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1A. FEWER THAN 12 DORSAL FIN SPINES (FIG. 1B) ....................MASTACEMBELUS PAUCISPINNIS
1B. 20 OR MORE DORSAL FIN SPINES (FIG. 1A)....................2
2A. PREORBITAL SPINE PRESENT (FIG. 3B) ....................3
2B. PREORBITAL SPINE ABSENT (FIG. 3A) .................... 7
3A. SNOUT BLUNT AND MOUTH SMALL, VERTICAL FROM CORNER OF MOUTH NOT REACHING NOSTRIL (FIG. 3A) ....................MASTACEMBELUS ROBERTSI
3B. SNOUT ELONGATE AND MOUTH LARGE, VERTICAL FROM CORNER OF MOUTH REACHING LEVEL OF NOSTRIL OR BEYOND (FIG. 3B) ....................4
4A. SMALL, SLENDER-BODIED, WITH AN ELEVATED NUMBER OF DORSAL FIN SPINES (MEAN 30 OR MORE) (FIG. 1A) ....................5
4B. LARGER, DEEPER-BODIED, USUALLY WITH FEWER DORSAL FIN SPINES (MEAN 28 OR FEWER) (FIG. 1B)....................6
5A. CAUDAL FIN BROAD AND WITH LARGE, IRREGULAR DARK BLOTCHES (FIG. 2B)....................MASTACEMBELUS NIGER
5B. CAUDAL FIN POINTED AND WITH NUMEROUS SMALL BLACK SPOTS (FIG. 2A)....................MASTACEMBELUS GRESHOFFI
6A. 20-22 (mean 21) DORSAL FIN SPINES, FIRST DORSAL FIN SPINE SITUATED ABOVE THE PECTORAL FIN ....................MASTACEMBELUS TRAVERSI
6B. 25-30 (mean 28) DORSAL FIN SPINES, FIRST DORSAL FIN SPINE SITUATED WELL BEHIND THE PECTORAL FIN ....................MASTACEMBELUS CONGICUS *
7A. 18-24 DORSAL FIN SPINES, NO PREOPERCULAR SPINES (FIG. 3B), BODY WITHOUT SCALES (FIG. 1A)....................8
7B. 26-32 DORSAL FINS SPINES, PREOPERCULAR SPINE PRESENT (FIG. 3A), BODY COVERED WITH SMALL SCALES (FIG. 1B)....................10
8A. NO EYES VISIBLE EXTERNALLY, BODY ROUNDED AND NOT LATERALLY COMPRESSED ....................MASTACEMBELUS CRASSUS
8B. EYES VISIBLE EXTERNALLY, BODY LATERALLY COMPRESSED ....................9
9A. EYES SMALL, 21-24 (mean 23) DORSAL FIN SPINES, VERTICAL FROM CORNER OF MOUTH JUST REACHING TO LEVEL OF NOSTRIL ....................MASTACEMBELUS LATENS
9B. EYES LARGE, 18-21 (mean 20) DORSAL FIN SPINES, VERTICAL FROM CORNER OF MOUTH REACHING TO LEVEL OF EYE ....................MASTACEMBELUS AVICEPS
10A. EYES VISIBLE EXTERNALLY, 28-32 (mean 31) DORSAL FIN SPINES, BODY PIGMENTATION RANGES FROM DARK TO PALE ....................MASTACEMBELUS BRACHYRHINUS
10B. EYES NOT VISIBLE EXTERNALLY, 26-29 (mean 28) DORSAL FIN SPINES, BODY USUALLY LACKING PIGMENTATION AND USUALLY PALE CREAMY WHITE OR PINK ....................MASTACEMBELUS BRICHARDI
Key modified after Vreven, E. 2001. A systematic revision of the African spiny-eels (Mastacembelidae; Synbranchiformes).
Vols I-VII, Unpublished PhD thesis, Katholieke Universiteit, Leuven, Belgium. 231pp.
Vreven (2001) also records the presence of Mastacembelus marchei in the lower Congo region. Differentiation between M. congicus and M. marchei is extremely difficult, and these two species appear to overlap in both morphometric and meristic features. A single specimen from the lower Congo River collected by us has recently been identified as Mastacembelus marchei by Emmanuel Vreven. The specimen was collected (along with many individuals of M. congicus) from a site above Luozi at Lenga Lenga.
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