Regional comparisons of Codium (Chlorophyta) assemblages in the northern versus southern English Channel

The cryptic invasion of the Asian macroalga Codium fragile (Suringar) Hariot ssp. fragile on north-eastern Atlantic shores has been long considered a classical example of a successful invader that has competitively displaced native congeners. Yet, the lack of quantitative information about morphologically similar native congeners, namely Codium tomentosum Stackhouse and C. vermilara (Olivi) Delle Chiaje, has hindered interspecific comparisons and ecological predictions. From September 2002 to 2005, we made extensive intertidal surveys on 12 northern and 26 southern rocky shores of the English Channel, specifically documenting the abundance, distribution and identity of Codium assemblages. On the north side of the English Channel, the native C. tomentosum and alien C. fragile were both sparsely distributed in intertidal pools in Devon. In contrast, the natives were absent from and the alien was locally abundant in shallow lagoons and rocky reefs around Bembridge and Whitecliff on the Isle of Wight. Finally, in the Channel Islands off the Atlantic coast of France, Codium spp. were abundant in pools and on low-shore emergent substrata with native species predominating. Patterns Of distribution varied substantially among Guernsey, Jersey and Alderney, despite their close proximity. The regional variation in the algal distributions merits further investigation to determine whether the pattern is produced by: (1) anthropogenic activities and effects; (2) ecological interactions; (3) oceanographic factors; or (4) some combination of these.