The shell can grow up 18 cm long. An off-white to yellow or bluish grey in colour, the shell often has deep purple patches. The left valve is deeply cupped with 6 or 7 bold ribs making the shell margin rough. The right valve is flat or slightly convex and has ribs corresponding to channels of left valve.Similar to Crassostrea virginica although this species lacks the crenulate shell margin and bold ribs of Crassostrea gigas. Crassostrea gigas was introduced from Portugal to Essex in 1926 as a commercial crop and has since established itself in the wild. It occurs naturally in Japan and south-east Asia. Crassostrea gigas is also known as the Portuguese or Japanese oyster.