Interstitial annelids in the family Saccocirridae live in the extremely turbulent and dynamic swash zone of exposed sandy beaches. We examine herein the relationship between distribution patterns of Saccocirrus pussicus du Bois-Reymond Marcus 1948 and morphodynamics, hydrodynamic zones, and environmental variables at beaches sampled along the Brazilian Atlantic coast. The occurrence and the abundance of S. pussicus at regional scale were positively correlated with the presence of a steep slope, large waves and coarse sand, which are characteristic of reflective beaches. On a local scale, S. pussicus occurred at the swash zone and breaking surf zone of reflective beaches. On a microscale, it preferred the upper 20 cm of the swash zone sediment. Saccocirrus pussicus prefers hydrodynamic zones, which change temporally, suggesting dependence with the dynamics of the surging and plunging waves. 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