Black corals (Cnidaria: Hexacorallia: Antipatharia) occur in all oceans of the globe, especially at depths greater than 50 m. However, their richness is underestimated due to the scarcity of studies on this group, especially in deep waters. The south Atlantic is one of the most depauperate regions in terms of our knowledge of antipatharians. Herein, we report 34 antipatharian species for the Southwestern Atlantic. Additionally, based on the examination of museum specimens, three species are new records in the Atlantic (Parantipathes laricides; Stichopathes paucispina and S. spiessi); and 17 had their distribution expanded, representing six families (Antipathidae, Cladopathidae, Leiopathidae, Myriopathidae, Schizopathidae and Stylopathidae). Additionally, the richness, distribution and associations of antipatharians are briefly discussed. An artificial key to South Atlantic antipatharians is included.

Polynoid scale-worms have been found living as commensals with deep-water antipatharians (commonly known as black corals) in the Potiguar Basin, off Rio Grande do Norte State, Northeastern Brazil. In this paper two polychaete species and four black corals species are redescribed. Benhamipolynoecf.antipathicola and Parahololepidellacf.greeffi, and the black coral Stylopathesadinocrada Opresko, 2006 are recorded for the Southwestern Atlantic. Benhamipolynoecf.antipathicola was first described from off New Zealand and the Malay Archipelago, as symbiont with the black coral Stylopathestenuispina (Silberfeld, 1909). It was later reported for the North Atlantic, off Florida, associated with Stylopathescolumnaris (Duchassaing, 1870). In our study, B.cf.antipathicola was found in association with the black coral S.adinocrada. Parahololepidellacf.greeffi was first described as a free-living from shallow waters off São Tomé and Cabo Verde Islands, West Africa, and later reported as symbiont with the black coral Tanacetipathescf.spinescens in the same location. Our data expand both the geographical distribution and the host range of this species which is reported for the first time as symbiont with Tanacetipathesbarbadensis (Brook, 1889), T.tanacetum (Pourtalès, 1880) and T.thamnea (Warner, 1981) in Brazil. The aim of this study is to discuss commensal associations between two species of scale-worm polynoids and black corals found in the Southwestern Atlantic, and also reporting their global distribution. Finally, we provided an updated list of the commensal polynoids and their black coral hosts.