|Keyhole Urchin (Mellita quinquiesperforata)
up to about 6 inches
The keyhole urchin, sometimes called the "keyhole sand dollar," is a relative of the common sand dollar (Echinarachnius parma); the five keyhole-shaped slots are distinguishing features of the keyhole urchin.
The skeleton (called a "test") of this flat sea urchin is rarely found on the Assateague beach. When the tan keyhole urchin dies, the soft tissues inside the test decompose, the spines fall off, and the test is bleached white. Each chipped test in the photos originally had the five oblong slots.
Keyhole urchins live on sandy bottoms in shallow water below the tide line where they borrow into the sand for protection. In the living urchin's mouth, located in the center of the underside (only the mouth hole remains in the test - bottom photo), is a structure known as "Aristotle's lantern," a set of five teeth, shaped something like the beak of a bird, that can be used for scraping algae off rocks. The rattle you hear when you shake the test may be caused by the dried teeth inside.
A live keyhole urchin's test is covered with skin, muscle tissue, and short, fine spines that are used for burrowing. Rows of tube feet extend through holes forming the five "petals" on the top side. In the middle photo, you can distinguish the fused plates that make up the test.
While there are male and female urchins, the sexes cannot be recognized by external features.
Keyhole urchins may be found from Cape Cod southwards, though they are most common south of the Virginia coast. Cod, flounder, and haddock feed on these urchins.
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