Diagnosis: Marcusenius desertus can be distinguished from its congeners by a combination of the following characters: short snout length, 40.8-45.8% of head length; small eye diameter, 11.9-19.5% of head length; nares separation 12.7-17.6 times in head length; short preanal distance, 59.2-62.7% of standard length; short head length, 17.6-20.8% of standard length; and long anal fin, 23.7-27.4% of standard length (Ref. 113652).
Description: Head with terminal mouth well in front of eye, mental lobe on lower jaw protruding beyond upper jaw; head and body dorso-laterally compressed (Ref. 113652). Dorsal fin situated about two thirds of standard length from snout, obliquely orientated, anteriorly higher and posteriorly lower, distal margin slightly crescentic with anterior two or three rays longer than posterior rays, number of rays 21-25: anal fin opposite dorsal fin with distinctly more anterior origin, obliquely orientated, anteriorly lower and posteriorly higher, anterior rays longer than posterior ones, especially in males where they also appear stronger and often darkened, distal margin crescentic, number of rays 26-32 (Ref. 113652). Scales cycloid with reticulate striae, scales extending anteriorly to operculum and pectoral fin; number of circumpeduncular scales 11-15 (Ref. 113652). Caudal peduncle relatively deep, subcylindrical entire length, 34.5-42.6% in its length (Ref. 113652). Electric organ discharge (EOD) biphasic with pre-potential so weak it is not always noticeable; significant sex difference in EOD, with male EODs usually of longer duration and weaker N-phase amplitude and P-phase amplitude than females (Ref. 113652).
Colouration: Colour in life: brownish grey with many distinct dark-brown blotches, except on head and belly, sometimes purple hue depending on the angle of light incidence, pelvic fins light and transparent, pectorals darker (Ref. 113652). Colour in preservation: brown (Ref. 113652).