AKA Eastern Oyster, Virginia Oyster, Atlantic Oyster, Common Oyster
These are the native oyster on the eastern coast of North America, with a range from the Gulf of St. Lawrence (Canada) as far south as Brazil, with the majority of farmed production being limited almost entirely to these same areas and a small production in the Hood Canal in Washington. Some attempts have been made to establish populations in Australia, British Columbia, the United Kingdom, and Japan, among others, but these attempts have resulted in limited commercial interest.
Because of their range and adaptability to different ecosystem conditions, this species shows marked variations in size, shape, and flavor. Also worth noting is that this species only makes up (by most estimates) about 5% of the world oyster production.
AKA Pacific Oyster, Japanese Oyster, Miyagi Oyster
These are the most common oyster species in the world. Originally native to the Pacific coast of Asia (most notably Japan), they are currently cultured in large numbers in Australia, British Columbia, Pacific Mexico, the United Kingdom, and France. These have been the go-to oysters when humans wipe out a native population of oysters (e.g. in the U.K. and France) and start farming to supplement. One reason for this broad range of production is the fact that they are very tolerant to different temperatures, thriving in waters as warm as those of Baja California and as cold as those in the bays of Alaska.
Originally from the Kumamoto prefecture of Japan, these were first imported to North America to supplement the dwindling populations of Olympia and gigas species. They met with varied enthusiasm of oyster farmers because they grow relatively slowly and stay quite small, which is why they were unpopular in Japan -- the Japanese culture favored oysters of larger size. However, they are considered by some to be the best oyster variety for their subtle yet complex flavors, and they often sell for a premium price.
AKA European Flat, Belon
Native to the Atlantic coast of Europe from Norway to Morocco, and currently grown commercially in France, the United Kingdom, Maine, Washington, and California. Most native populations were wiped out long ago by over-harvesting and pollution, though the species still has a few wild populations in Europe, the U.K., and Maine. Like Champagne, "Belon" is an AOC protected name, and oysters bearing this name in France must come from the Belon River estuary in Britany, France.
These are a very different kind of oyster, in pretty much every regard. Their shape is almost perfectly round, very flat, and appear much more similar to certain varieties of clams than the oysters most people are accustomed to. Their flavor is polarizing, to say the least. In fact, most people who try them immediately do not like them; the flavor is often very metallic (zinc, lead, and copper), fishy (like canned sardines or anchovies), and tannic. Even their breeding method differs from other oysters, but I'll go into that another time.
Ostrea conchaphila (formerly lurida)
(AKA Olympia Oyster)
This species is the only oyster native to the Pacific coast of North America, but over-harvesting and pollution pushed them almost to extinction. These are what gigas and sikamea oysters were imported to replace. Now farmed almost exclusively in the Olympia region of Puget Sound in Washington, these are another species that people either love or hate. The flavor is similar to that of O. edulis, but much more tame and without the fishiness, instead being strongly coppery balanced with a mild sweetness. These are one of my favorite oysters, but many people do not agree with me.
Like the Kumamoto, these are a very diminutive species, rarely exceeding 2.5" across, and often about the size of a half-dollar. One estimate stated that it would take 250 shucked Olympias to fill a pint. Small in size, big in flavor.