Epipelagic cnidarian fauna in the Western Equatorial Atlantic Ocean off the Amazon River Delta

We surveyed the epipelagic cnidarian fauna from the Western Equatorial Atlantic Ocean. A characteristic feature in this area is the freshwater discharge of the Amazon River, the largest continental inflow in the world's oceans. We analyzed 87 plankton and 90 neuston samples collected at the neritic and oceanic provinces of the Western Tropical Atlantic under the influence of the Amazon River Plume off the north coast of Brazil. We studied 34,772 organisms corresponding to 7 orders, 31 families, and 93 species. Among them, Forskalia tholoides was a new record to the coast off South America, and 20 taxa were new regional records. Species accumulation curves and diversity estimators did not reach an asymptote and clearly suggested the occurrence of a considerably higher number of species. Sampling in different seasons, with different gears, and including deeper waters certainly will further increase the number of known pelagic cnidarian species in the area. Open ocean had higher diversity than the continental shelf, both for holoplanktonic and some meroplanktonic taxa. This trend can be related to complex circulation and topography. In conclusion, we observed high diversity in the epipelagic cnidarian community. The heterogeneity of habitats in the area is likely the main factor responsible for the high biodiversity, with samples from estuarine, coastal, neritic (with a large reef system), and oceanic waters. The wide salinity range caused by the Amazon River Plume and other oceanographic mesoscale processes such as eddies and opposite currents add further complexity to the environment.