Remarks: The description of Tanystylum isthmiacum difficile Stock, 1966 was based on a single female collected at one of nine RV Calypso 1961–1962 cruise stations between north Brazil and Buenos Aires Province (Argentina). Owing to a mix-up in material (see Stock 1966), the exact station is not known, and thus its presence in Uruguayan waters is uncertain (Stock 1966). Fage & Stock (1966) and Stock (1975) recorded this subspecies from Cape Verde Islands (eastern Atlantic, 40 m depth) and northern Brazil (at depth between 51–93 m) respectively. It was also doubtfully recorded from shallow subtidal waters (6–9 m depth) on the Caribbean coast of Panama (Child 1979). Stock (1992) recorded this subspecies from southeast Brazil (19°S, 28 m depth). Müller & Krapp (2009) considered T. isthmiacum Stock, 1955 to be a morphologically variable species and did not follow the separation of that species into two subspecies. They also compiled the known distribution of this species in the Eastern Pacific, Western Atlantic and Eastern Atlantic.
Tanystylum orbiculare Wilson, 1878 has been recorded from the littoral zone of the Buenos Aires Province (Argentina) by several authors (Stock 1966; Bremec et al. 1986; Genzano 2002). Achelia assimilis (Haswell, 1885) has been recorded from the Buenos Aires Province (Argentina) (e. g. Stock 1966; Bremec et al. 1986; Genzano 2002; Albano et al. 2006) from near-shore locations; elsewhere it has been recorded for Subantarctic localities (e. g. Child 1994a). The taxonomy of species belonging to this genus remains problematic (see Child 1994a).