Uncispionidae
***Suborder Terebellida
****Family Acrocirridae (sometimes placed in Spionida)
****Family Alvinellidae
****Family Ampharetidae
****Family Cirratulidae (sometimes placed in Spionida)
****Family Ctenodrilidae (sometimes own suborder Ctenodrilida)
****Family Fauveliopsidae (sometimes own suborder Fauveliopsida)
****Family Flabelligeridae (sometimes suborder Flabelligerida)
****Family Flotidae (sometimes included in Flabelligeridae)
****Family Pectinariidae
****Family Poeobiidae (sometimes own suborder Poeobiida or included in Flabelligerida)
****Family Sternaspidae (sometimes own suborder Sternaspida)
****Family Terebellidae
****Family Trichobranchidae
*Subclass Scolecida
**Family Aeolosomatidae
**Family Arenicolidae
**Family Capitellidae
**Family Cossunidae
**Family Maldanidae
**Family Ophelidae
**Family Orbiniidae
**Family Paraonidae
**Family Parergodrilidae
**Family Potamodrilidae
**Family Psammodrilidae
**Family Questidae
**Family Scalibregmatidae

ee also

*Epitoky, a form of reproduction of "Polychaetae".

References

* Campbell, Reece, and Mitchell. Biology. 1999.
* cite journal
author = Rouse, Greg W.; Fauchald, Kristian
year = 1998
title = Recent views on the status, delineation, and classification of the Annelida
journal = American Zoologist
volume = 38 | pages = 953–964
doi = 10.1093/icb/38.6.953

External links

* Special issue dedicated to polychaete published in "Marine Ecology". [http://www.blackwell-synergy.com/toc/mae/26/3-4 Read the article abstracts online]
* [http://www.tafi.org.au/zooplankton/imagekey/annelida/index.html Marine Polychaete Larva - Guide to the Marine Zooplankton of south eastern Australia ]