Diagnosis: This species of Paretroplus is distinguished from all congeners except its sister taxon, P. damii, by the presence of a triangular, black pectoral-axil patch in combination with the absence of a series of prominent vertical bars on the flank; in life, unstressed individuals of these two species can easily be distinguished from all congeners by the presence of a broad, vertical brick red or pale yellow band on the anterior half of the flank (Ref. 86507). Paretroplus loisellei is distinguished from P. damii by the presence of a more or less straight gular region in lateral view, vs. rounded and convex in P. damii, such that the ventral margins of the preopercle and interopercle are essentially horizontal; in P. loisellei the oral jaws are horizontally oriented, vs. strongly oblique; the lips are enlarged and fleshy, vs. thin; and the snout is elongate and curved, creating a beaked appearance, vs. short and straight; the enlarged symphyseal teeth in the upper jaw possess distinct, sharp lateral cusps, vs. poorly developed and rounded; owing to convex premaxillary pedicels, Paretroplus loisellei exhibits a prominent indentation marking the transition from the dorsal margin of the premaxillary ascending process to the supraoccipital crest (Ref. 86507). The base colouration in P. loisellei is uniform dark brownish-gray in preservation; whereas P. damii is considerably lighter golden brown overall and frequently retains a faint, pale broad band on the anterior region of the flank; Paretroplus loisellei exhibits a distinctive bright golden breeding colouration, vs. brick red (Ref. 86507).

Description: A relatively shallow bodied Paretroplus belonging to the 'Paretroplus damii clade', which also includes P. damii, P. nourissati, P. tsimoly, and P. lamenabe (Ref. 86507). Snout pronounced and beak-like owing to elongate and convex premaxillary ascending process; head pointed, snout curved/convex, and predorsal profile moderately to strongly curved/convex; prominent indentation present just anterior to orbit, owing to curved premaxillary pedicels; dorsal body outline slightly curved, and ventral outline mostly straight, except posteriorly; caudal peduncle short, deep, and strongly laterally compressed (Ref. 86507). Total vertebral count 31 or 32, with formulae of: 15+16, 15+17 and 16+16 precaudal and caudal vertebrae, respectively (Ref. 86507). Jaws isognathous and horizontally oriented; lips of both upper and lower jaws enlarged and fleshy; single row of anteroposteriorly flattened, spatulate, unicuspid teeth in both upper and lower jaws; premaxillaery symphyseal teeth greatly enlarged with small, yet distinct and sharp lateral cusps; other upper jaw teeth graded in size and becoming progressively much smaller posterolaterally; lower-jaw teeth at symphysis reduced in size compared to adjacent lateral teeth, presumably to accomodate enlarged upper symphyseal teeth when jaws closed; teeth flattened at crown and procumbently implanted; teeth in upper jaw number six or seven on each side, and total 12 to 13; teeth in lower jaw number six or seven on each side, and total 12 to 13; teeth in both upper and lower jaws in general irregularly spaced, particularly posteriorly in both jaws (Ref. 86507). Upper and lower pharyngeal tooth plates well developed and robust; dentition on lower pharyngeal tooth plates hooked and bicuspid both laterally and anteriorly, becoming progressively enlarged medially; molariform teeth present only posteromedially on lower pharyngeal jaws; lower pharyngeal jaw well sutured, with numerous interdigitating sutures on posteroventral margin; seven to nine robust tooth plates cover majority of dorsal surface of fourth ceratobranchial bones; fourth ceratobranchial tooth plates not confluent with outer-row gill rakers of these elements; bases of individual tooth plates exhibit varrying degree of fusion depending on the individual; dentition on fourth ceratobranchial tooth plates unicuspid or weakly hooked and bicuspid laterally, hooked and bicuspid medially; dentition on third upper pharyngobranchial tooth plates molariform only posteromedially, hooked and bicuspid laterally and anteromedially; dentition on second pharyngobranchial tooth plates hooked and bicuspid, and arrayed in three, rarely four, rows (Ref. 86507). Twelve or 13 triangular and moderately elongate gill rakers arrayed along lower limb of firt gill arch; rakers of first arch weakly denticulate dorsomedially; all other lower-limb rakers short, triangular in shape, and strongly denticulate dorsally; epibranchial rakers elongate, numbering 11 to 12 (Ref. 86507). Body covered with large, regularly imbricate, cycloid scales; caudal margin of flank scales thin, unossified and lacking circuli, creating a thin, flexible 'flap'; well-developed ridges of scales present along dorsal- and anal-fin bases; scale ridges free from spiny dorsal and anal fins, but becoming weakly attached to both soft dorsal and anal fins, such that three or four rows of smaller scales present caudally on both soft dorsal and anal fins; numerous rows of smaller scales extend onto caudal fin, posterior of hypural flexure, for approximately 1/3 of its length; Pelvic axillary scale well developed and elongate; lateral-line scales number 38 or 39; chest scales reduced in size and embedded, with scales along ventrum extremely reduced in size; four to five rows of scales on cheek; opercle, subopercle, and interopercle fully scaled; snout, lacrimal, and rostral portion of interorbital region asquamate; preopercle asquamate ventrally, scaled only on anterodorsal portion of shaft where posteriormost cheek scales overlap (Ref. 86507). Dorsal fin with XVIII-XX spines and 11-13 soft rays; anal fin with VIII-X spines and 9-11 soft rays; origin of dorsal fin at level of vertical through pectoral-fin insertion at its dorsal insertion point; caudal fin weakly emarginate; upper and lower lobes quite broad and rounded; pectoral fin large, broad, and rounded at distal margin; distal margins of soft dorsal and anal fins becoming increasingly pointed in larger specimens; distal margins of both fins extend well past caudal origin; pelvic fin extending to about level of anal-fin origin or just beyond origin (Ref. 86507).

Colouration: Colouration in life: base body colouration golden brown, grayish, or dark grayish-black; triangular black patch present in pectoral-fin axil and extending dorsal to pectoral-fin base; unpaired fins olive-grey to grayish-black; pectoral fin brown to grayish-black; pelvic fin gray, grayish-black, or black; breeding colouration bright golden yellow, particularly ventral to dorsal midline (Ref. 86507). Colouration in preservative: dark grayish brown to dark rusty brown; body somewhat darker dorsally, particularly along base of dorsal fin and dorsal portion of head; triangular black patch characteristic of clade present in pectoral-fin axil and extending dorsal to pectoral-fin base; lips pale yellow to light olive, particularly ventrally; dorsal margin and posterior portion of upper lip dark gray to grayish-black; unpaired fins gray to grayish-black; pectoral fin pale yellow to olive; pelvic fin grayish-black to black (Ref. 86507).