sea urchin

Sea urchin
Strongylocentrotus purpuratus in a tide pool in California
Scientific classification
Kingdom:	Animalia
Phylum:	Echinodermata
Subphylum:	Echinozoa
Class:	Echinoidea Leske, 1778
Subclasses
Subclass Perischoechinoidea Order Cidaroida (pencil urchins) Subclass Euechinoidea Superorder Atelostomata Order Cassiduloida Order Spatangoida (heart urchins) Superorder Diadematacea Order Diadematoida Order Echinothurioida Order Pedinoida Superorder Echinacea Order Arbacioida Order Echinoida Order Phymosomatoida Order Salenioida Order Temnopleuroida Superorder Gnathostomata Order Clypeasteroida (sand dollars) Order Holectypoida

Wikispecies has information related to: Echinoidea

Sea urchins or urchins are small, spiny, globular animals that compose part of class Echinoidea. They are found in oceans all over the world. Their shell, or "test", is round and spiny, typically from 3 to 10 cm across. Common colors include black and dull shades of green, olive, brown, purple, and red. They move slowly, feeding mostly on algae. Sea otters, wolf eels, and other predators feed on urchins. Sea urchins are also harvested by humans and their roe is served as a delicacy.

Sea urchins are members of the phylum Echinodermata, which also includes sea stars, sea cucumbers, brittle stars, and crinoids. Like other echinoderms they have fivefold symmetry (called pentamerism) and move by means of hundreds of tiny, transparent, adhesive "tube feet". The pentamerous symmetry is not obvious at a casual glance but is easily seen in the dried shell or test of the urchin.

Together with sea cucumbers (Holothuroidea), they make up the subphylum Echinozoa, which is defined by primarily having a globoid shape without arms or projecting rays. Sea cucumbers and the irregular echinoids have secondarily evolved different shapes. Although many sea cucumbers have branched tentacles surrounding the oral opening, these have originated from modified tube feet and are not homologous to the arms of the crinoids, sea stars (sea star) and brittle stars.

Anatomy and physiology

At first glance, a sea urchin often appears sessile, i.e. inert or incapable of moving. Sometimes the most visible sign of life is the spines, which are attached at their bases to ball-and-socket joints and can be pointed in any direction. In most urchins, a light touch elicits a prompt and visible reaction from the spines, which converge toward the point that has been touched. A sea urchin has no visible eyes, legs, or means of propulsion, but it can move freely over surfaces by means of its adhesive tube feet, working in conjunction with its spines.

On the oral surface of the sea urchin is a centrally located mouth made up of five united calcium carbonate teeth or jaws, with a fleshy tongue-like structure within. The entire chewing organ is known as Aristotle's lantern, which name comes from Aristotle's accurate 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. 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. (Tr. D'Arcy Thompson)

The sea urchin builds its spicules, the sharp crystalline “bones” that constitute the animal’s endoskeleton, in the larval 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 logo.^[1]

The spines, which in some species are long and sharp, serve to protect the urchin from predators. The spines can inflict a painful wound on a human who steps on one, but they are not seriously dangerous, and it is not clear that the spines are truly venomous (unlike the pedicellariae between the spines, which are venomous).^[2]

Typical sea urchins have spines that are 1 to 3 cm in length, 1 to 2 mm thick, and not terribly sharp. Diadema antillarum, familiar in the Caribbean, has thin, potentially dangerous spines that can be 10 to 30 cm long. Some sea urchins spines are poisonous.

Ecology

Echinothrix calamaris, a species of sea urchin. The sphere in the middle of a sea urchin is its anus

Sea urchins feed mainly on algae, but can also feed on a wide range of invertebrates such as mussels, sponges, brittle stars and crinoids.^[3] Sea urchin is one of the favorite foods of sea otters and are also the main source of nutrition for wolf eels. Left unchecked, urchins will devastate their environment, creating what biologists call an urchin barren, devoid of macroalgae and associated fauna. Where sea otters have been re-introduced into British Columbia, the health of the coastal ecosystem has improved dramatically.^[4]

Evolutionary history

Fossil sea urchin Lovenia woodsi from the Pliocene of Australia.

The earliest known echinoids are found in the rock of the upper part of the Ordovician period (c 450 MYA), and they have survived to the present day, where they are a successful and diverse group of organisms. In well-preserved specimens the spines may be present, but usually only the test is found. Sometimes isolated spines are common as fossils. Some echinoids (such as Tylocidaris clavigera, which is found in the Cretaceous period Chalk Formation of England) 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 are also good for walking on the soft sea-floor.

Cretaceous echinoids from Castle Hayne quarry, North Carolina, USA

Complete fossil echinoids from the Paleozoic era are generally rare, usually consisting of isolated spines and small clusters of scattered plates from crushed individuals. Most specimens occur in rocks from the Devonian and Carboniferous periods. The shallow water limestones from the Ordovician and Silurian periods of Estonia are famous for the echinoids found there. The Paleozoic echinoids probably inhabited relatively quiet waters. Because of their thin test, they would certainly not have survived in the turbulent wave-battered coastal waters inhabited by many modern echinoids today. During the upper part of the Carboniferous period, there was a marked decline in echinoid diversity, and this trend continued to the Permian period. They neared extinction at the end of the Paleozoic era, with just six species known from the Permian period. Only two separate lineages survived the massive extinction of this period and into the Triassic: the genus Miocidaris, which gave rise to the modern cidaroids (pencil urchins), and the ancestor that gave rise to the euechinoids. By the upper part of the Triassic period, their numbers began to increase again. The cidaroids have changed very little since their modern design was established in the Late Triassic and are today considered more or less as living fossils.

Saddle Wrasse, Thalassoma duperrey is feeding on sea urchin

The euechinoids, on the other hand, diversified into new lineages throughout the Jurassic period and into the Cretaceous period, and from them emerged the first irregular echinoids (superorder Atelostomata) during the early Jurassic, and when including the other superorder (Gnathostomata) or irregular urchins which evolved independently later, they now represent 47% of all present species of echinoids thanks to their adaptive breakthroughs in both habit and feeding strategy, which allowed them to exploit habitats and food sources unavailable to regular echinoids. During the Mesozoic and Cenozoic eras the echinoids flourished. While most echinoid fossils are restricted to certain localities and formations, where they do occur, they are quite often abundant. An example of this is Enallaster, which may be collected by the thousands in certain outcrops of limestone from the Cretaceous period in Texas. Many fossils of the Late Jurassic Plesiocidaris still have the spines attached.

Some echinoids, such as Micraster which is found in the Cretaceous period Chalk Formation of England and France, serve as zone or index fossils. Because they evolved rapidly over time, such fossils are useful in enabling geologists to date the rocks in which they are found. However, most echinoids are not abundant enough and may be too limited in their geographic distribution to serve as zone fossils.

In the early Tertiary (c 65 to 1.8 MYA), sand dollars (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.

Systematics and taxonomy

Within the echinoderms, sea urchins are classified as echinoids (class Echinoidea). Specifically, the term "sea urchin" refers to the "regular echinoids," which are symmetrical and globular. The ordinary phrase "sea urchin" actually includes several different taxonomic groups: the Echinoida and the Cidaroida or "slate-pencil urchins", which have very thick, blunt spines (see image at right), and others (see taxonomic box on the right). Besides sea urchins, the Echinoidea also includes three groups of "irregular" echinoids: flattened sand dollars, sea biscuits, and heart urchins.

The name urchin is an old name for the round spiny hedgehogs that sea urchins resemble.

Importance to humans

Sea urchin (uni) served Japanese style as sashimi, with a dab of wasabi

Sea urchins are one of the traditional model organisms in developmental biology. The use of sea urchins in this context originates from the 1800s, when the embryonic development of the sea urchins was noticed to be particularly easily viewed by microscopy. Sea urchins were the first species in which the sperm cells were proven to play an important role in reproduction by fertilizing the ovum.

With the recent sequencing of the sea urchin genome, homology has been found between sea urchin and vertebrate immune system-related genes. Sea urchins code for at least 222 Toll-like receptor (TLR) genes and over 200 genes related to the Nod-like-receptor (NLR) family found in vertebrates^[5]. This has made the sea urchin a valuable model organism for immunologists to study the evolution of innate immunity.

As food

The ovaries of the sea urchin, called corals or roe, are culinary delicacies in many parts of the world.^[6]

In cuisines around the Mediterranean, Paracentrotus lividus is often eaten raw, with a squeeze of lemon.^[7] It can also flavor omelettes, scrambled eggs, fish soup,^[8] mayonnaise, Bechamel sauce for tartlets,^[9], the boullie for a soufflé,^[10], or Hollandaise sauce to make a sauce for fish.^[11] In Chile, it is served raw with lemon, onions, and olive oil.

Though the edible Strongylocentrus droebachiensis is found in the North Atlantic, it is not widely eaten, though it is exported, mostly to Japan;^[12] in Maine, sea urchins are known as whores' eggs. It was formerly considered a delicacy in the Orkney Islands, used instead of butter.^[13]

In the West Indies, Cidaris tribuloides is eaten.^[14]

On the Pacific Coast of North America, Strongylocentrotus franciscanus was praised by Euell Gibbons; Strongylocentrotus purpuratus is also eaten.

In New Zealand, Evechinus chloroticus, known as kina in Maori, is a delicacy, traditionally eaten raw. Though New Zealand fishermen would like to export them to Japan, their quality is too variable.^[15]

In Japan, sea urchin coral is known as uni (ウニ^?), and can retail for as much as $450/kg.^[16]; it is served raw as sashimi or in sushi, with soy sauce and wasabi. The demand for sea urchin is such that Japan imports large quantities from the United States, South Korea, and other producers. Japanese demand for sea urchin corals has raised concerns about overfishing.^[17]

See also

• Sea urchin injury

Gallery

Two purple urchins found in the tide pools of Cape Arago, Oregon, USA	Group of black, long-spined Caribbean sea urchins, Diadema antillarum (Philippi)	Sea urchin roe.	Sea urchin test. Each white band is the location of a row of tube feet; each pair of white bands is called an ambulacrum. There are five such ambulacra; the fivefold symmetry reveals a kinship with sea stars.
Closeup of a sea urchin test. In life, a tube foot or gill extends through each of the small holes, and a spine is supported by each of the raised tubercles.	Sea urchins have adhesive tube feet.	Sea urchin in a reef off the Florida coast.	Two Heterocentrotus trigonarius on a Hawaiian reef
Chilean sea urchins for sale in Feria fluvial, Valdivia. Three sea urchins are sold for 1000 Chilean Pesos.	Three dead specimens of Sterechinus neumayeri

References

Bibliography

• Smith, Andrew B. (1984), Echinoid Palaeobiology (Special topics in palaeontology). London: Allen & Unwin. ISBN 0-04-563001-1
• Schultz, Heinke. (2005), Sea-Urchins, a guide to worldwide shallow water species . hpsp scientific publications, Germany. ISBN 3-9809868-1-0
• http://animaldiversity.ummz.umich.edu/site/accounts/classification/Echinoidea.html#Echinoidea Animal Diversity Web Classification of the Echinoidea]
• http://whale.wheelock.edu/archives/ask99/0388.html#The Ocean Alliance giving advice on sea urchin cleaning]

External links

Wikimedia Commons has media related to: Sea urchin

• Sea Urchin Harvesters Association - California Also, (604) 524-0322.
• The Echinoid Directory from the Natural History Museum.
• Echinoids of the North Sea
• 70% of Sea Urchin Genes Have a Human Counterpart -- Sequencing confirms that sea urchins are more closely related to humans than fruit flies (LiveScience.com, November 2006).
• Spiny creature's genome insight
• Echinoids.nl
• lantern.jpg A labeled diagram of the sea urchin's Aristotle's lantern.
• aristotle.htm Who is this person Aristotle and what about this lantern?
• www.emilydamstra.com Illustration of the musculature of an Aristotle's lantern.
• Urchin Anatomy a flash about the anatomy of the sea urchin
• www.sea-urchins.com An article about sea-urchin parasites.
• Eating fresh sea urchin An article about eating fresh sea urchin, or uni, in Japan
• Further research on sea urchins
• Photographic Database of Cambodian Sea Urchins