ABSTRACT
The status of C. fragile ssp. atlanticum is a bit ambiguous. Silva (1955) suggested that the alga may have been introduced; later, he suggested that the alga might be native to Norway (Silva 1957). Subsequent authors have not explicitly acknowledged this uncertainty. Given the criteria for recognizing introduced species (Chapman & Carlton 1991, Boudouresque 1994, Ribera & Boudouresque 1995), what is the evidence that ssp. atlanticum was actually introduced? The primary fact (Table 4) is that numerous phycologists reported the appearance of a new species on the shore (e.g. Cotton 1912). The alga’s large size and occurrence in high intertidal pools made the alga’s appearance and subsequent spread easier to record than those of the other two introduced subspecies. C. fragile ssp. atlanticum was also highly localized at individual sites. Given that C. fragile apparently did not occur in the North Atlantic or Mediterranean at that time (based on archived herbarium specimens), the appearance of a mucronate species (pointed utricle apices) of Codium in the British Isles represented a large geographic discontinuity and exotic evolutionary origins (Table 4). Finally, ssp. atlanticum is a noteworthy alga as it often occurs on wave-exposed rocky shores; this habitat is unusual for introduced species (but common for the native conspecifics).
