Our mystery critter is an oyster, a two-valved mollusk found in oceans and along coastlines in mild climates throughout the world. At one time, Maryland's Chesapeake Bay reportedly had the largest population of oysters on Earth, though overfishing and pollution have depleted many of its oyster beds. Oysters begin life as males and become females within a few years. Often they change back again, as needed, to balance out their population. Fertilized oyster eggs hatch into larvae known as spat, which resembles grains of gray sand. Oysters cannot move on their own. They lie on their valve (the bigger, thicker shell) on the sea bottom or attach themselves to rocks. They're sometimes pried free and carried around by waves. Oysters breathe through gills covered with tiny hairs that wave food particles in. An oyster's shells are connected by a strong muscle called an adductor, which keeps the shells closed tight and protects the oyster's body and palps (lips). The shells are usually white and purple on the inside. Not all oysters produce pearls.

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