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Sea urchin

Sea urchin

Overview
Sea urchins or urchins are small, spiny
Spine (zoology)
A spine is a hard, thorny or needle-like structure which occurs on various animals. Animals such as porcupines and sea urchins grow spines as a self-defense mechanism. Spines are often formed of keratin...

, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm
Echinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....

 phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from 3 to 10 cm (1.2 to 3.9 ) across. Common colors include black and dull shades of green, olive, brown, purple, and red. They move slowly, feeding mostly on algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

.
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Encyclopedia
Sea urchins or urchins are small, spiny
Spine (zoology)
A spine is a hard, thorny or needle-like structure which occurs on various animals. Animals such as porcupines and sea urchins grow spines as a self-defense mechanism. Spines are often formed of keratin...

, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm
Echinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....

 phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from 3 to 10 cm (1.2 to 3.9 ) across. Common colors include black and dull shades of green, olive, brown, purple, and red. They move slowly, feeding mostly on algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

. Sea otters, wolf eel
Wolf eel
The wolf eel is a member of the family Anarhichadidae together with the wolffishes of the genus Anarhichas. The wolf eel is monotypic within the genus Anarrhichthys. This superficially eel-like fish feeds on crustaceans, sea urchins, mussels, clams and some fishes, crushing them with its strong jaws...

s, triggerfish
Triggerfish
Triggerfishes are about 40 species of often brightly colored fishes of the family Balistidae. Often marked by lines and spots, they inhabit tropical and subtropical oceans throughout the world, with the greatest species richness in the Indo-Pacific...

, and other predators feed on them. Their "roe
Roe
Roe or hard roe is the fully ripe internal egg masses in the ovaries, or the released external egg masses of fish and certain marine animals, such as shrimp, scallop and sea urchins...

" (actually the gonads) is a delicacy in many cuisines.

The name urchin
Urchin
Urchin or urcheon is the Middle English term for "hedgehog". As such, it is applied to many things that take a similar form to a hedgehog:* Street children, homeless children who live on the street...

is an old name for the round spiny hedgehog
Hedgehog
A hedgehog is any of the spiny mammals of the subfamily Erinaceinae and the order Erinaceomorpha. There are 17 species of hedgehog in five genera, found through parts of Europe, Asia, Africa, and New Zealand . There are no hedgehogs native to Australia, and no living species native to the Americas...

s that sea urchins resemble.

Taxonomy


Sea urchins are members of the phylum
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....

 Echinoderm
Echinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....

ata, which also includes sea star
Sea star
Starfish or sea stars are echinoderms belonging to the class Asteroidea. The names "starfish" and "sea star" essentially refer to members of the class Asteroidea...

s, sea cucumbers, brittle star
Brittle star
Brittle stars or ophiuroids are echinoderms in the class Ophiuroidea closely related to starfish. They crawl across the seafloor using their flexible arms for locomotion. The ophiuroids generally have five long slender, whip-like arms which may reach up to in length on the largest specimens...

s, and crinoid
Crinoid
Crinoids are marine animals that make up the class Crinoidea of the echinoderms . Crinoidea comes from the Greek word krinon, "a lily", and eidos, "form". They live both in shallow water and in depths as great as 6,000 meters. Sea lilies refer to the crinoids which, in their adult form, are...

s. Like other echinoderm
Echinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....

s they have fivefold symmetry (called pentamerism) and move by means of hundreds of tiny, transparent, adhesive "tube feet
Tube feet
Tube feet are the many small tubular projections found most famously on the oral face of a sea star's arms, but are characteristic of the water vascular system of the echinoderm phylum which also includes sea urchins, sand dollars and sea cucumbers and many other sea creatures.Tube feet function in...

". The symmetry is not obvious in the living animal, but is easily visible in the dried test. "Echinodermate" means "spiny skin" in Greek.

Specifically, the term "sea urchin" refers to the "regular echinoids," which are symmetrical and globular. The term includes several different taxonomic groups: the order Echinoida
Echinoida
Echinoida is an order of sea urchins in the class Echinoidea. They are distinguished from other sea urchins by simultaneously possessing both an un-sculpted test and a feeding lantern with large plates fused across the top of each pyramid.-Taxonomy:...

, the order Cidaroida
Cidaroida
Cidaroida is an order of primitive sea urchins, the only living order of the subclass Perischoechinoidea. All other orders of this subclass became extinct during the Mesozoic, and were even more primitive than the living forms....

 or "slate-pencil urchins", which have very thick, blunt spines, and others. Besides sea urchins, the class Echinoidea also includes three groups of "irregular" echinoids: flattened sand dollar
Sand dollar
The term Sand dollar refers to species of extremely flattened, burrowing echinoids belonging to the order Clypeasteroida. Some species within the order, not quite as flat, are known as sea biscuits...

s, sea biscuits, and heart urchin
Echinocardium
Echinocardium is a genus of sea urchins of the family Loveniidae, known as heart urchins.-Species:* Echinocardium australe Gray, 1855* Echinocardium cordatum * Echinocardium fenauxi Péquignat, 1963...

s.

Together with sea cucumbers (Holothuroidea), they make up the subphylum Echinozoa
Echinozoa
Echinozoa is a subphylum of free-living echinoderms in which the body is essentially globoid with meriodional symmetry. They lack arms, brachioles, or other appendages, and do not at any time exhibit pinnate structure....

, 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 tentacle
Tentacle
A tentacle or bothrium is one of usually two or more elongated flexible organs present in animals, especially invertebrates. The term may also refer to the hairs of the leaves of some insectivorous plants. Usually, tentacles are used for feeding, feeling and grasping. Anatomically, they work like...

s surrounding the oral opening, these have originated from modified tube feet and are not homologous to the arms of the crinoids, sea stars, and brittle stars.

Anatomy


Urchins typically range in size from 6 to 12 cm (2.4 to 4.7 in), although the largest species can reach up to 36 centimetres (14.2 in).

Five-fold symmetry


Like other echinoderms, sea urchins are bilaterans. Their early larvae have bilateral symmetry but they develop fivefold 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 the central axis. Several sea urchins, however, including the sand dollar
Sand dollar
The term Sand dollar refers to species of extremely flattened, burrowing echinoids belonging to the order Clypeasteroida. Some species within the order, not quite as flat, are known as sea biscuits...

s, 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 material.

Organs and test


The lower half of a sea urchin's body is referred to as the oral surface, because it contains the mouth, while the upper half is the aboral surface. The internal organs are enclosed in a hard test composed of fused plates of calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

 covered by a thin dermis
Dermis
The dermis is a layer of skin between the epidermis and subcutaneous tissues, and is composed of two layers, the papillary and reticular dermis...

 and epidermis. The test is rigid, and divides into five ambulacral areas separated by five inter-ambulacral areas. Each of these areas consists of two rows of plates, so that the test includes twenty rows in total. The plates are covered in rounded tubercles, to which the spines are attached. The inner surface of the test is lined by peritoneum
Peritoneum
The peritoneum is the serous membrane that forms the lining of the abdominal cavity or the coelom — it covers most of the intra-abdominal organs — in amniotes and some invertebrates...

.

Feet


Urchins have tube feet
Tube feet
Tube feet are the many small tubular projections found most famously on the oral face of a sea star's arms, but are characteristic of the water vascular system of the echinoderm phylum which also includes sea urchins, sand dollars and sea cucumbers and many other sea creatures.Tube feet function in...

, which arise from the five ambulacral areas. ( The tube feet are moved by the water vascular system
Water vascular system
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...

.)

Mouth/anus


The mouth lies in the centre of the oral surface in regular urchins, or towards one end of irregular urchins. It is surrounded by lips of softer tissue, with numerous small bony pieces embedded in it. This area, called the peristome, also includes five pairs of modified tube feet and, in many species, five pairs of gills. On the upper surface, opposite the mouth, is a region termed the periproct, which surrounds the anus
Anus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...

. The periproct contains a variable number of hard plates, depending on species, one of which contains the madreporite
Madreporite
The madreporite is a lightcolored calcerous 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...

.

Endoskeleton


The sea urchin builds its spicule
Spicule
Spicules are tiny spike-like structures of diverse origin and function found in many organisms, such as the copulatory spicules of certain nematodes or the grains on the skin of some frogs.In sponges, spicules perform a structural function....

s, the sharp crystalline "bones" that constitute the animal’s endoskeleton
Endoskeleton
An endoskeleton is an internal support structure of an animal, composed of mineralized tissue. Endoskeleton develops within the skin or in the deeper body tissues. The vertebrate is basically an endoskeleton made up of two types of tissues . During early embryonic development the endoskeleton is...

, in the larval
Larvae
In Roman mythology, lemures were shades or spirits of the restless or malignant dead, and are probably cognate with an extended sense of larvae as disturbing or frightening...

 stage. The fully formed spicule is composed of a single crystal with an unusual morphology. It has no facets and within 48 hours of fertilization assumes a shape that looks very much like the Mercedes-Benz
Mercedes-Benz
Mercedes-Benz is a German manufacturer of automobiles, buses, coaches, and trucks. Mercedes-Benz is a division of its parent company, Daimler AG...

 logo.

In other echinoderms, the endoskeleton is associated with a layer of muscle that allows the animal to move its arms or other body parts. This is entirely absent in sea urchins, which are unable to move in this way.

Spines



The spine
Spine (zoology)
A spine is a hard, thorny or needle-like structure which occurs on various animals. Animals such as porcupines and sea urchins grow spines as a self-defense mechanism. Spines are often formed of keratin...

s, long and sharp in some species, protect the urchin from predators. The spines inflict a painful wound when they penetrate human skin, but are not dangerous. It is not clear if the spines are venomous (unlike the pedicellariae between the spines, which are venomous).

Typical sea urchins have spines that are 1 to 3 cm (0.393700787401575 to 1.2 ) in length, 1 to 2 mm (0.0393700787401575 to 0.078740157480315 ) thick, and not terribly sharp. Diadema antillarum
Diadema antillarum
Diadema antillarum, also known as the lime urchin, black sea urchin or the long-spined sea urchin, is a species of sea urchin in the Family Diadematidae.This sea urchin is characterized by its exceptionally long black spines....

, familiar in the Caribbean, has thin, potentially dangerous spines that can reach 10 to 30 cm (3.9 to 11.8 ) long.

Reproductive organs



Sea urchins are dioecious
Dioecious
Dioecy is the property of a group of biological organisms that have males and females, but not members that have organs of both sexes at the same time. I.e., those whose individual members can usually produce only one type of gamete; each individual organism is thus distinctly female or male...

, having separate male and female sexes, although there is generally no easy way to distinguish the two, except for their location on the sea bottom. Males generally choose an elevated and exposed location so that their milt can be broadcast by sea currents. Females generally choose a low-lying location in sea bottom crevices, presumably so that the tiny larvae can have better protection from predators. Indeed, very small sea urchins are always found hiding underneath rocks. Regular sea urchins have five gonad
Gonad
The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...

s, lying underneath the interambulacral regions of the test, while the irregular forms have only four, with the hindmost gonad being absent. Each gonad has a single duct, rising from the upper pole to open at a gonopore
Gonopore
A gonopore, sometimes called a gonadopore, is a genital pore in many invertebrates. Hexapods, including insects have a single common gonopore, except mayflies, which have a pair of gonopores...

 lying in one of the genital plates surrounding the anus. The gonads are lined with muscles underneath the peritoneum
Peritoneum
The peritoneum is the serous membrane that forms the lining of the abdominal cavity or the coelom — it covers most of the intra-abdominal organs — in amniotes and some invertebrates...

, and these allow the animal to squeeze its gamete
Gamete
A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...

s through the duct and into the surrounding sea water, where fertilization takes place.

Digestion


The mouth of most sea urchins is made up of five calcium carbonate teeth or jaws, with a fleshy tongue-like structure within. The entire chewing organ is known as Aristotle's lantern (image), from Aristotle
Aristotle
Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...

's description in his History of Animals:
…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
Tongue
The tongue is a muscular hydrostat on the floors of the mouths of most vertebrates which manipulates food for mastication. It is the primary organ of taste , as much of the upper surface of the tongue is covered in papillae and taste buds. It is sensitive and kept moist by saliva, and is richly...

. Next to this comes the esophagus
Esophagus
The esophagus is an organ in vertebrates which consists of a muscular tube through which food passes from the pharynx to the stomach. During swallowing, food passes from the mouth through the pharynx into the esophagus and travels via peristalsis to the stomach...

, and then the stomach
Stomach
The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...

, divided into five parts, and filled with excretion, all the five parts uniting at the anal
Anus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...

 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
Lantern
A lantern is a portable lighting device or mounted light fixture used to illuminate broad areas. Lanterns may also be used for signaling, as 'torches', or as general light sources outdoors . Low light level varieties are used for decoration. The term "lantern" is also used more generically to...

 with the panes of horn left out. (Tr. D'Arcy Thompson)


Recent research has shown that the sea urchin's teeth are self-sharpening; it can chew through stone.

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 the pharynx
Pharynx
The human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...

. 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
Intestine
In human anatomy, the intestine is the segment of the alimentary canal extending from the pyloric sphincter of the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large 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
Rectum
The rectum is the final straight portion of the large intestine in some mammals, and the gut in others, terminating in the anus. The human rectum is about 12 cm long...

 ascends towards the anus. Despite the names, the small and large intestine of sea urchins are in no way homologous
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...

 to the similarly named structures in vertebrates.

Digestion occurs in the intestine, with the caecum producing further digestive enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

s. 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.

Circulation


Sea urchins possess both a water vascular system
Water vascular system
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...

 and a hemal system, the latter containing blood. However, the main circulatory fluid fills the general body cavity, or coelom
Coelom
The coelom is a fluid-filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size...

. This fluid contains phagocytic
Phagocyte
Phagocytes are the white blood cells that protect the body by ingesting harmful foreign particles, bacteria, and dead or dying cells. Their name comes from the Greek phagein, "to eat" or "devour", and "-cyte", the suffix in biology denoting "cell", from the Greek kutos, "hollow vessel". They are...

 coelomocytes which move through the vascular and hemal systems. The coelomocytes are an essential part of blood clotting, but also collect waste products and actively remove them from the body through the gills and tube feet.

Respiration


Most sea urchins possess five pairs of external gills, located around the mouth. These are thin-walled projections of the body cavity, and are the main organs of respiration in those urchins that possess them. Fluid can be pumped through the gills' interior by muscles associated with the lantern, but this is not continuous, and occurs only when the animal is low on 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.

Nervous system


The nervous system of sea urchins has a relatively simple layout. There is no true brain. The 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, and pedicellariae
Pedicellariae
A pedicellaria is a small wrench or claw-shaped structure commonly found on Echinoderms, particularly in sea stars and sea urchins...

.

Senses


Sea urchins are sensitive to touch, light, and chemicals. Although they do not have eyes or eye spots, recent research suggests that their entire body might function as one compound eye. They also have statocyst
Statocyst
The statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, echinoderms, cephalopods, and crustaceans. A similar structure is also found in Xenoturbella. The statocyst consists of a sac-like structure containing a mineralised mass and numerous...

s, called spheridia, that are located within the ambulacral plates and help the animal remain upright.

Ingression of primary mesenchyme cells



During early development, the sea urchin embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...

 undergoes ten cycles of cell division
Cell division
Cell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...

 resulting in a single epithelial
Epithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...

 layer enveloping a blastocoel
Blastocoele
A blastocoel or blastocele is the fluid-filled central region of a blastocyst...

. The embryo must then begin gastrulation
Gastrulation
Gastrulation is a phase early in the embryonic development of most animals, during which the single-layered blastula is reorganized into a trilaminar structure known as the gastrula. These three germ layers are known as the ectoderm, mesoderm, and endoderm.Gastrulation takes place after cleavage...

, a multipart process which involves the dramatic rearrangement and invagination
Invagination
Invagination means to fold inward or to sheath. In biology, this can refer to a number of processes.* Invagination is the morphogenetic processes by which an embryo takes form, and is the initial step of gastrulation, the massive reorganization of the embryo from a simple spherical ball of cells,...

 of cells to produce the three germ layer
Germ layer
A germ layer, occasionally referred to as a germinal epithelium, is a group of cells, formed during animal embryogenesis. Germ layers are particularly pronounced in the vertebrates; however, all animals more complex than sponges produce two or three primary tissue layers...

s.

The first step of gastrulation is the epithelial to mesenchymal transition
Epithelial-mesenchymal transition
Epithelial-mesenchymal transition or transformation is a hypothesized program of development of biological cells characterized by loss of cell adhesion, repression of E-cadherin expression, and increased cell mobility...

 and ingression of primary mesenchyme cells into the blastocoel. Primary mesenchyme
Mesenchyme
Mesenchyme, or mesenchymal connective tissue, is a type of undifferentiated loose connective tissue that is derived mostly from mesoderm, although some are derived from other germ layers; e.g. some mesenchyme is derived from neural crest cells and thus originates from the ectoderm...

 cells, or PMCs, are cells located in the vegetal plate that are specified to become mesoderm
Mesoderm
In all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...

. Prior to ingression, PMCs exhibit all the features of other epithelial cells that comprise the embryo. Cells of the epithelium are bound basally to a laminal matrix and apically to an extra-embryonic matrix. The apical microvilli
Microvillus
Microvilli are microscopic cellular membrane protrusions that increase the surface area of cells, and are involved in a wide variety of functions, including absorption, secretion, cellular adhesion, and mechanotransduction....

 of these cells reach into the hyaline layer, a component of the extra-embryonic matrix. Neighboring epithelial cells are also connected to each other through apical junctions
Cell junction
A cell junction is a type of structure that exists within the tissue of a some multicellular organism . Cell junctions consist of protein complexes and provide contact between neighbouring cells or between a cell and the extracellular matrix...

, protein complexes containing adhesion molecules such as cadherin
Cadherin
Cadherins are a class of type-1 transmembrane proteins. They play important roles in cell adhesion, ensuring that cells within tissues are bound together. They are dependent on calcium ions to function, hence their name.The cadherin superfamily includes cadherins, protocadherins, desmogleins, and...

s linked to catenin
Catenin
Catenins are proteins found in complexes with cadherin cell adhesion molecules of animal cells. The first two catenins that were identified became known as alpha-catenin and beta-catenin. Alpha-catenin can bind to beta-catenin and can also bind actin. Beta-catenin binds the cytoplasmic domain of...

s.
As PMCs begin to undergo an epithelial to mesenchymal transition, the lamina which binds them dissolves to begin the mechanical release of the cells. Expression of the membrane protein that binds laminin, integrin
Integrin
Integrins are receptors that mediate attachment between a cell and the tissues surrounding it, which may be other cells or the ECM. They also play a role in cell signaling and thereby regulate cellular shape, motility, and the cell cycle....

, also becomes irregular at the beginning of ingression. The microvilli which secure PMCs to the hyaline layer shorten, as the cells reduce their affinity for the extra-embryonic matrix. These cells concurrently increase their affinity for other components of the basal matrix, such as fibronectin
Fibronectin
Fibronectin is a high-molecular weight glycoprotein of the extracellular matrix that binds to membrane-spanning receptor proteins called integrins. In addition to integrins, fibronectin also binds extracellular matrix components such as collagen, fibrin and heparan sulfate proteoglycans...

, in part driving the movement of cells inward. The apical junctions which bind PMCs to their neighboring epithelial cells become disrupted during this transition, and are absent in cells that have fully ingressed into the blastocoel. Because staining for cadherins and catenins in ingressing cells decreases and develops as intracellular accumulations, apical junctions are thought to be cleared by endocytosis
Endocytosis
Endocytosis is a process by which cells absorb molecules by engulfing them. It is used by all cells of the body because most substances important to them are large polar molecules that cannot pass through the hydrophobic plasma or cell membrane...

 during ingression.

Once the PMCs disrupt all attachment to their former location, the cells themselves change their morphology by contracting their apical surface, apical constriction
Apical constriction
Apical constriction describes the process in which contraction of the apical side of a cell causes the cell to take on a wedged shape. Generally, this shape change is coordinated across many cells of an epithelial layer, generating forces that can bend or fold the cell sheet .-Morphogenetic...

, and enlarging their basal surface; acquiring a “bottle cell” phenotype. Cytoskeletal
Cytoskeleton
The cytoskeleton is a cellular "scaffolding" or "skeleton" contained within a cell's cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton...

 rearrangements mediate the shape changes of PMCs; and though the cytoskeleton assists in the mechanics of ingression, other mechanisms drive the process. Experimentally disrupting microtubule
Microtubule
Microtubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...

 dynamics in the species Strongylocentrotus pupuratus by applying colchicine
Colchicine
Colchicine is a medication used for gout. It is a toxic natural product and secondary metabolite, originally extracted from plants of the genus Colchicum...

 stalls the ingression of PMCs but does not inhibit it. Similarly, experimentally disrupting actin-myosin contraction using inhibitors slows down ingression, but does not arrest the process.
The morphogenetic movements of the PMCs are an autonomous cellular behavior. Experimentally grafting PMCs into heterotopic tissue does not prevent the cells from ingressing. In studies where PMCs are cultured in insolation, the cells were observed to gain affinity for fibronectin and simultaneously lose affinity for extra-embryonic matrix independent of the embryonic environment.

Life history


At first glance, sea urchins often appear sessile
Sessility (zoology)
In zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...

, i.e. incapable of moving. Sometimes the most visible life sign is the spines, which attach to ball-and-socket joints and can point in any direction. In most urchins, touch elicits a prompt reaction from the spines, which converge toward the touch point. Sea urchins have no visible eyes, legs, or means of propulsion, but can move freely over hard surfaces using adhesive tube feet, working in conjunction with the spines.

Reproduction


In most cases, the eggs float freely in the sea, but some species hold onto them with their spines, affording them a greater degree of protection. The fertilized egg develops into a free-swimming blastula
Blastula
The blastula is a hollow sphere of cells formed during an early stage of embryonic development in animals . The blastula is created when the zygote undergoes the cell division process known as cleavage. The blastula is preceded by the morula and is followed by the gastrula in the developmental...

 embryo in as little as twelve hours. Initially a simple ball of cells, the blastula soon transforms
Gastrulation
Gastrulation is a phase early in the embryonic development of most animals, during which the single-layered blastula is reorganized into a trilaminar structure known as the gastrula. These three germ layers are known as the ectoderm, mesoderm, and endoderm.Gastrulation takes place after cleavage...

 into a cone-shaped echinopluteus larva. In most species, this larva has twelve elongated arms, but in a few it contains supplies of nutrient yolk and lacks arms, since it has no need to feed. The arms are lined with bands of cilia that capture food particles and transport them to the mouth.

It may take several months for the larva to complete its development, which begins with the formation of the test plates around the mouth and anus. Soon the larva sinks to the bottom and metamorphoses
Metamorphosis
Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation...

 into adult form in as little as one hour. In some species, adults reach their maximum size in about five years.

Ecology



Sea urchins feed mainly on algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

, but can also feed on sea cucumbers, and a wide range of invertebrates such as mussels, polychaete
Polychaete
The Polychaeta or polychaetes are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. Indeed, polychaetes are sometimes referred to as bristle worms. More than 10,000...

s, sponges, brittle stars and crinoids. Population density varies by habitat with more dense populations being found in barren areas as compared to kelp
Kelp
Kelps are large seaweeds belonging to the brown algae in the order Laminariales. There are about 30 different genera....

 stands. Even in these barren areas, greatest densities are also found in shallow water. Populations are also generally found in deeper water if waves action is present. Density also decrease in winter when storms cause them to seek protection in cracks and around larger underwater structures. The shingle urchin
Shingle urchin
The shingle urchin or helmet urchin is a species of sea urchin in the family Echinometridae. In Hawaii it is known as "kaupali" which translates as "cliff-clinging"...

 (Colobocentrotus atratus) which lives on exposed shorelines is particularly resistant to wave action.

Sea urchin is one of the favorite foods of sea otter
Sea Otter
The sea otter is a marine mammal native to the coasts of the northern and eastern North Pacific Ocean. Adult sea otters typically weigh between 14 and 45 kg , making them the heaviest members of the weasel family, but among the smallest marine mammals...

s and is also the main source of nutrition for wolf eel
Wolf eel
The wolf eel is a member of the family Anarhichadidae together with the wolffishes of the genus Anarhichas. The wolf eel is monotypic within the genus Anarrhichthys. This superficially eel-like fish feeds on crustaceans, sea urchins, mussels, clams and some fishes, crushing them with its strong jaws...

s. Left unchecked, urchins devastate their environment, creating what biologists call an urchin barren
Urchin barren
An urchin barren is an area of the subtidal where the population growth of sea urchins has gone unchecked, causing destructive grazing of kelp beds or kelp forests ....

, devoid of macroalgae and associated fauna
Fauna
Fauna or faunæ is all of the animal life of any particular region or time. The corresponding term for plants is flora.Zoologists and paleontologists use fauna to refer to a typical collection of animals found in a specific time or place, e.g. the "Sonoran Desert fauna" or the "Burgess shale fauna"...

. Sea otters have re-entered British Columbia
British Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...

, dramatically improving coastal ecosystem health.

Evolutionary history



The earliest echinoid fossils date to the upper part of the Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

 period (c 450 MYA), and the taxon has survived to the present day as a successful and diverse group of organisms. Spines may be present in well-preserved specimens, but usually only the test remains. Isolated spines are common as fossils. Some echinoids (such as Tylocidaris clavigera, from the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 period's English Chalk Formation
Chalk Formation
The Chalk Group is a lithostratigraphic unit in the northwestern part of Europe. It is characterised by thick deposits of chalk, a soft porous white limestone, deposited in a marine environment during the Upper Cretaceous period.Chalk is a limestone that consists of coccolith biomicrite...

) had very heavy club-shaped spines that would be difficult for an attacking predator to break through and make the echinoid awkward to handle. Such spines simplify walking on the soft sea-floor.


Most of the fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

 echinoids from the Paleozoic
Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...

 era are incomplete, consisting of isolated spines and small clusters of scattered plates from crushed individuals. Most specimens occur in Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

 and Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...

 rocks. The shallow water limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

s from the Ordovician and Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...

 periods of Estonia
Estonia
Estonia , officially the Republic of Estonia , is a state in the Baltic region of Northern Europe. It is bordered to the north by the Gulf of Finland, to the west by the Baltic Sea, to the south by Latvia , and to the east by Lake Peipsi and the Russian Federation . Across the Baltic Sea lies...

 are famous for echinoids. Paleozoic echinoids probably inhabited relatively quiet waters. Because of their thin test, they would certainly not have survived in the wave-battered coastal waters inhabited by many modern echinoids. During the upper part of the Carboniferous period, there was a marked decline in echinoid diversity, and this trend continued to the Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...

 period. They neared extinction at the end of the Paleozoic era, with just six species known from the Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...

 period. Only two lineages survived this period's massive extinction of and into the Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

: the genus Miocidaris, which gave rise to modern cidaroida
Cidaroida
Cidaroida is an order of primitive sea urchins, the only living order of the subclass Perischoechinoidea. All other orders of this subclass became extinct during the Mesozoic, and were even more primitive than the living forms....

 (pencil urchins), and the ancestor that gave rise to the euechinoids
Euechinoidea
The subclass Euechinoidea includes almost all living species of sea urchin, and fossil forms going back as far as the Triassic....

. By the upper part of the Triassic period, their numbers began to increase again. Cidaroids have changed very little since the Late Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

 and are today considered to be living fossils.

The euechinoids, on the other hand, diversified into new lineages throughout the Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

 period and into the Cretaceous period, and from them emerged the first irregular echinoids (superorder Atelostomata) during the early Jurassic, and later the other superorder (Gnathostomata
Gnathostomata (echinoid)
The Gnathostomata are a superorder of sea urchins, including the familiar sand dollars.Gnathostomatans are irregular in shape, but unlike other irregular sea urchins, possess a feeding lantern. The mouth is located in the centre of the lower surface, as it is in most other sea urchins, but the anus...

) of irregular urchins which evolved independently. These two superorders are today representing 47% of all extant species of echinoids thanks to their adaptive breakthroughs, which allowed them to exploit habitats and food sources unavailable to regular echinoids. During the Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...

 and Cenozoic
Cenozoic
The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...

 eras the echinoids flourished. Most echinoid fossils are often abundant in the restricted localities and formations where they occur. An example of this is Enallaster, which exists by the thousands in certain outcrops of limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

 from the Cretaceous period in Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...

. Many fossils of the Late Jurassic
Late Jurassic
The Late Jurassic is the third epoch of the Jurassic Period, and it spans the geologic time from 161.2 ± 4.0 to 145.5 ± 4.0 million years ago , which is preserved in Upper Jurassic strata. In European lithostratigraphy, the name "Malm" indicates rocks of Late Jurassic age...

 Plesiocidaris
Plesiocidaris
Plesiocidaris durandi is an extinct species of sea urchin. Including its spines, the creature was 13 cm wide. Its remains have been found in Drâ el-Ahmar, near the Atlas Mountains in Oran, Algeria. A Commander Durand found the fossils there in 1870 and sent them to French paleontologists...

still have the spines attached.

Some echinoids, such as Micraster
Micraster
Micraster is an extinct genus of echinoid from the Late Cretaceous. Its remains have been found in Africa, Antarctica, Europe, and North America. Micraster was an infaunal echinoid living in a burrow below the sediment surface. the test is clearly bilateral and there is a deep anterior groove to...

which is found in the Cretaceous period Chalk Formation
Chalk Formation
The Chalk Group is a lithostratigraphic unit in the northwestern part of Europe. It is characterised by thick deposits of chalk, a soft porous white limestone, deposited in a marine environment during the Upper Cretaceous period.Chalk is a limestone that consists of coccolith biomicrite...

 of England and France, serve as zone or index
Index fossil
Index fossils are fossils used to define and identify geologic periods . They work on the premise that, although different sediments may look different depending on the conditions under which they were laid down, they may include the remains of the same species of fossil...

 fossils. Because they evolved rapidly, they aid geologists in dating the surrounding rocks. However, most echinoids are not abundant enough and are of too limited range to serve as zone fossils.

In the early Tertiary
Tertiary
The Tertiary is a deprecated term for a geologic period 65 million to 2.6 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary...

 (c 65 to 1.8 MYA), sand dollar
Sand dollar
The term Sand dollar refers to species of extremely flattened, burrowing echinoids belonging to the order Clypeasteroida. Some species within the order, not quite as flat, are known as sea biscuits...

s (order Clypeasteroida) arose. Their distinctive flattened test and tiny spines were adapted to life on or under loose sand. They form the newest branch on the echinoid tree.

In biology


Sea urchins are traditional model organisms in developmental biology
Developmental biology
Developmental biology is the study of the process by which organisms grow and develop. Modern developmental biology studies the genetic control of cell growth, differentiation and "morphogenesis", which is the process that gives rise to tissues, organs and anatomy.- Related fields of study...

. This use originates from the 1800s, when their embryonic development became easily viewed by microscopy. Sea urchins were the first species in which sperm
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...

 cells were proven to fertilize the ovum
Ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...

.

The recent sequencing of the sea urchin genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....

 establishes homology between sea urchin and vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

 immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...

-related genes. Sea urchins code for at least 222 Toll-like receptor
Toll-like receptor
Toll-like receptors are a class of proteins that play a key role in the innate immune system. They are single, membrane-spanning, non-catalytic receptors that recognize structurally conserved molecules derived from microbes...

 (TLR) genes and over 200 genes related to the vertebrates' Nod-like-receptor
Nod-like-receptor
The NOD-like receptors , in short for Nucleotide Oligomerization Domain receptors, are cytoplasmic proteins that may have a variety of functions in regulation of inflammatory and apoptotic responses. NLRs are encoded by genes from a large gene family present in many different animal species; there...

 (NLR) family found in vertebrates. This increases its usefulness as a valuable model organism for studying the evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

 of innate immunity.

As food


The gonads of both the male and female sea urchin, usually called sea urchin roe or corals, are culinary delicacies in many parts of the world.

In cuisines around the Mediterranean, Paracentrotus lividus
Paracentrotus lividus
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.-Description:...

is often eaten raw, with lemon. It can also flavor omelette
Omelette
In cuisine, an omelette or omelet is a dish made from beaten eggs quickly cooked with butter or oil in a frying pan, sometimes folded around a filling such as cheese, vegetables, meat , or some combination of the above...

s, scrambled eggs
Scrambled eggs
Scrambled eggs is a dish made from beaten whites and yolks of eggs . Beaten eggs are put into a hot pot or pan and stirred frequently, forming curds as they coagulate.-Sample preparation:...

, fish soup
Fish soup
Fisherman's Soup or halászlé ; , ribena chorba; , ribina čorba) is a hot, spicy paprika-based river fish soup, originating as a dish of Hungarian cuisine, a bright red hot soup prepared with generous amounts of hot paprika and carp or mixed river fish, characteristic for the cuisines of the...

, mayonnaise
Mayonnaise
Mayonnaise, , often abbreviated as mayo, is a sauce. It is a stable emulsion of oil, egg yolk and either vinegar or lemon juice, with many options for embellishment with other herbs and spices. Lecithin in the egg yolk is the emulsifier. Mayonnaise varies in color but is often white, cream, or pale...

, Béchamel sauce
Béchamel sauce
Béchamel sauce , also known as white sauce, is one of the mother sauces of French cuisine and is used in many recipes of Italian cuisine, for example lasagne. It is used as the base for other sauces . It is traditionally made by whisking scalded milk gradually into a white roux...

 for tartlets, the boullie for a soufflé
Soufflé
A soufflé is a light baked cake made with egg yolks and beaten egg whites combined with various other ingredients and served as a savoury main dish or sweetened as a dessert...

, or Hollandaise sauce
Hollandaise sauce
Hollandaise sauce is an emulsion of egg yolk and butter, usually seasoned with lemon juice. In appearance it is light yellow and opaque, smooth and creamy. The flavor is rich and buttery, with a mild tang added by the lemon juice, yet not so strong as to overpower mildly-flavored foods.Hollandaise...

 to make a fish sauce. In Chile, it is served raw with lemon, onions, and olive oil.

Though the edible Strongylocentrotus droebachiensis
Strongylocentrotus droebachiensis
Strongylocentrotus droebachiensis is commonly known as the green sea urchin because of its characteristic green color. It has the longest genus-species names in the animal kingdom...

is found in the North Atlantic, it is not widely eaten, though it is exported, mostly to Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

; in Maine, sea urchins are known as whores' eggs. It was formerly a delicacy in the Orkney Islands
Orkney Islands
Orkney also known as the Orkney Islands , is an archipelago in northern Scotland, situated north of the coast of Caithness...

, used instead of butter.

In the West Indies, Cidaris tribuloides is eaten.

On the Pacific Coast of North America, Strongylocentrotus franciscanus was praised by Euell Gibbons
Euell Gibbons
Euell Theophilus Gibbons was an outdoorsman and proponent of natural diets during the 1960s.He was born in Clarksville, Texas, on September 14, 1911, and spent much of his youth in the hilly terrain of New Mexico during the dust bowl era. His mother taught him about foods available in the wild...

; Strongylocentrotus purpuratus
Strongylocentrotus purpuratus
The purple sea urchin, Strongylocentrotus purpuratus, lives along the eastern edge of the Pacific Ocean extending from Ensenada, Mexico to British Columbia, Canada. This sea urchin species is deep purple in color and lives in lower intertidal and nearshore subtidal communities...

is also eaten.

In New Zealand, Evechinus chloroticus
Evechinus chloroticus
Evechinus chloroticus, better known as kina , is a sea urchin endemic to New Zealand. This echinoderm belongs to the family Echinometridae and it can reach a maximum diameter of 16–17 cm ....

, known as kina in Maori
Maori language
Māori or te reo Māori , commonly te reo , is the language of the indigenous population of New Zealand, the Māori. It has the status of an official language in New Zealand...

, is a delicacy, traditionally eaten raw. Though New Zealand fishermen would like to export them to Japan, their quality is too variable.

In Japan, sea urchin is known as , and its roe can retail for as much as $450/kg.; it is served raw as sashimi
Sashimi
Sashimi is a Japanese delicacy. It consists of very fresh raw meat, most commonly fish, sliced into thin pieces.-Origin:The word sashimi means "pierced body", i.e...

 or in sushi
Sushi
is a Japanese food consisting of cooked vinegared rice combined with other ingredients . Neta and forms of sushi presentation vary, but the ingredient which all sushi have in common is shari...

, with soy sauce
Soy sauce
Soy sauce is a condiment produced by fermenting soybeans with Aspergillus oryzae or Aspergillus sojae molds, along with water and salt...

 and wasabi
Wasabi
, also known as Japanese horseradish, is a member of the Brassicaceae family, which includes cabbages, horseradish, and mustard. Its root is used as a condiment and has an extremely strong flavor. Its hotness is more akin to that of a hot mustard rather than the capsaicin in a chili pepper,...

. Japan imports large quantities from the United States, South Korea
South Korea
The Republic of Korea , , is a sovereign state in East Asia, located on the southern portion of the Korean Peninsula. It is neighbored by the People's Republic of China to the west, Japan to the east, North Korea to the north, and the East China Sea and Republic of China to the south...

, and other producers. Japanese demand for sea urchin corals has raised concerns about overfishing.

External links