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.
Sea urchins are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin are distributed on the seabeds 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 covered in spines. Most urchin spines range in length from 3 to 10 cm, with outliers such as the black sea urchin possessing spines as long as 30 cm (12 in). 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. Predators that eat sea urchins include a wide variety of fish, starfish, crabs, marine mammals, and humans.
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 live 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 known 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.
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.
The red sea urchin is a sea urchin found in the northeastern Pacific Ocean from Alaska to Baja California. It lives in shallow waters from the low-tide line to greater than 280 m (920 ft) deep, and is typically found on rocky shores sheltered from extreme wave action in areas where kelp is available.
Tube feet are small active tubular projections on the oral face of an echinoderm, whether the arms of a starfish, or the undersides of sea urchins, sand dollars and sea cucumbers; they are more discreet though present on brittle stars, and have only a feeding function in feather stars. They are part of the water vascular system.
Keyhole sand dollar refers to three species of sand dollars in the genus Mellita. They are found on the Atlantic coasts of the Americas, ranging from Caribbean Islands, such as Jamaica and Puerto Rico, to the southern areas of the United States at the north to the southern coasts of Brazil at the south. Their range includes Bermuda and the Pacific coasts of equatorial countries, such as Mexico and Costa Rica also found in Honduras.
Astropyga radiata, the red urchin, fire urchin, false fire urchin or blue-spotted urchin, is a species of sea urchin in the family Diadematidae. It is a large species with long spines and is found in the tropical Indo-Pacific region. It was first described in 1778 by the German naturalist Nathaniel Gottfried Leske.
Lytechinus variegatus, commonly called the green sea urchin or the variegated sea urchin, is a species of sea urchin that can be found in the warm waters of the western Atlantic Ocean and Caribbean Sea.
Dendraster excentricus, also known as the eccentric sand dollar, sea-cake, biscuit-urchin, western sand dollar, or Pacific sand dollar, is a species of sand dollar in the family Dendrasteridae. It is a flattened, burrowing sea urchin found in the north-eastern Pacific Ocean from Alaska to Baja California.
Toxopneustes pileolus, commonly known as the flower urchin, is a widespread and commonly encountered species of sea urchin from the Indo-West Pacific. It is considered highly dangerous, as it is capable of delivering extremely painful and medically significant stings when touched. It inhabits coral reefs, seagrass beds, and rocky or sandy environments at depths of up to 90 m (295 ft). It feeds on algae, bryozoans, and organic detritus.
Paracentrotus lividus is a species of sea urchin in the family Parechinidae commonly known as the purple sea urchin. It is the type species of the genus and occurs in the Mediterranean Sea and eastern Atlantic Ocean.
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 are the only parts of the animal likely to be fossilized after an echinoderm dies.
Marthasterias is a genus of starfish in the family Asteriidae. It is monotypic and the only species in the genus is Marthasterias glacialis, commonly known as the spiny starfish. It is native to the eastern Atlantic Ocean.
Heliophora orbicularis, also known as the West African Sand Dollar, is a small sand dollar in to the family Rotulidae, and the only species in the genus Heliophora. It, and other members of Rotulidae have been found in West African marine strata from the Late Miocene onward. Like the related Rotula, it is still extant.
Strongylocentrotus fragilis is a species of sea urchin of the family Strongylocentrotidae.
Clypeaster rosaceus, the fat sea biscuit, is a species of sea urchin in the family Clypeasteridae. It occurs in shallow water in the western Atlantic Ocean and was first scientifically described in 1758 by Carl Linnaeus.
