sea urchin

 

spherical-shaped echinoderm with movable spines covering the body. The body wall is a firm, globose shell, or test, made of fused skeletal plates and marked by regularly arranged tubercles to which the movable spines are attached. Five rows of the skeletal plates are pierced by pores for the tube feet of the water-vascular system; these are typical of echinoderms and are used for locomotion. The mouth is centered on the lower side of the body and in many species is surrounded by a whorl of gills. A complex jaw and tooth apparatus in the mouth, known as Aristotle's lantern, is used to fragment food. Long, sharp spines are used for protection, and in some species are poisonous. The spines are also used as levers, aiding the tube feet in locomotion and, along with the teeth, are used by some species to dig burrows in hard rock. Sea urchins feed on all kinds of plant and animal material; some eat sand or mud, digesting out organic material that is present. Entirely marine, they occur in all seas and at all depths but prefer shallower waters and rocky bottoms. Arbacia and Strongylocentrotus are the most familiar American genera; one species of the latter, the red sea urchin (S. franciscanus) of the Pacific coast, is estimated to live for 200 years or more. Eggs and sperm are shed into the sea. After fertilization, a characteristic, free-swimming larva, called the pluteus larva, develops; it undergoes a profound metamorphosis to assume the adult form. Sea urchins have some economic significance. The roe is considered a delicacy, especially in Mediterranean regions and Japan, and burrowing species may damage sea walls. Sea urchins also are used in embryological studies. Sea urchins are classified in the phylum Echinodermata, class Echinoidea, subclass Regularia.

Sea Urchin
The sea urchin Strongylocentrotus purpuratus in an aquarium
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

Sea urchins are small, spiny sea creatures of the class Echinoidea found in oceans all over the world. (The name urchin is an Old English name for the round spiny hedgehogs sea urchins resemble.) Their shell, which is also called the "test", is globular in shape and covered with spines. The size of an adult test is typically from 3 to 10 cm.

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 spines that can be 10 to 20 cm long. Common colors include black and dull shades of green, olive, brown, purple, and red.

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, even if the 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, starfish and brittle stars.

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.

Physiology

At first glance, a sea urchin often appears to be an inanimate object, or one that is 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. The name comes from Aristotle's accurate description in his History of Animals:

...the urchin has what we may 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).

Diet

Sea urchins feed mainly on algae, but can also feed on a wide range of invertebrates such as mussels, sponges and brittle stars. Sea urchin is one of the favorite foods of sea otters and are also the main source of nutrition for wolf eels. Recently, the population of sea otters in the Monterey Bay of California has diminished. As a result, the population of sea urchins has multiplied, and they are quickly consuming the kelp forest in the area and upsetting the ecosystem. ^{[citation needed]} Left unchecked, urchins will devastate their environment, creating what biologists call an urchin barren, devoid of macroalgae and associated fauna.

Uses

Scientific

The sea urchin occupies a special place in biology due to its long-time use as a standard subject for studies in embryology. The sea urchin, particularly Arbacia punctulata, is the source of textbook descriptions of "the" egg, "the" embryo, and their early development. Theodor Boveri studied two species of sea urchin and concluded that all chromosomes were needed for normal embryonic development. At the Marine Biological Laboratory at Woods Hole, the Arbacia egg achieved almost the status of a standard "living cell" for physiological, biochemical and cytological work, resulting in overfishing and, in 1945, the near-extinction of the local Arbacia population. Sea urchins are a favored organism for studies of development using a systems biology [1]approach, often in conjunction with gene knockdown studies using Morpholino antisense oligos [2]. A purple sea urchin genome was sequenced [3] in November 2006 and it was discovered that 70% of their genes have a human counterpart.

High-school biology classes often demonstrate fertilization on a microscope slide with sea-urchin egg and sperm cells. This is literally in vitro fertilization, but it is of no use to infertile humans.

Since the sea urchin is globular and radially symmetrical, and since like other organisms, its early embryological stages are globular and radially symmetrical, it is surprising that its larval stage, known as a pluteus, is not. The pluteus exhibits only bilateral symmetry. (Pluteus is Latin for easel to which the larvae of some species really do show a close resemblance). During development, the sea urchin must transform first itself from having radial to bilateral symmetry, and then again from having bilateral to radial symmetry.

A group of pluteus larvae viewed under a dissecting microscope between crossed polarizers is a dramatic sight. The entire larva, including the calcareous skeleton, is transparent. However, the skeleton is birefringent. The result is that only the skeleton becomes visible--in glowing rainbow colors which change as the swimming larva changes its orientation with respect to the polarizers.

Culinary

Humans consume sea urchin ("roe") either raw or briefly cooked. Sea urchin "roe" is not actually roe, but rather the organs that produce the roe (the gonads). Five strips of roe reside within the structure of the urchin, a yellowish or orange substance resembling a rather firm custard. Sea urchin roe is a popular food in Korean cuisine, and it is called uni in Japanese sushi cuisine. It is a traditional food in Chile, where it is known as an erizo. Sea urchins are highly appreciated in Spain, Greece, where they are known as achinos (αχινός), and also in Italy, where they are known as ricci di mare. Sea urchin (Toutia in Lebanese) roe is also highly popular in northern Lebanon, where it is eaten directly from the Urchin with a spoon, or some people prefer it on a piece of Lebanese bread with a twist of lemon and raw white onion. Apart from domestic consumption, Chile and a number of other countries export the sea urchin to Japan in order to meet its demand throughout the country. Traditionally considered an aphrodisiac, sea urchin "roe" has been found to contain the cannabinoid anandamide [4].

Keepsakes

The tests of sea urchins found on beaches are often sold from seaside souvenir shops. Dropping a sea urchin into dilute household bleach helps remove the spines and flesh substance, leaving a clean test [5].

Geological history

The earliest known echinoids are found in the rock of the upper part of the Ordovician period, 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 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. 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.

The order of clypeasteroids arose in the early Tertiary and is the newest branch on the echinoid tree.

Gallery

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 starfish.	Closeup of 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 of the Florida coast.

References and further reading

• Smith, Andrew B. (1984), Echinoid Palaeobiology (Special topics in palaeontology). London: Allen & Unwin. ISBN 0-04-563001-1
• Animal Diversity Web Classification of the Echinoidea
• Ocean Alliance giving advice on sea urchin cleaning

External links

• Sea Urchin Harvesters Association - California
• The Echinoid Directory from the Natural History Museum.
• 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 Aristole'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.

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