Sea urchins or urchins are small, spiny, globular animals that compose part of class Echinoidea. They are found in all oceans. Their shell, or "test", is round and spiny, typically from 3 to 10 centimetres (1.2 to 3.9 in) 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. Humans harvest them and serve their roe 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 symmetry is not visually obvious, but is easily visible in the dried test.
Together with sea cucumbers (Holothuroidea), they make up the subphylum Echinozoa, which is characterized by a globoid shape without arms or projecting rays. Sea cucumbers and the irregular echinoids have secondarily evolved diverse 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, and brittle stars.
Anatomy and physiology
At first glance, sea urchins often appear sessile, i.e., incapable of moving. Sometimes the most visible life sign is the spines, which attach to ball-and-socket joints and can point in any direction. In most urchins, touch elicits a prompt reaction from the spines, which converge toward the touch point. Sea urchins have no visible eyes, legs, or means of propulsion, but can move freely over hard surfaces using the adhesive tube feet, working in conjunction with the spines.
On the oral surface 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 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.
The spines, which in some species are long and sharp, serve to protect the urchin from predators. The spines inflict a painful wound when they penetrate human skin, but are not seriously dangerous. It is not clear that the spines are venomous (unlike the pedicellariae between the spines, which are venomous).
Typical sea urchins have spines that are 1 to 3 centimetres (0.39 to 1.2 in) in length, 1 to 2 millimetres (0.039 to 0.079 in) thick, and not terribly sharp. Diadema antillarum , familiar in the Caribbean, has thin, potentially dangerous spines that can reach 10 to 30 centimetres (3.9 to 12 in) long.
Ecology
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 devastate their environment, creating what biologists call an urchin barren, devoid of macroalgae and associated fauna. Sea otters have re-entered British Columbia]], dramatically improving coastal ecosystem health.
Evolutionary history
The earliest echinoid fossils date to the upper part of the Ordovician period ( c 450 MYA), and the species has survived to the present day, where they are a successful and diverse group of organisms. Spines may be present in well-preserved specimens, but usually only the test remains. Isolated spines are common as fossils. Some echinoids (such as Tylocidaris clavigera , from the Cretaceous period's English Chalk Formation) 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 simplify walking on the soft sea-floor.
Complete fossil echinoids from the Paleozoic era are rare, usually consisting of isolated spines and small clusters of scattered plates from crushed individuals. Most specimens occur in Devonian and Carboniferous rocks. The shallow water limestones from the Ordovician and Silurian periods of Estonia are famous for echinoids. Paleozoic echinoids probably inhabited relatively quiet waters. Because of their thin test, they would certainly not have survived in the wave-battered coastal waters inhabited by many modern echinoids. 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 this period's massive extinction of 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 extant species of echinoids thanks to their adaptive breakthroughs, 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, such fossils are useful in enabling geologists to date the surrounding rocks. 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 urchins are a traditional model organisms in developmental biology. This use originates from the 1800s, when their embryonic development was noticed to be particularly easily viewed by microscopy. Sea urchins were the first species in which sperm cells were proven to play an important role in reproduction by fertilizing the ovum.
The recent sequencing of the sea urchin genome, establishes homology 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 vertebrates' Nod-like-receptor (NLR) family found in vertebrates. This has made it a valuable model organism for immunologists to study the evolution of innate immunity.
As food
Sea urchin - Wikipedia, the free encyclopedia
A sea urchin has no visible eyes, legs, or means of propulsion, but it can move freely ... dangerous spines that can be 10 to 30 cm long. Some sea urchins spines are poisonous.
Marley Retrieves a Poisonous Sea Urchin
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sea urchin definition of sea urchin in the Free Online Encyclopedia.
sea urchin, spherical-shaped echinoderm with movable spines covering the body. ... Long, sharp spines are used for protection, and in some species are poisonous.
sea urchin Facts, information, pictures | Encyclopedia.com articles ...
sea urchin Spiny echinoderm animal found in marine tidal pools along rocky shores. Round with long, radiating (often poisonous) moveable spines, its skeletal plates fuse to form a ...
Sea Urchin Videos
Some sea urchin spines are also poisonous, to further protect against predators. Sea urchins have rows of tiny tube feet used to move along the ocean floor and collect food.
Marty Snyderman Stock Collection : Sea Urchins : URCHIN-29
Flower sea urchin covers itself with rocks, shell bits and other debris; these urchins lack spines but can be poisonous if handled; Mexico's Sea of Cortez; Scientific Name ...
Poisonous sea urchin
Damien Douxchamps's Photography ... Poisonous sea urchin Complete with bright blue dots. No, these are not LEDs: it's naturally blue.
Sea urchins definition of Sea urchins in the Free Online Encyclopedia.
sea urchin, spherical-shaped echinoderm with movable spines covering the body. ... Long, sharp spines are used for protection, and in some species are poisonous.
Sea Urchin Facts and Information - About Saltwater Aquariums ...
Sea Urchin Puncture Wounds First-Aid The spines of some species of Sea Urchins are harmless, while others are poisonous, but no matter which type you get poked by, they are quite ...
Marty Snyderman Stock Collection : Sea Urchins : URCHIN-22
Flower sea urchin covers itself with rocks, shell bits and other debris; these urchins lack spines but can be poisonous if handled; Mexico's Sea of Cortez; Scientific Name ...
