True oysters, pearl oysters, and a dinner-plate giant
“Oyster” is a kitchen word covering several unrelated families of saltwater clams. The true oysters we eat belong to one family, the pearl oysters to another, and the biggest pearl producer, Pinctada maxima, grows to about the size of a dinner plate.
Edible true oysters, family Ostreidae, fall mainly into four genera, Ostrea, Crassostrea, Magallana and Saccostrea, and include the European flat, eastern, Olympia, Pacific and Sydney rock oysters. The family goes back to the Early Triassic, when members of the genus Liostrea attached themselves to the shells of living ammonoids. Crassostrea and Saccostrea mostly live between the tides, while Ostrea stays below the low-water mark, and the tropical mangrove oyster grows on mangrove roots. The English name arrived from Old French in the fourteenth century and traces back to a Greek word related to osteon, meaning bone.
Inside the shell sits a surprisingly complete body. Oysters breathe through gills and across a mantle rich in fine blood vessels, and a small three-chambered heart beneath the adductor muscle pumps colourless blood while two kidneys clear waste. There is no sign of a brain. They filter plankton from the water with beating cilia, feeding hardest between about 20 and 26 °C, and they close their valves to rest on rhythms tied to the tides and the day.
Most oysters mature within a year and change sex as they grow, spawning first as males and then, once they build reserves over the next two or three years, as females. A warm spell sets off a few spawners, which triggers the rest, and one female may release up to 100 million eggs. The larvae drift for two to three weeks before settling.
Pearls form when nacre coats a tiny intruding particle, layer upon layer; farmers insert a bead of polished mussel shell and wait three to seven years. Cultured pearls have far outsold natural ones since the early twentieth century. Reefs matter too, multiplying the surface area of a flat seabed tenfold and sheltering anemones, barnacles and the fish that hunt them.
Source: Oyster
