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Brief Summary

    Brief Summary
    provided by EOL staff

    Polychaeta, composed of about 10,000 species, is the larger (and apparently not monophyletic) of the two generally recognized major groups of segmented worms (phylum Annelida) – the other being the Clitellata (earthworms and leeches).

    Polychaete worms are characterized by an elongated, metameric body usually bearing a pair of appendages called parapodia on each metamere (segment), as well as tufts of chaetae (spines served by muscles which typically can be extended and retracted; often the polychaetes are called bristle worms, and their name derives from the Latin for many bristles). Parapodia show vast diversity of form and function, serving purposes such as locomotion, gas exchange, protection, attachment, controlling water flow within a tube, or can be reduced or lost altogether. The polychaete head can be adorned with a multitude of sensory structures such as tentacular palps, antennae, and cirri. Predatory carnivores often have large pharyngeal jaws. At the end of the segmented body is the tail, called the pygidium, which houses the anus (Brusca and Brusca 2003)

    Some polychaetes are free-living, with well developed muscles that allow them to move by swimming, crawling, or burrowing, often aided by parapodia adapted as paddles or legs. Burrowers often have a muscular proboscis to aid in digging. In contrast, sedentary polychaetes feed from permanent tubes or burrows, often by suspension feeding, selective deposit feeding, or feeding on detritus. Their parapodia are often adapted for circulating water in the tube. Permanent tube-dwellers have softer and less muscular bodies and frequently lose the septa between segments. This allows for adjustment of hydrostatic pressure within the worm, which is important for functions such as anchoring the end of the body housed in its tube. As well as providing protection, tubes also function as external support for these worms. Tubes can be soft, parchment-like forms constructed from sand and mucus, or hard calcareous tubes, which when many worms are together, form reef structures (Brusca and Brusca 2003)

    Reflecting the large diversity of lifestyles and degree of independence of body segments, polychaete circulatory and respiratory systems also show many variations among taxa. Gas exchange in many polychaetes is facilitated by distinct gills, but in others it occurs across the entire body surface (especially in small or sedentary taxa with no parapodia, or in worms with no or partial internal body septa to separate coelomic spaces). Some taxa increase surface respiratory areas with feathery protrusions of the body surface through which the coelom extends. In these taxa, the circulatory system is reduced, and most oxygen and nutrients are distributed in the coelomic fluid. Some taxa have pumping structures to increase blood flow, especially sedentary worms that do not use body movements to circulate the blood. Most (but not all) polychaetes have oxygen-carrying pigments in their circulatory fluid and coelomic fluid, usually a form of haeomoglobin. Polychaetes display a large array of different sensory structures, including touch receptors; photoreceptors which may be developed into one or more pairs of anteriorly positioned eyes or distributed around the body; chemoreceptors, and statocysts.

    (Brusca and Brusca 2003; Kozloff 1990)

    Polychaete: Brief Summary
    provided by wikipedia

    The Polychaeta /ˌpɒlɪˈkiːtə/, also known as the bristle worms or polychaetes, are a paraphyletic 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. As such, 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 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 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 deepest known spot in the Earth's oceans. Only 168 species (less than 2% of all polychaetes) are known from fresh waters.

Comprehensive Description

    Comprehensive Description
    provided by EOL staff

    Polychaete worms are characterized by an elongated, metameric body usually bearing a pair of appendages called parapodia on each metamere (segment), as well as tufts of chaetae (spines served by muscles which typically can be extended and retracted; often the polychaetes are called bristle worms, and their name derives from the Latin for many bristles). The body segmentation is visible as lateral lines around the worm’s body, reflecting the internal separation of segments with septa (although septa are lost or reduced in some groups, especially tube dwellers, see below). Parapodia show vast diversity of form and function, serving purposes such as locomotion, gas exchange, protection, attachment, controlling water flow within a tube, or can be reduced or lost altogether. The polychaete head can be adorned with a multitude of sensory structures such as tentacular palps, antennae, and cirri. Predatory carnivores often have large pharyngeal jaws. At the end of the segmented body is the tail, called the pygidium, which houses the anus (Brusca and Brusca 2003)

    A common organization of the polychaetes is to divide them into sedentary forms and free-living forms. Although this organization does not reflect their genetic relationships, it does illustrate the adaptation of their body form to their habitat and lifestyle. The free-living forms, which include families of carnivorous predators as well as direct deposit feeders – (free-living worms that burrow ingest the sediment sift out food particles in their gut) are commonly composed of a series of identical body segments (a homonomous body plan). They have well developed muscles and move by swimming, crawling, or burrowing with their parapodia adapted as paddles or legs. Burrowers often have a muscular proboscis to aid in digging. In contrast, the body segments of sedentary, tube-dwelling polychaetes show specializations for different functions (heterotomous form). These worms feed from permanent tubes or burrows, often by suspension feeding, selective deposit feeding or feeding on detritus. Their parapodia are often adapted for circulating water in the tube. Permanent tube-dwellers have softer and less muscular bodies, and frequently lose the septa between segments. This allows for adjustment of hydrostatic pressure within the worm, which is important for functions such as anchoring the end of the body housed in its tube. As well as providing protection, tubes also function as external support for these worms. Tubes can be soft, parchment-like forms constructed from sand and mucus, or hard calcareous tubes, which when many worms are together, form reef structures (Brusca and Brusca 2003)

    Reflecting the large diversity of lifestyles and degree of independence of body segments, polychaete circulatory and respiratory systems also show many variations among taxa. Almost all polychaetes have a closed circulatory system. Many have distinct gills, usually adapted as highly vascularized parts of the parapodia, and circulatory systems are well-developed with a pair longitudinal vessels carrying blood in the anterior (dorsal vessel) and posterior (ventral vessel) directions along the full body of the worm. Gas exchange in others occurs across the entire body surface (especially in small or sedentary taxa with no parapodia, or in worms with no or partial internal body septa to separate coelomic spaces). Some taxa increase surface respiratory areas with feathery protrusions of the body surface through which the coelom extends. In these taxa the circulatory system is reduced, and most oxygen and nutrients are distributed in the coelomic fluid. Some taxa have pumping structures to increase blood flow, especially sedentary worms that do not use body movements to circulate the blood. Most (but not all) polychaetes have oxygen-carrying pigments in their circulatory fluid and coelomic fluid, usually a form of haeomoglobin. Some taxa have more than one pigment. The pigments that are present often have adaptive value for the animal’s lifestyle, for example intertidal dwellers have the ability to hold oxygen during high tides and release it during low tides. Almost all polychaetes have metanephridia allowing for each coelomic space to eliminate waste, osmoregulate, and spawn gametes. The nervous system includes a cerebral ganglion at the head and one or more longitudinal nerves running the length of the body with an associated pair of ganglia in each segment. Polychaetes display a large array of different sensory structures, including touch receptors; photoreceptors which may be developed into one or more pairs of anteriorly positioned eyes or distributed around the body; chemoreceptors, and statocysts.

    (Brusca and Brusca 2003; Kozloff 1990)

    Polychaete
    provided by wikipedia
    Included groups

    Palpata
    Scolecida

    Cladistically included but traditionally excluded groups

    Chaetopteridae[1]

    The Polychaeta /ˌpɒlɪˈktə/, also known as the bristle worms or polychaetes, are a paraphyletic[1] 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. As such, 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 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 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 deepest known spot in the Earth's oceans.[2] Only 168 species (less than 2% of all polychaetes) are known from fresh waters.[3]

    Description

    Polychaetes are segmented worms, generally less than 10 cm (4 in) in length, although ranging at the extremes from 1 mm (0.04 in) to 3 m (10 ft), 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 setae, project from the parapodia.[4]

    However, polychaetes vary widely from this generalised 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 niches, including burrowing, swimming, pelagic life, tube-dwelling or boring, commensalism, and parasitism, 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.[4]

    The head also includes a pair of antennae, tentacle-like palps, and a pair of pits lined with cilia, known as "nuchal organs". These latter appear to be chemoreceptors, and help the worm to seek out food.[4]

    Internal anatomy and physiology

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    General anatomy of a polychaete

    The outer surface of the body wall consists of a simple columnar epithelium covered by a thin cuticle. Underneath this, in order, are a thin layer of connective tissue, a layer of circular muscle, a layer of longitudinal muscle, and a peritoneum surrounding the 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's 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 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. The digestive tract is a simple tube, usually with a stomach part way along.

    The smallest species, and those adapted to burrowing, lack gills, 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.[4]

    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 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, statocysts, and numerous additional sensory nerve endings, most likely in involved with the sense of touch, also occur on the body.[4]

    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.[4]

    The cuticle is constructed from cross-linked fibres of collagen and may be 200 nm to 13 mm thick. Their jaws are formed from sclerotised collagen, and their setae from sclerotised chitin.[5]

    Ecology

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    Christmas tree worms (Spirobranchus giganteus) from East Timor.

    Polychaetes 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 commensals. A few are parasitic. 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., fanworms.

    Underwater polychaetes have eversible mouthparts used to capture prey.[6] 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".[7]
    • 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 animal, 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 Challenger Deep, the greatest depth in the oceans, near 10,902 m (35,768 ft) in depth. It was about an inch long visually, but the probe failed to capture it, so it could not be studied in detail.[8]
    • The Bobbit worm (Eunice aphroditois) is a predatory species that can achieve a length at 3 m (9.8 feet), with an average diameter of 25 mm (0.98 in).

    Reproduction

    Most polychaetes have separate sexes, rather than being hermaphroditic. The most primitive species have a pair of gonads in every segment, but most species exhibit some degree of specialisation. The gonads shed immature gametes 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 plankton, 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.[4]

    Some polychaetes exhibit remarkable reproductive strategies. Some species in the genus Eunicie 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.[9]

    Another strange strategy is employed by the deep sea worm Syllis ramosa which lives inside a sponge. The rear end of the worm develops into a "stolon" containing the eggs or sperm, and this stolon becomes detached from the parent worm, and rises to the sea surface where fertilisation takes place.[10]

    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).[11] Many of the more famous Burgess Shale organisms, such as Canadia, may also have polychaete affinities. Wiwaxia, long interpreted as an annelid,[12] is now considered to represent a mollusc.[13][14] An even older fossil, Cloudina, dates to the terminal Ediacaran period; this has been interpreted as an early polychaete, although consensus is absent.[15][16]

    Being soft-bodied organisms, the fossil record of polychaetes is dominated by their fossilized jaws, known as scolecodonts, and the mineralized tubes that some of them secrete.[17] 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.[5] 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.[5] Their preservation potential is similar to that of jellyfish.[5]

    Taxonomy and systematics

    Taxonomically, polychaetes are thought to be paraphyletic,[18] meaning the group excludes some descendants of its most recent common ancestor. Groups that may be descended from the polychaetes include the oligochaetes (earthworms and 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.

    These divisions were shown to be mostly paraphyletic in recent years.

    See also

    References

    Bibliography

    • Campbell, Reece, and Mitchell. Biology. 1999.
    • Rouse, Greg W.; Fauchald, Kristian (1998). "Recent views on the status, delineation, and classification of the Annelida" (PDF). American Zoologist. 38 (6): 953–964. doi:10.1093/icb/38.6.953..mw-parser-output cite.citation{font-style:inherit}.mw-parser-output q{quotes:"""""'"'"}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/6/65/Lock-green.svg/9px-Lock-green.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-limited a,.mw-parser-output .cs1-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/d/d6/Lock-gray-alt-2.svg/9px-Lock-gray-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Lock-red-alt-2.svg/9px-Lock-red-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{font-size:100%}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}

    Notes

    1. ^ a b Struck, T. H.; Paul, C.; Hill, N.; Hartmann, S.; Hösel, C.; Kube, M.; Lieb, B.; Meyer, A.; Tiedemann, R.; Purschke, G. N.; Bleidorn, C. (2011). "Phylogenomic analyses unravel annelid evolution". Nature. 471 (7336): 95–98. doi:10.1038/nature09864. PMID 21368831.
    2. ^ Geography of Guam ns.gov.gu Accessed Oct. 8, 2009
    3. ^ Glasby, Cristopher; Timm, Tarmo (2008). E. V. Balian; C. Lévêque; H. Segers; K. Martens, eds. "Global diversity of polychaetes (Polychaeta: Annelida) in freshwater". Hydrobiologia. 595 (1: Freshwater Animal Diversity Assessment): 107–115. CiteSeerX 10.1.1.655.4467. doi:10.1007/s10750-007-9008-2.
    4. ^ a b c d e f g Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 469–525. ISBN 0-03-056747-5.
    5. ^ a b c d Briggs, D. E. G.; Kear, A. J. (1993). "Decay and preservation of polychaetes; taphonomic thresholds in soft-bodied organisms". Paleobiology. 19 (1): 107–135. doi:10.1017/S0094837300012343. JSTOR 2400774.
    6. ^ "Bristleworm". mesa.edu.au.
    7. ^ "'Zombie worms' found off Sweden". BBC News. 18 October 2005. Retrieved 12 February 2010.
    8. ^ Accessed Oct. 8, 2009 Geography of the ocean floor near Guam with some notes on exploration of the Challenger Deep.
    9. ^ Piper, Ross (2007). Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals. Greenwood Press.
    10. ^ Frost, Emily; Waters, Hannah (1 July 2015). "Some polychaetes have sex lives out of a science fiction movie". 14 fun facts about marine bristle worms. Smithsonian.com. Retrieved 9 August 2017.
    11. ^ Morris, S. C.; Peel, J. S. (2008). "The Earliest Annelids: Lower Cambrian Polychaetes from the Sirius Passet Lagerstätte, Peary Land, North Greenland". Acta Palaeontologica Polonica. 53: 137. doi:10.4202/app.2008.0110.
    12. ^ Butterfield, N. J. (1990). "A reassessment of the enigmatic Burgess Shale fossil Wiwaxia corrugata (Matthew) and its relationship to the polychaete Canadia spinosa Walcott". Paleobiology. 16 (3): 287–303. doi:10.2307/2400789. JSTOR 2400789.
    13. ^ Smith, M. R. (2012). "Mouthparts of the Burgess Shale fossils Odontogriphus and Wiwaxia: Implications for the ancestral molluscan radula". Proceedings of the Royal Society B. 279 (1745): 4287–4295. doi:10.1098/rspb.2012.1577. PMC 3441091. PMID 22915671.
    14. ^ Smith, M. R. (2014). "Ontogeny, morphology and taxonomy of the soft-bodied Cambrian 'mollusc' Wiwaxia". Palaeontology. 57 (1): 215–229. doi:10.1111/pala.12063.
    15. ^ Miller, A. J. (2004). "A revised morphology of Cloudina with ecological and phylogenetic implications" (PDF). CiteSeerX 10.1.1.526.5035. Retrieved 2007-04-24.
    16. ^ Vinn, O.; Zatoń, M. (2012). "Inconsistencies in proposed annelid affinities of early biomineralized organism Cloudina (Ediacaran): structural and ontogenetic evidences". Carnets de Géologie (CG2012_A03): 39–47. doi:10.4267/2042/46095. Retrieved 2012-08-29.
    17. ^ Vinn, O.; Mutvei, H. (2009). "Calcareous tubeworms of the Phanerozoic" (PDF). Estonian Journal of Earth Sciences. 58 (4): 286–296. doi:10.3176/earth.2009.4.07. Retrieved 2011-09-15.
    18. ^ Westheide, W. (1997). "The direction of evolution within the Polychaeta". Journal of Natural History. 31 (1): 1–15. doi:10.1080/00222939700770011.

Morphology

    Sexual Dimorphism
    provided by Fairbairn 2013
    Females generally larger than males and sometimes thicker-bodied; Sexual Dimorphism also in number of nephridial papillae, genital papillae, position of the genital opening and shape of the pygidial appendages; Sexual dimorphism can be extreme with dwarf males showing great reduction in size and simplification of morphology; males may live within the female’s body.
    Morphology
    provided by Animal Diversity Web

    Other Physical Features: ectothermic ; bilateral symmetry

Habitat

    Habitat
    provided by EOL staff

    The polychaete worms (in the traditional inclusion of taxa) are mostly marine. They are commonly found burrowing in sediments on beaches, or live in tubes, which in cases where many worms live together, form calcareous reef structures. Some species are free-swimming, some live as commensals or parasites. Polychaetes are the most abundant macrofauna of the deep sea, and inhabit the world’s oceans at all different depths and water temperatures.

Life Cycle

    Life Cycle
    provided by EOL staff

    Polychaete eggs undergo spiral cleavage. Many develop into a free-swimming trochophore larva. This larva grows at the “growth zone” on the posterior end, by forming sequential serial segments in a process called teloblastic growth. After a period of larval elongation, the larvae settles from the plankton and becomes a juvenile worm. Other polychaetes undergo direct development (without a larval stage), or short, non-feeding trochophore stage (Brusca and Brusca 2003)

Reproduction

    Reproduction
    provided by EOL staff


    Most polychaetes can regenerate to some degree – from regenerating lost appendages to posterior body segments. Some polychaetes reproduce asexually by breaking into two or more groups of segments, and some reproduce very efficiently this way by “multiple fragmentation”, in which each segment becomes a new individual (for example Dodecaceria (Cirratulidae)).

    Polychaetes are almost all dioecious, although they do not have distinct gonads. Instead, patches of the peritoneum lining the coelem (in one or many segments, depending on the taxon) divide to produce prospective gametes, which then break off and fully mature into eggs or sperm in the coelom. Eggs and sperm are released from the worm’s coelom through nephridial or coelomic ducts to the outside, or through breaks in the worm’s body wall during spawning. Most polychaetes have external fertilization, although some species brood their young. In order to maximize fertilization some primarily-benthic polychaetes (characteristic in the families Nereidae, Eunicidae, and Sillidae) spawn while swarming at the top of the water column. To do this, these worms transform into a swimming, sexual form quite different from the benthic form (called epitoky). One way this is done, as seen in members of the families Nereidae and Eunicidae, is by completely transforming the whole body into a sexual individual. Some notable modifications include enlarging swimming parapodia at the anterior end and often developing large eyes. Epitoke formation is stimulated by hormones in the brain, which are found only in older worms. Another method is to bud off the posterior portion of the body to form the sexual epitoke. This happens in the Syllidae. A third method of forming a sexual form is for a hind portion with the gametes to break off. This is not a true epitoke because the swimming section is not a complete worm. Palola viridis (Eunicidae) is an example of a species that makes this type of swarmer. Lunar cycles trigger the swarming event, and this species swarms in such huge numbers that natives of the samoan islands, where they are found, collect and feast on them (Kozloff 1990; Brusca and Brusca 2003)

Evolution

    Classification
    provided by World Register of Marine Species
    Several minor orders presented in Fauchald (1977)are now regarded as superfluous. All orders so tagged do not have child taxa that are currently in use. Basically the classification of Rouse & Pleijel (2001: Fig.1.1) is presented here, with old name Errantia substituting for their Aciculata and old name Sedentaria including their Canalipalpata and Scolecida, but if the indications of placements in Zrzavý et al (2009) are implemented, then Scolecida is polyphyletic and its families should disperse into Terebellida and Sabellida. Further, Struck (2011) has introduced Clade Pleistoannelida defined as the last common ancestor of a revived Errantia and Sedentaria, with the latter containing Clitellata (leeches and earthworms). This view is partly integrated into the WoRMS arrangement of Annelida taxa, although his new Pleistoannelida may be superfluous if Polychaeta is retained. Palpata as a subclass appears likely to be superfluous and is not currently used here. Some interstitial annelid families (including former 'archiannelids') are under 'Polychaeta incertae sedis' awaiting assignment, together with some other taxa of obscure or Polychaeta-basal affinities.
    Systematics and Taxonomy
    provided by EOL staff

    The polychaete worms are problematic in that molecular phylogenetic analyses carried out the last 20 years indicate that they are not a natural (monophyletic) group; rather, this clade includes the phylums Sipunculata and Echiura, as well as a small enigmatic group of worms called the beard worms (pogonophorans+vestimentiferans; about 100 species). In addition, many analyses also now suggest that Clitellata, which has long been considered the sister group to the polychaetes, is actually a derived group of worms within the polychaetes (McHugh 1997, 2005, Struck et al 2007).

    Fossil polychaetes have been found dating back to the early Cambrian. They are known mostly from fossilized jaws and mineralized tubes (Brusca and Brusca 2003).