Hippopotamyrus castor Pappenheim 1906

Body moderately deep, 20.5-23.7% SL, greatest depth near origin of dorsal fin. Head long, 23.5-24.9% SL, its depth 73-78% HL. Head may be slightly elongated in adult males. Dorsal profile continuously convex from the origin of the dorsal fin to the snout. Ventral profile moderately concave on the underside of the head, then straight to the origin of the anal fin. Snout rounded and protruding, snout length 8.1-9.4% SL, 18.1-26.4% HL. Snout may be sexually dimorphic, being higher and less rounded in the largest males (greater than 250 mm SL). Mouth subterminal, below the nostrils, and large, its posterior margin slightly anterior to anterior margin of eye, its width 25% HL. Chin with a moderate submental swelling. Teeth slightly notched in juveniles, worn flat or conical in larger individuals, five in upper jaw, six in lower. Median lower teeth much larger than others, protruding from lower jaw like the incisors of a beaver, Castor canadiensis, for which this species receives its name. Nostrils close to one another, level with the lower margin of the eye. Posterior nostril, 5.4-5.8% HL away from eye. Eye large, 14.1-20.2% HL. Dorsal fin base length 24.5-27.5% SL with 31-32 rays, height 14.6-15.7% SL. Predorsal distance, 58.6-61.1% SL. Anal fin base length 26.4-27.6% SL with 31-34 rays, height 13.9-14.3% SL. Pectoral fin: length 15.1-18.2% SL with 12 rays. Prepectoral length, 24.2-24.4% SL. Pelvic fin length 9.1-10.5% SL, prepelvic distance, 35.8-36.6% SL. Caudal peduncle depth 22.9-29.5% of its length with 16 circumpeduncular scales; scales along the lateral line, 80-81; 21-22 rows of scales above the lateral line, 19-21 between lateral line and pelvic fin. The caudal fin is deeply forked. Maximum size: 305 mm.

Coloration: deep chocolate-brown with a dark band running from the most anterior rays of the dorsal fin to the most anterior rays of the anal fin, flanked on both sides by lighter coloured bands. Dark bands run to the ends of the longest dorsal and anal rays. Younger specimens (up to 120 mm SL) have pronounced light bands in the form of parentheses ‘()’ extending from the origin of the dorsal to the origin of the anal, flanking the darker band that remains prominent. Width of dark band eight to nine scales at the lateral line.