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45-612: See text. Sorberacea were a monoorder and monofamilial class of benthic Tunicates . The single order of the class was Aspiraculata , and the single family of that order was Hexacrobylidae . In a taxonomic revision of the Tunicata in 2007, the name "Hexacrobylidae" was declared invalid and members of the family were included in the family Molgulidae in the class Ascidiacea . They superficially resemble sea squirts but prey on invertebrates such as crustaceans . They also have some unique physical features that distinguish them from

90-625: A dormant state. Some Actinomycetota found in Siberia are estimated to be half a million years old. Zoobenthos, prefix from Ancient Greek zôion  'animal', animals belonging to the benthos. Examples include polychaete worms , starfish and anemones. Phytobenthos , prefix from Ancient Greek phutón  'plant', plants belonging to the benthos, mainly benthic diatoms and macroalgae ( seaweed ). Endobenthos (or endobenthic), prefix from Ancient Greek éndon  'inner, internal', lives buried, or burrowing in

135-527: A few in terrestrial environments. They 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. 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

180-838: A food source for fish , such as the California sheephead , and humans . Benthic macro-invertebrates play a critical role in aquatic ecosystems . These organisms can be used to indicate the presence, concentration , and effect of water pollutants in the aquatic environment. Some water contaminants—such as nutrients, chemicals from surface runoff , and metals —settle in the sediment of river beds, where many benthos reside. Benthos are highly sensitive to contamination, so their close proximity to high pollutant concentrations make these organisms ideal for studying water contamination. Benthos can be used as bioindicators of water pollution through ecological population assessments or through analyzing biomarkers . In ecological population assessments,

225-1113: A higher diversity of benthic species. The number of benthic animal species exceeds one million. This far exceeds the number of pelagic animal species (about 5000 larger zooplankton species, 22,000 pelagic fish species and 110 marine mammal species). Macrobenthos, prefix from Ancient Greek makrós  'long', comprises the larger, visible to the naked eye, benthic organisms greater than about 1 mm in size. In shallow waters, seagrass meadows , coral reefs and kelp forests provide particularly rich habitats for macrobenthos. Some examples are polychaete worms , bivalves , echinoderms , sea anemones , corals , sponges , sea squirts , turbellarians and larger crustaceans such as crabs , lobsters and cumaceans . Meiobenthos , prefix from Ancient Greek meîon  'less', comprises tiny benthic organisms that are less than about 1 mm but greater than about 0.1 mm in size. Some examples are nematodes , foraminiferans , tardigrades , gastrotriches and smaller crustaceans such as copepods and ostracodes . Microbenthos, prefix from

270-557: 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

315-401: 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. 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

360-466: A polychaete's death. 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. Their preservation potential is similar to that of jellyfish . Taxonomically, polychaetes are thought to be paraphyletic , meaning the group excludes some descendants of its most recent common ancestor. Groups that may be descended from

405-507: A relative value of water pollution can be detected. Observing the number and diversity of macro-invertebrates in a waterbody can indicate the pollution level. In highly contaminated waters, a reduced number of organisms and only pollution-tolerant species will be found. In biomarker assessments, quantitative data can be collected on the amount of and direct effect of specific pollutants in a waterbody. The biochemical response of macro-invertebrates' internal tissues can be studied extensively in

450-421: 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

495-477: Is absorbed before it can reach deep ocean water, the energy source for deep benthic ecosystems is often organic matter from higher up in the water column that drifts down to the depths. This dead and decaying matter sustains the benthic food chain ; most organisms in the benthic zone are scavengers or detritivores . The term benthos , coined by Haeckel in 1891, comes from the Greek noun βένθος 'depth of

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540-423: Is a paraphyletic class of generally marine annelid worms , commonly called bristle worms or polychaetes ( / ˈ p ɒ l ɪ ˌ k iː t s / ). 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

585-399: 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

630-426: 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,

675-412: 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. A similar strategy is employed by

720-555: Is present. In coastal waters and other places where light reaches the bottom, benthic photosynthesizing diatoms can proliferate. Filter feeders , such as sponges and bivalves , dominate hard, sandy bottoms. Deposit feeders, such as polychaetes , populate softer bottoms. Fish, such as dragonets , as well as sea stars , snails , cephalopods , and crustaceans are important predators and scavengers. Benthic organisms, such as sea stars , oysters , clams , sea cucumbers , brittle stars and sea anemones , play an important role as

765-425: 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

810-472: The ascidiaceans , including a severely reduced pharynx , the retention of the dorsal nerve cord as adults, the superficial position of their ganglion and the unique histology of the cells of their digestive tracts . The branchial syphon is large and surrounded by six large lobes; the cloacal syphon is small. They live exclusively in deep water and range in size from less than one inch (2 cm) to 2.4 inches (6 cm). Contrary to almost all tunicates,

855-818: The biological pump . In the long-term or at steady-state, i.e., the biomass of benthic organisms does not change, the benthic community can be considered a black box diverting organic matter into either metabolites or the geosphere (burial). The macrobenthos also indirectly impacts carbon cycling on the seafloor through bioturbation . Benthos are negatively impacted by fishing , pollution and litter, deep-sea mining , oil and gas activities, tourism , shipping , invasive species , climate change (and its impacts such as ocean acidification , ocean warming and changes to ocean circulation ) and construction such as coastal development , undersea cables , and wind farm construction. Polychaete Chaetopteridae Polychaeta ( / ˌ p ɒ l ɪ ˈ k iː t ə / )

900-438: The coelomic fluid that fills their body cavities. 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

945-410: 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. However, some polychaetes exhibit remarkable reproductive strategies. Some species reproduce by epitoky . For much of

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990-481: 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

1035-474: The Alciopids' complex eyes which rival cephalopod and vertebrate eyes. Many species show bioluminescence ; eight families have luminous species. 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. The outer surface of the body wall consists of

1080-457: The Greek mikrós 'small', comprises microscopic benthic organisms that are less than about 0.1 mm in size. Some examples are bacteria , diatoms , ciliates , amoeba , flagellates . Marine microbenthos are microorganisms that live in the benthic zone of the ocean – that live near or on the seafloor, or within or on surface seafloor sediments. Microbenthos are found everywhere on or about

1125-405: The benthos are phytoplankton and organic detrital matter. In coastal locations, organic run off from land provides an additional food source. Meiofauna and bacteria consume and recycle organic matter in the sediments, playing an important role in returning nitrate and phosphate to the pelagic. The depth of water, temperature and salinity, and type of local substrate all affect what benthos

1170-656: The body. 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 . 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 . Polychaetes are predominantly marine, but many species also live in freshwater, and

1215-456: The bottom of the ocean. Regardless of form, their shells sink to the seafloor after they die. These shells are widely used as climate proxies . The chemical composition of the shells are a consequence of the chemical composition of the ocean at the time the shells were formed. Past water temperatures can be also be inferred from the ratios of stable oxygen isotopes in the shells, since lighter isotopes evaporate more readily in warmer water leaving

1260-701: 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, including

1305-546: 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. Stem-group polychaete fossils are known from the Sirius Passet Lagerstätte , a rich, sedimentary deposit in Greenland tentatively dated to

1350-413: The heavier isotopes in the shells. Information about past climates can be inferred further from the abundance of forams and diatoms, since they tend to be more abundant in warm water. The sudden extinction event which killed the dinosaurs 66 million years ago also rendered extinct three-quarters of all other animal and plant species. However, deep-sea benthic forams flourished in the aftermath. In 2020 it

1395-435: The laboratory. The concentration of a chemical can cause many changes, including changing feeding behaviors, inflammation , and genetic damage, effects that can be detected outside of the stream environment. Biomarker analysis is important for mitigating the negative impacts of water pollution because it can detect water pollution before it has a noticeable ecological effect on benthos populations. Organic matter produced in

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1440-439: The late Atdabanian (early Cambrian ). The oldest found is Phragmochaeta canicularis . 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

1485-542: The members of this group are not filter-feeders, and instead capture and eat nematodes and small crustaceans . They are solitary animals, found only in the deep sea. Order Aspiraculata This tunicate-related article is a stub . You can help Misplaced Pages by expanding it . Benthos Many organisms adapted to deep-water pressure cannot survive in the upper parts of the water column . The pressure difference can be very significant (approximately one atmosphere for every 10 metres of water depth). Because light

1530-465: 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

1575-449: 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

1620-429: 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

1665-627: 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. 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

1710-473: The sea'. Benthos is used in freshwater biology to refer to organisms at the bottom of freshwater bodies of water , such as lakes, rivers, and streams. There is also a redundant synonym, Benton . Compared to the relatively featureless pelagic zone , the benthic zone offers physically diverse habitats. There is a huge range in how much light and warmth is available, and in the depth of water or extent of intertidal immersion. The seafloor varies widely in

1755-418: The seafloor of continental shelves, as well as in deeper waters, with greater diversity in or on seafloor sediments. In photic zones benthic diatoms dominate as photosynthetic organisms. In intertidal zones changing tides strongly control opportunities for microbenthos. Both foraminifera and diatoms have planktonic and benthic forms, that is, they can drift in the water column or live on sediment at

1800-418: The sediment, often in the oxygenated top layer, e.g., a sea pen or a sand dollar . Epibenthos (or epibenthic), prefix from Ancient Greek epí  'on top of', lives on top of the sediments, e.g., sea cucumber or a sea snail. Hyperbenthos (or hyperbenthic), prefix from Ancient Greek hupér  'over', lives just above the sediment, e.g., a rock cod . The main food sources for

1845-790: The shells of mollusks. These "boring" polychaetes may be parasitic, but may be opportunistic or even obligate symbionts (commensals). 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. 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

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1890-403: The sunlit layer of the ocean and delivered to the sediments is either consumed by organisms or buried. The organic matter consumed by organisms is used to synthesize biomass (i.e. growth) converted to carbon dioxide through respiration , or returned to the sediment as faeces. This cycle can occur many times before either all organic matter is used up or eventually buried. This process is known as

1935-422: The types of sediment it offers. Burrowing animals can find protection and food in soft, loose sediments such as mud , clay and sand . Sessile species such as oysters and barnacles can attach themselves securely to hard, rocky substrates. As adults they can remain at the same site, shaping depressions and crevices where mobile animals find refuge. This greater diversity in benthic habitats has resulted in

1980-409: 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

2025-429: Was reported that researchers have examined the chemical composition of thousands of samples of these benthic forams and used their findings to build the most detailed climate record of Earth ever. Some endoliths have extremely long lives. In 2013 researchers reported evidence of endoliths in the ocean floor, perhaps millions of years old, with a generation time of 10,000 years. These are slowly metabolizing and not in

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