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Crinoid
Crinoid

Crinoids are marine animals that make up the class Crinoidea, one of the classes of the phylum Echinodermata, which also includes the starfish, brittle stars, sea urchins and sea cucumbers.[3] Those crinoids which, in their adult form, are attached to the sea bottom by a stalk are commonly called sea lilies, while the unstalked forms are called feather stars or comatulids, being members of the largest crinoid order, Comatulida.

Adult crinoids are characterised by having the mouth located on the upper surface.

There are only about 600 living species of crinoid,[4] but the class was much more abundant and diverse in the past.

Etymology


The name "Crinoidea" comes from the Greek word κρίνος, "a lily", with the suffix –oid meaning "like".[5][6] They live in both shallow water[7] and in depths as great as 9,000 meters (30,000 ft).[8] Those crinoids which in their adult form are attached to the sea bottom by a stalk are commonly called sea lilies.[9] The unstalked forms are called feather stars[10] or comatulids, being members of the largest crinoid order, Comatulida.[11]

Morphology


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.

The stem of sea lilies is composed of a column of highly porous ossicles which are connected by ligamentary tissue.

The theca is pentamerous (has five-part symmetry) and is homologous with the body or disc of other echinoderms. The base of the theca is formed from a cup-shaped set of ossicles (bony plates), the calyx, while the upper surface is formed by the weakly-calcified tegmen, a mebranous disc. The tegmen is divided into five "ambulacral areas", including a deep groove from which the tube feet project, and five "interambulacral areas" between them. The mouth is near the centre or on the margin of the tegmen, and ambulacral grooves lead from the base of the arms to the mouth. The anus is also located on the tegmen, often on a small elevated cone, in an interambulacral area. The theca is relatively small and contains the crinoid's digestive organs.[3]

The arms are supported by a series of articulating ossicles similar to those in the stalk.

Biology


Crinoids are passive suspension feeders, filtering plankton and small particles of detritus from 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.[3]

The tube feet are covered with sticky mucus that traps any particles which come in contact. Once they have caught a particle of food, the tube feet flick it into the ambulacral groove, 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 a Japanese sea lily with 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.[3]

The mouth descends into a short oesophagus. There is no true stomach, so the oesophagus connects directly to the intestine, which runs in a single loop right around the inside of the calyx. The intestine often includes numerous diverticulae, some of which may be long or branched. The end of the intestine opens into a short muscular rectum. This ascends towards the anus, 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.[3]

Specimens of the sea urchin Calocidaris micans found in the vicinity of the crinoid Endoxocrinus 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 are predators of the crinoids, and that the crinoids flee, offering part of their stem in the process.[15]

Various crinoid fossils hint at possible prehistoric predators.

Like other echinoderms, crinoids possess a water vascular system that maintains hydraulic pressure in the tube feet. This is not connected to external sea water via a madreporite, as in other echinoderms, but only connected through a large numbers of pores to the coelom (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 by mesenteries. 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.[3]

These various fluid-filled spaces, in addition to transporting nutrients around the body, also function as both a respiratory and an excretory system.

The crinoid nervous system is divided into three parts, with numerous connections between them.

Crinoids are not capable of clonal reproduction as are some starfish and brittle stars, but are capable of regenerating lost body parts. Arms torn off by predators or damaged by adverse environmental conditions can regrow, and even the visceral mass can regenerate over the course of a few weeks. This regeneration may be vital in surviving attacks by predatory fish.[3]

Crinoids are dioecious, with individuals being either male or female. In most species, the gonads are 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. The gametes are produced in genital canals enclosed in genital coeloms. The pinnules eventually rupture to release the sperm and eggs into the surrounding sea water. In certain genera, such as Antedon, the fertilised eggs are cemented to the arms with secretions from epidermal glands; in others, especially cold water species from Antarctica, the eggs are brooded in specialised sacs on the arms or pinnules.[3]

The fertilised eggs hatch to release free-swimming vitellaria larvae. The bilaterally symmetrical larva is barrel-shaped with rings of cilia running 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 a bottleneck after the Permian extinction, at that time losing the feeding larval stage.[20] 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 of metamorphoses into a stalked juvenile, 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.[3]

Locomotion


Most modern crinoids, i.e., the feather stars, are free-moving and lack a stem as adults.

In 2005, a stalked crinoid was recorded pulling itself along the sea floor off the Grand 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 (144 to 180 metres per hour).[23]

Evolution


If one ignores the enigmatic Echmatocrinus of the Burgess Shale, the earliest known unequivocal crinoid groups date back to the Ordovician. There are two competing hypotheses pertaining to the origin of the group: the traditional viewpoint holds that crinoids evolved from within the blastozoans (the eocrinoids and their derived descendants, the blastoids and the cystoids), whereas the most popular alternative suggests that the crinoids split early from among the edrioasteroids.[24] 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.[24]

Echinoderms with mineralized skeletons entered the fossil record in the early Cambrian (540 mya), and during the next 100 million years, the crinoids and blastoids (also stalked filter-feeders) were dominant.[25] At that time, the Echinodermata included twenty taxa of class rank, 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.[3]

The crinoids underwent two periods of abrupt adaptive radiation, the first during the Ordovician (485 to 444 mya), and the other during the early Triassic (around 230 mya).[26] 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.[27] This radiation occurred somewhat earlier than the Mesozoic marine revolution, possibly because it was mainly prompted by increases in benthic predation, specifically of echinoids.[28] There then followed a selective mass extinction at the end of the Permian period, during which all blastoids and most crinoids became extinct.[26] 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.[26]

Some fossil crinoids, such as Pentacrinites, 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 of Pentacrinites can be several metres long. Modern relatives of Pentacrinites live 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.[29]

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...] aromatic or polyaromatic quinones", 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.[30]

Taxonomy


Crinoidea has been accepted as a distinct clade of echinoderms since the definition of the group by Miller in 1821.[31] According to the World Register of Marine Species, Articulata, the only extant subclass of Crinoidea, includes the following families:-[32]

  • order Comatulida Clark, 1908 super-family Antedonoidea Norman, 1865 family Antedonidae Norman, 1865 family Pentametrocrinidae AH Clark, 1908 family Zenometridae AH Clark, 1909 super-family Atelecrinoidea Bather, 1899 family Atelecrinidae Bather, 1899 super-family Comatuloidea Fleming, 1828 family Comatulidae Fleming, 1828 super-family Himerometroidea AH Clark, 1908 family Colobometridae AH Clark, 1909 family Eudiocrinidae AH Clark, 1907 family Himerometridae AH Clark, 1907 family Mariametridae AH Clark, 1909 family Zygometridae AH Clark, 1908 super-family Notocrinoidea Mortensen, 1918 family Aporometridae HL Clark, 1938 family Notocrinidae Mortensen, 1918 super-family Paracomatuloidea Hess, 1951 † super-family Tropiometroidea AH Clark, 1908 family Asterometridae Gislén, 1924 family Calometridae AH Clark, 1911 family Charitometridae AH Clark, 1909 family Ptilometridae AH Clark, 1914 family Thalassometridae AH Clark, 1908 family Tropiometridae AH Clark, 1908 Comatulida incertae sedis family Atopocrinidae Messing, 2011 (in Hess & Messing, 2011) family Bathycrinidae Bather, 1899 family Bourgueticrinidae Loriol, 1882 family Guillecrinidae Mironov & Sorokina, 1998 family Phrynocrinidae AH Clark, 1907 family Septocrinidae Mironov, 2000
  • order Cyrtocrinida Sub-order Cyrtocrinina family Sclerocrinidae Jaekel, 1918 Sub-order Holopodina family Eudesicrinidae Bather, 1899 family Holopodidae Zittel, 1879
  • order Encrinida †
  • order Hyocrinida family Hyocrinidae Carpenter, 1884
  • order Isocrinida Sub-order Isocrinina family Cainocrinidae Simms, 1988 family Isocrinidae Gislén, 1924 family Isselicrinidae Klikushkin, 1977 family Proisocrinidae Rasmussen, 1978 Sub-order Pentacrinitina † family Pentacrinitidae Gray, 1842 †
  • order Millericrinida

The phylogeny, geologic history, and classification of the Crinoidea was discussed by Wright et al. (2017).[33] These authors presented new phylogeny-based and rank-based classifications based on results of recent phylogenetic analyses.[31][34][35][36] Their rank-based classification of crinoid higher taxa (down to Order), not fully resolved and with numerous groups incertae sedis (of uncertain placement), is illustrated in the cladogram.

In culture


Fossilised crinoid columnal segments extracted from limestone quarried on Lindisfarne, or found washed up along the foreshore, were threaded into necklaces or rosaries, and became known as St. Cuthbert's beads in the Middle Ages.[37] Similarly, in the Midwestern United States, fossilized segments of the columns of crinoids are sometimes known as Indian beads.[38] Crinoids are the state fossil of Missouri.[39]

Fossil crinoids


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