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.
Polychaetes are extremely variable in both form and lifestyle and include a few taxa that swim among the plankton
. Most burrow or build tubes on the bottom, and some live as commensals
. A few are parasitic. The mobile forms or Errantia tend to have well-developed sense organs and jaws, while the Sedentaria (or stationary forms) lack them but may have specialized gills or tentacles used for respiration and deposit or filter feeding, e.g., fanworms
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.
The oldest crown group polychaetes fossils come from the Sirius Passet Lagerstatte
, which is tentatively dated to the lower-middle Atdabanian
(early Cambrian). Many of the more famous Burgess Shale
organisms, such as Canadia
, 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.
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
, 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.
- Epitoky, a form of reproduction of Polychaetae.
- Campbell, Reece, and Mitchell. Biology. 1999.
- Rouse, Greg W.; Fauchald, Kristian (1998). "Recent views on the status, delineation, and classification of the Annelida". American Zoologist 38 953–964.