کرینوئیدا - Crinoidea

Crinoidea

A class of exclusively suspension-feeding echinoderms with long, slender arms arranged radially around the calyx, a rigid cuplike structure composed of calcareous plates. The radial arm arrangement gives crinoids a flowerlike appearance (Fig. 1). Two basic adult body types are recognized: the sea lilies, with a long, anchored stem vertically supporting the calyx and arms above the sea bottom; and the stemless featherstars, or comatulids, with a whorl of flexible appendages on the calyx. Crinoids have a worldwide distribution and can be found in all seas except the Black and Baltic. They occupy depths ranging from just below sea level to a depth of over 9000 m (29,500 ft). Sea lilies are found only at depths greater than 100 m (330 ft), whereas comatulids are most abundant and diverse in shallow, tropical coral reef environments.  See also: Blastoidea; Pelmatozoa

 

 

Fig. 1  Schematic diagram of a stemmed crinoid in feeding posture.

 

 

 

 

 

 

Over 500 species of living comatulids have been described, and many of these can be extremely abundant locally, yet fewer than 100 species of living sea lilies are known. These patterns contrast starkly with the fossil record of crinoids, which is dominated by stemmed crinoids: of the more than 5000 described fossil species, more than 90% have stems.

Living crinoids have no economic importance and are not used for food by humans. However, their fossil remains are often the dominant constituent of building limestone (for example, Indiana limestone) which is highly valued.

 

Morphology

 

Adult crinoids range in size from a few centimeters for some of the stemless forms to several meters from base of the stem to tip of the arms for the largest stemmed crinoids. They also vary in color from the largely bland whites and grays of the deep-water forms to the brilliant reds, yellows, and purples of the shallow-water tropical featherstars. In spite of the size range, color differences, and stemmed or stemless condition as adults, all crinoids share a suite of morphological traits that can be traced back to the Ordovician age, nearly 500 million years ago.  See also: Ordovician

As in other echinoderms, the crinoid skeleton is composed of many calcareous plates, often numbering in the thousands. Each plate is an intricate meshwork of interconnected microscopic rods and bars of calcium carbonate. The skeletal plates are joined to each other by fibers of connective tissues that penetrate and attach to the array of rods and bars. Plates can be bound to adjacent plates by ligaments, muscles, or both, and the strength and flexibility of each articulation is determined largely by the type and arrangement of the soft tissues.

The calyx consists of tightly sutured plates arranged in circlets of five with arm plates articulated to the uppermost circlet (Fig. 2). A flat to domelike structure, called the tegmen, forms a lid over the cuplike calyx. The tegmen may be armored with calcareous plates or, as in most living crinoids, may consist only of a leathery skin.

 

 

Fig. 2  Calyx and proximal stem and arm morphology, vertical section.

 

 

 

 

 

 

The arms are constructed of one or two series of plates; in arms consisting of two series of plates, adjacent plates interdigitate with each other. Each of the five arms may remain undivided, or may branch one or numerous times producing as many as 200 branches. Arm plates may bear short, lateral branches, called pinnules, composed of serially arranged plates (Fig. 3).

 

 

Fig. 3  Arm morphology, back (aboral) view.

 

 

 

 

 

 

As juveniles, all crinoids possess a stem. However, comatulids lose the stem early in ontogeny, such that in adults the lowest plate of the calyx bears only cirri, a set of long, slender articulated appendages with a clawlike terminal element used for grasping the substrate. In adult sea lilies, the stem is retained. One end of the stem articulates to the base of the calyx, and the opposite end anchors the animal to the bottom. The stem is composed of flat, disklike plates, called columnals, that are circular to pentagonal in outline, perforated at the center, and stacked plate on plate. Ligament fibers connect adjacent columnals. The anchoring end of the stem may consist of a rootlike structure that penetrates soft bottom sediments or that cements the stem to a hard substrate, or, most commonly, a set of grasping cirri. In extant sea lilies with cirri, called isocrinids, these appendages are arranged in whorls of five that are regularly spaced along the entire length of the stem.  See also: Ontogeny

Crinoid viscera are contained in the coelom that forms the body cavity in the calyx and also extends into the arms, pinnules, stem, and cirri. The digestive system consists of a U-shaped gut with its terminal ends, the mouth and anus, penetrating the tegmen. A water vascular system, consisting of an array of fluid-filled canals in the calyx and all appendages, performs many functions associated with feeding and respiration. Its slender movable podia-like structures, called tube feet, extend along the arms and pinnules; they are sites of respiratory gas exchange and the primary food capturing organs.

 

Ecology and function

 

Crinoids are exclusively passive suspension feeders, extracting food particles from the ambient water. Food capture occurs when a microscopic particle carried by the current strikes and adheres to a mucus-coated tube foot extended from the pinnule (Fig. 4). Subsequently, the tube foot deposits the particle into the food groove that lines the oral side of pinnules and arms. In the food groove, other tube feet and ciliary currents transport the particle toward the mouth.

 

 

Fig. 4  Arm morphology, side view.

 

 

 

 

 

 

Crinoids are indiscriminate feeders, and the tube feet capture organic and inorganic particles with a median size of about 50 micrometers and rarely larger than 500 μm. The organic food component consists primarily of phytoplankton, protozoa, and crustacea.

Crinoid morphology and behavior strongly reflect their total reliance on water movement for nutrient supply. Crinoids avoid slack-water environments, living in areas dominated by currents, wave action, or multidirectional flows. Comatulids typically occupy exposed sites on reef crests, and in deep water they are often perched on corals, sponges, and sea lily stems, away from the lower velocities characterizing the boundary layer. However, some live semicryptically within the reef infrastructure. Sea lilies are found in areas dominated by currents, even in ocean depths.

Among comatulids, tube-foot morphology and spacing along pinnules varies in relation to flow velocities: species with longer, more widely spaced tube feet live in environments with lower velocities, and those with shorter, more closely spaced tube feet occupy exposed sites with higher velocities. Crinoid feeding postures vary depending on flow velocities and flow patterns. When exposed to unidirectional currents, crinoid arms, pinnules, and tube feet are aligned into a nearly planar array oriented perpendicular to flow. To avoid having the ambient current directly impinge on food grooves in which particles are transported toward the mouth, crinoids arrange all their arms with the food groove, oral side facing downstream and the opposite, aboral side facing upstream.

Under oscillating water movements produced by wave action, arms are arranged in a bidirectional fan with half the arms facing one direction and the others facing in the opposite direction. The downstream orientation of the pinnular food groove is often maintained by the weathervanelike swiveling of pinnules with their extended tube feet.

Semicryptic comatulids living within the reef infrastructure experience multidirectional flows. These crinoids do not form planar fans with their arms, but extend their arms into three-dimensional bushlike structures. Pinnules are held in two or three planes along each arm, and swiveling of pinnules occurs in response to changes in current direction.

The coordination and flexibility required for achieving the appropriate feeding posture is attained with the complex nervous system and the ligamentary and muscular arm articulations. Arm movements are generated by muscles and viscoelastic ligaments acting antagonistically across a fulcrum that lies between two adjoining arm plates. In comatulids, arm movement can be highly coordinated and rapid, allowing crawling and, in some, short bursts of swimming. Crawling also has been observed among the dominant group of living stemmed crinoids, the isocrinids, which can move slowly along the bottom by pulling with their arms and dragging their stem behind.

Some comatulids are nocturnal and use their crawling abilities to move between reef crevices and exposed sites on the reef that they occupy at night. In general, crawling allows crinoids to move to better feeding microhabitats and to avoid disturbance.

Crinoid ligaments can undergo rapid and reversible changes in stiffness, a property that is important to crinoid function and behavior. In their compliant state, ligaments allow movement of body parts with a minimum amount of muscular force. In their stiff state, ligaments can withstand high external forces, such as those due to current drag, with little deformation, allowing crinoids to maintain stable feeding postures. Through the rapid loss of stiffness and strength, ligaments can irreversibly disintegrate, allowing the animal to shed body parts. The shedding of body parts, called autotomy, can occur in response to external physicochemical stimuli, such as rapid temperature changes or predatory attacks.

Several interactions between crinoids and other animals are known to occur in the modern seas. The most common are parasitic and commensal associations between polychaete worms (Myzostomida), small snails (Eulimidae, Styliferidae), and crustaceans (Ascothoracida) that infest crinoids. These parasites can feed from the ambulacral grooves or the tegmen and can induce malformations such as cysts. Clingfishes (Gobiesocidae) are also known to feed on comatulid pinnules and on worms and copepods associated with crinoids.

 

Geologic history

 

Crinoids have a long and rich fossil record, and at times in Earth history they were numerically one of the dominant members of the benthic marine ecosystem. The first undisputed crinoids were found in rocks of the Ordovician Period, although an enigmatic fossil, Echmatocrinus, of Middle Cambrian age has been described as a crinoid. Crinoid diversity waxed and waned throughout the roughly 250 million years of the Paleozoic. The Mississippian Period, known as the Age of Crinoids, represents the peak in their abundance and diversity. Thick, extensive deposits consisting almost exclusively of crinoid remains, called encrinites or criquinites, are especially common in the Mississippian rock record, when crinoid diversity reached its peak of over 100 genera. Crinoids remained an important component of marine communities until the Permo-Triassic extinction event that signaled the end of the Paleozoic Era. This event led to a great reduction in crinoid diversity. A single genus, Holocrinus, found in Lower Triassic rocks, is the most primitive member of the Articulata, a subclass that includes all post-Paleozoic crinoids.  See also: Cambrian; Mississippian; Paleozoic; Triassic

Crinoids rediversified rapidly, and by the Late Triassic their range of morphologies and ecologies was as high as at any time in the Paleozoic, although the number of Mesozoic or Cenozoic genera never approached that of the Mississippian. The Mesozoic also represents an interval during which several lineages lost the stem. Some of these lineages became planktonic or pseudoplanktonic, while the comatulids, which first appeared in the latest Triassic, achieved a great level of vagility but remained in the benthos. Overall, levels of vagility not known in the Paleozoic characterize post-Paleozoic crinoids, a consequence of the highly muscularized arms and stem-shedding abilities of the latter group.

After the Mesozoic, crinoids became a much less common and less diverse part of the fossil record. Cenozoic rocks contain only bottom-dwelling crinoids, with the stemless comatulids and the cirri-bearing isocrinids most common. Whereas stemmed crinoids had been an important element of shallow-water environments throughout the Paleozoic and Mesozoic, they became restricted to bathyal and abyssal depths during the Cenozoic, which is still true today. The timing of the displacement of stemmed crinoids into deeper waters during the Cenozoic was coincident with the diversification of shell-crushing fishes. Today significant fish predation on crinoids does not occur. The disappearance of stemmed crinoids from shallow water environments and the success there of the stemless comatulids that are highly mobile, often semicryptic, and possess certain other antipredatory features, argue for fish predation as an important ecological and evolutionary agent.  See also: Articulata (Echinodermata); Camerata; Cenozoic; Cretaceous; Crinozoa; Echinodermata; Flexibilia; Inadunata; Jurassic; Mesozoic; Regenerative biology

Tomasz K. Baumiller

 

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