Dictionary:

polychete

  (pŏl'ĭ-kēt')

Any of various annelid worms of the class Polychaeta, including mostly marine worms such as the lugworm, and characterized by fleshy paired appendages tipped with bristles on each body segment.

[New Latin Polychaeta, class name, from Greek polukhaitēs, with much hair : polu-, poly- + khaitē, long hair.]

(Clam, sand, and tubeworms)

Phylum: Annelida

Class: Polychaeta

Number of families: 86

Thumbnail description
Segmented worms with numerous bristles and one pair of parapodia per segment

Evolution and systematics

Polychaetes do not fossilize very well as they are soft-bodied animals. There are few fossil records from an entire worm; these have been found from the Pennsylvanian fauna. The oldest fossil records, dating to the middle Cambrian of the Burgess Shale, include the aciculates Wiwaxia and Canadia, which have a prostomium with appendages, well-developed parapodia, and different kinds of chaetae.

A considerable diversification among polychaetes occurred in the Middle Cambrian, with six genera represented in Burgess Shale (Canada). The genera Wiwaxia and Canadia do not have jaws, but some later forms possess hard jaws that could mineralize with iron oxide. Such polychaete fossils are known as scolecodonts and have been described from the Ordovician, Silurian, and Devonian periods. Other polychaete fossils include tubes and burrow structures produced by some sedentary forms that secreted a mucous lining for their burrow.

Polychaeta and Clitellata, which includes the classes Oligochaeta and Hirudinoidea, are the two great lineages within the phylum Annelida. As the marine polychaetes fossils that appeared in Middle Cambrian are the earliest record of Annelida, they are considered to be the earliest derived group within the phylum. The exact nature of the ancestral group from which Polychaeta and Clitellata arose is still obscure, but it was likely a homonomous, metameric burrower that possessed a compartmentalized coelom, paired epidermal chaetae, and a head composed of a presegmental prostomium and a peristomium much like those found in modern polychaetes. Due to some recent molecular data and anatomical and developmental evidence, many scientists believe the pogonophorans and vestimentiferans should be placed within the Annelida as a specialized polychaete family, either Pogonophoridae or Siboglionidae. The class Polychaeta is divided into 24 orders and 86 families, with more than 10,000 described species.

Physical characteristics

Polychaetes range in length from <0.078 in (<2 mm) to >9.8 ft (>3 m). The majority of them are <3.9 in (<10 cm) long and between 0.078–0.39 in (2–10 mm) wide. The morphology is greatly variable. The majority of polychaetes have a cylindrical and elongated body. The parapodia can be unirramous or biramous, with a dorsal lobe (notopodium) and ventral lobe (neuropodium).

The body morphology of polychaetes usually reflects their habits and habitats. Often, active forms have a more homonomous body construction than sedentary ones, which possess some degree of heteronomy, with differentiation in body regions that are utilized for particular functions.

Mobile forms usually have well-developed parapodia, eyes, and sensory organs. Additionally, in some species, the mouth has chitinous jaws and an eversible pharynx.

Sessile forms live in permanent burrows or tubes and have the parapodia reduced or absent; some of them possess special tentaclelike appendages projecting from the tube to collect food from the surface and also for aeration.

Polychaetes are extremely variable in color, varying from light tan to opaque, but most are colored red, pink, green, yellow, or a combination of colors. Some species are iridescent.

Polychaetes of the family Aphroditidae have the dorsum covered with scales (elytra) that can be overlaid by a hard hair-like layer. Because of these characteristics, one species is commonly called the "sea-mouse." Some planktonic species are adapted to live in the water column by usually being transparent and flattened with fin parapodia.

It is very common to observe the gas exchange structures of some polychaetes, especially the colorful ones. The branchial morphology is greatly variable: filaments in cirratulids, anterior gills in terebellids, and a tentacular or branchial crown on the heads of sabellids, serpulids, and spirorbids. The more active polychaetes possess a highly vascularized portion of the parapodia, utilizing it for gas exchange.

Distribution

Polychaetes are found worldwide, living in every marine habitat from tropical to polar regions. Some species occur in brackish or freshwater environments. A few live on land, but in habitats completely inundated with water.

Habitat

Due to morphological variations, polychaetes occupy different kinds of habitats, both planktonic and benthic. They are found from the intertidal zone to the deepest depths of the ocean in sandy and muddy sediments, digging or in temporary or permanent mucous tubes that are part of the infaunal community, or crawling on the surface of the substrate. Some species live above the sediment surface as part of the epifaunal community; for example, some nereid species live between mussel beds attached to piers.

Polychaetes are also found in coral and rocky reefs, occupying crevices or beneath stones, and some construct sandy or calcareous tubes that are often attached to coral.

Planktonic forms have adapted structures to swim, and spend all their lives in the water column. Some species tolerate the low salinity of estuaries and a few live in freshwater environments. Some spionids and a group of nereids inhabit semi-terrestrial habitats where they can be covered by freshwater during the wet season and during tidal inundations.

There are some polychaetes that have been found in deep-sea thermal vents. Some species can be commensal or even parasitic. Histriobdella homari eats incrusted bacteria and blue-green algae found on the gills and branchial chamber of Homarus americanus and Homarus gammarus. One Stratiodrilus species feeds on microorganisms in the gill chamber of freshwater crayfish or anomuran crustaceans. In the family Oenonidae, there are parasitic forms that live in the coelomic cavity of other polychaetes, echiurans, or bivalves.

Behavior

Polychaetes exhibit few behavior patterns. Certain species are gregarious, forming dense aggregations, while others are solitary. Horizontal and vertical partitioning of space in the sediment occurs between conspecifics and closely-related species or species from the same trophic group (species that feed on the same food resources).

Some polychaetes respond to shadows passing overhead and retreat rapidly back into their tubes. Some species "smell" their prey in the sediment through chemical sensory organs. Species that are prey of birds and fishes can discard their posterior end to avoid predation and can regenerate lost parts.

Some species pair during the breeding season; however, following this phase, they are so aggressive that they often eat each other. During spawning, several species are luminescent in response to light as a reproductive strategy. Others react to changes in temperature, day length, and Moon phases. All these responses are coordinated to ensure successful spawning and reproduction.

Feeding ecology and diet

The morphological and functional diversity of polychaetes enables them to exploit food resources in almost all marine environments in different ways. Polychaetes are usually categorized into raptorial (including carnivores, herbivores, and scavengers), omnivores, surface and subsurface deposit feeders, suspension feeders, and filter feeders.

Polychaetes can be non-selective or selective deposit feeders. The non-selective deposit feeders ingest sand or mud grains, showing little or no discrimination for the size and nutritional value of the particles, assimilating any organic material in the ingested sediment. Selective deposit feeders, however, utilize structures such as palps, tentacles, or buccal organs to select particles with high nutritional value.

The raptorial species usually have a homonomous construction, well-developed parapodia, sensory organs on the head, and a pharynx armed with hard jaws (nereids, glycerids, phyllodocids, syllids). They make rapid movements across the substratum. They often prey on small invertebrates. The prey can be located through sensorial or chemical means. Certain forms (glycerids) have poison glands associated with the jaws. Some raptorial species are not active hunters; they lie in wait for passing prey.

Herbivorous polychaetes feed by scraping or tearing plant material with their pharyngeal structures. Some scavengers feed on any dead or organic material they encounter. Omnivorous species feed on any material they find, and there are some carnivores that feed on deposits when prey are scarce.

Surface and subsurface deposit feeders ingest sandy or muddy particles, feeding on organic material attached to them. They usually have a saclike pharynx that sucks the grains from the sediment when burrowing (capitellids). Some surface deposit feeders have grooved mucous tentacles that collect particles from the surface and carry them directly into the mouth (terebellids, cirratulids).

Suspension feeders possess specialized structures such as tentacular sulcated crowns, or palps on the head, that enable them to collect suspended material in the water column. Some forms are very active, frequently moving their palps (spionids), and others simply expose their crown, waiting for particles to fall onto their surface (Oweniidae).

Filter feeders have specialized crowns with pinnated radioles that create a water current through the pinnules, collecting the particles in suspension (sabellids, sabelariids, spirorbids, serpulids).

Reproductive biology

Polychaetes have different degrees of regeneration. They regenerate lost appendages such as palps, tentacles, cirri, and parapodia. The regeneration of posterior ends is common, but regeneration of a lost head end is uncommon. Many polychaetes utilize regeneration during asexual reproduction, producing a series of individuals, a bud that grows from an individual, or new individuals that develop from an isolated fragment.

The majority of polychaetes are dioecious (gonochoristic); hermaphroditism occurs in relatively few species. Gametes usually mature inside the coelom and are released by gonoducts, coelomoducts, nephridia, or through the rupture of the parental body wall. The majority of species release their gametes into the water, where fertilization takes place. The larvae are planktotrophic, but some species have lecitotrophic larvae, and a few have both. Species with internal fertilization brood their eggs or produce encapsulated eggs that float or are attached to the substratum.

As a rule, segments are generated from a posterior growth zone, arising and developing sequentially from the anterior to the posterior.

Some polychaetes have evolved methods to increase the chances of fertilization. Some sexually reproductive nereids, syllids, and eunicids form an epitokous individual, wherein various body parts or the whole body become a gamete-carrying bag capable of swimming from the bottom upward into the water column, where the gametes are spread. In many polychaetes, the larva is a trochophore that possesses a locomotory ciliary band near the mouth region. The lifespan can range from a few weeks to several years, depending on the reproductive strategy of the particular species.

Conservation status

Polychaetes are part of many investigations that contribute to the conservation and knowledge of the biodiversity of the marine environment. No species of polychaete are listed by the IUCN. However, the Palolo worm, Eunice viridis, could potentially be in need of conservation efforts.

Significance to humans

Polychaetes play an important role in the marine benthic food chain, not only serving as food for other organisms, but also recycling organic matter within the sediment and breaking down plant material. Some polychaetes, such as nereids, are known to be important food sources for birds and for economically important fishes; polychaetes are also used as bait for recreational fishing.

Polychaetes play an important role in monitoring marine environmental quality. They respond quickly to changes in the environment, promoted by anthropogenic compounds or chemical contaminants because of their direct contact with the sediments and water column.

Some species can provide an indication of the condition and health of the sediment they live in and often occur at high densities in polluted habitats.

Some polydorids (Spionidae) bore into oyster shells, affecting their appearance and hence their market value, and cause a decline in oyster cultures or become pests. There are numerous significant fouling species that can settle and grow on the hulls of ships.

Species accounts

Resources

Blake, James A., B. Hilbig, and P. H. Scott. The Annelida Part 2—Polychaeta: Phyllodocida (Syllidae and Scale-bearing families), Amphinomida and Eunicida. Taxonomic Atlas of the Benthic Fauna of the Santa Maria Basin and the Western Santa Barbara Channel series, Vol. 5. Santa Barbara, CA: Santa Barbara Museum of Natural History, 1995. ——. The Annelida Part 4—Polychaeta: Flabelligerida to Sternaspidae. Taxonomic Atlas of the Benthic Fauna of the Santa Maria Basin and the Western Santa Barbara Channel series, Vol. 7. Santa Barbara, CA: Santa Barbara Museum of Natural History, 2000.

Brusca, N. C., and G. J. Brusca. Invertebrates. 2nd edition. Sunderland: MA: Sinauer Associates Inc. Publishers, 2003.

Giese, Arthur. C., and John S. Pearse. Reproduction of Marine Invertebrates. London: Academic Press, Inc. LTD, 1975.

Glasby, Christopher J., et al. "Class Polychaeta." In Polychaetes and Allies. The Southern Synthesis. Fauna of Australia. Polychaeta, Myzoztomida, Pogonophora, Echiura, Sipuncula, Vol. 4. Melbourne, Australia: CSIRO Publishing, 2000.

Rouse, Greg, and Fredrik W. Pleijel. Polychaetes. New York: Oxford University Press Inc., 2001.

Arndt, C., and D. Schiedek. "Nephtus hombergii, A Free-living Predator in Marine Sediments: Energy Production under Environmental Stress." Marine Biology 129 (1998): 643–540.

Bat, L., and D. Raffaelli. "Sediment Toxicity Testing: A Bioassay Approach Using the Amphipod Corophium volutator and the Polychaete Arenicola marina." Journal of Experimental Marine Biology and Ecology 226 (1998): 217–239.

Bridges, T. S. "Effects of Organic Additions to Sediment, and Maternal Age and Size, on Patterns of Offspring Investment and Performance in Two Opportunistic Deposit-feeding Polychaetes." Marine Biology 125 (1996): 345–357.

Dill, L. M., and A. H. G. Fraser. "The Worm Returns: Hiding Behavior of a Tube-dwelling Marine Polychaete, Serpula vermicularis." Behavioral Ecology 8, no. 2 (1997): 186–193.

Fauchald, J., and P. A. Jumars. "The Diet of Worms: A Study of Polychaete Feeding Guilds." Oceanography and Marine Biology: An Annual Review 17 (1979): 193–284.

Giangrande, Adriana "Polychaete Reproductive Patterns, Life Cycles and Life Histories: An Overview." Oceanography and Marine Biology: An Annual Review 35 (1997): 323–389.

Gamenick, I., and O. Giere. "Ecophysiological Studies on the Capitella capitata Complex: Respiration and Sulfide Exposure." Bulletin of Marine Science 60 (1997): 613.

Grassle, J. F., and J. P. Grassle. "Sibling Species in the Marine Pollution Indicator (Capitella capitata) (Polychaete)." Science 192 (1976): 567–569.

Hardege, J. D., M. G. Bentley, and L. Snape. "Sediment Selection by Juvenile Arenicola marina (Polychaete)." Marine Ecology Progress Series 166 (1998): 187–195.

Mendez, N., J. Romero, and J. Flos. "Population Dynamics and Production of the Polychaete Capitella capitata in the Littoral Zone of Barcelona (Spain, NW Mediterranean)." Journal of Experimental Marine Biology and Ecology 218 (1997): 263–284.

Pearson, T. H., and R. Rosenberg. "Macrobenthic Succession in Relation to Organic Enrichment and Pollution of the Marine Environment." Oceanography and Marine Biology: An Annual Review 16 (1978): 229–311.

Riisgard, H. U., and G. T. Banta. "Irrigation and Deposit Feeding by the Lugworm Arenicola marina, Characteristics and Secondary Effects on the Environment. A Review of Current Knowledge." Vie Milieu 48 (1998): 243–257.

Suadicani, S. O., J. C. Freitas, and M. I. Sawaya. "Pharmacological Evidence for the Presence of a Beta-adrenoceptor-like Agonist in the Amphinomid Polychaete Eurythoe complanata." Comparative Bichemistry and Physiology 104C, no. 2 (1993): 327–332.

Tsutsumi, H., and T. Kikuchi. "Study of the Life History of Capitella capitata Polychaeta: (Capitellidae) in Amakusa, South Japan, Including a Comparison with Other Geographical Regions." Marine Biology 80 (1984): 315–321.

Wilson, D. P. "Additional Observations on Larval Growth and Settlement of Sabellaria alveolata." Journal of the Marine Biological Association of the United Kingdom 50, no. 1 (1970): 1–32.

Zebe, E., and D. Schiedek. "The Lugworm Arenicola marina: A Model of Physiological Adaptation to Life in Intertidal Sediments." Helgoländer Meeresuntersuchungen 50 (1996): 37–68.

[Article by: Erica Veronica Pardo, PhD]

The largest class of the phylum Annelida, containing 68–70 families. About 1600 genera and 10,000 species have been named from worldwide areas; about one-fourth of this number may be synonymous. Polychaeta (meaning “many setae”) is conveniently though not clearly divisible into the Errantia, or free-moving annelids, and Sedentaria, or tubicolous families. See also Errantia; Sedentaria.

The body may be long, cylindrical, and multisegmented, or short and compact, with a limited number of segments. It consists of prostomium (Fig. 1), or head; peristomium, or first segment around the mouth; trunk, or body proper; and tail region, or pygidium. Most segments have highly diagnostic paired, lateral fleshy appendages called parapodia. These are provided with secreted supporting rods and spreading fascicles of setae, or hooks, which display remarkable specificity.

Orbiniidae).">
Terminology of the anterior parts of the body, based on Phylo (Orbiniidae).

The anterior end, or prostomium, may be a simple lobe derived from the larval trochophore, modified as a pseudoannulated cone, or covered by peristomial structures so as to be invisible. Oral tentacles for food gathering may be eversible from the buccal cavity; they may be long, slender, or thick and their surface smooth or papillated.

The anterior, preoral end may be developed as a thick, fleshy papillated, nonretractile proboscis (Fig. 2), or the prostomium may be completely retractile into the first several segments and protected by a cage formed of setae directed forward, or concealed by a compact operculum formed of setae of the first several segments. The anterior end of the alimentary tract is muscular or epithelial; it may be covered with soft papillae or hard structures. These structures function for secretion, food gathering, and maintaining traction; they are named for their form or function.

Terebellidae).">
Nonretractile proboscis organ preceding prostomium in Artacama (Terebellidae).

The trunk is the main body region and is composed of metameres numbering few to many. They may be similar to one another (homonomous) as in Errantia, or different (heteronomous) resulting in anterior thoracic and posterior abdominal regions.

Reproduction is highly evolved and diversified; it can be sexual or asexual. Sexual reproduction is usually dioecious, with the two sexes similar. In rare cases it is dimorphic.

Polychaetes range in length from a fraction of 0.04 in. (1 mm) to more than 144 in. (360 cm). Colors and patterns are varied and specific, due to pigment and refraction of light. Littoral, warm-water species may be brilliant and multicolored, whereas polar and deep-water species tend to be drab or sometimes melanistic to almost black.

Most polychaetes are free-living; some of the remaining members are commensal with another animal for attachment surface, for food, or for protection. Polychaetes are distributed in all marine habitats and show remarkable specificity according to latitude, depth, and kinds of substrata. Most of the families tend to be represented in any major geographic area, although taxa may differ with place. See also Annelida.

For more information on polychaete, visit Britannica.com.

Polychaetes
"A variety of marine worms": plate from Das Meer by M. J. Schleiden (1804–1881).
Scientific classification
Kingdom: Animalia Phylum: Annelida Class: Polychaeta Grube, 1850
Subclasses
Palpata Scolecida

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. Polychaeta means "many-bristled" (as opposed to the Oligochaeta which are "few-bristled"), and 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.

General anatomy of a polychaete

Tomopteris from plankton

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, 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.

Sabellastarte indica

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

Cloudina is an Ediacaran fossil that may be an early Serpulid worm. The oldest definite polychaetes are from the Cambrian Period, such as Canadia from the Burgess Shale.

Polychaetes are more commonly known from their fossilized jaws, known as scolecodonts, and from the mineralized tubes that some of them secrete.

Taxonomy

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Polychaeta

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, 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.

• Subclass Palpata
  • Order Aciculata
    • Suborder uncertain
      • Family Aberrantidae
      • Family Nerillidae
      • Family Spintheridae
    • Suborder Eunicida
      • Family Amphinomidae
      • Family Diurodrilidae
      • Family Dorvilleidae
      • Family Eunicidae
      • Family Euphrosinidae
      • Family Hartmaniellidae
      • Family Histriobdellidae
      • Family Lumbrineridae
      • Family Oenonidae
      • Family Onuphidae
    • Suborder Phyllodocida
      • Family Acoetidae
      • Family Alciopidae
      • Family Aphroditidae
      • Family Chrysopetalidae
      • Family Eulepethidae
      • Family Glyceridae
      • Family Goniadidae
      • Family Hesionidae
      • Family Ichthyotomidae
      • Family Iospilidae
      • Family Lacydoniidae
      • Family Lopadorhynchidae
      • Family Myzostomatidae
      • Family Nautillienellidae
      • Family Nephtyidae
      • Family Nereididae
      • Family Paralacydoniidae
      • Family Pholoidae
      • Family Phyllodocidae
      • Family Pilargidae
      • Family Pisionidae
      • Family Polynoidae
      • Family Pontodoridae
      • Family Sigalionidae
      • Family Sphaeodoridae
      • Family Syllidae
      • Family Typhloscolecidae
      • Family Tomopteridae
  • Order Canalipalpata
    • Suborder uncertain
      • Family Polygordiidae
      • Family Protodrilidae
      • Family Protodriloididae
      • Family Saccocirridae
    • Suborder Sabellida
      • Family Oweniidae
      • Family Siboglinidae (formerly the phyla Pogonophora & Vestimentifera)
      • Family Serpulidae
      • Family Sabellidae
      • Family Sabellariidae
      • Family Spirorbidae
    • Suborder Spionida
      • Family Apistobranchidae
      • Family Chaetopteridae
      • Family Longosomatidae
      • Family Magelonidae
      • Family Poecilochaetidae
      • Family Spionidae
      • Family Trochochaetidae
      • Family Uncispionidae
    • Suborder Terebellida
      • Family Acrocirridae
      • Family Alvinellidae
      • Family Ampharetidae
      • Family Cirratulidae
      • Family Ctenodrilidae
      • Family Fauveliopsidae
      • Family Flabelligeridae
      • Family Flotidae
      • Family Pectinariidae
      • Family Poeobiidae
      • Family Sternaspidae
      • Family Terebellidae
      • Family Trichobranchidae
• Subclass Scolecida
  • Family Aeolosomatidae
  • Family Arenicolidae
  • Family Capitellidae
  • Family Cossunidae
  • Family Maldanidae
  • Family Ophelidae
  • Family Orbiniidae
  • Family Paraonidae
  • Family Parergodrilidae
  • Family Potamodrilidae
  • Family Psammodrilidae
  • Family Questidae
  • Family Scalibregmatidae

See also

• Epitoky, a form of reproduction of Polychaetae.

References

• 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. 

External links

• Special issue dedicated to polychaete published in Marine Ecology. Read the article abstracts online

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Polychaeta

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