Around 1500 species of polychaetes have been recorded in Mexico, while Oaxaca, Southern Mexican Pacific, has 315 recorded species. The sedentary polychaetes encrusted themselves in artificial substrates such as dock piles, submerged ropes, buoys, and ship hulls, forming microhabitats that allow other invertebrates to establish themselves in these substrates. Artificial substrates in ports and marinas have been studied a little in Oaxaca, so a systematic sampling was carried out on the port of Salina Cruz and the Chahué marina. In addition, specimens from the LABSIM Scientific Collection were reviewed, mainly from Puerto Ángel. Thus, 1063 polychaete specimens were reviewed, of which 48 species were identified belonging to 32 genera and 14 families. Fourteen species are new records for Oaxaca, 14 are possible new species, and three non-native species were found: Hydroides dirampha, H. elegans, and Naineris setosa; and two invasive species: Branchiomma bairdi and H. sanctaecrucis. Hydroides elegans was the species with the largest number of specimens (518, representing 48.7%), followed by H. sanctaecrucis (105, 9.9%) and B. bairdi (101, 9.5%). The port with the greatest species richness was Salina Cruz with 23 species (47.9%), followed by the Chahué marina with 19 species (39.6%) and Puerto Ángel with 18 species (37.5%). With this work, the number of polychaete species recorded on the coast of Oaxaca increased to 329, demonstrating that ports and marinas are important habitats for biodiversity while shipping traffic is the main route of the dispersion of non-native species.
