Elacatinus phthirophagus differs from its congeners of the putative “randalli-evelynae” cleaning clade (sensu Taylor & Hellberg 2005) by the following combination of characters: mouth subterminal, pale (bright yellow in life) elliptical spot on dark snout, width of lateral pale (bright yellow in life) stripe almost equal as eye diameter (slightly narrower in live individuals), light purplish sheen (in life) extending laterally from below eye to tail origin, no blue line (in life) from below eye to end of opercular margin, teeth

multiserial on the distal portion of both jaws, males with 3 enlarged and recurved teeth on dentary inner row (Ref. 79614).

Body naked, elongate and slightly laterally compressed. Mouth subterminal, U-shaped. Teeth conical and slightly curved on both jaws (Ref. 79614).

Color pattern: bright yellow postocular stripe extending to the middle of caudal-fin; bright

yellow elliptical spot on blackish snout (the latter less pigmented in small juveniles); upper portion of eye bright yellow and lower part black; black lateral stripe from lower half of eye’s posterior edge to the middle of caudal-fin and more or less coincident with lateral septum (fading towards the end of caudal-fin); black dorsal stripe from middle of interorbital space extending in a curve to the anterior third of caudal-fin (in some large specimens the anterior part of this stripe is divided by a paler, grayish stripe); lower jaw and belly whitish, the anterior third of belly with a light purplish sheen; cheek and lower part of operculum to preoperculum rosy with purplish sheen; dorsal, pectoral and anal-fins, and outer border of caudal-fin pale with scattered dark chromatophores; pelvic-fin pale with no or fewer scattered chromatophores (Ref. 79614).