Colpomenia peregrina Sauvageau
Oyster Thief, Bladder Weed; Ballons, Voleuse d'huîtres.
Description: Plants baloon-like when young, becoming contorted and collapsed with age, brown when young to yellow-brown when older, to 250 mm. Frond thin, papery, delicate, and filled with seawater when young, hollow and empty when older.
Habitat: Generally growing epiphytically on Corallina officinalis and other algae, and on mussels and oysters. It most obvious in spring, reaching its maximum size in May in Ireland.
Key characteristics: Not solid and gelatinous like Leathesia marina, which persists all year around although it most obvious in summer.
Note: An introduced species, perhaps by means of shellfish movements. Plants are said to inflate with gas and to float away with small oysters; thus the common name.