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DescriptionColpomenia peregrina is a non-gelatinous green alga. It is greenish-olive in colour with fine brown dots. It forms a thin-walled hollow sphere, usually 1-7 cm in diameter. It may be confused with the native Leathesia difformis which is lobed with a gelatinous surface. Colpomenia peregrina is dry and papery to the touch and can be torn easily.Colpomenia peregrina occurs naturally in the Pacific Ocean. It was introduced to France with with juvenile Crassostrea virginica from the Pacific coast of North America. Colpomenia peregrina was introduced in 1907 from France into Cornwall and Dorset. It has negligible effects on the environment. In other countries, it grows attached to oysters and floats away with the oyster when the air-filled thalli grow large enough. Hence the common name of oyster thief.