| Since the first collection in 1989 from Mahone Bay (NS), the highly invasive green macroalga Codium fragile has been spreading in Atlantic Canada and has become established along the coasts of Nova Scotia, Prince Edward Island and New Brunswick. Considerable morphological variation had been found between plants from different collection sites. This variation may be caused by the presence of different subspecies, or by phenotypic plasticity of one of the subspecies. Morphometric and molecular investigations were performed on Codium collected from seven sites of Maritime Canada and one from Maine to evaluate these hypotheses. Based on both mucron length and ISSR markers, plants separated into three groups, suggesting multiple introductions of C. fragile into Canada. One group, currently growing on eastern shore of Nova Scotia and near the coast of New Brunswick, is clearly ssp. tomentosoides that likely originates from the United States. A second group, growing in Malpeque Bay (PEI), is possibly an asexual variant of ssp. atlanticum, another invasive subspecies of C. fragile. A third group, found in Caribou Harbour only, is an intermediate in terms of both mucron length and molecular data, suggesting a hybrid of ssp. tomentosoides and ssp. atlanticum. The origin of this intermediate group remains to be established. |
