Polychaete - WikiHQ

Polychaeta () is a paraphyletic class of generally marine annelid worms,

However, polychaete body plans 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 niches, including burrowing, pelagic swimming, dwelling in self-created tubes or ones bored out of a substrate, commensalism, and parasitism; such varied lifestyles requires a divergence from the basic body plan of the common ancestor.

The head, or prostomium, is relatively well developed, compared with other annelids. It projects forward over the mouth, which is located on the succeeding section; the peristomium. The mouthparts vary in form depending on their diets, since the group includes predators, herbivores, filter feeders, scavengers, and parasites. In general, however, they possess a pair of jaws and a pharynx that can be rapidly everted, allowing the worms to grab food and pull it into their mouths. In some species, the pharynx is modified into a lengthy proboscis. Their jaws are formed from sclerotised collagen. The head also includes a pair of antennae, tentacle-like palps, and a pair of pits lined with cilia known as nuchal organs, which are chemoreceptors that help the worm to seek out food.

The papillae of polychaetes are tiny, fleshy projections on the worm's body or parapodia, often associated with sensory and locomotive functions. Some papillae have sensory receptors to help the worm detect environmental changes, such as touch, water currents, or chemical signals, and support movement by working with parapodia to aid grip and friction during crawling or burrowing. Other papilla types play a protective role, specifically in the secretion of mucus used to deter predators.

Underneath the cuticle, in order, are a thin layer of connective tissue, a layer of circular muscle, a layer of longitudinal muscle, and a peritoneum surrounding the coelom (body cavity). Additional oblique muscles move the parapodia. In most species the body cavity is divided into separate compartments by sheets of peritoneum between each segment, but in some species it is more continuous.

Physiology

A simple but well-developed circulatory system is usually present. The two main blood vessels furnish smaller vessels to supply the parapodia and the gut. Blood flows forward in the dorsal vessel, above the gut, and returns down the body in the ventral vessel, beneath the gut. The blood vessels themselves are contractile, helping to push the blood along, so most species have no need of a heart. In a few cases, however, muscular pumps analogous to a heart are found in various parts of the system. Conversely, some species have little or no circulatory system at all, transporting oxygen in the coelomic fluid that fills their body cavities.

Ecology

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 species, roughly 80 (less than 0.5% of species), are parasitic. They are predominantly marine, but 168 species (nearing 2% of total species) also live in freshwater, and a few in semiterrestrial environments and even in caves. 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. Polychaete mouthparts are eversible and used to capture prey.

The parasitic forms 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.

Hesiocaeca methanicola 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 able to live to over 250 years old.
A still unclassified predatory polychaete worm around an inch long was observed from the ROV Nereus at the bottom Challenger Deep, the greatest depth in the oceans, near in depth. 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; 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; with just 68 neurons and no digestive system, they live around just a week before dying.
The Namanereidinae may inhabit semi-terrestrial environments, though they are restricted to humid areas with associated plant matter. Some have even adapted to live in caves, becoming stygofaunas.

File:Alvinella pompejana01.jpg|The Pompeii worm lives at great depths by hydrothermal vents at temperatures up to .

File:Hesiocaeca methanicola noaa.jpg|Hesiocaeca methanicola lives at great depths on methane ice

File:Lamellibrachia luymesi1.png|The cold seep tube worm Lamellibrachia can live over 250 years

File:Eunice aphroditois.jpg|The predatory bobbit worm

File:Tomopteriskils.jpg|Pelagic gossamer worm

File:Spirobrancheus giganteus.jpg|Christmas tree worms bore into living coral

File:Nereis pelagica.jpg|Rag worms are used as fishing bait

File:Nereis virens.jpg|Sandworms eat seaweed and microorganisms and can be over long

File:Namanereis canariarum (10.3897-subtbiol.36.55090) Figure 1 (cropped).jpg|Namanereis canariarum is one of the multiple polychaete species which inhabit caves

File:Riftia tube worm colony Galapagos 2011.jpg|Giant tube worms are another hydrothermal vent specialist

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Reproduction<span class="anchor" id="Epitoky"></span><span class="anchor" id="Epitoke"></span>

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.

A similar strategy is employed by the branching deep sea worm Syllis ramosa, which lives inside a sponge; the worm develop "stolons" containing eggs or sperm from one of their many rear ends; these stolons detach from the parent worm and rise to the sea surface, where fertilisation takes place.

Evolution

Stem-group polychaete fossils are known from the Sirius Passet Lagerstätte, a rich, sedimentary deposit in Greenland tentatively dated to the late Atdabanian (early Cambrian). The oldest known polychaete as of 2025 is Dannychaeta tucolus, dated to approximately 514 million years ago. Many of the more famous Burgess Shale organisms, such as Canadia, may also have polychaete affinities. Wiwaxia, long interpreted as an annelid, is now considered to represent a mollusc. An even older fossil, Cloudina, dates to the terminal Ediacaran period; this has been interpreted as an early polychaete, although consensus is absent.

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

Below is a phylogenetic tree of annelids from a 2021 review of annelid diversity (clades labeled × are not considered polychaetes);

References

Bibliography

Campbell, Reece, and Mitchell. Biology. 1999.

Notes

External links

World Polychaeta Database
Special issue of Marine Ecology dedicated to polychaetes
Marine Polychaete Larva, a guide to the marine zooplankton of south eastern Australia
Key to Families of Polychaetes, Natural History Museum