Polychaeta () is a

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of

chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is the second most abundant polysaccharide in nature (behind only cell ...

. More than 10,000 species are described in this class. Common representatives include the lugworm (''Arenicola marina'') and the sandworm or clam worm ''Alitta''. Polychaetes as a class are robust and widespread, with species that live in the coldest ocean temperatures of the abyssal plain, to forms which tolerate the extremely high temperatures near hydrothermal vents. Polychaetes occur throughout the Earth's oceans at all depths, from forms that live as

plankton Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...

near the surface, to a 2- to 3-cm specimen (still unclassified) observed by the robot ocean probe ''Nereus'' at the bottom of the

Challenger Deep The Challenger Deep is the List of submarine topographical features#List of oceanic trenches, deepest known point of the seabed of Earth, located in the western Pacific Ocean at the southern end of the Mariana Trench, in the ocean territory o ...

, the deepest known spot in the Earth's oceans. Only 168 species (less than 2% of all polychaetes) are known from fresh waters.

Description

Polychaetes are segmented worms, generally less than in length, although ranging at the extremes from to , in '' Eunice aphroditois''. They can sometimes be brightly coloured, and may be iridescent or even luminescent. Each segment bears a pair of paddle-like and highly vascularized parapodia, which are used for movement and, in many species, act as the worm's primary respiratory surfaces. Bundles of bristles, called chaetae, project from the parapodia. However, polychaetes vary widely from this generalized pattern, and can display a range of different body forms. The most generalised polychaetes are those that crawl along the bottom, but others have adapted to many different

ecological niche In ecology, a niche is the match of a species to a specific environmental condition. Three variants of ecological niche are described by It describes how an organism or population responds to the distribution of Resource (biology), resources an ...

s, including burrowing, swimming,

pelagic The pelagic zone consists of the water column of the open ocean and can be further divided into regions by depth. The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or water column between the sur ...

life, tube-dwelling or boring, commensalism, and

parasitism Parasitism is a close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The en ...

, requiring various modifications to their body structures. The head, or prostomium, is relatively well developed, compared with other annelids. It projects forward over the mouth, which therefore lies on the animal's underside. The head normally includes two to four pair of eyes, although some species are blind. These are typically fairly simple structures, capable of distinguishing only light and dark, although some species have large eyes with lenses that may be capable of more sophisticated vision, including the Alciopids' complex eyes which rival cephalopod and vertebrate eyes. Many species show bioluminescence; eight families have luminous species. The head also includes a pair of antennae, tentacle-like palps, and a pair of pits lined with

cilia The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...

, known as "nuchal organs". These latter appear to be chemoreceptors, and help the worm to seek out food.

Internal anatomy and physiology

The outer surface of the body wall consists of a simple columnar epithelium covered by a thin

cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non- homologous, differing in their origin, structu ...

. Underneath this, in order, are a thin layer of connective tissue, a layer of circular muscle, a layer of longitudinal muscle, and a

peritoneum The peritoneum is the serous membrane forming the lining of the abdominal cavity or coelom in amniotes and some invertebrates, such as annelids. It covers most of the intra-abdominal (or coelomic) organs, and is composed of a layer of mesotheli ...

surrounding the

body cavity A body cavity is any space or compartment, or potential space, in an animal body. Cavities accommodate organs and other structures; cavities as potential spaces contain fluid. The two largest human body cavities are the ventral body cavity, a ...

. 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 mouth of polychaetes is located on the peristomium, the segment behind the prostomium, and varies 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 The pharynx (: pharynges) is the part of the throat behind the human mouth, mouth and nasal cavity, and above the esophagus and trachea (the tubes going down to the stomach and the lungs respectively). It is found in vertebrates and invertebrates ...

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 A proboscis () is an elongated appendage from the head of an animal, either a vertebrate or an invertebrate. In invertebrates, the term usually refers to tubular arthropod mouthparts, mouthparts used for feeding and sucking. In vertebrates, a pr ...

. The digestive tract is a simple tube, usually with a stomach part way along. The smallest species, and those adapted to burrowing, lack

gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...

s, breathing only through their body surfaces. Most other species have external gills, often associated with the parapodia. 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. The nervous system consists of a single or double ventral nerve cord running the length of the body, with

ganglia A ganglion (: ganglia) is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system, this includes dorsal root ganglia and trigeminal ganglia among a few others. In the autonomic nervous system, there a ...

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 The endocrine system is a network of glands and organs located throughout the body. Along with the nervous system, it makes the neuroendocrine system, which controls and regulates many of the body's functions. Endocrine glands are ductless 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, statocysts, 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 oligochaetes, which appears to function in metabolism, in a similar fashion to that of the vertebrate

liver The liver is a major metabolic organ (anatomy), organ exclusively found in vertebrates, which performs many essential biological Function (biology), functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of var ...

. The cuticle is constructed from cross-linked fibres of

collagen Collagen () is the main structural protein in the extracellular matrix of the connective tissues of many animals. It is the most abundant protein in mammals, making up 25% to 35% of protein content. Amino acids are bound together to form a trip ...

and may be 200 nm to 13 mm thick. Their jaws are formed from sclerotised collagen, and their setae from sclerotised

Ecology

Polychaetes are predominantly marine, but many species also live in freshwater, and a few in terrestrial environments. They are extremely variable in both form and lifestyle, and include a few taxa that swim among the

or above the abyssal plain. Most burrow or build tubes in the sediment, and some live as commensals. A few species, roughly 80 (less than 0.5% of species), are parasitic. 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). The mobile forms (

Errantia Errantia is a diverse group of marine life, marine polychaete worms in the phylum Annelida. Traditionally a subclass (biology), subclass of the paraphyletic class Polychaeta, it is currently regarded as a monophyletic group within the larger Pleis ...

) 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., fanworms. 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.

Notable polychaetes

*One notable polychaete, the Pompeii worm (''Alvinella pompejana''), is endemic to the hydrothermal vents of the Pacific Ocean. Pompeii worms are among the most heat-tolerant complex animals known. *A recently discovered genus, '' Osedax'', includes a species nicknamed 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 m and may be the most long-lived annelid, being over 250 years old. *A still unclassified multilegged predatory polychaete worm was identified only by observation from the underwater vehicle ''Nereus'' at the bottom of the

, the greatest depth in the oceans, near in depth. It was about an inch long visually, but the probe failed to capture it, so it could not be studied in detail. *The Bobbit worm ('' Eunice aphroditois'') is a predatory species that can achieve a length of ), with an average diameter of . *''Dimorphilus gyrociliatus'' has the smallest known genome of any annelid. The species shows extreme

sexual dimorphism Sexual dimorphism is the condition where sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...

. Females measure ~1 mm long and have simplified bodies containing six segments, a reduced coelom, and no appendages, parapodia, or chaetae. The males are only 50 μm long and consist of just a few hundred cells. They lack a digestive system and have just 68 neurons, and only live for roughly a week.

Reproduction

Most polychaetes have separate sexes, rather than being hermaphroditic. The most primitive species have a pair of

gonad A gonad, sex gland, or reproductive gland is a Heterocrine gland, mixed gland and sex organ that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gon ...

s in every segment, but most species exhibit some degree of specialisation. The gonads shed immature

gamete A gamete ( ) is a Ploidy#Haploid and monoploid, haploid cell that fuses with another haploid cell during fertilization in organisms that Sexual reproduction, reproduce sexually. Gametes are an organism's reproductive cells, also referred to as s ...

s 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. The fertilized eggs typically hatch into trochophore larvae, which float among the

, and eventually metamorphose 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 deep sea worm '' Syllis ramosa'', which lives inside a

sponge Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a basal clade and a sister taxon of the diploblasts. They are sessile filter feeders that are bound to the seabed, and a ...

. The rear ends of the worm develop into "stolons" containing the eggs or sperm; these stolons then become detached from the parent worm and rise to the sea surface, where fertilisation takes place.

Fossil record

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 Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

). The oldest found is '' Phragmochaeta canicularis''. 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 The Ediacaran ( ) is a geological period of the Neoproterozoic geologic era, Era that spans 96 million years from the end of the Cryogenian Period at 635 Million years ago, Mya to the beginning of the Cambrian Period at 538.8 Mya. It is the last ...

period; this has been interpreted as an early polychaete, although consensus is absent. Being soft-bodied organisms, the fossil record of polychaetes is dominated by their fossilized jaws, known as scolecodonts, and the

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...

ized tubes that some of them secrete. Most important biomineralising polychaetes are serpulids, sabellids, and cirratulids. 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 Jellyfish, also known as sea jellies or simply jellies, are the #Life cycle, medusa-phase of certain gelatinous members of the subphylum Medusozoa, which is a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animal ...

Taxonomy and systematics

Taxonomically, polychaetes are thought to be

, meaning the group excludes some descendants of its most recent common ancestor. Groups that may be descended from the polychaetes include the clitellates (

earthworm An earthworm is a soil-dwelling terrestrial invertebrate that belongs to the phylum Annelida. The term is the common name for the largest members of the class (or subclass, depending on the author) Oligochaeta. In classical systems, they we ...

s and

leech Leeches are segmented parasitism, parasitic or Predation, predatory worms that comprise the Class (biology), subclass Hirudinea within the phylum Annelida. They are closely related to the Oligochaeta, oligochaetes, which include the earthwor ...

es), 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 In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...

have been subject to

cladistic Cladistics ( ; from Ancient Greek 'branch') is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is ...

analysis, some groups which are usually considered invalid today may eventually be reinstated. These divisions were shown to be mostly paraphyletic in recent years. * Basal or ''

incertae sedis or is a term used for a taxonomy (biology), taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertainty ...

'' **Family Diurodrilidae **Family Histriobdellidae **Family Nerillidae **Family Parergodrilidae **Family Potamodrilidae **Family Psammodrilidae **Family Spintheridae **Family Protodriloididae **Family Saccocirridae **Order Haplodrili **Order Myzostomida ***Family Endomyzostomatidae ***Family Asteromyzostomatidae ***Family Myzostomatidae *Subclass Palpata **Family Protodrilidae **Family Polygordiidae *Subclass

Aciculata Errantia is a diverse group of marine life, marine polychaete worms in the phylum Annelida. Traditionally a subclass (biology), subclass of the paraphyletic class Polychaeta, it is currently regarded as a monophyletic group within the larger Pleis ...

**Family Levidoridae **Order Amphinomida ***Family Amphinomidae ***Family Euphrosinidae **Order Eunicida ***Family Dorvilleidae ***Family Eunicidae ***Family Hartmaniellidae ***Family Ichthyotomidae ***Family Lumbrineridae ***Family Oenonidae ***Family Onuphidae **Order Phyllodocida ***Suborder Aphroditiformia ****Family Acoetidae ****Family Aphroditidae ****Family Eulepethidae ****Family Iphionidae ****Family Pholoidae ****Family Polynoidae ****Family Sigalionidae ***Suborder Glyceriformia ****Family Glyceridae ****Family Goniadidae ****Family Lacydoniidae ****Family Paralacydoniidae ***Suborder Nereidiformia ****Family Antonbruunidae ****Family Chrysopetalidae ****Family Hesionidae ****Family Nereididae ****Family Pilargidae ****Family Syllidae ***Suborder Phyllodocida incertae sedis ****Family Iospilidae ****Family Nautiliniellidae ****Family Nephtyidae ****Family Typhloscolecidae ****Family Tomopteridae ***Suborder Phyllodociformia ****Family Alciopidae ****Family Lopadorrhynchidae ****Family Phyllodocidae ****Family Pontodoridae *Subclass Sedentaria **Family Chaetopteridae **Infraclass Canalipalpata ***Order Sabellida ****Family Caobangidae ****Family Fabriciidae ****Family Oweniidae ****Family Sabellariidae ****Family Sabellidae ****Family Serpulidae ****Family Siboglinidae (formerly the phyla Pogonophora & Vestimentifera) ***Order Spionida ****Suborder Spioniformia *****Family Apistobranchidae *****Family Longosomatidae *****Family Magelonidae *****Family Poecilochaetidae *****Family Spionidae *****Family Trochochaetidae *****Family Uncispionidae ***Order Terebellida ****Suborder Cirratuliformia *****Family Acrocirridae (sometimes placed in Spionida) *****Family

Cirratulidae Cirratulidae is a family of marine polychaete worms. Members of the family are found worldwide, mostly living in mud or rock crevices. Most are deposit feeders, but some graze on algae or are suspension feeders. Although subject to multiple re ...

(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 Poeobiidae (sometimes own suborder Poeobiida or included in Flabelligerida) *****Family Sternaspidae (sometimes own suborder Sternaspida) ****Suborder Terebellomorpha *****Family Alvinellidae *****Family Ampharetidae *****Family Pectinariidae *****Family Terebellidae *****Family Trichobranchidae **Infraclass Scolecida ***Family Arenicolidae ***Family Capitellidae ***Family Cossuridae ***Family Maldanidae ***Family Opheliidae ***Family Orbiniidae ***Family Paraonidae ***Family Scalibregmatidae ***Order Capitellida (

nomen dubium In binomial nomenclature, a ''nomen dubium'' (Latin for "doubtful name", plural ''nomina dubia'') is a scientific name that is of unknown or doubtful application. Zoology In case of a ''nomen dubium,'' it may be impossible to determine whether a ...

) ***Order Cossurida (nomen dubium) ***Order Opheliida (nomen dubium) ***Order Orbiniida (nomen dubium) ***Order Questida (nomen dubium) ***Order Scolecidaformia (nomen dubium) *Subclass Echiura ** Order Bonelliida *** Family Bonelliidae *** Family Ikedidae ** Order Echiurida *** Family Echiuridae *** Family Thalassematidae *** Family Urechidae

References

Bibliography

* Campbell, Reece, and Mitchell. Biology. 1999. *

Notes

External links

World Polychaeta Database

Special issue of ''Marine Ecology''
dedicated to polychaetes

a guide to the marine zooplankton of south eastern Australia

Natural History Museum {{Authority control * Extant Cambrian first appearances Paraphyletic groups