Gymnolebias Costa, 2008 genus stat. nov.

Gymnolebias Costa, 2008 as a subgenus of Austrolebias .

Type species: Cynolebias gymnoventris Amato, 1986 .

Diagnosis: Gymnolebias can be distinguished from other Austrolebias genus group genera by the following unique combination of characters: (1) scales absent on ventral region; (2) absence of conspicuous suborbital and supraorbital dark grey marks in living specimens; and (3) flanks dark brownish grey to black, with wide dark grey bars on anterior portion (modified from Costa 2008).

Differential diagnosis: In addition to the unique combination of character states mentioned in the Diagnosis above, the genus Gymnolebias can be distinguished from all other genera of the Austrolebias genus group by having <40 scales in the longitudinal series (vs.> 40 scales in Titanolebias); by the lack of ossified contact organs in the anal fin of males (vs. present in Acantholebias); by the basal and medial region of the pectoral fins being hyaline in adult males (vs. non-hyaline in Acrolebias, Cypholebias, Matilebias, Argolebias and Megalebias); anal fin rounded in females (vs. nearly triangular in Austrolebias and Garcialebias); and male pectoral fins without black margin (vs. black distal margin in Amatolebias)

Synapomorphies: Gymnolebias was recovered as a monophyletic clade supported by 26 molecular characters and the following five morphological characters (see Supporting Information, Appendix S5): wide border of most scales on ventral half of flank, above anal fin, in dominant mature males (not considering bands, bars and iridescent lines) dark grey to black (3:0); centre of most scales in mid-dorsal portion of flank, anterior to dorsal fin, in dominant mature males (not considering bands, bars and iridescent lines) dark grey to black (11:1); dark suborbital bar absent (diffused and inconspicuous) (25:0); scales in the preopercular region absent (78:1); and <14 abdominal vertebrae (169:0)

Included species: Gymnolebias gymnoventris Amato, 1986 comb. nov. and Gymnolebias jaegari Costa & Cheffe, 2002 comb. nov.

Distribution: Basins associated with the Patos-Merín lagoon system, in Brazil and Uruguay.