Sea Urchin (Arbacia punctulata Lamarck, 1816)

Purple Sea Urchin
Arbacia punctulata Lamarck, 1816

Sea Urchin (Arbacia punctulata Lamarck, 1816)

Aristotle's Lantern

Porcupine or Pin Cushion?
No, Sea Urchin

By Patricia B. Mitchell.

The Sea Urchin is not a mollusk, but rather a member of the phylum Echinodermata. Starfish, Sea Cucumbers, Sea Lilies, and Sand Dollars are its close relatives.

The kinship to Starfish (also called Sea Stars) is especially evident, in that members of the phylum Echinodermata all exhibit five-rayed symmetry. (Picture a Sea Star with its “arms” held together up over its head, and you can see the similarity to an Urchin.)

The echinoderm body is divided into five parts around a central axis. This arrangement is termed pentamerous radial symmetry.*

The Sea Urchin has a flattened, globular, calcareous shell (“test”) made up of fused ossicles (skeletal plates). From these plates protrude long, moveable spines. These spines can be tilted on ball-and-socket joints at their bases. The test is actually an internal skeleton, for tissue does cover the test and spines of a living Sea Urchin.

On the oral surface of the Sea Urchin (the big opening at the bottom of the test) is its chewing apparatus/mouth. The mouth has five sharp, lever-operated teeth (actually tooth-like plates). This chewing organ is known as Aristotle's Lantern because Aristotle accurately described it, and the overall anatomy of the creature, in his History of Animals. The entire Lantern can be extended outwards in order to chew food. The anus of the Urchin is at the aboral pole (the top).

The Sea Urchin moves by siphoning water through the hole on top of its dome-shaped shell, and pumping the water through its tube feet, which have suction disks at the ends. These tentacle-like feet protrude through holes perforating the five vertical, evenly-spaced, lighter-colored bands on the surface of the shell.

Besides their use in its “hydraulic” technique of movement, the Sea Urchin's feet also are used as sense organs, and as “arms and hands” to draw food to its mouth. In addition, some Sea Urchins have pedicellariae (small pincers) which are used for defense and also for gathering and grasping food. Specific species of Sea Urchins have venomous pedicellariae. (These types are not native to waters off the coasts of the continental United States.) The Sea Urchin can also use its pedicellariae to draw bits of seaweed, debris, etc., to itself. The “trash” sticks to its spines, effectively camouflaging it.

The Sea Urchin can use its spines to push itself along; picture a balled-up, rolling, stiff-spined porcupine. As mentioned, the spines are mounted in ball-and-socket joints, and can point in any direction. (Some species have spines as long as 10-14 inches.) The bumps on the surface of the de-spined shell indicate a “spine mount.” These bumps are called tubercles.

Some Urchin tests are as big as 10cm (4½ inches) in diameter. The test and spines may be tan, brown, purplish, or almost black. The Red Sea Urchin (S. franciscanus) of the U.S. Pacific Coast may live for 200 or more years. It can excavate into solid rock.

On the southeastern coast of the United States one is most likely to find the Atlantic Purple Sea Urchin (Arbacia punctulata) or the Slate-Pencil Irchin. The Atlantic Purple has purplish-brown or reddish-gray spines. The longest spines are near the top of the dome-shaped test, but there are no spines at the very top. It is found from Cape Cod to the Gulf of Mexico. (An interesting fact: When a shadow falls on Arbacia, it points its spines in the direction of the object, in case it is a predator.)

The Slate-Pencil Urchin has fewer spines, and they are much heavier and more blunt than those of the Atlantic Purple. The area around the top pore of the Slate-Pencil Urchin has smaller, flattened spines.

If you find a living Sea Urchin, let it live. If you find a dead one, it may or may not have all its spines. If some are missing and you want to, you can scrape off the remaining spines. Assuming the living tissue inside the cavity has already been eaten or has otherwise disappeared, just wash the shell and rinse it in a little bleach before adding it to your collection. If the dead invertebrate is inside the shell, you may use tweezers to extract it before cleaning. After this process, you will have a shell resembling an old-fashioned pincushion. (Incidentally, when you find a dead Sea Urchin on the beach, if you shake it, you might hear its five little “teeth” rattling around inside.)

Fossilized Sea Urchins are especially interesting to collectors. In England these “skeletons” are known as Shepherd's Crowns. By the way, the name Urchin is an Old English name for the European mammal now known as the hedgehog, which is a cute, furry creature with spines on its back.

In addition to being popular with shell collectors, Sea Urchins, in their living state, are much appreciated by diners in Japan, Italy, France and elsewhere because the “roe” of the female urchin is deemed a delicacy. (Even though people usually refer to the “roe,” it is actually the organs which produce the roe — the gonads — which are consumed.)

In the Marselles region of France, a popular way to enjoy roe is to eat it raw, with a spoon, straight out of the shell. Another serving idea is to spread the roe onto a slice of French bread and sprinkle it with fresh lemon juice.

Another French recipe for “Oursin” recommends lightly boiling the urchin, as you would an egg. Then you drain it, and, using scissors, cut it open on the concave side. You throw out the “excremental part” and then dip buttered bits of bread into the Urchin to extract the contents, which taste similar to crayfish meat.

In Japan, Sea Urchin is called uni, and is extremely popular in sushi preparation.

The Sea Urchin does, obviously, reproduce by eggs, which are released through five genital pores (“genopores”) on the upper surface of the doorknob-sized test. The eggs are fertilized externally by urchin sperm which have also been dispersed into the sea. When fertilization occurs, a free-swimming larva, called the pluteus larva, develops. It soon metamorphoses into the adult form.


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