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Crassostrea gigas (most commonly known as the Pacific Oyster) is native to Japan, but has been introduced in many parts of the world for aquacultural and ecological purposes. Its shell is usually grey to purple and between 8-20 cm in length. Normally, the oyster is found in shallow waters at depths of around three meters and in temperatures ranging from 4-35°C. In its adult form, it is cemented to a hard substrate by one of its two valves. Individuals are usually male at first, but can change between sexes throughout their lifetime. The species is a filter-feeding, heterotrophic consumer, feeding mostly on bacteria, protozoa, various types of phytoplankton, larval forms of other invertebrates, detritus, and inorganic material.
C. gigas is used extensively by humans. It is cultured and harvested extensively for food. Also, it can be used to replace dying out organisms in an ecosystem. Introducing the species in vast quantities throughout the world has had a negative effect, however. C. gigas is now considered an invasive species. The aggressive manner in which it spreads can be very detrimental to an ecosystem.