The Oyster thief, Colpomenia peregrina is a non-gelatinous olive-green coloured alga which forms a thin-walled hollow sphere, usually 3 – 9 cm. The young are brown and balloon-like, becoming yellow-brown, contorted and collapsed with age. It may be confused with the native Leathesia difformis which is lobed with a gelatinous surface in contrast to Colpomenia peregrina which is dry and papery and can be torn easily. The oyster thief is usually epiphytic, growing on a variety of seaweeds in mid to lower shore rock pools and in the shallow subtidal region, particularly in sheltered areas.
The oyster thief was introduced to France from the USA with imports of oysters at the end of the 19th century. It was first recorded in southern England in 1907 either as a result of natural spread or introduced with commercial oyster imports. By the 1940s it had spread to the Orkneys and has been recorded in the Outer Hebrides since the 1970s.
No significant impacts have been reported on native species and although economic losses were reported from French oyster beds in the early 20th century there have been no recent reports.
Information and Identification Sheets: Oyster Thief