Polychaetes as a class are robust and widespread, with species that live in the coldest ocean temperatures of theabyssal plain, to forms which tolerate the extremely high temperatures nearhydrothermal vents. Polychaetes occur throughout the Earth's oceans at all depths, from forms that live asplankton near the surface, to a 2- to 3-cm specimen (still unclassified) observed by the robot ocean probeNereus at the bottom of theChallenger 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]
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), inEunice aphroditois. They can sometimes be brightly coloured, and may beiridescent or evenluminescent. Each segment bears a pair of paddle-like and highly vascularizedparapodia, which are used for movement and, in many species, act as the worm's primaryrespiratory surfaces. Bundles of bristles, calledchaetae, 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 differentecological niches, including burrowing, swimming,pelagic life, tube-dwelling or boring,commensalism, andparasitism, requiring various modifications to their body structures.
The head, orprostomium, 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]
The head also includes a pair ofantennae, tentacle-likepalps, and a pair of pits lined withcilia, known as "nuchal organs". These latter appear to bechemoreceptors, and help the worm to seek out food.[4]
The outer surface of the body wall consists of a simplecolumnar epithelium covered by a thincuticle. Underneath this, in order, are a thin layer of connective tissue, a layer of circular muscle, a layer of longitudinal muscle, and aperitoneum surrounding thebody 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 theperistomium, the segment behind theprostomium, 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 apharynx 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 lengthyproboscis. The digestive tract is a simple tube, usually with a stomach part way along.
The smallest species, and those adapted to burrowing, lackgills, 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 thecoelomic 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 ishaemoglobin, but some groups havehaemerythrin or the green-colouredchlorocruorin, instead.
The nervous system consists of a single or double ventral nerve cord running the length of the body, withganglia 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. Anendocrine 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 ofprotonephridia ormetanephridia for excreting waste, which in some cases can be relatively complex in structure. The body also contains greenish "chloragogen" tissue, similar to that found inoligochaetes, which appears to function in metabolism, in a similar fashion to that of the vertebrateliver.[4]
The cuticle is constructed from cross-linked fibres ofcollagen and may be 200 nm to 13 mm thick. Their jaws are formed fromsclerotised collagen, and theirsetae from sclerotisedchitin.[9]
Polychaetes are predominantly marine, but many species also live in freshwater, and a few in terrestrial environments.[10] They are extremely variable in both form and lifestyle, and include a few taxa that swim among theplankton or above theabyssal plain. Most burrow or build tubes in the sediment, and some live ascommensals. A few species, roughly 80 (less than 0.5% of species), are parasitic.[11][12] These include bothectoparasites andendoparasites. 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 (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.[14][self-published source?] A few groups have evolved to live in terrestrial environments, likeNamanereidinae with many terrestrial species, but are restricted to humid areas. Some have even evolved cutaneous invaginations for aerial gas exchange.
One notable polychaete, thePompeii worm (Alvinella pompejana), is endemic to thehydrothermal vents of the Pacific Ocean. Pompeii worms are among the most heat-tolerant complex animals known.
A still unclassified multilegged predatory polychaete worm was identified only by observation from the underwater vehicleNereus at the bottom of theChallenger 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.[16]
The Bobbit worm (Eunice aphroditois) is a predatory species that can achieve a length of 3 m (10 ft)), with an average diameter of 25 mm (1 in).
Dimorphilus gyrociliatus has the smallest known genome of any annelid. The species shows extremesexual 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. They lack a digestive system and have just 68 neurons, and only live for roughly a week.[17][18]
Most polychaetes have separate sexes, rather than being hermaphroditic. The most primitive species have a pair ofgonads in every segment, but most species exhibit some degree of specialisation. The gonads shed immaturegametes 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 speciescopulate, but most fertilize their eggs externally.
The fertilized eggs typically hatch intotrochophore larvae, which float among theplankton, and eventuallymetamorphose 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 byepitoky. 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 wormSyllis ramosa, which lives inside asponge. 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]
Beingsoft-bodied organisms, the fossil record of polychaetes is dominated by their fossilized jaws, known asscolecodonts, and themineralized tubes that some of them secrete.[27] Most importantbiomineralising polychaetes areserpulids,sabellids, andcirratulids. Polychaete cuticle does have somepreservation 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 ofjellyfish.[9]
Taxonomically, polychaetes are thought to beparaphyletic,[28] meaning the group excludes some descendants of its most recent common ancestor. Groups that may be descended from the polychaetes include theclitellates (earthworms andleeches),sipunculans, andechiurans. The Pogonophora andVestimentifera were once considered separate phyla, but are now classified in the polychaete familySiboglinidae.
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 polychaetetaxa have been subject tocladistic analysis, some groups which are usually considered invalid today may eventually be reinstated.
These divisions were shown to be mostly paraphyletic in recent years.
