Everything about Polychaete totally explained
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
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, for example,
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.
Evolutionary history
The oldest crown group polychaetes fossils come from the
Sirius Passet, which is tentatively dated to the lower-middle
Atdabanian (early Cambrian). 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.
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 doesn't 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 doesn't 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.
Further Information
Get more info on 'Polychaete'.
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