HABITAT GEOMETRY OF BENTHIC SUBSTRATA - EFFECTS ON ARRIVAL AND SETTLEMENT OF MOBILE EPIFAUNA

The effect of substratum complexity on the early stages of colonizatio n by mobile epifauna was assessed through a comparative study based on the architecture of artificial substrata. We conducted field observat ions over 4 years, on six types of small plastic substrata placed in t he 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 ov er 300 sample units was recorded, and their densities compared as a fu nction of substrata type and immersion time. Microcrustaceans predomin ated during the initial stages in all substrata. In this category, har pacticoid copepods and amphipods were the most abundant taxa. The effe ct of the original substratum complexity seemed to be restricted to th e early stages of colonization, since after 12 wk of immersion the ori ginal 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 foldi ng, a one-dimensional measure that evaluates the amount of filaments a nd folds in the substratum's surface. Folding was correlated with high faunal densities and high initial colonization rates, and proved to b e a better density predictor than total substratum area, or volume. Th is correlation was especially well defined for amphipods.