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Polychaeta () is a paraphyletic class of generally marine Annelid, common name bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia which bear many bristles called , hence their name.
More than 10,000 species have been described in this diverse and widespread class; in addition to inhabiting all of the world's oceans, polychaetes occur at all ocean depths, from species living near the Sea surface, to a small undescribed species observed through ROV at the deepest region in the Earth's oceans, Challenger Deep. In addition, many species live on the , , Parasitism, and a few within fresh water.
Commonly encountered representatives include the , bloodworms, and species of Alitta such as the Alitta succinea and Alitta virens; these species inhabit shallow water marine environments and of Subtropics and temperate regions around the world and may be used as fishing bait. More exotic species include the stinging fireworms, the predatory and large-bodied bobbit worm, the culturally important palolo worm, the , and giant tube worms, which are that tolerate near-boiling water near hydrothermal vents.
However, polychaete vary widely from this generalized pattern, and can display a range of different body forms. The most generalised polychaetes are those that Benthos, but others have adapted to many different , including burrowing, pelagic swimming, Tube worm or ones bored out of a substrate, , and parasitism; such varied lifestyles requires a divergence from the basic body plan of the common ancestor.
The head, or prostomium, is relatively well developed, compared with other annelids. It projects forward over the mouth, which is located on the succeeding section; the peristomium. The mouthparts vary in form depending on their diets, since the group includes predators, herbivores, filter feeders, scavengers, and parasites. In general, however, they possess a pair of jaws and a pharynx that can be rapidly everted, allowing the worms to grab food and pull it into their mouths. In some species, the pharynx is modified into a lengthy proboscis. Their jaws are formed from sclerotised collagen. The digestive tract is a simple tube, usually with a stomach partway along.
The head may include two to four pairs of eyes, although some species are eyeless. The eyes are typically fairly simple structures, capable of distinguishing only light and dark, although some species have large eyes with that may be capable of more sophisticated vision, an example being the complex eyes of Alciopidae, which rival those of Cephalopod eye and Tetrapod eye. High-resolution vision in pelagic polychaetes The head also includes a pair of antennae, tentacle-like , and a pair of pits lined with cilia known as , which are that help the worm to seek out food.
The outer surface of the body wall consists of a simple columnar epithelium covered by a thin cuticle, constructed from and may be thick. Sclerotized collagen makes up their setae.
Underneath the cuticle, in order, are a thin layer of connective tissue, a layer of circular muscle, a layer of longitudinal muscle, and a peritoneum surrounding the coelom (body cavity). Additional oblique muscles move the parapodia. In most species the body cavity is divided into separate compartments by sheets of peritoneum between each segment, but in some species it is more continuous.
The smallest species, and those adapted to burrowing, lack , breathing only through their body surfaces (by diffusion). Most other species have external gills, often associated with the parapodia.
The nervous system consists of a single or double ventral nerve cord running the length of the body, with ganglion and a series of small nerves in each segment. The brain is relatively large, compared with that of other annelids, and lies in the upper part of the head. An endocrine gland is attached to the ventral posterior surface of the brain, and appears to be involved in reproductive activity. In addition to the sensory organs on the head, photosensitive eye spots, , and numerous additional sensory nerve endings, most likely involved with the sense of touch, also occur on the body.
Polychaetes have a varying number of protonephridia or metanephridia for excreting waste, which in some cases can be relatively complex in structure. The body also contains greenish "chloragogen" tissue, similar to that found in , which appears to function in metabolism, in a similar fashion to that of the vertebrate liver.
Many species exhibit bioluminescence; eight families have luminous species.
The mobile forms (Errantia) tend to have well-developed sense organs and jaws, while the stationary forms (Sedentaria) lack them, but may have specialized gills or tentacles used for respiration and deposit or filter feeding, e.g., .
Underwater polychaetes have eversible mouthparts used to capture prey. 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.
The fertilized eggs typically hatch into trochophore larvae, which float among the plankton, and eventually metamorphosis into the adult form by adding segments. A few species have no larval form, with the egg hatching into a form resembling the adult, and in many that do have larvae, the trochophore never feeds, surviving off the yolk that remains from the egg.
However, some polychaetes exhibit remarkable reproductive strategies. Some species reproduce by epitoky. For much of the year, these worms look like any other burrow-dwelling polychaete, but as the breeding season approaches, the worm undergoes a remarkable transformation as new, specialized segments begin to grow from its rear end until the worm can be clearly divided into two halves. The front half, the atoke, is asexual. The new rear half, responsible for breeding, is known as the epitoke. Each of the epitoke segments is packed with eggs and sperm and features a single eyespot on its surface. The beginning of the last lunar quarter is the cue for these animals to breed, and the epitokes break free from the atokes and float to the surface. The eye spots sense when the epitoke reaches the surface and the segments from millions of worms burst, releasing their eggs and sperm into the water.
A similar strategy is employed by the branching deep sea worm Syllis ramosa, which lives inside a sponge; the worm develop "stolons" containing eggs or sperm from one of their many rear ends; these stolons detach from the parent worm and rise to the sea surface, where fertilisation takes place.
Being soft-bodied organisms, the fossil record of polychaetes is dominated by their fossilized jaws, known as , and the tubes that some of them secrete. Most important biomineralising polychaetes are Serpulidae, Sabellidae, and Cirratulidae. Polychaete cuticle does have some preservation potential; it tends to survive for at least 30 days after a polychaete's death. Although biomineralisation is usually necessary to preserve soft tissue after this time, the presence of polychaete muscle in the nonmineralised Burgess shale shows this need not always be the case. Their preservation potential is similar to that of jellyfish.
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.
These divisions were shown to be mostly paraphyletic in recent years.
Below is a phylogenetic tree of annelids from a 2021 review of annelid diversity (clades labeled × are not considered polychaetes);
Physiology
A simple but well-developed circulatory system is usually present. The two main blood vessels furnish smaller vessels to supply the parapodia and the gut. Blood flows forward in the dorsal vessel, above the gut, and returns down the body in the ventral vessel, beneath the gut. The blood vessels themselves are contractile, helping to push the blood along, so most species have no need of a heart. In a few cases, however, muscular pumps analogous to a heart are found in various parts of the system. Conversely, some species have little or no circulatory system at all, transporting oxygen in the coelomic fluid that fills their body cavities. The blood may be colourless, or have any of three different respiratory pigments. The most common of these is haemoglobin, but some groups have haemerythrin or the green-coloured chlorocruorin, instead.
Ecology
Polychaetes are predominantly marine, but 168 species (nearing 2% of total species) also live in freshwater, and a few in terrestrial environments. Annelids in Extreme Aquatic Environments: Diversity, Adaptations and Evolution They are extremely variable in both form and lifestyle, and include a few taxa that swim among the plankton or above the abyssal plain. Most burrow or build tubes in the sediment, and some live as . A few species, roughly 80 (less than 0.5% of species), are parasitic. (1998). 9780429210600, CRC Press. ISBN 9780429210600 These include both ectoparasites and endoparasites. Ectoparasitic polychaetes feed on skin, blood, and other secretions, and some are adapted to bore through hard, usually calcerous surfaces, such as the shells of mollusks. These "boring" polychaetes may be parasitic, but may be opportunistic or even obligate symbionts (commensals).
Reproduction
Most polychaetes have Dioecy, rather than being Hermaphrodite. The most primitive species have a pair of in every segment, but most species exhibit some degree of specialisation. The gonads shed immature directly into the body cavity, where they complete their development. Once mature, the gametes are shed into the surrounding water through ducts or openings that vary between species, or in some cases by the complete rupture of the body wall (and subsequent death of the adult). A few species copulate, but most fertilize their eggs externally.
Evolution
Stem-group polychaete fossils are known from the Sirius Passet Lagerstätte, a rich, sedimentary deposit in Greenland tentatively dated to the late Atdabanian (early Cambrian). The oldest known polychaete as of 2025 is Dannychaeta tucolus, dated to approximately 514 million years ago. Many of the more famous Burgess Shale organisms, such as Canadia, may also have polychaete affinities. Wiwaxia, long interpreted as an annelid, is now considered to represent a mollusc. An even older fossil, Cloudina, dates to the terminal Ediacaran period; this has been interpreted as an early polychaete, although consensus is absent.
Taxonomy and systematics
Taxonomically, polychaetes are thought to be paraphyletic, meaning the group excludes some descendants of its most recent common ancestor. Groups that may be descended from the polychaetes include the ( and ), , and . The Pogonophora and Vestimentifera were once considered separate phyla, but are now classified in the polychaete family Siboglinidae.
See also
Bibliography
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