Gymnoxenisthmus Gill, Bogorodsky & Mal, 2014 Gymnoxenisthmus Gill et al., 2014: 492 (type species G. tigrellus Gill, Bogorodsky & Mal, 2014, by original designation and monotypy). Diagnosis. The following combination of characters distinguishes Gymnoxenisthmus from other xenisthmid genera: head pores absent; scales lacking; first dorsal fin with five spines; second dorsal-fin rays I,13; most segmented dorsal- and anal-fin rays branched; pelvic fin with a spine and five unbranched rays. Remarks. The discovery of Gymnoxenisthmus flavicinctus new species necessitates revision of the original diagnosis for the genus. Previously we listed the presence of branched pectoral-fin rays as a diagnostic character of the genus, but the new species has entirely unbranched pectoral-fin rays. Entirely unbranched pectoral-fin rays is one of several characters that we proposed as synapomorphies of a clade consisting of Tyson Springer (1983), Rotuma Springer (1988) and Allomicrodesmus Schultz in Schultz et al. (1966). The new species lacks the remaining proposed synapomorphies of this clade (all segmented second dorsal-fin rays unbranched; all segmented anal-fin rays unbranched; ceratohyals represented by a single ossification). This distribution of derived characters suggests that the new species should be placed in its own genus as sister-group to the Tyson + Rotuma + Allomicrodesmus clade. However, the general similarity in form and coloration suggests the two species are closely related. We are also reluctant to erect additional monotypic genera when the osteology of neither species has been adequately studied. We note, however, that synapomorphies linking the two species are lacking, and the possibility remains that the genus as recognised here is paraphyletic. Further testing must await the discovery of additional specimens for more detailed anatomical study.

Published as part of Gill, Anthony C., Bogorodsky, Sergey V. & Mal, Ahmad O., 2019, Description of a second species of Gymnoxenisthmus from the Red Sea (Teleostei: Gobioidei: Xenisthmidae), pp. 177-183 in Zootaxa 4590 (1) on pages 177-178, DOI: 10.11646/zootaxa.4590.1.8, http://zenodo.org/record/2651588