The composition and structure of the sessile macroinvertebrates associated to pelagic Sargassum species at the northeast coast of Pará was the inedited objective of this study. Sampling was performed at the municipality of São Caetano de Odivelas during July 2014, Atalaia beach (Salinópolis municipality) in April and May 2015 and May 2016 and from 100 km of the coast of Maracanã during April 2016. Samples were randomly collected and a boat was used when necessary. The algae were identified, weighted and measured. The associated fauna was identified at the species level when possible and quantified. Richness, diversity (H’), dominance (D’) and evenness (J’) were compared from different localities, Sargassum species and morphological regions of the algae. Cluster analysis were executed in search of similarity patterns, followed by significance tests (ANOSIM). The algae were smaller than those observed for the North Atlantic, probably due to the higher temperature from the low latitudes. The free-living, sessile fauna showed a higher richness and differed from the described at the Sargasso sea and the Golf current, except by Spirorbis sp., Actiniidae and Ascidacea. The difference is probably due to algal recolonization inside the North Equatorial Recirculation Region (NERR). Smaller turbulence and different hydrological characteristics are probably responsible for the higher richness and density of associated invertebrates in open water samples, when compared to beach samples. Sargassum natans showed higher ecological indexes for the free-living associated community when compared to S. fluitans. Overall, the free-living epizooids colonized mostly the leaves, since they are suspension feeders and the particle flux is higher when compared to the other anatomical regions. The sessile, colonial species were represented by Obelia dichotoma and Membranipora sp., both commonly found associated to Sargassum from the North Atlantic. Membranipora sp. was dominant on S. natans, while Obelia dichotoma dominated over S. fluitans, which may indicate a mechanism to avoid competition between these epizooids. Membranipora colonized the stems with more intensity, probably due to its higher hardness and the absence of the tannin effect. Obelia dichotoma colonized mostly the stems for S. fluitans and the leaves for S. natans, as consequence of the spread, linear colonies. This study demonstrate a different pattern of the associated fauna, when compared to other regions like the North Atlantic and reports that the Sargassum slicks reaching the amazon coast has a low functionality as transport vector of exotic invertebrates species to the Amazon region.
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