Oysters are bivalves that filter feed, requiring food designed for these types of invertebrates.
This makes them somewhat more demanding in the home aquarium than their cousins the Tridacna Clams. They require supplementation with phytoplankton, and organic foods (Marine Snow or Selcon) provided by using a small pipette/straw to dispense the food upstream of its location. Each oyster needs to be fed individually.
You're right Scott, most of the pearls on the market today are cultured pearls produced in Japan
and they are formed by an irritant introduced into the oyster.