Oyster farmers who rely upon seed do not depend upon the oyster bed they farm for reproducing their crop. The numbers and sizes of the oysters in a crop can, therefore, be carefully controlled. Our Pacific or West Coast oysters are good examples. For the last thirty years or so, seed from oysters of Japanese origin have been available from hatcheries on the West Coast of the United States. Prior to that time, such seed came directly from Japan.
The most obvious difference between an oyster of Japanese descent and one that originated in the US is the shell. The shape, thickness, and coloration of the oyster shells are quite different. These are all Kumamotos.
Oysters imported to the US from Japan have been of two types: Crassostrea gigas and Kumamotos. The gigas was the dominant species before WWII. The Kumamoto has increased in numbers more recently. The Kumamoto shell is generally smaller and deeper than the gigas. Both are easily identifiable as being different from our Crassostrea virginica oysters.
The Japanese mastered the farming of oysters from seed and were able to ship seed very long distances. It was extremely important to oysterculture to start with the seed rather than use the natural processes of oysters themselves. The major difference was the ability to transport vast numbers of seed that were completely clean and free of disease. Prior to that time, oysters that were moved bodily from one oyster bed to another often died and/or contaminated the new bed with various microbial diseases and organisms.
Another substantial advantage from using the seed from hatcheries is the ability to grow oysters in waters that are otherwise too cold for normal oyster procreation. The colder waters are themselves an effective deterrent to many diseases that also require warm temperatures.
Kumamotos were first imported to the West Coast of the U.S. after World War II. The U.S. hatcheries soon learned how to produce their own Kumamoto seed. Sadly, in the fifty years since it came to the U.S., the Kumamoto oyster all but disappeared in the bay that bears its name in Japan. But, it is alive and well and living in Seattle. Seafood markets and restaurants on the East Coast import Kumamotos regularly. Efforts are now also underway in Japan to revive them where they originally once flourished.