Sicyopterus lagocephalus, the red-tailed goby or blue stream goby, is a species of goby native to islands of the Indian Ocean from the Comoros to the Mascarene Islands to the Pacific Ocean where it reaches French Polynesia and can be found as far north as Japan. It is an amphidromous species: adults can be found in swift-flowing streams with rocky beds but the eggs hatch at sea and the larval stage remains in marine waters, migrating to freshwaters when they reach the postlarval stage. This species can reach a total length of 13 cm (5 in). In some places it is an important species for local consumption with the post-larvae being caught as they mass in estuaries.