Hisonotus megaloplax differs from its congeners by the arrangement of abdominal plates,

which is composed by greatly enlarged lateral abdominal plates, which contact each other at the belly midline, without a median abdominal series or with a single triangular plate located posteriorly. All other species of Hisonotus, with the exception of Hisonotus montanus, possess a complete median series between the lateral abdominal plates or a naked area between them. This feature is polymorphic in H. montanus, some individuals having enlarged lateral abdominal plates contacting at the midline and leading to misidentification. Hisonotus megaloplax is further distinguished from Hisonotus montanus by the caudal-fin color pattern which is almost completely dark vs. color pattern composed by vertical dark stripes, and by the infraorbital canal entering infraorbital series via sphenotic vs. infraorbital canal entering infraorbital series via compound pterotic (Ref. 80996).