The term “oyster” is used broadly for various bivalve mollusks, making the exact number of species complex to determine. Oysters are bivalve mollusks, meaning they have two hinged shells, and they typically live attached to a substrate in brackish or marine environments. Determining a single number is difficult due to ongoing taxonomic revisions and the difference between edible oysters and other similar-looking species. The most accurate count depends on the biological family being considered, as classification methods continue to evolve.
The Definitive Global Count
Estimates for the total number of recognized oyster species globally range from 30 to over 200, depending on the taxonomic criteria used. This variance is often due to phenotypic plasticity, where one species can look extremely different based on its environment. This characteristic makes visual identification unreliable, leading scientists to rely on genetic analysis to distinguish species accurately.
The count is more consistent when focusing on true oysters, which are the ones generally consumed as food. These edible species belong to the family Ostreidae, which currently accepts about 16 genera. While the total number of species within Ostreidae is debated, the commercial market relies on a small fraction of this global diversity.
Biological Classification: True Oysters vs. Pearl Oysters
The primary distinction affecting the species count is the difference between true oysters and pearl oysters, which belong to separate biological families. True oysters, harvested for consumption, are classified under the family Ostreidae. These mollusks have an irregular, rough shell shape and a single, central adductor muscle scar.
Pearl oysters, cultivated for their nacre to produce pearls, belong to the family Pteriidae. They are not closely related to edible oysters despite the shared name. Pearl oysters typically have a strong inner layer of mother-of-pearl and often possess a characteristic “wing” shape on their hinge line.
The Five Most Commercially Important Species
The vast majority of oysters consumed worldwide belong to just five major species that dominate the commercial market.
The Pacific Oyster (Crassostrea gigas) is the most cultivated species globally, making up approximately 90% of the world’s production. Native to the Pacific coast of Asia, it is known for its relatively sweet flavor profile and fluted, ruffled shell edges.
The Eastern Oyster (Crassostrea virginica) is the only oyster native to the North American Atlantic coast, ranging from Canada to the Gulf of Mexico. This species is characterized by a teardrop or crescent shape and a distinctly briny, mineral-forward flavor. Many popular regional varieties, such as Blue Points and Malpeques, are simply different examples of this single species grown in specific locations.
The Kumamoto Oyster (Crassostrea sikamea) originated in Japan’s Kumamoto Prefecture. It is prized for its deep, bowl-shaped shell and its creamy, sweet flavor that often features a nutty finish. The Kumamoto is particularly popular among those new to oyster eating due to its pleasant taste and manageable size.
The European Flat Oyster (Ostrea edulis) is the native species of Europe and is often marketed under the name “Belon.” This species has a smooth, nearly circular shell and a bold, coppery, and metallic flavor. It is generally more difficult to cultivate than the Crassostrea species, making it rarer and more expensive.
The Olympia Oyster (Ostrea lurida or Ostrea conchaphila) is the only oyster species native to the Pacific coast of North America. This tiny oyster was nearly wiped out by overharvesting during the Gold Rush era. Cultivation efforts have brought it back, and it is known for its strong, coppery, and sometimes celery-salt-like flavor profile.