Diagnosis: Amphilius krefftii is distinguished from A. chalei, A. grandis, A. athiensis, and A. uranoscopus by its emarginate caudal fin, vs. forked, and from A. grandis and A. athiensis by the presence of distinct pale patches at the origin and insertion of the dorsal fin; it also differs from A. chalei, A. athiensis, A. grandis, and A. uranoscopus by having a short body with usually 36 vertebrae, vs. elongate body with 37-40 vertebrae; it also differs from A. cryptobullatus, A. athiensis, and A. uranoscopus by the absence of dark marking on the body, vs. body mottled in A. cryptobullatus, finely spotted in A. athiensis and with diffuse black stripe along side in A. uranoscopus; it also differs from A. athiensis by the absence of spots on the fins, vs. fins heavily spotted; it also differs from A. chalei by having a deeper body, body depth at anus 11.1-16.5% of standard length vs. 8.9-10.8%, and a deeper caudal peduncle, caudal peduncle depth 10.2-13.5% of standard length vs. 7.1-9.7%; it also differs from A. cryptobullatus by having normally developed bilateral bony swimbladder capsules, vs. bilateral bony swimbladder capsules extremely large (Ref. 85045).
Description: Body elongate, ventral profile flattened ventrally to anal-fin base, then tapered dorsally to end of caudal peduncle; dorsal profile rising gently from tip of snout to origin of dorsal fin, then nearly horizontal to end of caudal peduncle; greatest body depth at dorsal-fin origin; caudal peduncle laterally compressed, without crenellated epidermal fold; anus and urogenital openings located at insertion pelvic fin; skin smooth (Ref. 85045). Lateral line complete, extending from dorsal edge of opercular cavity to base of caudal fin (Ref. 85045). Total vertebrae 36-38, abdominal vertebrae 20-21, caudal vertebrae 16-17; first dorsal pterygiophore intercept count 1 or 2 (Ref. 85045). Head and anterior part of body depressed and broad; head wedge-shaped in lateral view; snout broad, blunt when viewed from above; head becoming wider from tip of snout to pectoral-fin base; branchiostegal membranes moderately joined at isthmus forming a V-shaped or deeply concave connection (Ref. 85045). Mouth broad, gently curved, subterminal; lips moderately fleshy, papillate; rictal lobe large and papillate; anterior portion of premaxillary tooth band exposed with mouth closed; premaxillary tooth patches joined, forming U-shaped band, with short conical teeth; dentary teeth short and conical, tooth patches forming U-shaped band, separated medially (Ref. 85045). Three pairs of simple, tapered circumoral barbels; maxillary barbel large, fleshy, and flattened with pointed tip; barbel extending posterolaterally from corner of mouth to middle of pectoral-fin base; outer mandibular barbel thin with pointed tip, origin at posterior corner of lower jaw, extending to edge of branchiostegal membrane; inner mandibular barbel originates anterolaterally of outer mandibular barbel, extending to edge of branchiostegal membrane; branchiostegal membrane with 6-7 rays (Ref. 85045). Gill rakers on first epibranchial 2-4; rakers on first ceratobranchial 5-8; total gill rakers on first arch 7-11 (Ref. 85045). Eyes small, positioned dorsolaterally approximately midway between tip of snout and posterior margin of operculum; horizontal diameter of eye slightly wider than vertical diameter; eye without free orbit; covered with skin confluent with dorsal surface of head; anterior and posterior nares with prominent tubular rims; nares separate but relatively close to each other; posterior nare located about midway between eye and tip of snout (Ref. 85045). Dorsal-fin origin at point over tip of pectoral fin; dorsal fin with 1 unbranched and 5-6 branched rays, and fin margin slightly convex; pectoral fin with 1 unbranched and 8-11 branched rays with first ray unbranched and greatly thickened; pectoral fin with 4 or 5 innermost rays progressively shorter making posterior fin margin rounded; pelvic fin inserted posteriorly to dorsal-fin base; pelvic fin with 1 unbranched and 5 branched rays, with first ray unbranched and greatly thickened; pelvic fin with straight posterior margin; adipose-fin base longer than anal-fin base; origin anterior to origin of anal-fin base, fin extending past anal-fin insertion; margin strongly convex with sharply rounded edge, not deeply incised posteriorly; caudal fin emarginate with tips of lobes rounded, with i,5-7,6-8,i principal rays; anal fin with short base, origin posterior to origin of adipose-fin base; fin with 3 unbranched and 5-7 branched rays; anal fin margin almost straight (Ref. 85045).
Colouration: Dorsal and lateral surfaces of head and body brown; ventral region light brown; light dorsal saddles at origin and insertion of dorsal-fin and adipose-fin; dorsal, adipose, caudal, and anal fins brown; pectoral and pelvic fins positioned horizontally with upper surfaces brown and lower surfaces light yellow; maxillary and mandibular barbels brown; caudal fin with dark crescent-shaped band at base (Ref. 85045). Juvenile colouration similar to that of adult, except all fins light yellow, with small blotches of brown pigment on branched rays; pectoral and pelvic fins with first unbranched ray dark brown dorsally (Ref. 85045).