Thriving populations of the native macroalga Codium tomentosum on Guernsey rocky shores

Since the incursions of the introduced green macroalgae Codium fragile ssp. tomentosoides and ssp. atlanticum on European shores, the abundance and distribution of the native congeners C. toznentosum and C. vermilara have come tinder closer scrutiny. Hypotheses regarding the reported declines in the native(s) are largely speculative due to the paucity of quantitative information on the natives, the cryptic nature of the invaders and the unknown processes regulating Codium assemblages. As part of a field-based Codium study on north-eastern Atlantic shores, our surveys on Guernsey shores, Channel Islands, in September 2002 have revealed two unexpected and noteworthy results: (1) thriving populations of C. tonzenlosum are quite abundant on mid to low littoral shores; and (2) in marked contrast, sparse populations of C fragile ssp. tonzentosoides are restricted primarily to upper inid-littoral rock pools. Native Codium has not been displaced or eliminated from the southern end of the British Isles.