Ossicles are small calcareous elements embedded in the dermis of the body wall of echinoderms. They form part of the endoskeleton and provide rigidity and protection. They are found in different forms and arrangements in sea urchins, starfish, brittle stars, sea cucumbers, and crinoids. The ossicles and spines (which are specialised sharp ossicles) are the only parts of the animal likely to be fossilized after an echinoderm dies.
Ossicles are created intracellularly by specialised secretory cells known as sclerocytes in the dermis of the body wall of echinoderms. Each ossicle is composed of microcrystals of calcite arranged in a three-dimensional lattice known as a stereom. Under polarized light the ossicle behaves as if it were a single crystal because the axes of all the crystals are parallel. The space between the crystals is known as the stroma and allows entry to sclerocytes for enlargement and repair. The honeycomb structure is light but tough and collagenous ligaments connect the ossicles together. The ossicles are embedded in a tough connective tissue which is also part of the endoskeleton. When an ossicle becomes redundant, specialised cells known as phagocytes are able to reabsorb the calcareous material. [1] All the ossicles, even those that protrude from the body wall, are covered by a thin layer of epidermis but functionally they act more like an exoskeleton than an endoskeleton. [2]
Ossicles have a variety of forms including flat plates, spines, rods and crosses, and specialised compound structures including pedicellariae and paxillae.
Plates are tabular ossicles that fit neatly together in a tessellated manner. They form the main skeletal covering for sea urchins and sea stars. [3]
Spines are ossicles that project from the body wall and articulate with other ossicles through ball and socket joints mounted on tubercles. [4] They are formed from crystals of calcite and can be solid or hollow, long or short, thick or thin and sharp or blunt. [3] [5] The spines serve a protective function and are also used for locomotion. [1]
Pedicellariae are compound ossicles that articulate with other ossicles and protrude from the aboral (upper) surface of some sea stars (and also the test of sea urchins). They usually have short fleshy stalks and either two or three moveable ossicles forming a set of pincer-like jaws. They may be scattered over the surface or may be grouped around spines. Their function is to pick off debris so as to keep the surface clean and to prevent larvae of other invertebrates from settling and growing there. [1] [6]
Paxillae are small pillar-shaped ossicles with flat tops sometimes found covering the aboral surface of sea stars such as Luidia , Astropecten and Goniaster that live underneath sediment. Their stalks emerge from the body wall and their tops, each fringed with short spines, and abut each other to form a protective external false skin. Beneath this is a water-filled cavity which contains the madreporite and delicate gill structures known as papillae. [1]
Sea urchins are covered with plates which are usually fused together to give a rigid test, but in the order Echinothurioida, the test is leathery because the plates are separate. The test is divided into five segments that extend from the apex to the mouth. Each contains two ambulacral rows of plates alternating with two interambulacral rows. The ambulacral plates are each pierced by a pair of pores through which the active tube feet are connected to the water vascular system. Ossicles in the form of spines connect to tubercles on some of the plates. Sea urchins have several types of pedicellariae, some of which are toxic. A ring of specialised plates surround the aboral pole consisting of five genital plates, one of which is the madreporite, and five smaller ocular plates. Other large specialist plates surround the mouth in a set of jaws known as Aristotle's lantern. [4]
Sea stars have separate plates giving flexibility to the disc and arms. They are arranged into interambulacral and ambulacral regions and the arms have an ambulacral groove on the underside from which the tube feet project. Other ossicles that may be present include pedicellariae and paxillae. There is often a large row of marginal plates adjoining the ambulacral groove, sometimes bearing spines. [1]
Brittle stars do not have pedicellariae, and the plates that cover their surface are known as shields. On the arms these are in four rows with each segment having an aboral and oral shield and two lateral shields, usually with fringing spines. Other ossicles include spines, tubercles, small scales and vertebrae. The large central vertebrae in each arm segment provides the articulating element that joins it to the next. [7]
Several types of small ossicles are found in the body wall of sea cucumbers. Baskets are cup-shaped and usually have four projections. Buttons are disc-shaped and pierced by four holes and may be smooth or knobbed. Perforated plates are sieve-like and often widely distributed and rods provide support for the tube feet and tentacles. [3] In the order Apodida, members of which lack tube feet, there are anchor-shaped ossicles attached to anchor plates. The flukes project from the body wall and provide traction. [8]
Crinoids are supported by jointed stalks containing substantial compound ossicles. The crown has ossicles scattered throughout the connective tissue (crinoids have no distinct dermis). The arms contain columns of well-developed vertebrae-like ossicles. Each joint has limited movement but the whole arm can be coiled and uncoiled. [9]
An echinoderm is any member of the phylum Echinodermata. The adults are recognisable by their radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the sea lilies or "stone lilies". Adult echinoderms are found on the sea bed at every ocean depth, from the intertidal zone to the abyssal zone. The phylum contains about 7,000 living species, making it the second-largest grouping of deuterostomes, after the chordates. Echinoderms are the largest entirely marine phylum. The first definitive echinoderms appeared near the start of the Cambrian.
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. 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. They live in both shallow water and in depths as great as 9,000 meters (30,000 ft).
Sea urchins are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone — from the intertidal seashore down to 5,000 meters. The spherical, hard shells (tests) of sea urchins are round and spiny, ranging in diameter from 3 to 10 cm. Sea urchins move slowly, crawling with tube feet, and also propel themselves with their spines. Although algae are the primary diet, sea urchins also eat slow-moving (sessile) animals. In the food chain, the predators who eat sea urchins are the sea otter and the starfish, the wolf eel, the triggerfish, and human beings.
Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea. Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish are also known as asteroids due to being in the class Asteroidea. About 1,900 species of starfish occur on the seabed in all the world's oceans, from warm, tropical zones to frigid, polar regions. They are found from the intertidal zone down to abyssal depths, at 6,000 m (20,000 ft) below the surface.
Sea cucumbers are echinoderms from the class Holothuroidea. They are marine animals with a leathery skin and an elongated body containing a single, branched gonad. Sea cucumbers are found on the sea floor worldwide. The number of holothurian species worldwide is about 1,717, with the greatest number being in the Asia-Pacific region. Many of these are gathered for human consumption and some species are cultivated in aquaculture systems. The harvested product is variously referred to as trepang, namako, bêche-de-mer, or balate. Sea cucumbers serve a useful role in the marine ecosystem as they help recycle nutrients, breaking down detritus and other organic matter, after which bacteria can continue the decomposition process.
The water vascular system is a hydraulic system used by echinoderms, such as sea stars and sea urchins, for locomotion, food and waste transportation, and respiration. The system is composed of canals connecting numerous tube feet. Echinoderms move by alternately contracting muscles that force water into the tube feet, causing them to extend and push against the ground, then relaxing to allow the feet to retract.
A pedicellaria is a small wrench- or claw-shaped appendage with movable jaws, called valves, commonly found on echinoderms, particularly in sea stars and sea urchins. Each pedicellaria is an effector organ with its own set of muscles, neuropils, and sensory receptors and is therefore capable of reflex responses to the environment. Pedicellariae are poorly understood but in some taxa, they are thought to keep the body surface clear of algae, encrusting organisms, and other debris in conjunction with the ciliated epidermis present in all echinoderms.
Brittle stars, serpent stars, or ophiuroids are echinoderms in the class Ophiuroidea, closely related to starfish. They crawl across the sea floor using their flexible arms for locomotion. The ophiuroids generally have five long, slender, whip-like arms which may reach up to 60 cm (24 in) in length on the largest specimens.
The madreporite is a light colored calcareous opening used to filter water into the water vascular system of echinoderms. It acts like a pressure-equalizing valve. It is visible as a small red or yellow button-like structure, looking like a small wart, on the aboral surface of the central disk of a sea star or sea urchin or the oral surface of Ophiuroidea. Close up, it is visibly structured, resembling a "madrepore" colony. From this, it derives its name.
Strongylocentrotus droebachiensis is commonly known as the green sea urchin because of its characteristic green color. It is commonly found in northern waters all around the world including both the Pacific and Atlantic Oceans to a northerly latitude of 81 degrees and as far south as Maine and England. The average adult size is around 50 mm (2 in), but it has been recorded at a diameter of 87 mm (3.4 in). The green sea urchin prefers to eat seaweeds but will eat other organisms. They are eaten by a variety of predators, including sea stars, crabs, large fish, mammals, birds, and humans. The species name "droebachiensis" is derived from the name of the town Drøbak in Norway.
The Asterozoa are a subphylum in the phylum Echinodermata. Characteristics include a star-shaped body and radially divergent axes of symmetry. The subphylum includes the class Asteroidea, the class Ophiuroidea, and the extinct order Somasteroidea.
Eccentric sand dollar, also known as the sea-cake, biscuit-urchin, western sand dollar, or Pacific sand dollar, is a member of the order Clypeasteroida, better known as sand dollars, a species of flattened, burrowing sea urchins found in the northeast Pacific Ocean from Alaska to Baja California.
The Parechinidae are a family of sea urchins in the class Echinoidea.
Psychocidaridae is a family of sea urchins in the order Cidaroida. The genus Psychocidaris is extant while the other genera are only known from fossils. The family has been in existence since the Lower Jurassic (Toarcian) and the range includes Europe, Ukraine, North America, North Africa and the West Pacific.
Aspidodiadema jacobyi is a small sea urchin in the family Aspidodiadematidae. It lives in tropical seas at great depths. Aspidodiadema jacobyi was first scientifically described in 1880 by Alexander Emanuel Agassiz, an American scientist.
Comatulida is an order of crinoids. Members of this order are known as feather stars and mostly do not have a stalk as adults. The oral surface with the mouth is facing upwards and is surrounded by five, often divided rays with feathery pinnules. Comatulids live on the seabed and on reefs in tropical and temperate waters.
Echinaster spinulosus, the small spine sea star, is a species of sea star found in shallow parts of the western Atlantic Ocean, the Caribbean Sea and Gulf of Mexico.
A paxilla is a small umbrella-shaped structure sometimes found on Echinoderms, particularly in starfish such as Luidia, Astropecten and Goniaster that immerse themselves in sediment. They are ossicles composed of calcite microcrystals found on the aboral (upper) surface of the animal. Their stalks emerge from the body wall and their umbrella-like crowns, each fringed with short spines, meet edge-to-edge forming a protective external false skin. The water-filled cavity beneath contains the madreporite and delicate gill structures known as papullae.
Leodia sexiesperforata, commonly known as the six-holed keyhole urchin, is a species of sand dollar, in the echinoderm order Clypeasteroida. It is native to tropical and sub-tropical parts of the western Atlantic Ocean where it buries itself in soft sediment in shallow seas.
Freyella elegans is a species of deep-water starfish in the family Freyellidae in the order Brisingida, living at abyssal depths in the northwestern Atlantic Ocean.