Instruct: Given a question, retrieve documents that answer the question.
Query: How many species of sea urchin are there?
Sea urchins or urchins (/ˈɜːrtʃɪnz/) are typically spiny, globular animals, echinoderms in the class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal to 5,000 metres (16,000ft; 2,700 fathoms).[1] Their tests (hard shells) are round and spiny, typically from 3 to 10cm (1 to 4in) across. Sea urchins move slowly, crawling with their tube feet, and sometimes pushing themselves with their spines. They feed primarily on algae but also eat slow-moving or sessile animals. Their predators include sea otters, starfish, wolf eels, and triggerfish.
Skeletogenesis is a key morphogenetic event in the embryonic development of vertebrates and is of equal, although transient, importance in the development of the sea urchin, a marine invertebrate. The larval sea urchin does not resemble its adult form, because the sea urchin is an indirect developer, meaning its larva form must undergo metamorphosis to form the juvenile adult. Here, the focus is on skeletogenesis in the sea urchin species "Strongylocentrotus purpuratus", as this species has been most thoroughly studied and characterized.
Sea urchins, "Dotidae" and sea slugs eat seaweed. Some species of sea urchins, such as "Diadema antillarum", can play a pivotal part in preventing algae from overrunning reefs. Researchers are investigating the use of native collector urchins, "Tripneustes gratilla", for their potential as biocontrol agents to mitigate the spread of invasive algae species on coral reefs. "Nudibranchia" and sea anemones eat sponges.
The collector urchin ("Tripneustes gratilla") is a species of sea urchin. Collector urchins are found at depths of in the waters of the Indo-Pacific, Hawaii, the Red Sea, and The Bahamas. They can reach in size.
Phyllacanthus imperialis, also known as the Sputnik urchin, imperial lance urchin, imperial sea urchin, imperial urchin, pencil sea urchin, lance urchin, knobby sputnik sea urchin, mine urchin, and land mine sea urchin, is a species of sea urchins in the family Cidaridae.
Sea urchins are one of the favourite foods of many lobsters, crabs, triggerfish, California sheephead, sea otter and wolf eels (which specialise in sea urchins). All these animals carry particular adaptations (teeth, pincers, claws) and a strength that allow them to overcome the excellent protective features of sea urchins. Left unchecked by predators, urchins devastate their environments, creating what biologists call an urchin barren, devoid of macroalgae and associated fauna.[27] Sea otters have re-entered British Columbia, dramatically improving coastal ecosystem health.[28]
Observations have identified sea otters as a keystone species along the coasts of many of the Aleutian Islands. Their presence encourages the growth of kelp forests, as the otters control sea urchin populations (as large populations of sea urchins can create urchin barrens by clearing away kelp stands).
One of the sea otters’ primary food sources is the sea urchin. When hunters caused sea otter populations to decline, an ecological release of sea urchin populations occurred. The sea urchins then overexploited their main food source, kelp, creating urchin barrens, areas of seabed denuded of kelp, but carpeted with urchins. No longer having food to eat, the sea urchin became locally extinct as well. Also, since kelp forest ecosystems are homes to many other species, the loss of the kelp caused other cascade effects of secondary extinctions.
Many species of wildlife may be found here including bald eagles, great blue herons, peregrine falcons, gulls, crows, several species of sea anemone, many species of crab, snails, sea urchins, and barnacles.
A relatively small number of species shelter in sea urchins or crinoids. Whether this relationship is obligate (clingfish always with a sea urchin or crinoid) or facultative (clingfish sometimes with a sea urchin or crinoid) varies with species. In some, only young clingfish are obligate and gradually move away as they become adult. Three clingfish species, the Australian "Cochleoceps bicolor" and "C. orientalis", and the warm East Atlantic "Diplecogaster tonstricula", are cleaner fish that will cling onto the bodies of larger fish.
Purple sea urchins at low tide in California. They dig a cavity in the rock to hide from predators during the day. Dermechinus horridus, an abyssal species, at thousands of meters deep Antarctic sea urchin (Sterechinus neumayeri) inhabits frozen seas. The shape of the shingle urchin allows it to stay on wave-beaten cliffs.
The AMB's crest crafted by local Haida artist "Giitsxaa" represents the unique joint management relationship. 
The AMB chose the sea otter and the sea urchin because of the creatures' significance in the history and tradition of the protected area and because of their ecological significance.
Populations of sea urchin, a kelp-grazing species, were once kept in check by sea otters, ensuring an abundance of kelp and species that depend upon kelp communities. With the extirpation of sea otters during the Maritime Fur Trade, the natural balance between species in the community was disturbed. As a result, the sea urchin population has increased dramatically over time and the health of kelp forests is threatened. The loss of the sea otter is a powerful reminder of the vulnerability of individual species and entire ecosystems.
Some species of sea urchins, such as the slate pencil urchin (Eucidaris tribuloides), are commonly sold in aquarium stores. Some species are effective at controlling filamentous algae, and they make good additions to an invertebrate tank.[76]
"S. troglodytes" feed and live on the spines of sea urchins. It lives on the body of the slate pencil urchin "Eucidaris tribuloides". Unlike most of its gastropod relatives, "S. troglodyte" has lost its radula so they gain access to all that soft internal spine tissue of a sea urchin by secreting some kind of corrosive substance that eats through the tough walls of the spine. This parasitic snail is not content to simply just feed on the sea urchin, they also alter the urchin's spines to make it a more comfortable home. "Sabinella troglodytes" is one of many species of gall-forming snails that parasitize echinoderms. It is currently unknown how "S. troglodytes" alters the sea urchin's spines, but it could be due to some other components in the snail's saliva. In addition to a corrosive agent to erode the sea urchin's spine, it might also be spitting out growth factors that alter the tissue of the spine. In addition to being a cozy place to feed and hide from threats, these galls seem to be a bit of a love nest for "S. troglodytes" during the summer months.
Approximately 23 species of sea urchins, 3 species of heart urchins, and 9 species of sand dollars call the Gulf of California habitat their home. Where they are found throughout the Gulf of California, can be classified into three biogeographic regions called the Northern Gulf, Central Gulf, and Southern Gulf.
Diadema savignyi is a species of long-spined sea urchin belonging to the family Diadematidae. Common names include long-spined sea urchin, black longspine urchin and the banded diadem. It is native to the east coast of Africa, the Red Sea, the Indian Ocean and western Pacific Ocean. It was first described in 1829 by the French naturalist Jean Victoire Audouin. The specific epithet honours the French zoologist Marie Jules César Savigny who described many new marine species from the Mediterranean Sea and Red Sea. The type locality is Mauritius.
Diadema antillarum, also known as the lime urchin, black sea urchin, Grabaskey's bane or the long-spined sea urchin, is a species of sea urchin in the Family Diadematidae.
Kelp beds provide a nursery area for many marine species and a biodiverse habitat. The grazing behaviour of sea urchins, particularly the green sea urchin, "Strongylocentrotus droebachiensis", restricts the development of new growth of "L. hyperborea". If the urchins become too numerous, whole areas normally dominated by kelp may become "urchin barrens", denuded of kelp and supporting a much less biodiverse community based on encrusting coralline algae.
Sea urchins are established in most seabed habitats from the intertidal downwards, at an extremely wide range of depths.[33] Some species, such as Cidaris abyssicola, can live at depths of several thousands metres. Many genera are only found in the abyssal zone, including many cidaroids, most of the genera in the Echinothuriidae family, and the "cactus urchins" Dermechinus. One of the deepest-living families is the Pourtalesiidae,[34] strange bottle-shaped irregular sea urchins that live only in the hadal zone and have been collected as deep as 6850 metres beneath the surface in the Sunda Trench.[35] Nevertheless, this makes sea urchin the class of echinoderms living the least deep, compared to brittle stars, starfish and crinoids that remain abundant below 8,000m (26,250ft) and sea cucumbers which have been recorded from 10,687m (35,100ft).[35]
Sea urchins move by walking using their many flexible tube feet in a similar way to starfish; regular sea urchins do not have any favourite walking direction.[11] The tube feet protrude through pairs of pores in the test, and are operated by a water vascular system; this works through hydraulic pressure, allowing the sea urchin to pump water into and out of the tube feet. During locomotion, the tube feet are assisted by the spines which can be used for pushing the body along or to lift the test off the substrate. Movement is generally related to feeding, with the red sea urchin (Mesocentrotus franciscanus) managing about 7.5cm (3in) a day when there is ample food, and up to 50cm (20in) a day where there is not. An inverted sea urchin can right itself by progressively attaching and detaching its tube feet and manipulating its spines to roll its body upright.[2] Some species bury themselves in soft sediment using their spines, and Paracentrotus lividus uses its jaws to burrow into soft rocks.[12]
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.
Psammechinus miliaris is a species of sea urchin in the family Parechinidae. It is sometimes known as the green sea urchin or shore sea urchin. It is found in shallow areas of the eastern Atlantic Ocean and the North Sea.
Sea otters ("Enhydra lutris") are commonly cited as an example of a keystone species; because, they limit the density of sea urchins that feed on kelp. If sea otters are removed from the system, the urchins graze until the kelp beds disappear, and this has a dramatic effect on community structure. Hunting of sea otters, for example, is thought to have led indirectly to the extinction of the Steller's sea cow ("Hydrodamalis gigas"). While the keystone species concept has been used extensively as a conservation tool, it has been criticized for being poorly defined from an operational stance. It is difficult to experimentally determine what species may hold a keystone role in each ecosystem. Furthermore, food web theory suggests that keystone species may not be common, so it is unclear how generally the keystone species model can be applied.
Many facets of the interaction between sea otters and the human economy are not as immediately felt. Sea otters have been credited with contributing to the kelp harvesting industry via their well-known role in controlling sea urchin populations; kelp is used in the production of diverse food and pharmaceutical products. Although human divers harvest red sea urchins both for food and to protect the kelp, sea otters hunt more sea urchin species and are more consistently effective in controlling these populations. The health of the kelp forest ecosystem is significant in nurturing populations of fish, including commercially important fish species. In some areas, sea otters are popular tourist attractions, bringing visitors to local hotels, restaurants, and sea otter-watching expeditions.
Some species, called a "keystone species" form a central supporting hub unique to their ecosystem. The loss of such a species results in a collapse in ecosystem function, as well as the loss of coexisting species. Keystone species are usually predators due to their ability to control the population of prey in their ecosystem. The importance of a keystone species was shown by the extinction of the Steller's sea cow ("Hydrodamalis gigas") through its interaction with sea otters, sea urchins, and kelp. Kelp beds grow and form nurseries in shallow waters to shelter creatures that support the food chain. Sea urchins feed on kelp, while sea otters feed on sea urchins. With the rapid decline of sea otters due to overhunting, sea urchin populations grazed unrestricted on the kelp beds and the ecosystem collapsed. Left unchecked, the urchins destroyed the shallow water kelp communities that supported the Steller's sea cow's diet and hastened their demise. The sea otter was thought to be a keystone species because the coexistence of many ecological associates in the kelp beds relied upon otters for their survival. However this was later questioned by Turvey and Risley, who showed that hunting alone would have driven the Steller's sea cow extinct.