The macrofauna associated with individually collected clumps of pelagic Sargassurn was influenced strongly by the location of sampling. Algae collected in the Sargasso Sea carried more species with lower dominance and higher evenness and variation in abundance than algae collected in the Gulf Stream. Gulf Stream samples were dominated by the shrimp Latreutes fucorum (67.8 %) with little compositional variation whereas the gastropod Litiopa melanostoma was the most abundant but weakly dominant

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... ecies (25.1 %) in the Sargasso Sea. Generally, species endemic to pelagic Sargassum species were most abundant in the Sargasso Sea while numerous temporary associates were found in the Gulf Stream. Numbers of individuals and species increased with the weight of algal clumps but clump size had little effect on the species composition of algal associates. Variation in species composition and trophic organization of macrofaunal associates with geographic region and with algal age were related to differences in the epiphytization of the algae. Because of the dissimilarity in algal associates of the Gulf Stream and Sargasso Sea, the influx of littoral fauna from the West Indies to the Sargasso Sea is probably very slow and the present forms may have evolved in that water mass.