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Crinoids
Temporal range:Ordovician–recent[1]
Crinoid on the reef of Batu Moncho Island,Indonesia
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Echinodermata
Subphylum: Crinozoa
Class: Crinoidea
Miller,1821[2]
Major groups

Crinoidsare marine invertebrates that make up theclassCrinoidea.Crinoids that remain attached to thesea floorby a stalk in their adult form are commonly calledsea lilies,while the unstalked forms, calledfeather stars[3][4]orcomatulids,are members of the largest crinoidorder,Comatulida.Crinoids areechinodermsin thephylumEchinodermata,which also includes thestarfish,brittle stars,sea urchinsandsea cucumbers.[5]They live in both shallow water[6]and in depths as great as 9,000 meters (30,000 ft).[7]

Adult crinoids are characterised by having the mouth located on the upper surface. This is surrounded by feeding arms, and is linked to a U-shaped gut, with the anus being located on the oral disc near the mouth. Although the basic echinoderm pattern of fivefold symmetry can be recognised, in most crinoids the five arms are subdivided into ten or more. These have feathery pinnules and are spread wide to gatherplanktonicparticles from the water. At some stage in their lives, most crinoids have a short stem used to attach themselves to thesubstrate,but many live attached only as juveniles and become free-swimming as adults.

There are only about 700 living species of crinoid,[8]but the class was much more abundant and diverse in the past. Some thicklimestonebeds dating to the mid-PaleozoiceratoJurassicperiodare almost entirely made up of disarticulated crinoid fragments.[9][10][11]

Etymology

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The name "Crinoidea" comes from the Ancient Greek wordκρίνον(krínon), "a lily", with the suffix–oidmeaning "like".[12][13]Those crinoids which in their adult form are attached to the sea bottom by a stalk are commonly called sea lilies,[14]while the unstalked forms are called feather stars[15]or comatulids, being members of the largest crinoidorder,Comatulida.[16]

Morphology

[edit]
Anatomy of a stalked crinoid

The basic body form of a crinoid is a stem (not present in adult feather stars) and a crown consisting of a cup-like central body known as the theca, and a set of five rays or arms, usually branched and feathery. Themouthandanusare both located on the upper side of the theca, making thedorsal(upper) surface the oral surface, unlike in the other echinoderm groups such as thesea urchins,starfishandbrittle starswhere the mouth is on the underside.[17]The numerous calcareous plates make up the bulk of the crinoid, with only a small percentage of soft tissue. These ossicles fossilise well and there are beds of limestone dating from theLower CarboniferousaroundClitheroe,England, formed almost exclusively from a diverse fauna of crinoid fossils.[18]

Stalked crinoid drawn byErnst Haeckel

The stem of sea lilies is composed of a column of highly porous ossicles which are connected by ligamentary tissue. It attaches to the substrate with a flattenedholdfastor with whorls of jointed, root-like structures known ascirri.Further cirri may occur higher up the stem. In crinoids that attach to hard surfaces, the cirri may be robust and curved, resembling birds' feet, but when crinoids live on soft sediment, the cirri may be slender and rod-like. Juvenile feather stars have a stem, but this is later lost, with many species retaining a few cirri at the base of the crown. The majority of living crinoids are free-swimming and have only avestigialstalk. In those deep-sea species that still retain a stalk, it may reach up to 1 m (3 ft) in length (although usually much smaller), and fossil species are known with 20 m (66 ft) stems,[5]the largest recorded crinoid having a stem 40 m (130 ft) in length.[19]

The theca ispentamerous(has five-part symmetry) and ishomologouswith the body or disc of other echinoderms. The base of the theca is formed from a cup-shaped set of ossicles (bony plates), thecalyx,while the upper surface is formed by the weakly-calcifiedtegmen,a membranous disc. The tegmen is divided into five "ambulacral areas", including a deep groove from which thetube feetproject, and five "interambulacral areas" between them. The mouth is near the centre or on the margin of the tegmen, andambulacralgrooves lead from the base of the arms to the mouth. Theanusis also located on the tegmen, often on a small elevated cone, in aninterambulacralarea. The theca is relatively small and contains the crinoid's digestive organs.[5]

The arms are supported by a series of articulating ossicles similar to those in the stalk. Primitively, crinoids had only five arms, but in most modern forms these are divided into two at ossicle II, giving ten arms in total. In most living species, especially the free-swimming feather stars, the arms branch several more times, producing up to two hundred branches in total. Being jointed, the arms can curl up. They are lined, on either side alternately, by smaller jointed appendages known as "pinnules" which give them their feather-like appearance. Both arms and pinnules havetube feetalong the margins of the ambulacral grooves. The tube feet come in groups of three of different size; they have no suction pads and are used to hold and manipulate food particles. The grooves are equipped withciliawhich facilitate feeding by moving the organic particles along the arm and into the mouth.[5]


Online Model Organism Database

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Echinobaseis the model organism database for the feather star and a number of other echinoderms.

Biology

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Feeding

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Two arms with pinnules and tube feet outstretched

Crinoids are passivesuspension feeders,filteringplanktonand small particles ofdetritusfrom the sea water flowing past them with their feather-like arms. The arms are raised to form a fan-shape which is held perpendicular to the current. Mobile crinoids move to perch on rocks, coral heads or other eminences to maximise their feeding opportunities. The food particles are caught by the primary (longest) tube feet, which are fully extended and held erect from the pinnules, forming a food-trapping mesh, while the secondary and tertiary tube feet are involved in manipulating anything encountered.[5]

The tube feet are covered with stickymucusthat traps any particles which come in contact. Once they have caught a particle of food, the tube feet flick it into theambulacralgroove, where the cilia propel the mucus and food particles towards the mouth. Lappets at the side of the groove help keep the mucus stream in place. The total length of the food-trapping surface may be very large; the 56 arms of aJapanese sea lilywith 24 cm (9 in) arms, have a total length of 80 m (260 ft) including the pinnules. Generally speaking, crinoids living in environments with relatively little plankton have longer and more highly branched arms than those living in food-rich environments.[5]

The mouth descends into a shortoesophagus.There is no true stomach, so the oesophagus connects directly to theintestine,which runs in a single loop right around the inside of the calyx. The intestine often includes numerousdiverticulae,some of which may be long or branched. The end of the intestine opens into a short muscularrectum.This ascends towards theanus,which projects from a small conical protuberance at the edge of the tegmen. Faecal matter is formed into large, mucous-cemented pellets which fall onto the tegmen and thence the substrate.[5]

Predation

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Specimens of the sea urchinCalocidaris micansfound in the vicinity of the crinoidEndoxocrinus parrae,have been shown to contain large quantities of stem portions in their guts. These consist of articulated ossicles with soft tissue, whereas the local sediment contained only disarticulated ossicles without soft tissue. This makes it highly likely that these sea urchins arepredatorsof the crinoids, and that the crinoids flee, offering part of their stem in the process.[20]

Various crinoid fossils hint at possible prehistoric predators.Coprolitesof both fish andcephalopodshave been found containing ossicles of various crinoids, such as the pelagic crinoidSaccocoma,from theJurassiclagerstattenSolnhofen,[21]while damaged crinoid stems with bite marks matching the toothplates ofcoccosteidplacodermshave been found in LateDevonianPoland.[22]The calyxes of several Devonian toCarboniferous-aged crinoids have the shells of a snail,Platyceras,intimately associated with them.[23]Some have the snail situated over the anus, suggesting thatPlatyceraswas acoprophagouscommensal, while others have the animal directly situated over a borehole, suggesting a more pernicious relationship.[24]

Water vascular system

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Like other echinoderms, crinoids possess awater vascular systemthat maintainshydraulicpressure in the tube feet. This is not connected to external sea water via amadreporite,as in other echinoderms, but only connected through a large number of pores to thecoelom(body cavity). The main fluid reservoir is the muscular-walled ring canal which is connected to the coelom by stone canals lined with calcareous material. The coelom is divided into a number of interconnecting spaces bymesenteries.It surrounds the viscera in the disc and has branches within the stalk and arms, with smaller branches extending into the pinnules. It is the contraction of the ring canal that extends the tube feet. Three narrow branches of the coelom enter each arm, two on the oral side and one aborally, and pinnules. The action of cilia cause there to be a slow flow of fluid (1mm per second) in these canals, outward in the oral branches and inward in the aboral ones, and this is the main means of transport of nutrients and waste products. There is no heart and separate circulatory system but at the base of the disc there is a large blood vessel known as the axial organ, containing some slender blind-ended tubes of unknown function, which extends into the stalk.[5]

These various fluid-filled spaces, in addition to transporting nutrients around the body, also function as both a respiratory and an excretory system. Oxygen is absorbed primarily through the tube feet, which are the most thin-walled parts of the body, with further gas exchange taking place over the large surface area of the arms. There are no specialised organs for excretion while waste is collected byphagocyticcoelomocytes.[5]

Nervous system

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The crinoid nervous system is divided into three parts, with numerous connections between them. The oral or uppermost portion is the only onehomologouswith the nervous systems of other echinoderms. It consists of a central nerve ring surrounding the mouth, and radial nerves branching into the arms and is sensory in function. Below this lies an intermediate nerve ring, giving off radial nerves supplying the arms and pinnules. These nerves are motor in nature, and control the musculature of the tube feet. The third portion of the nervous system lies aborally, and is responsible for the fle xing and movement actions of the arms, pinnules and cirri. This is centred on a mass of neural tissue near the base of the calyx, and provides a single nerve to each arm and a number of nerves to the stalk.[5]

Reproduction and life cycle

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Crinoids are not capable of clonal reproduction as are somestarfishandbrittle stars,but are capable of regenerating lost body parts. Arms torn off by predators or damaged by adverse environmental conditions can regrow, and even thevisceral masscan regenerate over the course of a few weeks. This regeneration may be vital in surviving attacks by predatory fish.[5]

Crinoids aredioecious,with individuals being either male or female. In most species, thegonadsare located in the pinnules but in a few, they are located in the arms. Not all the pinnules are reproductive, just those closest to the crown. Thegametesare produced in genital canals enclosed in genital coeloms. The pinnules eventually rupture to release thespermandeggsinto the surrounding sea water. In certain genera, such asAntedon,the fertilised eggs are cemented to the arms with secretions from epidermal glands; in others, especially cold water species from Antarctica, the eggs arebroodedin specialised sacs on the arms or pinnules.[5]

The fertilised eggs hatch to release free-swimmingvitellaria larvae.The bilaterally symmetrical larva is barrel-shaped with rings ofciliarunning round the body, and a tuft of sensory hairs at the upper pole. While both feeding (planktotrophic) and non-feeding (lecithotrophic) larvae exist among the four other extant echinoderm classes, all present day crinoids appear to be descendants from a surviving clade that went through abottleneckafter thePermian extinction,at that time losing the feeding larval stage.[25]The larva's free-swimming period lasts for only a few days before it settles on the bottom and attaches itself to the underlying surface using an adhesive gland on its underside. The larva then undergoes an extended period ofmetamorphosesinto a stalkedjuvenile,becoming radially symmetric in the process. Even the free-swimming feather stars go through this stage, with the adult eventually breaking away from the stalk.[5]

Locomotion

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A stalked crinoid (white) and a comatulid (red) in deep sea, showing the differences between these two sister groups

Most modern crinoids, i.e., the feather stars, are free-moving and lack a stem as adults. Examples of fossil crinoids that have been interpreted as free-swimming includeMarsupites,SaccocomaandUintacrinus.[26]In general, crinoids move to new locations by crawling, using the cirri as legs. Such a movement may be induced in relation to a change in current direction, the need to climb to an elevated perch to feed, or because of an agonistic behaviour by an encountered individual.[27]Crinoids can also swim. They do this by co-ordinated, repeated sequential movements of the arms in three groups. At first the direction of travel is upwards but soon becomes horizontal, travelling at about 7 cm (2.8 in) per second with the oral surface in front. Swimming usually takes place as short bursts of activity lasting up to half a minute, and in the comatulidFlorometra serratissimaat least, only takes place after mechanical stimulation or as an escape response evoked by a predator.[27]

In 2005, a stalked crinoid was recorded pulling itself along the sea floor off theGrand Bahama Island.While it has been known that stalked crinoids could move, before this recording the fastest motion known for a stalked crinoid was 0.6 metres (2 feet) per hour. The 2005 recording showed one of these moving across the seabed at the much faster rate of 4 to 5 cm (1.6 to 2.0 in) per second, or 144 to 180 m (472 to 591 ft) per hour.[28]

Evolution

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See also:List of echinodermata orders

Origins

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Agaricocrinus americanus,a fossil crinoid from theCarboniferousof Indiana
MiddleJurassic(Callovian)Apiocrinitescrinoid pluricolumnals from theMatmor Formationin southern Israel

If one ignores the Enigma ticEchmatocrinusof theBurgess Shale,the earliest known unequivocal crinoid groups date back to theOrdovician,480 million years ago. There are two competing hypotheses pertaining to the origin of the group: the traditional viewpoint holds that crinoids evolved from within theblastozoans(theeocrinoidsand their derived descendants, theblastoidsand thecystoids), whereas the most popular alternative suggests that the crinoids split early from among theedrioasteroids.[29]The debate is difficult to settle, in part because all three candidate ancestors share many characteristics, including radial symmetry, calcareous plates, and stalked or direct attachment to the substrate.[29]

Diversity

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Echinoderms with mineralized skeletons entered the fossil record in the earlyCambrian(540 mya), and during the next 100 million years, the crinoids and blastoids (also stalked filter-feeders) were dominant.[30]At that time, the Echinodermata included twenty taxa ofclassrank, only five of which survived the mass extinction events that followed. The long and varied geological history of the crinoids demonstrates how well the echinoderms had adapted to filter-feeding.[5]

The crinoids underwent two periods of abruptadaptive radiation,the first during the Ordovician (485 to 444 mya), and the other during the early Triassic (around 230 mya).[31]This Triassic radiation resulted in forms possessing flexible arms becoming widespread;motility,predominantly a response to predation pressure, also became far more prevalent than sessility.[32]This radiation occurred somewhat earlier than theMesozoic marine revolution,possibly because it was mainly prompted by increases in benthic predation, specifically of echinoids.[33]There then followed a selectivemass extinctionat the end of thePermianperiod, during which all blastoids and most crinoids became extinct.[31]After the end-Permian extinction, crinoids never regained the morphological diversity and dominant position they enjoyed in the Paleozoic; they employed a different suite of ecological strategies open to them from those that had proven so successful in the Paleozoic.[31]

Fossils

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Some fossil crinoids, such asPentacrinites,seem to have lived attached to floating driftwood and complete colonies are often found. Sometimes this driftwood would become waterlogged and sink to the bottom, taking the attached crinoids with it. The stem ofPentacrinitescan be several metres long. Modern relatives ofPentacriniteslive in gentle currents attached to rocks by the end of their stem. The largest fossil crinoid on record had a stem 40 m (130 ft) in length.[19]

In 2012, three geologists reported they had isolated complex organic molecules from 340-million-year-old (Mississippian) fossils of multiple species of crinoids. Identified as "resembl[ing...]aromaticorpolyaromaticquinones",these are the oldest molecules to be definitively associated with particular individual fossils, as they are believed to have been sealed inside ossicle pores by precipitated calcite during the fossilization process.[34]

Crinoid fossils, and in particular disarticulated crinoid columnals, can be so abundant that they at times serve as the primary supporting clasts in sedimentary rocks.[citation needed]Rocks of this nature are calledencrinites.

Taxonomy

[edit]
Colorful crinoids in shallow waters in Indonesia
Multiple crinoids on a reef in Indonesia
Crinoid atWakatobi National Park,2018

Crinoidea has been accepted as a distinctcladeof echinoderms since the definition of the group by Miller in 1821.[35]It includes many extinct orders as well as four closely related living orders (Comatulida,Cyrtocrinida,Hyocrinida,andIsocrinida), which are part of the subgroupArticulata.Living articulates comprise around 540 species.

Phylogeny

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Thephylogeny,geologic history, and classification of theCrinoideawas discussed by Wright et al. (2017).[36]These authors presented new phylogeny-based and rank-based classifications based on results of recent phylogenetic analyses.[35][37][38][39]Their rank-based classification of crinoid higher taxa (down to Order), not fully resolved and with numerous groupsincertae sedis(of uncertain placement), is illustrated in thecladogram.

Crinoidea

In culture

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Fossilised crinoid columnal segments extracted fromlimestonequarried onLindisfarne,or found washed up along the foreshore, were threaded intonecklacesorrosaries,and became known asSt. Cuthbert's beadsin theMiddle Ages.[40]Similarly, in the Midwestern United States, fossilized segments of the columns of crinoids are sometimes known asIndian beads.[41]A species of crinoid,Eperisocrinus missouriensis,is thestate fossilofMissouri.[42]The aliens in the movie franchiseAlienwere inspired by crinoids.[43]

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References

[edit]
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  39. ^Rouse, Greg W.; Jermiin, Lars S.; Wilson, Nerida G.; Eeckhaut, Igor; Lanterbecq, Deborah; Oji, Tatsuo; Young, Craig M.; Browning, Teena; Cisternas, Paula; Helgen, Lauren E.; Stuckey, Michelle; Messing, Charles G. (2013). "Fixed, free, and fixed: the fickle phylogeny of extant Crinoidea (Echinodermata) and their Permian-Triassic origin".Molecular Phylogenetics and Evolution.66(6): 161–181.Bibcode:2013MolPE..66..161R.doi:10.1016/j.ympev.2012.09.018.PMID23063883.
  40. ^Lane, N. Gary; Ausich, William I. (2001). "The Legend of St Cuthbert's Beads: A Palaeontological and Geological Perspective".Folklore.112(1): 65–73.JSTOR1260865.
  41. ^"Identifying Unknown Fossils (by their shape)".Kentucky Geological Survey / University of Kentucky.Retrieved21 June2009.
  42. ^"Missouri's State Fossil".Office of the Secretary of State, Missouri.Retrieved31 March2019.
  43. ^Bressan, David (26 April 2019)."The Fossils That Inspired 'Alien'".Forbes.Retrieved1 February2024.
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