Seed:

Oyster seed are oysters which have been reared through the entire larval phase and allowed to set and complete metamorphosis on micro-cultch in the hatchery, the result being single growing oysters. Single oyster seed is usually reared in some sort of nursery system before being planted for grow-out, generally done in a containment unit such as a bag, cage, or float for predator protection, reduced risk of burial, and ease of handling.

OSH sells oyster seed at 1 or 2mm.  Seed sizes are defined by the size screen on which seed is retained.