The familiar is exotic: I. Codium fragile ssp. atlanticum on Scottish rocky intertidal shores

Summary

The green macroalga Codium fragile ssp. atlanticum appeared and spread around the British Isles in the last two centuries, generally inhabiting high intertidal pools. Described are (i) historical patterns and rates of spread on Scottish shores, (ii) human activities that may have facilitated establishment, and (iii) current status of established populations. In Scotland, the alga was first collected at Argyll in 1826. The alga's appearance coincided with the advent of steamships there, peak human population densities, and extensive harvesting of fucoids and kelps. Since 1826, it has spread around the Scottish coastlines at 13 km yr^-1, inhabiting shores exposed to the full force of the Atlantic Ocean or North Sea and sea lochs sheltered from wave action. The former is an unusual habitat for introduced species. Algae are typically <25 cm long with several axes or fronds per thallus. At St Andrews Bay, Fife, thalli grew up to 1 cm per month and persisted through mild winters. The introduced alga is readily consumed by the herbivorous snails Littorina littorea that coexist in the pools on wave-sheltered shores.