M. bertini differs from all other species of the genus by the great distance between anal fin origin and anus (13-19 myomeres). It is further distinguished by the following combination of characters: head short, snout relatively long, skull length 3.6-4.9% TL, rostral fang with dorsal profile slightly convex forming an almost straight line with frontal part of skull, dorsal fin origin above myomere number 5-7, pectoral fin 5.3-10.5% TL and no lateral, subdermal spots of pigmentation (Ref. 34506).