Other Physical Features: ectothermic ; bilateral symmetry
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 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)
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.
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)
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)
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).
Polychaeta (/ˌpɒlɪˈkiːtə/) is a paraphyletic[1] class of generally marine annelid worms, commonly called bristle worms or polychaetes (/ˈpɒlɪˌkiːts/). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. 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]
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 chaetae, 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] including the Alciopids' complex eyes which rival cephalopod and vertebrate eyes.[5]
Many species show bioluminescence; eight families have luminous species.[6][7]
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]
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 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 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.[8]
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.[9][10] These 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.[10] These "boring" polychaetes may be parasitic, but may be oppurtunistic or even obligate symbionts (commensals).[11][10][9]
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.[12] 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.
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]
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.[17]
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.[18]
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 found is Phragmochaeta canicularis.[19] Many of the more famous Burgess Shale organisms, such as Canadia, may also have polychaete affinities. Wiwaxia, long interpreted as an annelid,[20] is now considered to represent a mollusc.[21][22] An even older fossil, Cloudina, dates to the terminal Ediacaran period; this has been interpreted as an early polychaete, although consensus is absent.[23][24]
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.[25] 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.[8] 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.[8] Their preservation potential is similar to that of jellyfish.[8]
Taxonomically, polychaetes are thought to be paraphyletic,[26] 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.
{{cite book}}: CS1 maint: DOI inactive as of February 2023 (link) Polychaeta (/ˌpɒlɪˈkiːtə/) is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (/ˈpɒlɪˌkiːts/). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. 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.
