Assigned to the genus Hydrolagus based upon the absence of an anal fin. Hydrolagus alphus is distinguished by being medium in size (average PCL 321 mm) and uniform dark brown in color with a distinct white spot (4%-6% BDL) on the lateral side above the pectoral fins. Paired fins with bluish hue and white margins. The dorsal spine longer than triangular shaped first dorsal fin and extends beyond the origin of the second dorsal fin when depressed. Eyes large (40.8–44.5% HDL) and pectoral fins reaching to or beyond insertion of pelvic fins when depressed. Pelvic claspers small, not extending beyond distal edge of pelvic fin, and divided distally for one half their length with slender fleshy denticulate tips. The base of second dorsal fin is long, deeply depressed and light colored in the center, anterior and posterior regions dark, considerably greater in height than the white middle region (Ref. 57721).