RESUMEN
The Amazon Reef System (ARS) is one of the most important shallow and mesophotic reef ecosystems in the South Atlantic Ocean. The ARS consists mainly of extensive beds of calcareous algae interspersed by assemblages of octocorals and sponges. The enormous freshwater discharge from the Amazon River forms a plume along the extensive Amazon continental shelf, for which the hydroid community is still largely unknown. The aim of this study is to document the diversity and distribution of hydroids from the ARS, as well as to infer the influence of the plume on species composition in the different zones. Samples were collected at ninety-six stations between 15 and 240 m deep on the Amazon shelf. A total of 37 species were recorded in the studied area. Hydroid assemblages are richer in zones under lower river plume influence, and species composition differs significantly between zones with and without plume influence (PERMANOVA, p = 0.0025). The dissolved oxygen and nitrate ranges were the environmental variables significantly correlated with the hydroid distribution. This study is the first surveying the hydroid species composition and richness in the ARS, highlighting the presence of a typical reef biota and that further faunal studies in underexplored areas of the Atlantic should reveal the distribution of many poorly known hydroids species.