Sandy reefs constructed by Polychaeta are the main substrate for a variety of complex marine communities and among them we can quote the meiofauna, organisms with dimensions from 44 μm to 500 μm. A sandy reef by Sabellaria wilsoni was studied in the Algodoal/Maiandeua Island, Pará, Brazil. The samplings were performed on June/2008 (rainy month) and December/2008 (dry month). Biosedimentological samplings were done with a corer of 3,0 cm of diameter up to 10 cm depth, fixed with formol 4% and colored with Rose Bengal. A total of 32 samples were collected for each month, 16 from the sandy reef and 16 from the adjacent beach sediment (named as beach in the present work) and they were divided equally between protected and exposed areas beside the division in 2 blocks. Up to 30 nematodes were sorted out from each sample and identified up to genus level. The meiofauna was represented by 18 taxa: Hydrozoa, Turbellaria, Gastrotricha, Nemertea, Nematoda, Tardigrada, Polychaeta, Oligochaeta, Gastropoda, Bivalvia, Cladocera, Copepoda, Nauplius, Amphipoda, Isopoda, Acari, Insect Larvae and Tunicata. Nematoda was the dominant group in all months and habitats. The total mean density of the meiofauna varied from 309,80 (June-reef) to 1757,60 ind./10cm² (December-beach). The nematofauna was represented by 63 genera with the first records of Eumorpholaimus, Paranticoma and Thornia to Brazil. Among the factors studied here, the habitat had the major impact on the meiofauna and nematofauna associated, with the reefs presenting lower densities but higher genera richness. The study of the meiofauna associated to the reefs of S. wilsoni is the first study of this kind in Brazil, as well as the first one about nematofauna in the Amazonian region for sandy beaches contributing significantly to the art state of these fauna.
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