Feather Star

Any member of the crinoid invertebrates found in marine habitats is known as a feather star. Crinoidea is a class of living organisms with a total of 550 species. It is classified as Echinoderms without a stalk in the phylum Echinodermata. They usually have five limbs with apparent feathery fringes that they employ for swimming. Cirri, or their legs, are utilised to perch on sponges, corals, and other substrata. Drifting microbes get caught in the sticky grooves of their arms, which they eat. Because they belong to the single order Comatulida, a Feather star crinoid is also known as a "comatulid." However, rather than preserving these species in one order, some authorities have separated them into many.

Feather Star Species

Feather star species are a collection of 550 extant species that belong to the Crinoidea class. As a result, a feather star is also known as a Crinoid in scientific terms. A feather star is a crinoid that does not have a stalk or is one of the unstalked variants of a feather crinoid. Starfish, brittle stars, sea urchins, and sea cucumbers are also included. Sea lilies and feather stars are crinoids that have a stalk attached to their adult form and are members of the largest crinoid order, Comatulida. Feather stars or comatulids are stalked crinoids that are members of the largest crinoid order, Comatulida. These can be interestingly coloured such members as red feather starfish or black feather star.

Adult crinoids are distinguished by the presence of a mouth on the upper surface. The anus is placed on the oral disc near the mouth, encircled by feeding arms and connected to a U-shaped gut. Although most crinoids have the classic echinoderm pattern of fivefold symmetry, the five arms are subdivided into 10 or more. These feature feathery pinnules and are fanned out in the water to collect planktonic particles. Most crinoids have a stem that they utilise to attach themselves to the substrate at some point in their life, however many are only connected as juveniles and become free-swimming feather stars as adults.

Crinoid species are only approximately 600 alive today, but they used to be far more numerous and diversified. Disarticulated crinoid particles make up nearly all of some thick limestone strata from the mid-to late-Paleozoic age. They can be found in both shallow and deep water, up to a depth of 9,000 metres (30,000 ft). Sea lilies are crinoids that have a stem that attaches them to the seafloor in their adult state. Members of the biggest crinoid order, Comatulida, the unstalked forms are known as feather stars or comatulids.

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Fig: Feather Star

The Make Up Feather Star

A stem (not present in mature feather stars) and a crown consisting of a cup-like core body known as the theca and a pair of five rays or arms, frequently branching and feathery, make up the basic body structure of a crinoid. The mouth and anus are both on the upper side of the theca, making the dorsal (upper) surface the oral surface, in contrast to other echinoderm groups where the mouth is on the underside, such as sea urchins, starfish, and brittle stars. The crinoid is mostly made up of calcareous plates, with only a minor amount of soft tissue. These ossicles fossilise well, and there are limestone strata from the Lower Carboniferous around Clitheroe, England, that are almost entirely made up of crinoid fossils.

Sea lilies have a stem made up of a column of very porous ossicles joined by ligament tissue. For connecting to the substrate it normally uses the flat hold fasting technique with whorls of jointed and root-like structures known as the cirri. Cirri may appear further up the stem. The cirri of crinoids that connect to hard surfaces may be robust and curled, like bird's feet, whereas the cirri of crinoids that reside in soft silt may be slender and rod-like. A stem is present in juvenile feather stars, but it is eliminated subsequently, with many species maintaining a few cirri at the crown's base. The majority of living crinoids are free-swimming, with only a stub of a stalk. The stalks of deep-sea animals that still have them can grow up to 1 m in length (though they are normally much shorter), and fossil species have been found with stems as long as 20 m.

The theca is pentamerous (five-part symmetry) and analogous with other echinoderms' bodies or discs. The calyx, a cup-shaped group of ossicles (bony plates), forms the base of the theca, while the weakly-calcified tegmen, a membranous disc, forms the upper surface. The tegmen is divided into five "ambulacral sections," which include a deep groove from which the tube feet protrude, and five "interambulacral areas" in between. Ambulacral grooves run from the base of the arms to the mouth, which is near the centre or on the periphery of the tegmen. In an interambulacral area, the anus can also be seen on the tegmen, often on a little raised cone. The digestive organs of the crinoid are housed in the theca, which is a tiny chamber.

A system of articulating ossicles, similar to those found in the stalk, support the arms. Crinoids used to have only five arms, but they are now divided in half at ossicle II, giving them a total of ten. The arms of most of the live species that are particularly known for their unique feather star swimming, branch many times The arms of most of the species that are alive, especially the free-swimming feather star, branch out many times overall producing up to 200 branches in total. Because the arms are linked, they can curl up. On their edges on both sides, they have smaller and jointed appendages which are known as pinnules responsible for giving them their feather-like look. Along the ambulacral grooves' edges, both arms and pinnules have tube feet. The tube feet come in three sizes and are used to hold and manipulate food particles. They do not include suction pads. Cilia in the grooves help to assist feeding by transporting organic particles along the arm and into the mouth.

Biological Characteristics of Feather Star

Crinoids are suspension feeders that use their feather-like arms to filter plankton and microscopic pieces of detritus from the seawater flowing past them. The arms are raised to form a fan shape that is parallel to the current. To maximise their feeding possibilities, mobile crinoids move to perch on rocks, coral heads, or other eminences. The main tube feet, which are the longest in the body, catch the food particle by completely extending and holding upright from the pinnules, further forming a food-trapping mesh with secondary and tertiary tube feet. are involved in manipulating whatever is met.

The sea urchin Heliocidaris micans, which was discovered near the crinoid Endoxocrinus parrae, was found to have considerable amounts of stem parts in its guts. These ossicles are articulated with soft tissue. Whereas only the disarticulated ossicles without any soft tissue are generally found in the nearby sediments. This is an indication that these specific sea urchins are the crinoids’ or the feather sea star predators and the crinoids usually run away and escape by giving away a portion of their stem in the escape.

Crinoids, like other echinoderms, have a water vascular system that keeps hydraulic pressure in the tube feet constant. Unlike most echinoderms, it is not connected to external seawater via a madreporite, but rather through a great number of pores to the coelom (body cavity). The muscular-walled ring canal, which is connected to the coelom via stone canals coated with calcareous material, is the major fluid reservoir. Mesenteries divide the coelom into a number of interconnected spaces. It has branches within the stalk and arms, with smaller branches reaching into the pinnules, and it surrounds the viscera in the disc. The tube feet are extended by the ring canal contracting. Each arm has three slender branches of the coelom, two on the oral side and one aborally, as well as pinnules. Because of the cilia, the flow of the fluid is so slow, around 1mm per second, through the canals outward in the oral ones and inwards in the aboral ones and thus this is the main route of transporting nutrients and waste materials. There is no heart or distinct circulatory system, but there is a huge blood channel known as the axial organ near the base of the disc that continues into the stalk and contains some slender blind-ended tubes of unknown purpose.

The crinoid nervous system is organised into three divisions, each with a complex network of connections. The only part of the nervous system that is similar to that of other echinoderms is the oral or topmost portion. It is sensory and comprises a central nerve ring enclosing the mouth and radial nerves spreading into the arms. An intermediate nerve ring exists underneath this, feeding the arms and pinnules with radial nerves. These nerves are motor nerves that govern the tube feet' musculature. The flexing and movement activities of the arms, pinnules, and cirri are controlled by the third component of the nervous system, which is located aborally.  At the centre is the mass of neural tissue around the calyx’s base. It is responsible for supplying the single nerve to each arm and a number of nerves to the stalk.

Crinoids, unlike some starfish and brittle stars, are not capable of clonal reproduction, although they may regenerate destroyed body parts. Arms that have been ripped off by predators or damaged by harsh environmental circumstances can regrow, and even the visceral mass can heal in a matter of weeks. This regeneration could be crucial in surviving predatory fish attacks. Crinoids are dioecious, meaning they can be male or female. The gonads are found in the pinnules in most species, however, in a few, they are found in the arms. Only the pinnules nearest to the crown are capable of reproduction. In genital canals enclosed in genital coeloms, gametes are created. When the pinnules burst, the sperm and eggs are released into the surrounding seawater. The fertilised eggs are cemented to the arms with secretions from epidermal glands in certain genera, such as Antedon; in others, particularly cold-water species from Antarctica, the eggs are hatched in specialised sacs present on the arms or pinnules.

As adults, most modern crinoids, such as feather stars, are free-moving and lack a stem. Marsupial, Saccocoma, and Uintacrinus are examples of fossil crinoids that have been regarded as free-swimming feather stars. Crinoids generally move around by crawling and utilising their cirri as legs. A change in the current direction, the desire to climb to an upper perch to eat, or an agonistic behaviour by an encountered individual may all cause such a movement. Crinoids are also capable of swimming. They accomplish this by making three groups of coordinated, successive arm movements. The oral surface is in front and the direction of motion is first upwards, but it quickly turns horizontal, moving at around 7 cm per second. Swimming occurs in small bursts of activity lasting up to half a minute, and only following mechanical stimulation or as an escape response elicited by a predator in the comatulid Florometra serratissima.

Cultural Aspects of Feather Star

In the Middle Ages, fossilised crinoid columnal pieces taken from Lindisfarne limestone quarries or washed up along the foreshore were fashioned into necklaces or rosaries and became known as St. Cuthbert's beads. In the same way, fossilised fragments of crinoids' columns are frequently referred to as Indian beads in the Midwest. Crinoids are Missouri's state fossil.

Thus, feather stars are a species belonging to the group of Crinoid invertebrates which mostly grow underwater and will be normally found at the bottom or bed of the sea or ocean i.e. normally in marine habitat. The word Crinoid means ‘lily-like’ and is named so because of their unique resemblance in appearance to lilies.