Abstract This study evaluates the natural occurrence of inter- and intracrust fusions of patches of a common species of Codium in central Chile. Field experiments suggested a high capacity for vegetative growth in the species followed by fast colonization of new areas. When two crusts encounter each other on a new substratum, their respective rhizoidal filaments and utricles grow intermixed, forming a continuous crust. Growth is initiated by the production of colourless cytoplasmic projections from the borders of the utricles. Protoplasm and plastids then invade these syncytial outgrowths, which then elongate, growing as creeping filaments on the new substratum. Creeping filaments later differentiate into utricles and rhizoidal filaments. Inter- and intracrust contact areas then become covered with the intertwining utricles and rhizoidal filaments of the two neighbouring crusts. Measurements of intracrust genetic variability, screened with three organellar markers (chloroplast partial rbcL, mitochondrial ...