The present paper presents the first population data for the Southern Hemisphere of the rare seagrass Halophila baillonis. The population studied is located in a calm, reef-protected area at depths ≤ 5 m, covering 12,000 m2 (400 m long by 30 m wide, oriented parallel to the coastline). The population generally demonstrated low shoot density and biomass during the rainy season, with significant differences between seasons, being found only for aboveground biomass. Despite the identification of this new population, the species continues to be considered rare along the Brazilian coast.
vulnerable species; new population; seagrass; shoot density
