Three types of skeletal anomalies were observed on Gobius niger Linnaeus, 1758 caught in Sado estuary: dentary, vertebrae and hypural plates. Hypural plate deformities were the most common occurring in 19.8% of the specimens caught and 7.2% had deformed vertebrae. Dentary deformities were observed in only 0.2% of the fish examined. Vertebral deformities were more frequent in the 60/69 mm class decreasing thereafter with fish length, revealing an impact on fish survival. Deformities were more frequent in the industrialized bank of the estuary allowing the relationship between pollution and observed teratology. Gobius niger showed conditions to be considered a pollution indicator species. Further studies are needed to relate specific pollutants with the observed types of deformities.