Slate pencil urchin (cidaroid)
Group of black, long-spined Caribbean sea urchins,
Diadema antillarum (Philippi)
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 extends through each of the small holes, and a spine is supported by each of the raised tubercles
Sea urchins are spiny sea creatures found in oceans all over the world. (The name sea urchin means sea hedgehog, hedgehog being one meaning of the word urchin). Their shell, which biologists call the test, is globular in shape, and covered with spines. The size of the test in adults is typically in the range of 3 to 10 cm.
Typical sea urchins have spines 1-2 cm in length (e.g. "Sea urchin", right), a millimeter or two thick, and not terribly sharp. Diadema antillarum, familiar in the Caribbean, has thin spines that can be 10-20 cm long. Sea urchins are usually dull in color, common colors including green, olive, brown, purple, and black.
Sea urchins are echinoderms (phylum Echinodermata), which also includes starfish, sea cucumbers, brittle-stars, and crinoids. Like other echinoderms they have five-fold or pentamerous radial symmetry 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 of the urchin (see picture below, right).
Within the echinoderms, sea urchins are classified as echinoids (class Echinoidea). Specifically, the term "sea urchin" refer to the "regular echinoids," which are symmetrical and globular. The ordinary phrase "sea urchin" actually includes two different taxonomic classes: the Echinoida, and the Cidaroida or "slate-pencil urchins" which have very thick, blunt spines (see image at right). Besides sea urchins, the Echinodea include three classes of "irregular" echinoids: flattened sand dollars, sea biscuits, and heart urchins.
At first glance a sea urchin often appears to be an inanimate object, or one which 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.
The spiness, which in some species are long and sharp, serve to protect the urchin from predators. Sea urchins feed mainly on algae. 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).
Sea urchin is one of the favorite foods of sea otters. 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 chewing up the kelp forest in the area and upsetting the ecosystem.
The bare shells ("tests") of sea urchins are sometimes found on beaches, and are often sold in seaside souvenir shops. Dropping a sea urchin into ordinary household bleach quick removes the spines and flesh substance, leaving a clean test. The test has a dramatic geometrical beauty and looks utterly unlike the familiar molluscan seashells.
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. 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, of course, in overfishing and, in 1945, the near-extinction of the local Arbacia population.
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.