Sea urchin

Sea urchinsorurchins(/ˈɜːrtʃɪnz/) are typicallyspiny, globularanimals,echinodermsin 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).Theirtests(hard shells) are round and spiny, typically from 3 to 10 cm (1 to 4 in) across. Sea urchins move slowly, crawling with theirtube feet, and sometimes pushing themselves with their spines. They feed primarily onalgaebut also eat slow-moving orsessileanimals. Theirpredatorsincludesharks,sea otters,starfish,wolf eels, andtriggerfish.
Like all echinoderms, adult sea urchins have pentagonal symmetry with theirpluteus larvaefeaturingbilateral (mirror) symmetry; The latter indicates that they belong to theBilateria, along withchordates,arthropods,annelidsandmolluscs.  Sea urchins are found in every ocean and in every climate, from thetropicsto thepolar regions, and inhabit marine benthic (sea bed) habitats, from rocky shores tohadal zonedepths. The fossil record of theEchinoidsdates from theOrdovicianperiod, some 450 million years ago. The closest echinoderm relatives of the sea urchin are thesea cucumbers(Holothuroidea), which like them aredeuterostomes, a clade that includes thechordates. (Sand dollarsare a separate order in the sea urchin class Echinoidea.)
The animals have been studied since the 19th century asmodel organismsindevelopmental biology, as their embryos were easy to observe. That has continued with studies of theirgenomesbecause of their unusual fivefold symmetry and relationship to chordates. Species such as theslate pencil urchinare popular in aquaria, where they are useful for controlling algae. Fossil urchins have been used as protectiveamulets.
Sea urchins are members of thephylumEchinodermata, which also includesstarfish,sea cucumbers,sand dollars,brittle stars, andcrinoids. Like other echinoderms, they have five-fold symmetry (calledpentamerism) and move by means of hundreds of tiny, transparent, adhesive "tube feet". The symmetry is not obvious in the living animal, but is easily visible in the driedtest.
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) andCidaroidea, 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 the Euechinoidea, calledIrregularia, and includeAtelostomataandNeognathostomata. Irregular echinoids include flattenedsand dollars,sea biscuits, andheart urchins.
Together with sea cucumbers (Holothuroidea), they make up the subphylumEchinozoa, 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 branchedtentaclessurrounding their oral openings, these have originated from modified tube feet and are not homologous to 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 theclassthe nameEchinoidea(from the Greekἐχῖνοςekhinos'spine').The nameurchinis an old word forhedgehog, which sea urchins resemble; they have archaically been calledsea hedgehogs.The name is derived from the Old Frenchherichun, from Latinericius('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 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 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 ofcalcium carbonatecovered by a thindermisandepidermis.  The test is referred to as anendoskeletonrather 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 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 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 byperitoneum.Sea urchins convert aqueouscarbon dioxideusing acatalyticprocess involvingnickelinto the calcium carbonate portion of the test.
Most species have two series of spines, primary (long) and secondary (short), distributed over 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 ofpedicellaria, moveable stalked structures with jaws.
Sea urchins move by walking, using their many flexible tube feet in 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 awater vascular system; this works throughhydraulic 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 thered 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, andParacentrotus lividususes its jaws to burrow into soft rocks.
The mouth lies in 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 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 the test at the aboral pole is a membrane, theperiproct, which surrounds theanus. The periproct contains a variable number of hard plates, five of which, the genital plates, contain the gonopores, and one is modified to contain themadreporite, 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 is known as Aristotle's lantern fromAristotle's description in hisHistory of Animals(translated byD'Arcy Thompson):
However, this has recently been proven to be 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 urchinsare unusual in not having a lantern. Instead, the mouth is surrounded byciliathat 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 thepharynx. At the top of the lantern, the pharynx opens into the esophagus, which runs back down the outside of the lantern, to join the smallintestineand a singlecaecum. 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, arectumascends towards the anus. Despite the names, the small and large intestines of sea urchins are in no wayhomologousto the similarly named structures in vertebrates.
Digestion occurs in the intestine, with the caecum producing further digestiveenzymes. 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 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 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, orcoelom. This coelomic fluid containsphagocyticcoelomocytes, which move through the vascular and hemal systems and are involved in internal transport and gas exchange. The coelomocytes are an essential part ofblood clotting, but also collect waste products and actively remove them from 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 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 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, andpedicellariae.
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 the mouth.Although they do not have eyes or eye spots (except fordiadematids, 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 frompencil urchins, havestatocystsin globular organs called spheridia. These are stalked structures and are located within the ambulacral areas; their function is to help in gravitational orientation.
Sea urchins aredioecious, having separate male and female sexes, although no distinguishing features are visible externally. In addition to their role in reproduction, thegonadsare also nutrient storing organs, and are made up of two main type of cells:germ cells, andsomatic cellscalled nutritive phagocytes.Regular sea urchins have five gonads, lying underneath 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 agonoporelying 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 the sediment.The gonads are lined with muscles underneath the peritoneum, and these allow the animal to squeeze itsgametesthrough the duct and into the surrounding sea water, wherefertilizationtakes place.
During early development, the sea urchinembryoundergoes 10 cycles ofcell division,resulting in a singleepitheliallayer enveloping theblastocoel. The embryo then beginsgastrulation, a multipart process which dramatically rearranges its structure byinvaginationto produce the threegerm layers, involving anepithelial-mesenchymal transition; primarymesenchymecells move into the blastocoeland becomemesoderm.It has been suggested thatepithelial polaritytogether with planar cell polarity might be sufficient to drive gastrulation in sea urchins.
An unusual feature of sea urchin development is the replacement of the larva'sbilateral symmetryby the adult's broadly fivefold symmetry. During cleavage, mesoderm and small micromeres are specified. At the end of gastrulation, cells of these two types formcoelomicpouches. In the larval stages, the adult rudiment grows from the left coelomic pouch; after metamorphosis, that rudiment grows to become the adult. Theanimal-vegetal axisis 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 a free-swimmingblastulaembryo in as few as 12 hours. Initially a simple ball of cells, the blastula soontransformsinto a cone-shapedechinopluteuslarva. 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 nutrientyolkand lacks arms, since it has no need to feed.
Several months are needed for 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 andmetamorphosesinto a juvenile urchin in as little as one hour.In some species, adults reach their maximum size in about five years.Thepurple urchinbecomes sexually mature in two years and may live for twenty.
Red sea urchinswere 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 onalgae, so they are primarilyherbivores, but can feed on sea cucumbers and a wide range of invertebrates, such asmussels,polychaetes,sponges, brittle stars, and crinoids, making them omnivores, consumers at a range oftrophic 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 juvenilePseudocentrotus depressusandHemicentrotus pulcherrimus, both cultivated in Japan; 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 smallurchin clingfishlives among the spines of urchins such asDiadema; 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 manylobsters,crabs,triggerfish,California sheephead,sea otterandwolf 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 anurchin barren, devoid of macroalgae and associatedfauna.Sea urchins graze on the lower stems of kelp, causing the kelp to drift away and die. Loss of the habitat and nutrients provided bykelp forestsleads to profoundcascade effectson the marine ecosystem. Sea otters have re-enteredBritish Columbia, dramatically improving coastal ecosystem health.
Thespines, long and sharp in some species, protect the urchin frompredators. Some tropical sea urchins likeDiadematidae,EchinothuriidaeandToxopneustidaehave venomous spines. Other creatures also make use of these defences; crabs, shrimps and other organisms shelter among the spines, and often adopt the colouring of their host. Some crabs in theDorippidaefamily carry sea urchins, starfish, sharp shells or other protective objects in their claws.
Pedicellariaeare 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 the intertidal downwards, at an extremely wide range of depths.Some species, such asCidarisabyssicola, can live at depths of several kilometres. Many genera are found in only theabyssal zone, including manycidaroids, most of the genera in theEchinothuriidaefamily, and the "cactus urchins"Dermechinus. One of the deepest-living families is thePourtalesiidae,strange bottle-shaped irregular sea urchins that live in only thehadal zoneand have been collected as deep as 6850 metres beneath the surface in theSunda Trench.Nevertheless, this makes sea urchin the class of echinoderms living the least deep, compared tobrittle stars,starfishandcrinoidsthat remain abundant below 8,000 m (26,250 ft) andsea cucumberswhich have been recorded from 10,687 m (35,100 ft).
Population densities vary by habitat, with more dense populations in barren areas as compared tokelpstands.Even in these barren areas, greatest densities are found in shallow water. Populations are generally found in deeper water if wave action is present.Densities decrease in winter when storms cause them to seek protection in cracks and around larger underwater structures.Theshingle 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 the polar sea urchinSterechinus neumayerihave 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 tophotosyntheticfood sources.
The earliest echinoidfossilsdate to theMiddle Ordovicianperiod (circa465Mya).There is a rich fossil record, their hard tests made ofcalciteplates 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. SomeJurassicandCretaceousCidaroidahad very heavy, club-shaped spines.
Most fossil echinoids from thePaleozoicera are incomplete, consisting of isolated spines and small clusters of scattered plates from crushed individuals, mostly inDevonianandCarboniferousrocks. The shallow-waterlimestonesfrom the Ordovician andSilurianperiods ofEstoniaare famous for echinoids.Paleozoic echinoids probably inhabited relatively quiet waters. Because of their thin tests, they would certainly not have survived in 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 thePermianperiod. Only two lineages survived this period's massive extinction and into theTriassic: the genusMiocidaris, which gave rise to moderncidaroida(pencil urchins), and the ancestor that gave rise to theeuechinoids. By the upper Triassic, their numbers increased again. Cidaroids have changed very little since the LateTriassic, and are the only Paleozoic echinoid group to have survived.
The euechinoids diversified into new lineages in theJurassicandCretaceousperiods, and from them emerged the first irregular echinoids (theAtelostomata) during the early Jurassic.
Some echinoids, such asMicrasterin the chalk of the Cretaceous period, serve as zone orindexfossils. Because they are abundant and evolved rapidly, they enable geologists to date the surrounding rocks.
In thePaleogeneandNeogeneperiods (circa66 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 aredeuterostomeanimals, like thechordates. A 2014 analysis of 219 genes from all classes of echinoderms gives the followingphylogenetic tree.Approximate dates of branching of major clades are shown in millions of years ago (mya).
The phylogeny of the sea urchins is as follows:
The phylogenetic study from 2022 presents a different topology of theEuechinoideaphylogenetic tree.Irregulariaare sister group ofEchinacea(includingSalenioida) forming a common cladeCarinacea, basal groupsAspidodiadematoida,Diadematoida,Echinothurioida,Micropygoida, andPedinoidaare comprised in a common basal cladeAulodonta.
Sea urchin injuries are puncture wounds inflicted by the animal's brittle, fragile spines.These are a common source of injury to ocean swimmers, especially along coastal surfaces where coral with stationary sea urchins are present. Their stings vary in severity depending on the species. Their spines can be venomous or cause infection.Granulomaand staining of the skin from the natural dye inside the sea urchin can also occur. Breathing problems may indicate a serious reaction to toxins in the sea urchin.They inflict a painful wound when they penetrate human skin, but are not themselves dangerous if fully removed promptly; if left in the skin, further problems may occur.
Sea urchins are traditionalmodel organismsindevelopmental biology. This use originated in the 1800s, when their embryonic development became easily viewed by microscopy. The transparency of the urchin's eggs enabled them to be used to observe thatspermcells actually fertilizeova.They continue to be used for embryonic studies, asprenatal developmentcontinues to seek testing for fatal diseases. Sea urchins are being used in longevity studies for comparison between the young and old of the species, particularly for their ability to regenerate tissue as needed.Scientists at theUniversity of St Andrewshave discovered a genetic sequence, the '2A' region, in sea urchins previously thought to have belonged only to viruses likefoot-and-mouth disease virus.More recently,Eric H. DavidsonandRoy John Brittenargued for the use of urchins as a model organism due to their easy availability, high fecundity, and long lifespan. Beyondembryology, urchins provide an opportunity to researchcis-regulatory elements.Oceanography has taken an interest in monitoring the health of urchins and their populations as a way to assess overallocean acidification,temperatures, and ecological impacts.
The organism's evolutionary placement and unique embryology with five-fold symmetry were the major arguments in the proposal to seek the sequencing of itsgenome. Importantly, urchins act as the closest living relative to chordates and thus are of interest for the light they can shed on the evolution ofvertebrates.The genome ofStrongylocentrotus purpuratuswas completed in 2006 and established homology between sea urchin and vertebrateimmune system-related genes. Sea urchins code for at least 222Toll-like receptorgenes and over 200 genes related to theNod-like-receptorfamily found in vertebrates.This increases its usefulness as a valuable model organism for studying theevolutionofinnate immunity. The sequencing also revealed that while some genes were thought to be limited to vertebrates, there were also innovations that have previously never been seen outside the chordate classification, such as immune transcription factorsPU.1andSPIB.
Thegonadsof both male and female sea urchins, sometimes euphemized as sea urchin "roe" or "corals",are culinary delicacies in many parts of the world, especially Japan.In Japan, sea urchin is known asuni(うに), and its gonads (the only meaty, edible parts of the animal) can retail for as much as ¥40,000 ($360) per kilogram;they are served raw assashimior insushi, withsoy sauceandwasabi. Japan imports large quantities from the United States,South Korea, and other producers. Japan consumes 50,000 tons annually, amounting to over 80% of global production.Japanese demand for sea urchins has raised concerns about overfishing.
Sea urchins are commonly eaten stuffed with rice in the traditionaloko-okodish among theSama-Bajau peopleof thePhilippines.They were once foraged by coastal Malay communities ofSingaporewho call themjani.In New Zealand,Evechinus chloroticus, known askinainMāori, is a delicacy, traditionally eaten raw. Though New Zealand fishermen would like to export them to Japan, their quality is too variable.
InMediterranean cuisines,Paracentrotus lividusis often eaten raw, or with lemon,and known asriccion Italian menus where it is sometimes used in pasta sauces. It can also flavouromelettes,scrambled eggs,fish soup,mayonnaise,béchamel saucefor tartlets,theboulliefor asoufflé,orHollandaise sauceto make a fish sauce.In the region ofMarseille, sea urchin are commonly eaten in dedicated food festival calledoursinade.
On the Pacific Coast of North America,Strongylocentrotus franciscanuswas praised byEuell Gibbons;Strongylocentrotus purpuratusis also eaten.Native Americans in Californiaare also known to eat sea urchins.The coast of Southern California is known as a source of high qualityuni, with divers picking sea urchin from kelp beds in depths as deep as 24 m/80 ft.As of 2013, the state was limiting the practice to 300 sea urchin diver licenses.Though the edibleStrongylocentrotus droebachiensisis found in the North Atlantic, it is not widely eaten. However, sea urchins (calleduutukinAlutiiq) are commonly eaten by theAlaska Nativepopulation aroundKodiak Island. It is commonly exported, mostly toJapan.In the West Indies,slate pencil urchinsare eaten.
InChilean cuisine, it is served raw with lemon, onions, and olive oil.
Some species of sea urchins, such as the slate pencil urchin (Eucidaris tribuloides), are commonly sold in aquarium stores. Some species are effective at controllingfilamentous algae, and they make good additions to aninvertebratetank.
A folk tradition in Denmark and southern England imaginedsea urchin fossilsto be thunderbolts, able to ward off harm by lightning or by witchcraft, as anapotropaic symbol.Another version supposed they were petrified eggs of snakes, able to protect against heart and liver disease, poisons, and injury in battle, and accordingly they were carried asamulets. These were, according to the legend, created by magic from foam made by the snakes at midsummer.