This study aimed at describing the quantitative variations of epiphytic diatoms in Rhodophyta Galaxaura rugosa, collected in the Fernando de Noronha archipelago (north-eastern Brazil), during two annual periods (June 2006 and June 2007). The distribution of epiphytic diatoms in G. rugosa confirmed the occurrence of a quantitative variation/zoning along the thallus of the analysed host, with an increase in density (cells per gram wet weight of seaweed) of apical portions towards the basal parts of the seaweed, regardless of the collection points and the annual periods. The abundance was characterized by individuals with pennate symmetry with raphe, belonging to the Bacillariophyceae class. This fact is the result of the host being collected in an insular environment with great water movement under the direct action of trade winds and ocean currents for most of the year. The study confirmed that in marine ecosystems with strong hydrodynamics, epiphytic flora tends to comprise species with the largest adhesive strength.
