Abstract

Within twelve years of its discovery in 1989 on the south shore of Nova Scotia, Codium fragile (Suringar) Hariot is now dispersed in eastern Canada at sites along over 1200 km of shoreline in Nova Scotia, New Brunswick and Prince Edward Island. Collections of plants from eight sites in the summer of 1999 showed considerable morphological variation. Plants were described using 12 macroscopic features of whole thalli and nine microscopic features associated with utricles. Statistical analyses showed two groups of populations: one group with smaller, regularly dichotomous thalli (morphotype A), and the other group with larger, more irregularly branched thalli (morphotype B). The smaller thalli had flat-topped utricles with small mucrons, whereas the larger plants had utricles with more lanceolate tips and larger mucrons. Cluster analysis and principle component analysis largely distinguished these forms. The larger plants are typical of C. fragile ssp. tomentosoides (Van Goor) Silva whereas the smaller plants have features more characteristic of C. fragile ssp. atlanticum (Cotton) Silva. Whether these different morphologies represent independent introductions, or patterns of phenotypic plasticity adaptating to local environments remains to be established.