Colonisation and growth strategies in two Codium species (Bryopsidales, Chlorophyta) with different thallus forms

In clonal macroalgae, evidence of guerrilla and phalanx growth strategies has been related to a differential response due to a heterogeneous habitat. However, some species of the green algal genus Codium may exhibit different growth strategies at different times of their development. Since the crustose species C. bernabei and the erect C. fragile had different thallus forms as well as ecological and geographic distributions, we used them to test the idea that despite morphological and growth differences, both species exhibited a similar propagation strategy. We cultured, under controlled conditions, individuals of both species to determine, first, if isolated utricles can function as propagation units; second, the type of growth at different stages of development; and, third, if species show differences in specific growth rate. Our results indicated that isolated utricles could be used for propagation because they had the ability to regenerate young mat-forming thalli in both species. Thallus regeneration implied morphological modifications of utricles, which combined guerrilla and phalanx strategies. In C. bernabei, the dynamic of vegetative propagation was independent of water movement in restoring a young thallus, which combined guerrilla and phalanx growth to colonise all available substrate. In C. fragile, water movement was required to stimulate the phalanx growth strategy and to regenerate the upright thallus. However, the guerrilla phase of C. fragile grew twice as fast as that of C. bernabei. We suggest that guerrilla-type growth is required to generate the prostrate filamentous network that first colonises new substratum. Later, the phalanx-type growth develops, generating the young mat-forming thalli in both studied species. Therefore, in these species the guerrilla strategy is not only a response to environmental constraints but also an obligate early ontogenetic stage of development. Further studies are needed to explore whether both growth strategies also occur in other species, thereby conferring them adaptive plasticity in heterogeneous environments.