552 MARINE PRODUCTS OF COMMERCE 



Species 



Scientific classification of oysters is based primarily on the appearance and 

 structure of their shell. The latter is of simple shape and lacks any prominent 

 ornamental or sculptural characteristics, which are well developed in other 

 mollusks and are frequently used in distinguishing various species. Furthermore, 

 virtually all edible oysters vary greatly in size and shape, depending on the en- 

 vironment. They are long and narrow on soft bottom and round and flat on hard 

 ground. Crowded conditions cause them to form clusters of grossly misshapen indi- 

 viduals; when they are attached to rocks, piles, or other submerged objects, their 

 shells follow the curvature of the substratum. Because of this variabiUty identifi- 

 cation of the species presents considerable difficulty. 



Of the large number of species of oysters described by zoologists, only a few 

 are used in commerce. The principal edible oysters used in different countries are 

 as follows: Ostrea virginica Gmelin: American oyster of the Atlantic coast; Ostrea 

 edulis hinne: European edible oyster; Ostrea (Gryphaea) anguhta Lamarck: Por- 

 tuguese oyster introduced to France; Ostrea commercialis Iredale and Roughley, 

 Ostrea angasi Sowerby: from Australia and New Zealand; Ostrea denselamellosa 

 Lischke and Ostrea gigas Thunberg from Japan (introduced to the Pacific Coast 

 of the United States); Ostrea cucullata Born: in India; Ostrea chilensis Philippi: 

 found on the coast of Central and South America; Ostrea lurida Carpenter: so- 

 called Olympia oyster from the Pacific Coast of the United States and Canada; 

 Ostrea taurica Krynicki: cultivated in the Black Sea, Russia; Ostrea (Gryphaea) 

 rhizopJwrae Gmelin: from West Indies. 



The question whether various species of oysters may interbreed and produce 

 hybrids has not been definitely answered, but it is known that under laboratory 

 conditions cross-fertilization of eggs takes place between O. virginica, O. gigas, 

 O. angulata, and O. commercialis. It is quite possible, therefore, that hybrid oysters 

 occur in nature or that they may be produced by artificial cultivation. 



Biology 



The oyster belongs to the class of mollusks called by Cuvier as Bivalvia and 

 more recently renamed Pelecypoda. The animals of this group have their bodies 

 enclosed in a two-valve shell and have no head. Oysters differ from other members 

 of this group by the absence of a muscular foot, which in clams and mussels 

 serves as a powerful organ of locomotion. Having lost their foot, which is present 

 dm^ing the larval stages, in the process of evolution oysters became sessile crea- 

 tures. They live attached to rocks, pilings, and other submerged objects, or lie 

 motionless on the bottom of the sea. They are unable to move out of the unfavor- 

 able environment or avoid danger; their only means of defense is the ability to 

 shut their valves so tightly that the shell is hermetically sealed. A quantity of sea 

 water which is retained in the shell cavity keeps the body moist and prevents 

 desiccation when oysters are exposed at low water or when they are shipped long 

 distance to the market. The ability of oysters to remain closed for several days 

 or even weeks is a useful adaptation which helps the oyster to survive adverse 

 conditions, such as freshets, temporary pollution of water, or exposure to strong 

 sunlight at low tide. 



Loss of locomotion and the absence of organs of hearing and vision are com- 



