Diagnosis: Hisonotus vespuccii differs from the congeners by having a unique coloration pattern of caudal fin with 1 black spot extending from its origin to the ventral lobe and 2 dark spots at the end of the lobe`s rays and the following combinationof character states (none is unique): odontodes forming longitudinally aligned rows (1 odontode after the other, but not necessarily forming parallel series) on head and trunk; a functional V-shaped spinelet; the presence of a single rostral plate at tip of the snout; the lack of contrasting dark geometric spots on the anterodorsal region of the body; a low caudal peduncle (depth 6–8% SL); few lateral median plates (21–23); and numerous premaxillary and dentary teeth (13–21 and 11–21, respectively) (Ref. 114934).

Description: Maximum body size 35.7 mm SL; dorsal profile of head, in lateral view, slightly convex from snout tip to margin of posterior naris, strongly convex to posterior margin of parieto-supraoccipital and almost straight to dorsal-fin origin; dorsal profile of trunk, in lateral view, straight and descending from dorsal-fin origin to insertion of caudal-fin; ventral profile, in lateral view, straight from snout tip to anal-fin origin, concave and ascending to caudal-fin insertion; greatest body depth at dorsal-fin origin (14−18% SL); greatest body width at cleithral region (21−25% SL), progressively narrowing towards to both snout and caudal fin; cross-section of body between pectoral and pelvic fins dorsally rounded and ventrally flat; cross-section of caudal peduncle ellipsoid, rounded laterally and almost flat dorsally and ventrally; head rounded in dorsal view; snout round and slightly pointed; dorsal and ventral series of odontodes along anterior margin of snout completely covering its tip; odontodes larger than remaining ones on head; odontodes on head and trunk hypertrophied and arranged in longitudinal rows; head without conspicuous crests; some specimens with a poor developed tuft of odontodes in posterior portion of parieto-supraoccipital; eyes small (13–17% HL), dorsolaterally positioned; iris operculum present and developed; premaxillary teeth 13–21; dentary teeth 11–21; teeth bifid, major (medial) cusp large and rounded, minor (lateral) cusp minute and pointed; accessory patch of teeth absent on dentary and premaxilla; oral disk oval, covered with papillae uniformly distributed on base of dentary and premaxilla and slightly decreasing in size distally; lower lip larger than upper lip, its border fringed; maxillary barbel present and joined to lower lip; presence of conspicuous V-shaped buccal papilla located immediately anterior to buccal valve; tip of snout with large rostral plate; dorsal fin II,7, its origin slightly posterior to pelvic-fin origin; tip of adpressed dorsal fin surpassing vertically through end of anal-fin origin; dorsal-fin spinelet short and V-shaped; dorsal-fin lock functional; pectoral fin I,6, its tip reaching middle of pelvic-fin length when depressed; pectoral-axillary slit present between pectoral-fin insertion and lateral process of cleithrum; pectoral spine supporting sharp odontodes on dorsal and ventral surfaces (well developed posteriorly); pelvic fin I,5; its tip reaching anal-fin origin when depressed in males and far from reaching anal-fin origin in females; pelvic-fin unbranched ray with dermal flap along its dorsal surface in males; pectoral spine supporting sharp odontodes on ventral surface turned mesially; anal fin i,5, its tip reaching 7th or 8th plate from its origin; caudal-fin i,7–7,I, distal margin forked; adipose fin absent; total vertebrae 27 (in 7 c&s specimens); body almost entirely covered by bone plates, except on ventral portion of head, around pectoral- and pelvic-fin origins, on dorsal-fin base and area around anus; abdomen partially covered by bony plates randomly distributed and surrounded by naked areas (in some specimens abdomen is completely covered by bony plates); laterally, body completely covered by plates; mid-dorsal and mid-ventral plate series well developed reaching vertical through half of caudal peduncle; median plate series continuous in median portion of body; coracoid and cleithrum completely exposed, covered with odontodes; arrector fossae partially enclosed by ventral lamina of coracoids (Ref. 114934).

Color in alcohol: ground color of dorsal surface of head and body dark gray to lighter brown (juveniles lighter than adults); ventral surface light brown to yellow in juveniles; all body and fins covered by scattered chromatophores, more visible on ventral portions and around fins insertions; caudal-fin hyaline, except for 1 black spot at its origin extending to ventral lobe and 2 dark spots at end of rays; in some specimens, caudal-fin with chromatophores irregular distributed and sometimes badly forming 2 dark strips (more visible in juveniles); neither variation nor variability of caudal-fin coloration patter found in samples we examined (holotype and 249 specimens widely distributed in rio São Francisco basin) with specific emphasis to variability between populations and variation depending on feeding (Ref. 114934). Color in life: similar to pattern described for alcohol individuals, but with ground color light green (Ref. 114934).