Diagnosis: Marcusenius gracilis has notched teeth (Ref. 93659). A much smaller dorsal to anal fin base ratio distinguishes Marcusenius thomasi from M. gracilis, being 0.7 vs. 0.79; M. mento is characterized by a higher number of lateral line scales, more than 76 vs. less than 77 for M. gracilis; M. ussheri differs by its lower number of lateral line scales, less than 65 vs. more than 70 in M. gracilis, its smaller pectoral fin length to head ratio, which is less than 0.9, and its usually deeper caudal peduncle, the ratio caudal peduncle depth to length less than 0.3 vs. more than 0.32 in M. gracilis (Ref. 93659). Marcusenius furcidens is distinguished by a combination of its higher count of dorsal fin rays, being more than 29 vs. less than 30 in M. gracilis, a longer dorsal fin, a higher ratio of dorsal to anal fin base, being 0.78, and a greater body depth; males of M. furcidens have no distinct kink in the anal fin base, in contrast to the males of M. gracilis; relative to the P phase of its EOD, M. furcidens EODs have a stronger N phase amplitude than those of M. gracilis (Ref. 93659).
Description: Head with terminal mouth well in front of eye, a strongly swollen chin on lower jaw protruding beyond upper jaw; head and body dorsolaterally compressed (Ref. 93659). Body depth around one quarter of standard length, 21.4-26.6% (Ref. 93659). Diameter of the eye one-fifth to one-quarter of the length of the head (Ref. 93659). Teeth notched; lower/upper jaw, variation from 3-7/3-6 (Ref. 93659). Dorsal fin origin situated close to, but less than two thirds of standard length from snout, obliquely orientated, anteriorly higher and posteriorly lower, distal margin crescentic with anterior two or three rays longer longer than posterior rays, number of rays 26-30; anal fin opposite dorsal fin with more anterior origin than dorsal by 6 or 7 rays, 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 31-35; pectoral fins about as long as head length, extending beyond the origin of the base of the pelvic fins (Ref. 93659). Scales cycloid with reticulate striae, scales extending anteriorly to operculum and pectoral fins; 12 circumpeduncular scales (Ref. 93659). Caudal peduncle slender, subcylindrical entire length, usually 24.7-31.9% in caudal peduncle length (Ref. 93659). Scales on caudal peduncle circumference (Ref. 93659). EOD biphasic; EOD pulse duration 306-380 µs.
Colouration: Colour in life: dark silvery grey with brownish hue, paired fins light and transparent (Ref. 93659). Colour in preservation brown (Ref. 93659).