First in situ record of the medusa stage of Cladonema radiatum (Cnidaria: Anthoathecata) in the South Atlantic Ocean

Editor: Rubens M. Lopes Hydromedusae are important predators in marine habitats and information on their distribution is essential for understanding biogeographic patterns and marine ecosystem functioning (Colin et al., 2003; Tewksbury et al., 2014). Contrary to most hydromedusae, species of the family Cladonematidae Gegenbaur, 1857 (Anthoathecata) are characterized by a mostly or exclusively benthic mode of life with specialized adhesive structures on the tentacles used to adhere and move over the substrate (Schuchert, 2006). The medusae of the genus Cladonema Dujardin, 1843 are distinguished from other Cladonematidae by their tentacles being branched more than once, and by having two types of tentacle branches: some ending in an adhesive structure, and others having nematocyst clusters; and the presence of simple oral knobs of nematocysts. Cladonema species have undergone several groupings and splittings since they were first described. Currently, six valid species are recognized but species distinction is still somewhat controversial (Schuchert et al., 2006; Gershwin and Zeidler, 2008; Cedeño-Posso, 2014; Schuchert, 2020). To date, in the Southwestern Atlantic the genus has been recorded in subtropical waters (23oS) based on hydroids and laboratory-reared medusae (Migotto, 1996). In the present work, we report the first in situ observation of the medusa stage of Cladonema radiatum in the South Atlantic Ocean associated with two different substrates, collected with emergence traps. This record extends the known geographic occurrence of the species to northestern Brazil. We additionally provide insights on methodologies that can improve the collection efficiency of organisms with demersal habits. The specimens were collected in the Abrolhos Archipelago, which is part of the Abrolhos Bank. The Abrolhos Bank is an extension of the Brazilian continental shelf, off Bahia state. It occupies an area of approximately 46,000 km2, between 16o40’-19o40’S and 37o20’-39o10’W, composed of coral reefs, rhodolith beds, volcanic islands, shallow banks and channels (Leão and Kikuchi, 2001; Moura et al., 2013; Francini-Filho et al., 2013). The Abrolhos Archipelago is formed by five volcanic islands surrounded by fringing reefs and the sediment among the islands is mainly sandy and calcareous with the presence of rhodolith beds. The islands are surrounded by scleractinian coral colonies that dominate the substrate and a diverse seaweed community (Leão and Kikuchi, 2001; Torrano and Oliveira, 2013). The area is surrounded by unique mushroom-shaped structures formed by corals and calcareous algae known as “chapeirões” (Leão et al., 2019). The samplings were conducted in April 2016 (austral fall) during 3 consecutive days in triplicate. Emergence traps were set up 1m from the bottom at two different sites. One site was located on a rocky bottom, dominated by turf, calcareous algae and scleractinian coral. The second site was located on a https://doi.org/10.1590/s2675-28242020068349