The effect of substratum complexity on the early stages of colonization by mobile epifauna was assessed through a comparative study based on the architecture of artificial substrata. We conducted field observations over 4 years, on six types of small plastic substrata placed in the low intertidal zone of an exposed rocky shore, for varied immersion periods (1, 2, 4 and 12 wk). The use of artificial substrata allowed us to manipulate independently structural and spatial features of the habitat, such as total area, amount of folds, intercepting area, total volume, and interstitial volume. The invertebrate fauna colonizing over 300 sample units was recorded, and their densities compared as a function of substrata type and immersion time. Microcrustaceans predominated during the initial stages in all substrata. In this category, harpacticoid copepods and amphipods were the most abundant taxa. The effect of the original substratum complexity seemed to be restricted to the early stages of colonization, since after 12 wk of immersion the original geometry was greatly modified by fouling organisms, particularly ascidians and epiphytic algae. The geometric characteristic that most influenced epifaunal composition and density was the substratum folding, a one-dimensional measure that evaluates the amount of filaments and folds in the substratum's surface. Folding was correlated with high faunal densities and high initial colonization rates, and proved to be a better density predictor than total substratum area, or volume. This correlation was especially well defined for amphipods.2061-23954Barry, J.P., Dayton, P.K., Physical heterogeneity and the organization of marine communities (1991) Ecological Heterogeneity, pp. 270-320. , edited by J. Kolasa and S.T.A. 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