Neolebias trewavasae, Poll and Gosse, 1963

Moritz, Timo, Dayem, Zuheir Nour El, Abdallah, Mohammed Abakar & Neumann, Dirk, 2019, New and rare records of fishes from the White Nile in the Republic of the Sudan, Cybium 43 (2), pp. 137-151 : 143-144

Neolebias trewavasae differs from Nannaethiops unitaeniatus , Neolebias unifasciatus Steindachner, 1894 and Nannaethiops bleheri with a larger mouth and maxillary reaching posteriorly back to the level of the pupil (vs. a maxillary ending in front of the eye in the other three species). In our White Nile specimens, the maxillary terminates in front of the eye. According to Géry and Zarske, (2003), N. unitaeniatus is distinct from N. unifasciatus and N. bleheri by having a complete lateral line. Boulenger (1907) noted in the account for his species from the Nile: “the lateral line [is] sometimes restricted to a few of the anterior scales” (p. 137), but he likely based his view on specimens available at the Natural History Museum London to him at this time, including specimens from elsewhere in Africa and not only Nile specimens. Our assumption is based on the fact that he depicted the type specimen of Nannaethiops unitaeniatus from Gabon in his ‘Fishes of the Nile’ on plate 21 ( Boulenger, 1907). All specimens recorded from the White Nile during our surveys share an incomplete lateral line series, with no more than 4 to 8 pored scales, a feature that would be supportive for either Neolebias unifasciatus or Nannaethiops bleheri . Géry and Zarske (2003) confirm that

Neolebias unifasciatus and Nannaethiops bleheri are closely related and that the first species differs from the latter by a more slender body, a more pointed snout, a larger eye and slightly altering colouration; but their ratios of eye size are overlapping and the ratios of body length to height are very close (3.0-3.7 vs. 3.75-3.85). Given the fact that only six specimens each from the wide distribution area in the Sahel were investigated and the only useful character to diagnose both species seems to be the slenderness of the body, we feel the usefulness of the published diagnostic features has limitations: pointedness of the snout is not further quantified and the colouration pattern for Neolebias unifasciatus in our view (based on many specimens from several river systems in West Africa, TM Pers. Obs.) has a much higher variability than stated for the six specimens originally investigated by Géry and Zarske (2003). Comparison of our specimens with the paratype of Nannaethiops bleheri (MRAC 2001-60-P-1) in the Royal Museum for Central Africa in Tervuren, which is in poor condition, did not allow to clarify unambiguously the identity of our specimens. Until thorough revision of the genera Nannaethiops and Neolebias we refer to our White Nile specimens, which differ from Neolebias trewavasae , as Neolebias unifasciatus . It needs to be tested if Nannaethiops bleheri falls together with Neolebias unifasciatus , i.e. being synonymous, or if it represents a third Nilotic species probably restricted to the Gambela area.