Polychaete

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"Polychaeta" redirects here. For the genus of flies, see Polychaeta (fly).
Polychaetes
Temporal range: Cambrian (or earlier?) – present
"A variety of marine worms": plate from Das Meer by M. J. Schleiden (1804–1881)
"A variety of marine worms": plate from Das Meer by M. J. Schleiden (1804–1881)
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Annelida
Class: Polychaeta
Grube, 1850
Groups included
Cladistically included but traditionally excluded taxa

Chaetopteridae[1]

Polychaeta (

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

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.[4]

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

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] including the Alciopids' complex eyes which rival cephalopod and vertebrate eyes.[5][6]

Many species show bioluminescence; eight families have luminous species.[7][8]

The head also includes a pair of

cilia, known as "nuchal organs". These latter appear to be chemoreceptors, and help the worm to seek out food.[4]

Internal anatomy and physiology

General anatomy of a polychaete
Phyllodoce rosea

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 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 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

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 involved with the sense of touch, also occur on the body.[4]

Polychaetes have a varying number of

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

setae from sclerotised chitin.[9]

Ecology

Pompeii worm
lives at great depths by hydrothermal vents at temperatures up to 80 °C
methane ice
The cold seep tube worm Lamellibrachia can live over 250 years
Bobbit worm

Polychaetes are predominantly marine, but many species also live in freshwater, and a few in terrestrial environments.

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.[12] These "boring" polychaetes may be parasitic, but may be opportunistic or even obligate symbionts (commensals).[13][12][11]

The mobile forms (

fanworms
. Underwater polychaetes have eversible mouthparts used to capture prey.[14][self-published source?] 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

Reproduction

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

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]

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.[19]

A similar strategy is employed by the deep sea worm Syllis ramosa, which lives inside a sponge. 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.[20]

Fossil record

Cloudina, dates to the terminal Ediacaran period; this has been interpreted as an early polychaete, although consensus is absent.[25][26]

Being

preservation potential; it tends to survive for at least 30 days after a polychaete's death.[9] 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.[9] Their preservation potential is similar to that of jellyfish.[9]

Taxonomy and systematics

See also: List of annelid families
Head of Phyllodoce lineata
feather duster worm
are used to filter water
Planktonic bristleworm Tomopteris
Christmas tree worms
Rag worms
can be dangerous to touch, giving painful burns
Sandworms eat seaweed and microorganisms and can be longer than four feet
Giant tube worms
can tolerate extremely high hydrogen sulfide levels

Taxonomically, polychaetes are thought to be

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

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". American Zoologist. 38 (6): 953–964.
    doi:10.1093/icb/38.6.953
    .

Notes

  1. ^
    S2CID 4428998
    .
  2. ^ Geography of Guam ns.gov.gu Accessed Oct. 8, 2009
  3. S2CID 13143924
    .
  4. ^
    ISBN 978-0-03-056747-6
    .
  5. ^ High-resolution vision in pelagic polychaetes
  6. ^ "14 Fun Facts About Marine Bristle Worms".
  7. PMID 34580316
    .
  8. S2CID 2473677
    .
  9. ^
    S2CID 84073818
    .
  10. ^ Annelids in Extreme Aquatic Environments: Diversity, Adaptations and Evolution
  11. ^
    PMID 28584710
    .
  12. ^
    ISBN 9780429210600.{{cite book}}: CS1 maint: DOI inactive as of January 2024 (link
    )
  13. PMID 36095560
    .
  14. ^ "Bristleworm". MESA.[self-published source]
  15. ^ "'Zombie worms' found off Sweden". BBC News. 18 October 2005. Retrieved 12 February 2010.
  16. ^ Accessed Oct. 8, 2009 Geography of the ocean floor near Guam with some notes on exploration of the Challenger Deep.
  17. ^ A tiny worm sheds light into genome compaction
  18. PMID 33199869
    .
  19. ISBN 9780313339226
    .
  20. ^ 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.
  21. doi:10.4202/app.2008.0110
    .
  22. S2CID 88100863
    .
  23. PMID 22915671
    .
  24. S2CID 84616434
    .
  25. CiteSeerX 10.1.1.526.5035
    .
  26. doi:10.4267/2042/46095
    .
  27. doi:10.3176/earth.2009.4.07
    .
  28. doi:10.1080/00222939700770011
    .

External links

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