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66-846: Sea urchins or urchins ( / ˈ ɜːr tʃ ɪ n z / ) 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 (16,000 ft; 2,700 fathoms). Their tests (hard shells) are round and spiny, typically from 3 to 10 cm (1 to 4 in) 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 sharks , sea otters , starfish , wolf eels , and triggerfish . Like all echinoderms, adult sea urchins have fivefold symmetry with their pluteus larvae featuring bilateral (mirror) symmetry ; The latter indicates that they belong to

132-551: A distinct grade of organization—i.e. a 'level of complexity', measured in terms of how differentiated their organ systems are into distinct regions or sub-organs—with a distinct type of construction, which is to say a particular layout of organ systems. This said, the composition of each class is ultimately determined by the subjective judgment of taxonomists . In the first edition of his Systema Naturae (1735), Carl Linnaeus divided all three of his kingdoms of nature ( minerals , plants , and animals ) into classes. Only in

198-424: A distinct rank of biological classification having its own distinctive name – and not just called a top-level genus (genus summum) – was first introduced by French botanist Joseph Pitton de Tournefort in the classification of plants that appeared in his Eléments de botanique of 1694. Insofar as a general definition of a class is available, it has historically been conceived as embracing taxa that combine

264-528: A feature of the shell of several different species of gastropod and bivalve mollusks, including the venus clam Pitar lupanaria . Many species of arthropods also have spine-like protrusions on their bodies for defensive purposes. For example, the rostra on many shrimp species form a sharp spine that can be used against predators. The urticating bristles or setae on many caterpillars and New World tarantulas are essentially tiny detachable spines that can cause severe irritation upon contact. Those on

330-446: A free-swimming blastula embryo in as few as 12 hours. Initially a simple ball of cells, the blastula soon transforms into a cone-shaped echinopluteus larva. In most species, this larva has 12 elongated arms lined with bands of cilia that capture food particles and transport them to the mouth. In a few species, the blastula contains supplies of nutrient yolk and lacks arms, since it has no need to feed. Several months are needed for

396-503: A hemal system with a complex network of vessels in the mesenteries around the gut, but little is known of the functioning of this system. However, the main circulatory fluid fills the general body cavity, or coelom . This coelomic fluid contains phagocytic coelomocytes, which move through the vascular and hemal systems and are involved in internal transport and gas exchange. The coelomocytes are an essential part of blood clotting , but also collect waste products and actively remove them from

462-484: A jointed internal appendicular skeleton . The limbs of tetrapods , who descended from sarcopterygian ancestors , are homologous to the paired pectoral and pelvic fins . Some fish, such as scorpion fish and lionfish , has prominent sharp, venomous spines for anti-predator defense . The tail stinger on a stingray is also a type of barbed spine modified from dermal denticles . The acanthodians , an extinct class of ancient fish that are paraphyletic to

528-413: A mistranslation. Aristotle's lantern is actually referring to the whole shape of sea urchins, which look like the ancient lamps of Aristotle's time. Heart urchins are unusual in not having a lantern. Instead, the mouth is surrounded by cilia that pull strings of mucus containing food particles towards a series of grooves around the mouth. The lantern, where present, surrounds both the mouth cavity and

594-431: A multipart process which dramatically rearranges its structure by invagination to produce the three germ layers , involving an epithelial-mesenchymal transition ; primary mesenchyme cells move into the blastocoel and become mesoderm . It has been suggested that epithelial polarity together with planar cell polarity might be sufficient to drive gastrulation in sea urchins. An unusual feature of sea urchin development

660-436: 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 . Sea urchins graze on the lower stems of kelp, causing the kelp to drift away and die. Loss of the habitat and nutrients provided by kelp forests leads to profound cascade effects on

726-419: A taxonomy of the flowering plants up to the level of orders, many sources have preferred to treat ranks higher than orders as informal clades . Where formal ranks have been assigned, the ranks have been reduced to a very much lower level, e.g. class Equisitopsida for the land plants, with the major divisions within the class assigned to subclasses and superorders. The class was considered the highest level of

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792-409: A toothband with a hard tooth pointing towards the centre of the mouth. Specialised muscles control the protrusion of the apparatus and the action of the teeth, and the animal can grasp, scrape, pull and tear. The structure of the mouth and teeth have been found to be so efficient at grasping and grinding that similar structures have been tested for use in real-world applications. On the upper surface of

858-477: A warning to predators, much like rattlesnakes use their rattles. Because many species of fish and invertebrates carry venom within their spines, a rule of thumb is to treat every injury as if it were a snake bite. Venom can cause intense pain, and can sometimes result in death if left untreated. On the other hand, being pricked by a porcupine quill is not dangerous, and the quills are not poisonous. The quill can be removed by gently but firmly pulling it out of

924-405: A way similar to that of starfish; regular sea urchins do not have any favourite walking direction. 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

990-547: A wide range of invertebrates, such as mussels , polychaetes , sponges , brittle stars, and crinoids, making them omnivores, consumers at a range of trophic levels . Mass mortality of sea urchins was first reported in the 1970s, but diseases in sea urchins had been little studied before the advent of aquaculture. In 1981, bacterial "spotting disease" caused almost complete mortality in juvenile Pseudocentrotus depressus and Hemicentrotus pulcherrimus , both cultivated in Japan;

1056-441: Is a large nerve ring encircling the mouth just inside the lantern. From the nerve ring, five nerves radiate underneath the radial canals of the water vascular system, and branch into numerous finer nerves to innervate the tube feet, spines, and pedicellariae . Sea urchins are sensitive to touch, light, and chemicals. There are numerous sensitive cells in the epithelium, especially in the spines, pedicellaria and tube feet, and around

1122-471: Is derived from the Old French herichun , from Latin ericius ('hedgehog'). Like other echinoderms, sea urchin early larvae have bilateral symmetry, but they develop five-fold symmetry as they mature. This is most apparent in the "regular" sea urchins, which have roughly spherical bodies with five equally sized parts radiating out from their central axes. The mouth is at the base of the animal and

1188-408: Is established before the egg is fertilized. The oral-aboral axis is specified early in cleavage, and the left-right axis appears at the late gastrula stage. In most cases, the female's eggs float freely in the sea, but some species hold onto them with their spines, affording them a greater degree of protection. The unfertilized egg meets with the free-floating sperm released by males, and develops into

1254-409: Is known as Aristotle's lantern from Aristotle 's description in his History of Animals (translated by D'Arcy Thompson ): ... the urchin has what we mainly call its head and mouth down below, and a place for the issue of the residuum up above. The urchin has, also, five hollow teeth inside, and in the middle of these teeth a fleshy substance serving the office of a tongue . Next to this comes

1320-498: Is not obvious in the living animal, but is easily visible in the dried test . Specifically, the term "sea urchin" refers to the "regular echinoids", which are symmetrical and globular, and includes several different taxonomic groups, with two subclasses: Euechinoidea ("modern" sea urchins, including irregular ones) and Cidaroidea , or "slate-pencil urchins", which have very thick, blunt spines, with algae and sponges growing on them. The "irregular" sea urchins are an infra-class inside

1386-424: Is present. Densities decrease in winter when storms cause them to seek protection in cracks and around larger underwater structures. The shingle urchin ( Colobocentrotus atratus ), which lives on exposed shorelines, is particularly resistant to wave action. It is one of the few sea urchin that can survive many hours out of water. Sea urchins can be found in all climates, from warm seas to polar oceans. The larvae of

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1452-400: Is the replacement of the larva's bilateral symmetry by the adult's broadly fivefold symmetry. During cleavage, mesoderm and small micromeres are specified. At the end of gastrulation, cells of these two types form coelomic pouches. In the larval stages, the adult rudiment grows from the left coelomic pouch; after metamorphosis, that rudiment grows to become the adult. The animal-vegetal axis

1518-511: The Lonomia caterpillars are venomous and can cause lethal coagulopathy , hemolysis and kidney failure . Spines are also found in internal organs in invertebrates, such as the copulatory spines in the male or female organs of certain flatworms . In many cases, spines are a defense mechanism that help protect the animal against potential predators. Because spines are sharp, they can puncture skin and inflict pain and damage which may cause

1584-580: The Bilateria , along with chordates , arthropods , annelids and molluscs . Sea urchins are found in every ocean and in every climate, from the tropics to the polar regions , and inhabit marine benthic (sea bed) habitats, from rocky shores to hadal zone depths. The fossil record of the Echinoids dates from the Ordovician period, some 450 million years ago. The closest echinoderm relatives of

1650-477: The Middle Ordovician period ( circa 465 Mya ). There is a rich fossil record, their hard tests made of calcite plates surviving in rocks from every period since then. Spines are present in some well-preserved specimens, but usually only the test remains. Isolated spines are common as fossils. Some Jurassic and Cretaceous Cidaroida had very heavy, club-shaped spines. Most fossil echinoids from

1716-596: The Paleozoic era are incomplete, consisting of isolated spines and small clusters of scattered plates from crushed individuals, mostly in Devonian and Carboniferous rocks. The shallow-water limestones from the Ordovician and Silurian periods of Estonia are famous for echinoids. Paleozoic echinoids probably inhabited relatively quiet waters. Because of their thin tests, they would certainly not have survived in

1782-419: The cartilaginous fishes , have prominent bony spines in the front ( rostral ) edges of all fins except the tail . The primary function of these rigid spines are generally presumed to be defensive against predators, but other proposed roles are as cutwaters to reduce drag or as holdfasts against subsurface currents . Defensive spines are also found in invertebrate animals, such as sea urchins . They are

1848-515: The chordates . A 2014 analysis of 219 genes from all classes of echinoderms gives the following phylogenetic tree . Approximate dates of branching of major clades are shown in millions of years ago (mya). [REDACTED] Spine (zoology) Spines in mammals include the prickles of hedgehogs , and among rodents, the quills of porcupines (of both the New World and the Old ), as well as

1914-429: The esophagus , and then the stomach , divided into five parts, and filled with excretion, all the five parts uniting at the anal vent, where the shell is perforated for an outlet ... In reality the mouth-apparatus of the urchin is continuous from one end to the other, but to outward appearance it is not so, but looks like a horn lantern with the panes of horn left out. However, this has recently been proven to be

1980-423: The fins of most bony fishes , particularly actinopterygians ( ray-finned fishes ), who have folding fan -like fin made of spreading bony spines called lepidotrichia or "rays" covered by thin stretches of skin. In the other bony fish clade , the sarcopterygians ( lobe-finned fish ), the fin spines (if any at all) are significantly shorter and each fin is instead dominated by a muscular stalk ("lobe") with

2046-696: The hadal zone and have been collected as deep as 6850 metres beneath the surface in the Sunda Trench . Nevertheless, this makes sea urchin the class of echinoderms living the least deep, compared to brittle stars , starfish and crinoids that remain abundant below 8,000 m (26,250 ft) and sea cucumbers which have been recorded from 10,687 m (35,100 ft). Population densities vary by habitat, with more dense populations in barren areas as compared to kelp stands. Even in these barren areas, greatest densities are found in shallow water. Populations are generally found in deeper water if wave action

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2112-402: The pharynx . At the top of the lantern, the pharynx opens into the esophagus, which runs back down the outside of the lantern, to join the small intestine and a single caecum . The small intestine runs in a full circle around the inside of the test, before joining the large intestine, which completes another circuit in the opposite direction. From the large intestine, a rectum ascends towards

2178-471: The slate pencil urchin are popular in aquaria, where they are useful for controlling algae. Fossil urchins have been used as protective amulets . Sea urchins are members of the phylum Echinodermata , which also includes starfish , sea cucumbers , sand dollars , brittle stars , and crinoids . Like other echinoderms, they have five-fold symmetry (called pentamerism ) and move by means of hundreds of tiny, transparent, adhesive " tube feet ". The symmetry

2244-648: The Euechinoidea, called Irregularia , and include Atelostomata and Neognathostomata . Irregular echinoids include flattened sand dollars , sea biscuits , and heart urchins . Together with sea cucumbers ( Holothuroidea ), they make up the subphylum Echinozoa , which is characterized by a globoid shape without arms or projecting rays. Sea cucumbers and the irregular echinoids have secondarily evolved diverse shapes. Although many sea cucumbers have branched tentacles surrounding their oral openings, these have originated from modified tube feet and are not homologous to

2310-458: The ambulacral areas; their function is to help in gravitational orientation. Sea urchins are dioecious , having separate male and female sexes, although no distinguishing features are visible externally. In addition to their role in reproduction, the gonads are also nutrient storing organs, and are made up of two main type of cells: germ cells , and somatic cells called nutritive phagocytes. Regular sea urchins have five gonads, lying underneath

2376-411: The animal is low in oxygen. Tube feet can also act as respiratory organs, and are the primary sites of gas exchange in heart urchins and sand dollars, both of which lack gills. The inside of each tube foot is divided by a septum which reduces diffusion between the incoming and outgoing streams of fluid. The nervous system of sea urchins has a relatively simple layout. With no true brain, the neural center

2442-460: The animal kingdom are Linnaeus's classes similar to the classes used today; his classes and orders of plants were never intended to represent natural groups, but rather to provide a convenient "artificial key" according to his Systema Sexuale , largely based on the arrangement of flowers. In botany, classes are now rarely discussed. Since the first publication of the APG system in 1998, which proposed

2508-435: The animals to burrow through sand or other soft materials. The internal organs are enclosed in a hard shell or test composed of fused plates of calcium carbonate covered by a thin dermis and epidermis . The test is referred to as an endoskeleton rather than exoskeleton even though it encloses almost all of the urchin. This is because it is covered with a thin layer of muscle and skin; sea urchins also do not need to molt

2574-437: The anus at the top; the lower surface is described as "oral" and the upper surface as "aboral". Several sea urchins, however, including the sand dollars, are oval in shape, with distinct front and rear ends, giving them a degree of bilateral symmetry. In these urchins, the upper surface of the body is slightly domed, but the underside is flat, while the sides are devoid of tube feet. This "irregular" body form has evolved to allow

2640-449: The anus. Despite the names, the small and large intestines of sea urchins are in no way homologous to the similarly named structures in vertebrates. Digestion occurs in the intestine, with the caecum producing further digestive enzymes . An additional tube, called the siphon, runs beside much of the intestine, opening into it at both ends. It may be involved in resorption of water from food. The water vascular system leads downwards from

2706-544: The arms of the crinoids, sea stars, and brittle stars. Urchins typically range in size from 3 to 10 cm (1 to 4 in), but the largest species can reach up to 36 cm (14 in). They have a rigid, usually spherical body bearing moveable spines, which give the class the name Echinoidea (from the Greek ἐχῖνος ekhinos 'spine'). The name urchin is an old word for hedgehog , which sea urchins resemble; they have archaically been called sea hedgehogs . The name

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2772-589: 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 7.5 cm (3 in) a day when there is ample food, and up to 50 cm (20 in) 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. Some species bury themselves in soft sediment using their spines, and Paracentrotus lividus uses its jaws to burrow into soft rocks. The mouth lies in

2838-430: The body through the gills and tube feet. Most sea urchins possess five pairs of external gills attached to the peristomial membrane around their mouths. These thin-walled projections of the body cavity are the main organs of respiration in those urchins that possess them. Fluid can be pumped through the gills' interiors by muscles associated with the lantern, but this does not provide a continuous flow, and occurs only when

2904-422: The centre of the oral surface in regular urchins, or towards one end in irregular urchins. It is surrounded by lips of softer tissue, with numerous small, embedded bony pieces. This area, called the peristome, also includes five pairs of modified tube feet and, in many species, five pairs of gills. The jaw apparatus consists of five strong arrow-shaped plates known as pyramids, the ventral surface of each of which has

2970-631: The chalk of the Cretaceous period, serve as zone or index fossils. Because they are abundant and evolved rapidly, they enable geologists to date the surrounding rocks. In the Paleogene and Neogene periods ( circa 66 to 2.6 Mya), sand dollars (Clypeasteroida) arose. Their distinctive, flattened tests and tiny spines were adapted to life on or under loose sand in shallow water, and they are abundant as fossils in southern European limestones and sandstones. Echinoids are deuterostome animals, like

3036-435: The disease recurred in succeeding years. It was divided into a cool-water "spring" disease and a hot-water "summer" form. Another condition, bald sea urchin disease , causes loss of spines and skin lesions and is believed to be bacterial in origin. Adult sea urchins are usually well protected against most predators by their strong and sharp spines, which can be venomous in some species. The small urchin clingfish lives among

3102-406: The interambulacral regions of the test, while the irregular forms mostly have four, with the hindmost gonad being absent; heart urchins have three or two. Each gonad has a single duct rising from the upper pole to open at a gonopore lying in one of the genital plates surrounding the anus. Some burrowing sand dollars have an elongated papilla that enables the liberation of gametes above the surface of

3168-572: The intertidal downwards, at an extremely wide range of depths. Some species, such as Cidaris abyssicola , can live at depths of several kilometres. Many genera are found in only 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 , strange bottle-shaped irregular sea urchins that live in only

3234-803: The larva to complete its development, the change into the adult form beginning with the formation of test plates in a juvenile rudiment which develops on the left side of the larva, its axis being perpendicular to that of the larva. Soon, the larva sinks to the bottom and metamorphoses into a juvenile urchin in as little as one hour. In some species, adults reach their maximum size in about five years. The purple urchin becomes sexually mature in two years and may live for twenty. Red sea urchins were originally thought to live 7 to 10 years but recent studies have shown that they can live for more than 100 years. Canadian red urchins have been found to be around 200 years old. Sea urchins feed mainly on algae , so they are primarily herbivores , but can feed on sea cucumbers and

3300-434: The madreporite through the slender stone canal to the ring canal, which encircles the oesophagus. Radial canals lead from here through each ambulacral area to terminate in a small tentacle that passes through the ambulacral plate near the aboral pole. Lateral canals lead from these radial canals, ending in ampullae. From here, two tubes pass through a pair of pores on the plate to terminate in the tube feet. Sea urchins possess

3366-408: The marine ecosystem. Sea otters have re-entered British Columbia , dramatically improving coastal ecosystem health. The spines , long and sharp in some species, protect the urchin from predators . Some tropical sea urchins like Diadematidae , Echinothuriidae and Toxopneustidae have venomous spines. Other creatures also make use of these defences; crabs, shrimps and other organisms shelter among

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3432-472: The mouth. Although they do not have eyes or eye spots (except for diadematids , which can follow a threat with their spines), the entire body of most regular sea urchins might function as a compound eye. In general, sea urchins are negatively attracted to light, and seek to hide themselves in crevices or under objects. Most species, apart from pencil urchins , have statocysts in globular organs called spheridia. These are stalked structures and are located within

3498-453: The plates are covered in rounded tubercles to which the spines are attached. The spines are used for defence and for locomotion and come in a variety of forms. The inner surface of the test is lined by peritoneum . Sea urchins convert aqueous carbon dioxide using a catalytic process involving nickel into the calcium carbonate portion of the test. Most species have two series of spines, primary (long) and secondary (short), distributed over

3564-471: The polar sea urchin Sterechinus neumayeri have been found to use energy in metabolic processes twenty-five times more efficiently than do most other organisms. Despite their presence in nearly all the marine ecosystems, most species are found on temperate and tropical coasts, between the surface and some tens of meters deep, close to photosynthetic food sources. The earliest echinoid fossils date to

3630-454: The predator to avoid that species from that point on. The spine of some animals are capable of injecting venom . In the case of some large species of stingray, a puncture with the barbed spine and the accompanying venom has occasionally been fatal to humans. Animals such as porcupines are considered aposematic , because their spines warn predators that they are dangerous, and in some cases, potentially toxic. Porcupines rattle their quills as

3696-639: The prickly fur of spiny mice , spiny pocket mice , and of species of spiny rat . They are also found on afrotherian tenrecs of the family Tenrecinae (hedgehog and streaked tenrecs), marsupial spiny bandicoots , and on echidnas (a monotreme ). An ancient synapsid , Dimetrodon , had extremely long spines on its backbone that were joined together with a web of skin that formed a sail-like structure. Many mammalian species, like cats and fossas , also have penile spines . The Mesozoic eutriconodont mammal Spinolestes already displayed spines similar to those of modern spiny mice. Spines are found in

3762-489: The sea urchin are the sea cucumbers (Holothuroidea), which like them are deuterostomes , a clade that includes the chordates . ( Sand dollars are a separate order in the sea urchin class Echinoidea.) The animals have been studied since the 19th century as model organisms in developmental biology , as their embryos were easy to observe. That has continued with studies of their genomes because of their unusual fivefold symmetry and relationship to chordates. Species such as

3828-415: The sediment. The gonads are lined with muscles underneath the peritoneum, and these allow the animal to squeeze its gametes through the duct and into the surrounding sea water, where fertilization takes place. During early development, the sea urchin embryo undergoes 10 cycles of cell division , resulting in a single epithelial layer enveloping the blastocoel . The embryo then begins gastrulation ,

3894-551: The skin. The barbed tip sometimes breaks off, but it works its way out through the skin over time. Common uses for animal spines include: Class (biology) In biological classification , class ( Latin : classis ) is a taxonomic rank , as well as a taxonomic unit, a taxon , in that rank. It is a group of related taxonomic orders. Other well-known ranks in descending order of size are life , domain , kingdom , phylum , order , family , genus , and species , with class ranking between phylum and order. The class as

3960-443: The spines of urchins such as Diadema ; juveniles feed on the pedicellariae and sphaeridia, adult males choose the tube feet and adult females move away to feed on shrimp eggs and molluscs. 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

4026-469: The spines, and often adopt the colouring of their host. Some crabs in the Dorippidae family carry sea urchins, starfish, sharp shells or other protective objects in their claws. Pedicellariae are a good means of defense against ectoparasites, but not a panacea as some of them actually feed on it. The hemal system defends against endoparasites. Sea urchins are established in most seabed habitats from

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4092-540: The surface of the body, with the shortest at the poles and the longest at the equator. The spines are usually hollow and cylindrical. Contraction of the muscular sheath that covers the test causes the spines to lean in one direction or another, while an inner sheath of collagen fibres can reversibly change from soft to rigid which can lock the spine in one position. Located among the spines are several types of pedicellaria , moveable stalked structures with jaws. Sea urchins move by walking, using their many flexible tube feet in

4158-460: The test at the aboral pole is a membrane, the periproct , which surrounds the anus . The periproct contains a variable number of hard plates, five of which, the genital plates, contain the gonopores, and one is modified to contain the madreporite , which is used to balance the water vascular system. The mouth of most sea urchins is made up of five calcium carbonate teeth or plates, with a fleshy, tongue-like structure within. The entire chewing organ

4224-585: The upper Triassic, their numbers increased again. Cidaroids have changed very little since the Late Triassic , and are the only Paleozoic echinoid group to have survived. The euechinoids diversified into new lineages in the Jurassic and Cretaceous periods, and from them emerged the first irregular echinoids (the Atelostomata ) during the early Jurassic. Some echinoids, such as Micraster in

4290-582: The wave-battered coastal waters inhabited by many modern echinoids. Echinoids declined to near extinction at the end of the Paleozoic era, with just six species known from the Permian period. Only two lineages survived this period's massive extinction and into the Triassic : the genus Miocidaris , which gave rise to modern cidaroida (pencil urchins), and the ancestor that gave rise to the euechinoids . By

4356-420: The way invertebrates with true exoskeletons do, instead the plates forming the test grow as the animal does. The test is rigid, and divides into five ambulacral grooves separated by five wider interambulacral areas. Each of these ten longitudinal columns consists of two sets of plates (thus comprising 20 columns in total). The ambulacral plates have pairs of tiny holes through which the tube feet extend. All of

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