Everything about Echinoidea totally explained
Sea urchins are small, spiny sea creatures of the class
Echinoidea found in oceans all over the world. (The name
urchin is an old 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, potentially dangerous 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've fivefold symmetry (called
pentamerism) and move by means of hundreds of tiny, transparent, adhesive "
tube feet". The pentamerous symmetry isn't 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 a
sessile creature, for example one that's 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 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 isn't 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're not seriously dangerous, and it isn't 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, brittle stars and crinoids. 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.
The earliest known echinoids are found in the rock of the upper part of the
Precambrian period, and they've survived to the present day, where they're 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'd 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're 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're 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.
Culinary Usage/Fishery
Sea urchins are an important fishery and are harvested for food. Contrary to popular belief, the portion of the sea urchin sold and served as one of the ocean’s most opulent treasures isn't the roe. It is the gonads of this hermaphrodite sea creature that are scooped out of the urchin’s spiny shell in five custard-like, golden sections. Known in Japan as "uni" and traditionally considered an
aphrodisiac, gonads are the only edible part of the urchin.
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 1800's, 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. This has made the sea urchin a valuable model organism for immunologists to study the
evolution of
innate immunity.
Gallery
Image:urchin003.jpg|Group of black, long-spined Caribbean sea urchins (Diadema antillarum (Philippi)).
Image:Sea urchin eggs.jpg|Sea urchin roe.
Image:Zeeegel3.jpg|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.
Image:UrchinTestClose.jpg|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.
Image:Seeigel-Saugfuesse(Galicien2005).jpg|Sea urchins have adhesive tube feet.
Image:Sea Urchin off Florida coast 2007-08-06.JPG|Sea urchin in a reef off of the Florida coast.
Image:Sea urchins in california tide pools.jpg|Sea urchins in California tide pools
Image:Heterocentrotus trigonarius in Kona.jpg|Two Heterocentrotus trigonarius at Hawaiian reef
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]
Further Information
Get more info on 'Echinoidea'.
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