The intertidal environment is characterized by a constant submersion and immersion regime and these environmental variations are responsible for conditioning the zoning and abundance of individuals living there. These environments are also recognized for high primary productivity, which is performed by the epilithic microphytobenthos. This work was conducted on the beaches of Paraiso, Farol and Grande in Mosqueiro - Pará and aimed to evaluate the density effect of the gastropod Neritina zebra on epilithic chlorophyll-a of the intertidal consolidated substrate. The densities of recruits, intermediaries and adults were estimated to measure the influence of different size classes on the availability of the biofilm. For the density were used samplers squares of 25 x 25 cm and made 10 replicates at each beach, and for the samples for chlorophyll extraction, used as an indirect measure of primary production and t abundance of microphytobenthos, fragments of approximately 3 x 3 cm were removed from rocks, each collect was made in the months of May and October. The density differences for the three size classes of Neritina zebra were significant for the months and the beaches studied, as well as the chlorophyll. The effect of density on the concentration of chlorophyll appeared prominently in intermediaries, of which it was possible to observe a density-dependent relationship in which high densities of individuals are related to low chlorophyll concentrations, however this pattern is not constant, being observed a moment when this relationship ceases to exist. The effect of the recruits and adults was less evident when compared to intermediaries, suggesting that they have a greater role in structuring the local microalgae community.
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