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.

Marine specialist herbivores inhabiting Japanese rocky shores are significantly more diverse than in other biogeographical regions at comparable latitudes. The interaction webs of specialist sacoglossans were investigated on the Pacific shore of Honshu, Japan. The host use of the small stenophagous sacoglossan opisthobranch Placida sp. (sensu Baba, 1986) was investigated in association with the green macroalgae Codium spp. Based on field associations, Placida sp. attacked six congeneric algal host species out of eight investigated (75%) in and around Sagami Bay and ten of 20 species (50%) in Japan. In pairwise-choice feeding experiments, Placida sp. preferred the algal host from which it was collected (source alga) to alternate host and non-host algae. Preference rankings, however, could be altered based on the condition or identity of the choices (e.g. old versus young fronds of C. fragile). Connectance values were lower in these high-diversity assemblages than in their low-diversity counterparts on other temperate shores (but these values may increase with additional study). The sacoglossan occasionally coexisted with two other common sacoglossan species (Elysia trisinuata and E. setoensis) and several other less common ones. However, interspecific feeding interactions appeared minor, with no discernible evidence of interference or exploitation competition. Despite the diverse assemblage of specialist sacoglossans feeding on Codium spp., these Japanese sacoglossans (1) had low niche differentiation, (2) were weak interactors with their hosts, and (3) had negligible intra-guild interactions.