Ligia Collado-Vides, Lab. Ficologi'a, Fac. Ciencias, Universidad Nacional Auto'noma de Me'xico.

The capacity that plants have for marked variation in their phenotype as a result of environmental influences on their genotype during development, and the gradual and reversible adjustment of physiology and morphology to changing environmental conditions, have been the topic of numerous plant ecological studies. Plants are modular organisms, either clonal or non-clonal. I am interested on the form and growth of modular clonal marine macroalgae. Two contrasting growth forms have been found among clonal modular plants; the "guerrilla" or "diffuse" growth form, which allows plants to invade new space and is characterised by plants with long stolons and less branched modules; and the "phalanx" or compact growth form which is characterised by short, highly branched stolons. Following this theoretical background, my first objective was to describe and understand the invasive strategies in marine macroalgae. A study on a red clonal macroalgae (Bostrychia radicans) was done. Initially an architectural model was constructed in order to recognise the structural units of growth. Using L-Systems a simulation of its growth was done. A phalanx invasive strategy is shown as a result of the simulation. My next objective was to demonstrate that, as in the case of some clonal terrestrial plants, macroalgae can show the ability to modify their morphology in response to local environmental conditions, and that such modifications enhance their ability to colonise a heterogeneous area. To address this question a morphological and photosynthetic study was done in seven species of the clonal genus Caulerpa from an exposed coral reef, and a sheltered reef lagoon. The "Guerilla" growth form was characteristic for lagoon species and "phalanx" form for reef species. Differences in photosynthesis were found between Caulerpa species, with sun tolerant species at the reef although sun and shade tolerant species were present in lagoonal environment. C. cupressoides was the only species found in both environments, it demonstrated higher photosynthetic rates, and a compressed morphology when growing on the reef, and lower photosynthetic rates and expanded morphology for lagoon ramets. These results suggests that C. cupressoides has a broader phenotypic (morphological and physiological) ability in acclimation to lagoonal and reef settings than the rest of the species studied enhancing its ecological success on the studied system. Questions now are, can we measure and model the phenotypic plasticity of each structural unit? What tools can be used to evaluate morphological plasticity in clonal marine macroalgae?. Using allometric studies on Caulerpa prolifera conducted in a Mexican Caribbean reef system some inputs will be discussed.