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Colpomenia peregrina, sometimes referred to by its vernacular names Oyster Thief and Bladder Weed, is a "brown "seaweed not native to the "British Isles, but recorded in "Ireland since 1934. It appears to have been introduced from the "Pacific and was first noticed in "Europe in 1906 on "oyster beds. It has now been recorded throughout the eastern north "Atlantic from "Norway and "Sweden to "Portugal. It was first recorded in "Britain in 1908 and in "Ireland in 1934.
Colpomenia peregrina (syn. Colpomenia sinuosa (Mertens ex Roth) Derbès et Solier var. peregrina Sauvageau) is a small brown alga, bladder-like, hollow and membranous, up to 9 cm across. The surface is thin and smooth but often collapsed or torn when older. Olive brown in colour and attached by rhizoidal filaments to rock at the base. There are two species in Europe: C. sinuosa (Mert.) Derb. & Sol. and C. peregrina (Sauvageau) Hamel. C. sinuosa was present at least as far back as the 1840s in "Spain and C.peregrina was introduced and first noticed by oyster fishermen in the "Bay of Biscay in 1906. It was first noticed in Britain in 1907 in "Cornwall and "Dorset. The two species are superficially similar and in older texts, such as Knight and Parke (1931), C.peregrina is referred to as C.sinuosa.
"Leathesia difformis is similar, it is yellow brown in colour, fleshy and mucilaginous in texture. It is globose and smooth when young becoming hollow and convoluted with age and growing to 5 cm in diameter.  It is easily distinguished as it readily squashes when pressed under finger and thumb.
There are specimens stored in the "Ulster Museum Herbarium (BEL) from: "Co. Donegal in the "Republic of Ireland and "Northern Ireland and "Wales under the catalogue museum numbers: F11254; F3136; F7675; F6154; F1682; F1693; F7491; F7674; F4254; F4254 and F1832.
Colpomenia peregrina has been recorded in Ireland from Counties: "Down, "Donegal, "Kerry, "Galway, "Clare and "Cork. Apparently this alga was first recorded in Ireland by M.J.Lynn from "Strangford Lough in March 1934 and from Lough "Larne near Ballycarry and Magheramorne in 1935. It was also recorded from Portballintrae, on the north coast, and in the south at Lough Ine. In 1936 it was found at Rush ("County Dublin) and at Killough (County Down). There are further records of from: "Portstewart ("County Londonderry), cast ashore at Hood's Ferry, Islandmagee (opposite Larne), ("County Antrim). It is now abundant.
Generally around the British Isles. In Hardy and Guiry (2006) it is shown to be generally recorded around Ireland, south west "England, Wales and the west coast of "Scotland. Records from the east of Scotland are few and it not shown as present on the east or south east coast of England. It is noted as a recent addition to the flora (as C. sinuosa in Knight and Parke (1931).
Alaska to La Jolla, California.
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