Sea urchin
Sea urchin Temporal range:
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Tripneustes ventricosus and Echinometra viridis | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Echinodermata |
Subphylum: | Echinozoa |
Class: | Echinoidea Leske , 1778
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Subclasses | |
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Sea urchins (
Like all echinoderms, adult sea urchins have fivefold symmetry, but their
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 the slate pencil urchin are popular in aquaria, where they are useful for controlling algae. Fossil urchins have been used as protective amulets.
Diversity
Sea urchins are members of the
Specifically, the term "sea urchin" refers to the "regular echinoids", which are symmetrical and globular, and includes several different taxonomic groups, with two subclasses:
Together with sea cucumbers (
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A sand dollar, an irregular sea urchin (Irregularia)
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Cidaroidea)
Description
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).[4] They have a rigid, usually spherical body bearing moveable spines, which give the class the name Echinoidea (from the Greek ἐχῖνος ekhinos 'spine').[5] The name urchin is an old word for hedgehog, which sea urchins resemble; they have archaically been called sea hedgehogs.[6][7] The name is derived from the Old French herichun, from Latin ericius ('hedgehog').[8]
Like other echinoderms, sea urchin early larvae have bilateral symmetry,[9] 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".[a][2]
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.[4]
Systems
Musculoskeletal
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 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.[10]
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.[11] The inner surface of the test is lined by peritoneum.[4] Sea urchins convert aqueous carbon dioxide using a catalytic process involving nickel into the calcium carbonate portion of the test.[12]
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 of pedicellaria, moveable stalked structures with jaws.[2]
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.
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Test of an Echinus esculentus, a regular sea urchin
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Test of black sea urchin, showing tubercles and ambulacral plates (on right)
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Inner surface of test, showing pentagonal interambulacral plates on right, and holes for tube feet on left.
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Test of ansand dollar")
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Test of a Phyllacanthus imperialis, a cidaroid sea urchin. These are characterised by their big tubercles, bearing large radiola.
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Close-up of the test showing an ambulacral groove with its two rows of pore-pairs, between two interambulacra areas (green). The tubercles are non-perforated.
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Close-up of a cidaroid sea urchin apical disc: the 5 holes are the gonopores, and the central one is the anus ("periproct"). The biggest genital plate is the madreporite.[15]
Feeding and digestion
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.[4] 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.[2] 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.[16]
On the upper surface of 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.[2]
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 from
... 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 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 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.[17][18]
The lantern, where present, surrounds both the mouth cavity and the
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.[4]
Circulation and respiration
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.[2]
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.
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.[2]
Nervous system and senses
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, and
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.[2] 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.[19] 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 the ambulacral areas; their function is to help in gravitational orientation.[4]
Life history
Reproduction
Sea urchins are
Development
During early development, the sea urchin
An unusual feature of sea urchin development is the replacement of the larva's
Life cycle and development
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-swimming
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 and metamorphoses into a juvenile urchin in as little as one hour.[2] In some species, adults reach their maximum size in about five years.[4] The purple urchin becomes sexually mature in two years and may live for twenty.[26]
Longevity
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.[27][28]
Ecology
Trophic level
Sea urchins feed mainly on algae, so they are primarily herbivores, but can feed on sea cucumbers and a wide range of invertebrates, such as mussels, polychaetes, sponges, brittle stars, and crinoids, making them omnivores, consumers at a range of trophic levels.[29]
Predators, parasites, and diseases
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
Adult sea urchins are usually well protected against most predators by their strong and sharp spines, which can be venomous in some species.[32] The small urchin clingfish lives among 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.[33]
Sea urchins are one of the favourite foods of many
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Wolf eel, a highly specialized predator of sea urchins
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A sea otter feeding on a purple sea urchin.
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A marbled stone crab (Carpilius convexus) attacking a slate pencil sea urchin (Heterocentrotus mamillatus)
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Awrasse finishing the remains of a damaged Tripneustes gratilla
Anti-predator defences
The
Pedicellariae[37] are a good means of defense against ectoparasites, but not a panacea as some of them actually feed on it.[38] The hemal system defends against endoparasites.[39]
Range and habitat
Sea urchins are established in most seabed habitats from the intertidal downwards, at an extremely wide range of depths.[40] 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,[41] strange bottle-shaped irregular sea urchins that live in only the hadal zone and have been collected as deep as 6850 metres beneath the surface in the Sunda Trench.[42] 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).[42]
Population densities vary by habitat, with more dense populations in barren areas as compared to kelp stands.[43][44] Even in these barren areas, greatest densities are found in shallow water. Populations are generally found in deeper water if wave action is present.[44] Densities decrease in winter when storms cause them to seek protection in cracks and around larger underwater structures.[44] The
Sea urchins can be found in all climates, from warm seas to polar oceans.[40] The larvae of 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.[46] 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.[40]
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Purple sea urchins at low tide in California. They dig a cavity in the rock to hide from predators during the day.
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Dermechinus horridus, an abyssal species, at thousands of meters deep
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Antarctic sea urchin (Sterechinus neumayeri) inhabits frozen seas.
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The shape of the shingle urchin allows it to stay on wave-beaten cliffs.
Evolution
Fossil history
The earliest echinoid
Most fossil echinoids from 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.[52] 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.[52] 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 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.[52]
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.[53]
Some echinoids, such as
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.[52]
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Clypeasteroida, Miocene, c. 10 mya
Phylogeny
External
Echinoids are deuterostome animals, like the chordates. A 2014 analysis of 219 genes from all classes of echinoderms gives the following phylogenetic tree.[55] Approximate dates of branching of major clades are shown in millions of years ago (mya).
Internal
The phylogeny of the sea urchins is as follows:[56][57]
Echinoidea | |
450 mya |
The phylogenetic study from 2022 presents a different topology of the
Relation to humans
Injuries
Sea urchin injuries are puncture wounds inflicted by the animal's brittle, fragile spines.[59] 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. Granuloma and 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.[60] 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.[61]
Science
Sea urchins are traditional
The organism's evolutionary placement and unique embryology with five-fold symmetry were the major arguments in the proposal to seek the sequencing of its
As food
The gonads of both male and female sea urchins, sometimes euphemized as sea urchin "roe" or "corals",[69] are culinary delicacies in many parts of the world, especially Japan.[70][71][72] In Japan, sea urchin is known as uni (うに), and its gonads (the only meaty, edible parts of the animal) can retail for as much as ¥40,000 ($360) per kilogram;[73] they are served raw as sashimi or in sushi, with soy sauce and wasabi. 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.[74] Japanese demand for sea urchins has raised concerns about overfishing.[75]
Sea urchins are commonly eaten stuffed with rice in the traditional
In Mediterranean cuisines, Paracentrotus lividus is often eaten raw, or with lemon,[79] and known as ricci on Italian menus where it is sometimes used in pasta sauces. It can also flavour omelettes, scrambled eggs, fish soup,[80] mayonnaise, béchamel sauce for tartlets,[81] the boullie for a soufflé,[82] or Hollandaise sauce to make a fish sauce.[83]
On the Pacific Coast of North America,
In Chilean cuisine, it is served raw with lemon, onions, and olive oil.
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Japanese uni-don, or rice bowl with sea urchin roe
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Japanesenigirizushiwith sea urchin roe
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Sea urchin roe (uni) sashimi
Aquaria
Some species of sea urchins, such as the slate pencil urchin (
Folklore
A folk tradition in Denmark and southern England imagined sea urchin fossils to be thunderbolts, able to ward off harm by lightning or by witchcraft, as an apotropaic symbol.[89] 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 as amulets. These were, according to the legend, created by magic from foam made by the snakes at midsummer.[90]
Explanatory notes
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External links
- World Register of Marine Species link: Echinoidea Leske, 1778 (+species list)
- The sea urchin genome project
- Sea Urchin Harvesters Association – California Also, (604) 524-0322.
- The Echinoid Directory from the Natural History Museum.
- Virtual Urchin at Stanford
- California Sea Urchin commission
- Introduction to the Echinoidea at UCMP Berkeley