Asteroidea de Blainville, 1830
Starfish, or Sea Stars, are such fascinating creatures that a great deal has been written about them. For the purposes of this article, I will be fairly brief.
The animals are not, as you know, fish. Rather, they are echinoderms, a word which means “spiney” or “prickly skin.” Some Sea Stars have five arms, but others have four, six, seven, eight, ten, or even more. The Sunflower Star has up to 24 arms and a span of over two feet across. (The “Basket Star,” a Sea Star relative, has an almost uncountable number of appendages.) The largest Sea Star specimen ever found measured 54½ inches (138cm) across.
Sea Stars exhibit many pretty colors — pink, red, orange, yellow, brown, blue, purple, and greenish-black; and at the tip of each arm, they have a tiny pink, orange, or red eye.
The Sea Stars are kin to Sea Urchins and Sand Dollars. Like them, Sea Stars exhibit radial symmetry and move around by their tube feet. These “feet” are on the underside of the arms. If you study the bottom side of a live Sea Star, you will see the fleshy “suction cup” tube feet waving. Some species of Sea Stars use their tube feet to “shuck” oysters, clams, and other shellfish. The Sea Star will grasp a bivalve with its arms, and, using an incredible amount of suction, will pull until the oyster (or other bivalve) begins to weaken. A small opening will appear between the oyster shell's halves. Then a large part of the Sea Star's stomach is inserted inside the mollusk and the hapless oyster's soft body is digested alive.
The opening at the margin of the bivalve's shell need only be 1/25 of an inch wide for the Sea Star to win the battle. Moreover, the Star may exert up to 100 pounds of force for a short period of time, and apply a smaller amount of continuous force for up to two days. Also, rather odd about this process (or at least certainly different from human digestion!) is the fact that the Star sticks its hungry stomach out through its mouth (“everts” it).
Besides oysters, Stars like to eat clams, cockles, mussels, scallops, snails, worms, barnacles, and even fish. A Sea Star can eat as many as eight clams a day.
Some species of Sea Stars can use their arms to burrow in the sand. As you probably know, if a Star uses one or more of its arms, or even a part of the central disk of the body, the Star can quickly grow new parts. Whole new Sea Stars can even sometimes grow from a piece of an arm only 2/5's of an inch long.