Sea urchin


Sea urchin

Taxobox
name = Sea urchin


image_width = 250px
image_caption = Sea urchins, "Sterechinus neumayeri"
regnum = Animalia
phylum = Echinodermata
subphylum = Echinozoa
classis = Echinoidea
classis_authority=Leske, 1778
subdivision_ranks = Subclasses
subdivision =
*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

Sea urchins are small, globular, spiny sea cat animals, composing most 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 harvested and served as a delicacy.

Sea urchins are members of the phylum Echinodermata, which also includes starfish, 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 (starfish) and brittle stars.

Classification and nomenclature

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.

Physiology

At first glance, a sea urchin often appears sessile, i.e. 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 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).

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 20 cm long.

Diet

Sea urchins feed mainly on algae, but can also feed on a wide range of invertebrates such as mussels, sponges, brittle stars and crinoids. [doi ref|10.1146/annurev.earth.36.031207.12411] 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.cite web|url=http://www.dfo-mpo.gc.ca/species-especes/species/species_seaOtter_e.asp|title=Aquatic Species at Risk - Species Profile - Sea Otter|publisher=Fisheries and Oceans Canada|accessdate=2007-11-29]

Geologic history

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.

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 into the Permian period. They neared extinction at the end of the Paleozoic era, with just six species known from the 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.

Model Organism

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 [Rast, JP et al. "Genomic insights into the immune system of the sea urchin." Science. 2006 Nov 10;314(5801):952-6.] . This has made the sea urchin a valuable model organism for immunologists to study the evolution of innate immunity.

Gallery

References and further reading

* (1984), "Echinoid Palaeobiology (Special topics in palaeontology)". London: Allen & Unwin. ISBN 0-04-563001-1

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

* [http://seaurchin.org/ Sea Urchin Harvesters Association - California] Also, (604) 524-0322.
* [http://www.nhm.ac.uk/research-curation/projects/echinoid-directory The Echinoid Directory] from the Natural History Museum.
* [http://nlbif.eti.uva.nl/bis/echinodermata.php?menuentry=zoeken&selected=boom&echinodermata-taxon=class&echinodermata-node=Echinoidea&echinodermata-treepath=none,Animalia,Echinodermata,Echinoidea&collapsed=false#Echinoidea Echinoids of the North Sea]
* [http://www.livescience.com/animalworld/061109_urchin_relatives.html 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).
* [http://news.bbc.co.uk/2/hi/science/nature/6130244.stm Spiny creature's genome insight]
* [http://www.thesea.org/TheSea/purple_sea_urchin.html 80 gallon tank video] of purple sea urchin
* [http://www.echinoids.nl/ Echinoids.nl]
* [http://www.nhm.ac.uk/research-curation/projects/echinoid-directory/morphology/JPEG/LANTERN.jpglantern.jpg] A labeled diagram of the sea urchin's Aristotle's lantern.
* [http://askabiologist.asu.edu/research/urchins/aristotle.html aristotle.htm] Who is this person Aristotle and what about this lantern?
* [http://www.emilydamstra.com/portfolio2.php?illid=56 www.emilydamstra.com] Illustration of the musculature of an Aristotle's lantern.
* [http://virtualurchin.stanford.edu/urchinanatomy.swf Urchin Anatomy] a flash about the anatomy of the sea urchin


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Look at other dictionaries:

  • sea urchin — sea urchins N COUNT A sea urchin is a small round sea creature that has a hard shell covered with sharp points …   English dictionary

  • Sea urchin — Sea ur chin (Zo[ o]l.) Any one of numerous species of echinoderms of the order {Echinoidea}. Note: When living they are covered with movable spines which are often long and sharp. [1913 Webster] …   The Collaborative International Dictionary of English

  • sea urchin — sea .urchin n a small round sea animal with a hard shell covered in sharp points …   Dictionary of contemporary English

  • sea urchin — sea ,urchin noun count a small round animal that lives in the ocean and has a hard shell with sharp points …   Usage of the words and phrases in modern English

  • sea urchin — ► NOUN ▪ a marine animal which has a shell covered in mobile spines …   English terms dictionary

  • sea urchin — n. [so named because of its spines] any of various orders of echinoid echinoderms having a somewhat globular body of fused skeletal plates studded with long, calcareous, movable spines …   English World dictionary

  • sea urchin —   Wana, wanawana, wana kauila; ina.    ♦ Various kinds: hā uke uke (also, name of a tapa design); hā uke uke iwi loloa, hā uke, hā uka uka, hā ue ue, hāku eku e, hāwa e, hālula, pūnohu, hulu anai, niho, hailimoa.    ♦ Sea urchin meat, alelo, poke …   English-Hawaiian dictionary

  • sea urchin — UK / US noun [countable] Word forms sea urchin : singular sea urchin plural sea urchins a small round animal that lives in the sea and has a hard shell with sharp points …   English dictionary

  • sea urchin — noun shallow water echinoderms having soft bodies enclosed in thin spiny globular shells • Hypernyms: ↑echinoderm • Hyponyms: ↑edible sea urchin, ↑Echinus esculentus, ↑sand dollar, ↑heart urchin • Member Holonyms: ↑ …   Useful english dictionary

  • sea urchin — sea ur·chin (seґ urґchin) any of various marine animals of the class Echinoidea, having round bodies enclosed in a shell with spiny processes and pedicellariae protruding from it. Genera such as Diadema and Echinothrix may secrete venom from… …   Medical dictionary


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