Polyonyx gibbesi Haig, 1956: 28–30.
Material examined. Brazil, São Paulo: 1 ♀, CCDB 2391, Ubatuba, Enseada de Ubatuba, colls. F. Mantelatto et al., 26.ix.2002.
Distribution. Western Atlantic— USA (Massachusetts, Rhode Island, North Carolina, South Carolina, Florida, Texas), Panama, Cuba, Puerto Rico, Trinidad and Tobago, Venezuela, Brazil (Ceará, Pernambuco, Bahia, Espírito Santo, Rio de Janeiro, São Paulo, Paraná, Santa Catarina, Rio Grande do Sul), Uruguay, Argentina (Haig 1956; Gore 1968; Craig 1974; Melo 1999; Rodríguez et al. 2005; Coelho et al. 2007; Belleggia et al. 2010; Ferreira & Melo 2016; Spivak et al. 2019).
Remarks. Polyonyx gibbesi is a common commensal of the tubicolous polychaete, Chaetopterus variopedatus (Renier, 1804) (Haig 1956; Gore 1968; Craig 1974; Williams 1984), living in pairs, and rarely found outside their tubes. The species occurs from the intertidal zone down to depths of 50 m, on rocks, sand and mud of the sublittoral region (Veloso & Melo 1993).