| Summary: | Oysters are two-shelled molluscs belonging to the class Bivalvia. In the Atlantic region the main oyster species of economic importance are the Pacific cupped oyster (Magallana/Crassostrea gigas) and the native European flat oyster (Ostrea edulis, in Europe), the mangrove oyster (Crassostrea gasar, in Brazil), and the Eastern oyster (Crassostrea virginica, in the US). The Pacific oyster originates from Japan but has been introduced to all continents except Antarctica for aquaculture purposes (Padilla 2010), and is now one of the most widely introduced marine invertebrates (Ruesink et al. 2005, Sousa et al. 2009). Oysters are found in a wide range of habitats from sub-tidal areas (down to 40 m - O. edulis) to shallow, intertidal areas (e.g. M. gigas and C. gasar). Oysters can establish populations in a wide variety of habitat types, e.g. hard substrates, sandy or muddy areas, where they attach to small stones, shell fragments or other debris, or on conspecifics, forming reef-like structures at high densities, or on other bivalve species. Each species has its own optimal environmental range in which its performance is optimized, and can survive only limited periods of time when the environmental conditions approach the species’ critical tolerance limits. As an intertidal species, M. gigas is very tolerant to varying condition and has a very broad temperature range for survival, from sub-zero degrees to 30 C (Quayle 1969, Walne 1974, Le Gall & Raillard 1988, Bougrier et al. 1995, Diederich et al. 2005, Diederich 2006, Carrasco & Baron 2010, Strand et al. 2011). The optimal temperature for growth, however, ranges between 20-25 C (Pauley et al. 1988, Wiltshire 2007). Similarly, the tolerance level for salinity ranges between 2-42 ppt (Gunter & Geyer 1955, Pauley et al. 1988, Wiltshire 2007), but the optimal for growth is 15-35 (Gunter & Geyer 1955, Brown & Hartwick 1988, Pauley et al. 1988, Wiltshire 2007). Earlier life stages are more sensitive to temperature and salinity levels. Spawning will ...
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