The familiar is exotic: II. Codium fragile ssp. tomentosoides on Scottish rocky intertidal shores

Summary

The green macroalga Codium fragile ssp. tomentosoides has spread around the British Isles since it was introduced in 1939. The first verified Scottish collection of the alga was at Loch Sween, Argyll in 1953, shortly after the relaying of European oysters (Ostrea edulis) from Brittany to Scottish shores and the large-scale nutrient and predator-enhancement experiments conducted in Argyll. In Scotland, ssp. tomentosoides presently occurs in sea lochs and sheltered bays in Argyll, the Outer Hebrides, the Orkney Islands and the more semi-exposed east coast at Berwickshire. The overall rate of spread was ∼70 km yr^-1. At three marine sites in Argyll (Oban Esplanade, Loch na Cille and Loch Sween), the introduced alga exhibited markedly different population structure and dynamics. At one intertidal site, 30-85 % of the adult thalli had partially de-differentiated, and undifferentiated vaucherioid stages formed extensive epilithic and epiphytic mats. Ascoglossan (= sacoglossan) sea slugs (especially Elysia viridis) attacked ssp. tomentosoides at all sites, and their grazing often seriously damaged the alga. The occurrence, establishment and spread of C. fragile ssp. tomentosoides on Scottish shores has been insufficiently studied, given the alga's pest-like attributes on NW Atlantic shores.