Hisonotus acuen is distinguished from all congeners with the exception of H. bockmanni, H. chromodontus, H. insperatus, H. luteofrenatus, H. oliveirai and H. paresi by the possession of a functional V-shaped spinelet, (vs. non-functional spinelet, a square ossification, or spinelet absent). It can be diagnosed from H. insperatus, H. paresi, H. luteofrenatus, and H. oliveirai by having odontodes not forming longitudinally aligned rows on head and trunk (vs. odontodes forming longitudinally aligned rows on head and trunk). It differs from H. insperatus and H. luteofrenatus by having 4-5 lateral series of abdominal plates (vs. 6?8 and 7?8, respectively) and from H. insperatus by having 22-24 lateral median plates (vs. 25?26). It can be distinguished from H. luteofrenatus, H. oliveirai and H. paresi by having a single rostral plate at tip of snout (vs. presence of a pair of rostral plates at tip of snout); from H. bockmanni by the absence of unpaired platelets at typical adipose fin position (vs. presence of the unpaired platelets); from H. chromodontus by having yellowish tipped teeth (vs. reddish-brown teeth), caudal-fin color pattern mostly hyaline, except for dark blotch on origin of rays, and dark brown chromatophores largely concentrated on rays near lower caudal spine (vs. caudal-fin mostly dark brown with chromatophores largely concentrated on rays and membranes, and with two hyaline spots on middle of the fin); from H. paresi by the absence of conspicuous dark dorsal saddle and longitudinal stripe on the body (vs. inconspicuous dark saddles and stripe of the body) and from H. insperatus by having 14-27 premaxillary teeth (vs. 6?12) and 12-23 dentary teeth (vs. 5?11) (Ref. 106073).