OystersOstreida

A pearl is a wound healing in the wrong place. The inside of an oyster shell is nacre, laid down by the mantle in sheets of aragonite tablets about as thick as a wavelength of light, which is exactly why it shines: the layers split the light passing through them and put it back together. If a fragment of that shell making tissue ends up inside the body, carried in by a parasite or an injury, it goes on doing its only job where it lands, walling itself into a sac and coating the inside of it. Given years, that is a pearl. None of it is intended.

The pearl banks of the southern 🌊 Red Sea were worked for centuries by divers on ropes holding their breath, and the trade died out in the twentieth century when cultured pearls made natural ones ordinary. The animals are still here. Two pearl oysters are listed for this coast, along with winged and hammer oysters that hang from the same anchor: a beard of byssus threads, spun out as a liquid protein that sets hard underwater. Chemists have spent decades trying to copy that glue, because very little else will bond to a wet surface.

True oysters do not hang. The larva swims, picks a spot once, cements its lower valve to it and never moves again. The shell then grows into whatever it landed on, so a rock oyster is a slab, an oyster settled on a gorgonian branch grips the branch like a hand, and no two are the same shape. That is why they are so easy to swim past. What gives one away is not an oyster shape but a line: the frilled zigzag seam where two mismatched valves meet.

Fourteen species are listed here, from hooded oysters sitting exposed on shore rock at low tide to the giant honeycomb oyster on deep walls, whose thick layered shell becomes a lodging house for boring and encrusting animals long before the oyster inside it dies.