Diagnosis: Ancharius fuscus is distinguished from other species of Anchariidae by the following unique characters: shorter snout, 40.1-45.8% of head length vs. 47.1-54.3; longer maxillary barbels, reaching to midpoint of pectoral-fin base vs. not reaching to base of pectoral spine; pectoral spine serrated along its entire posterior margin; vs. along distal half only; and purplish-gray colouration vs. highly mottled dark gray and cream, lead gray or mottled brownish-gray (Ref. 56142). It further differs from Ancharius griseus in having a shorter dorsal-fin spine, 14.6-22.6% of standard length vs. 22.9-24.3%; and larger eye, 11.4-15.4% of head length vs. 9.3-10.1% (Ref. 56142).

Description: Body cylindrical, becoming more laterally compressed toward caudal peduncle (Ref. 56142). Dorsal profile gently convex from dorsal-spine base to anterior orbital margin, nearly straight anterior of orbit; dorsal profile horizontal from dorsal-fin origin to middle of adipose-fin base, then sloping gently ventrally to end of caudal peduncle; ventral profile almost horizontal to anal-fin base, then sloping gently dorsally to end of caudal peduncle (Ref. 56142). Head moderately depressed; dorsal profile slightly convex and ventral profile almost straight (Ref. 56142). Bony elements of dorsal surface of head covered with thick skin, although granular surface of neurocranium faintly visible in inter- and postorbital region; supraoccipital spine short and broad, with gently converging sides and blunt tip, i contact with anterior nuchal plate (Ref. 56142). Eye ovoid, horizontal axis longest, with free margin; located entirely in dorsal half of head (Ref. 56142). Gill openings wide, extending from ventral to posttemporal region to isthmus at vertical through anterior orbital margin; gill membranes broadly fused to each other and attached to isthmus, save for narrow flap along posterior margin at isthmus; gently notched at ventral midline (Ref. 56142). Snout not strongly produced, with broadly triangular profile when viewed laterally; mouth subterminal; oral teeth small and unicuspid, in irregular rows on all tooth-bearing surfaces; premaxillary tooth band broad and crescentic, of equal width throughout; premaxillary teeth conical; dentary tooth band in two semicrescentic patches, separated at midline; much narrower than premaxillary tooth band at symphysis; dentary teeth recurved (Ref. 56142). Barbels in three pairs; maxillary barbel short, thick and flattened, extending to middle of pectoral-fin base, with membranous extension of uniform width along entire posterior margin of barbel; inner mandibular-barbel origin proximal to midline, thicker and longer than nasal barbel and extending to vertical through posterior orbital margin; outer mandibular barbel originating posterolateral of inner mandibular barbel, extending to base of pectoral spine (Ref. 56142). Dorsal fin with spinelet, spine and 6-7 rays; dorsal-fin origin anterior to mid-body, at about two-fifths of body length; dorsal-fin margin straight, with first branched fin ray not significantly longer than other rays; dorsal-fin spine long, straight and slender, adpressed spine just reaching adipose-fin origin or overlapping up to anterior fifth of adipose fin; anterior margin of spine with asperities on proximal two thirds; posterior margin smooth; anterior nuchal plate rectangular, closely sutured to supraoccipital process; median nuchal plate with truncate anterior tip (Ref. 56142). Pectoral fin with stout spine, sharply pointed at tip, and 8-9 rays; anterior spine margin with 0-27 shallow, anteriorly-directed serrations along entire length of spine; posterior spine margin with 7-21 deep, retrorse serrations along entire length of spine; pectoral fin triangular, with straight anterior and posterior margins; humeral process short and slender, with concave dorsal edge and extending to midpoint of anterior nuchal plate (Ref. 56142). Pelvic-fin origin at vertical through midpoint between caudal end of dorsal-fin base and adipose-fin origin, with i,5 rays and slightly convex margin; tip of adpressed fin not reaching anal-fin origin; anus and urogenital openings located at vertical through midpoint of adpressed pelvic fin (Ref. 56142). Adipose fin with convex margin along entire length; deeply incised posteriorly; length of adipose base approximately 1.5 times length of anal-fin base (Ref. 56142). Anal-fin origin located at vertical through anterior half of adipose fin; fin with iv-v,9-11 rays and straight posterior margin (Ref. 56142). Caudal peduncle deep; caudal fin moderately forked, with i,6,7,i principal rays; upper and lower caudal lobes broad, pointed distally; upper lobe longer than lower; procurrent rays extending only slightly anterior to fin base (Ref. 56142). Skin smooth; lateral line complete; canal running along lateral myoseptum, with pores emanating from short ventrally and dorsally directed oblique branches (Ref. 56142).

Colouration: ground colouration in ethanol, similar to live colouration, purplish-gray on dorsal and lateral surfaces of head and body, fading to light gray or cream on ventral surfaces; small darker spots randomly and sparsely distributed on dorsal and lateral surfaces of head and body; adipose fin, and fin rays and membranes of median fins, purplish-gray; paired fins with similar colouration as body dorsally, light gray or cream ventrally; maxillary barbels purplish-gray dorsally, light gray or cream ventrally (Ref. 56142).