tending along the tips of 10–13 neural spines (Fig 3; vs. procurrent caudal-fin rays thin and flexible with the dorsal ones extending for less than eight neural spines tips). Trichomycterus ytororo can be further distinguished from T. crassicaudatus, T. igobi, and T. stawiarski by the number of branchiostegal rays (9 vs. 10–11). Trichomycterus ytororo can be further distinguished from T. igobi by the smaller head length (19.7–22.5% vs. 23.8–26.8% of SL), longest caudal peduncle (20.3–23.4% vs. 15.4–19.7% of SL) and 4–7 pores in the trunk canal of the laterosensory system (vs. two pores); from T. crassicaudatus by the caudal-fin distal margin rounded in adults (vs. forked), higher number of vertebrae (37–38 vs. 35–36), lower number of ventral procurrent caudal-fin rays (13 vs. 17–18), and the dorsal procurrent caudal-fin rays extending over the tips of 13 neural spines (vs. dorsal procurrent caudal-fin rays extending over the tips of 12 neural spines); and from T. stawiarski by the lower number of ventral procurrent caudal-fin rays (13 vs. 17), the higher number of pectoral-fin rays (i,7 vs. i,6), and lower number of vertebrae (37–38 vs. 39).
In addition to the above mentioned characters, Trichomycterus ytororo is distinguishable from congeners inhabiting the Paraná-Paraguay system by: 1) the number of pectoral-fin rays (i,7 vs. i,5 in T. mboycy Wosiacki & Garavello, T. naipi Wosiacki & Garavello, T. piratymbara Katz, Barbosa & Costa, T. taroba Wosiacki & Garavello; i,5–6 in T. davisi (Haseman), T. perkos Datovo, Carvalho & Ferrer; i,6 in T. castroi de Pinna, T. maracaya Bockmann & Sazima, T. paolence (Eigenmann), T. papilliferus Wosiacki & Garavello, T. pauciradiatus Alencar & Costa, T. pirabitira Barbosa & Azevedo-Santos, T. septemradiatus Katz, Barbosa & Costa; and i,8 in T. boylei (Nichols), T. dali Rizzato, Costa, Trajano & Bichuette, T. therma Fernández & Miranda). 2) First pectoral-fin ray not prolonged as filament (vs. first pectoral-fin ray prolonged as a long filament beyond the distal margin of the fin in T. aguarague Fernández & Osinaga and T. taroba. 3) The color pattern of lateral surface of body composed of black blotches over a pale yellow background extending to the mid-length of caudal fin (vs. lateral surface of body gray in T. plumbeus Wosiacki & Garavello and T. papilliferus; with well-defined spots in T. iheringi (Eigenmann); and proximal region of the caudal fin with a vertical unpigmented band in T. diabolus Bockmann, Casatti & de Pinna).
SD: standard deviation. Range, Mean and SD include holotype.
Body relative short, approximately rounded in transverse section at pectoral-fin level and laterally compressed towards tail. Dorsal profile from anterior margin of snout to dorsal-fin origin slightly convex; approximately straight from caudal peduncle to last caudal-fin rays. Ventral profile from tip of snout to pelvic fin slightly convex; straight from this point to last caudal-fin rays. Head relatively small, almost as long as wide, depressed from lateral view and trapezoidal from dorsal view. Snout broad, distal margin slightly convex from dorsal view. Dorsal and ventral profile of head slightly convex.
Eyes dorsally located on head but also visible from lateral view, anteroposteriorly elliptical, covered by thin and transparent skin. Anterior nostril smaller than eye, surrounded by fleshy flap of integument posterolaterally continuous with nasal barbel; distance between anterior nostrils slightly shorter than interorbital distance. Posterior nostril smaller than anterior one, partially surrounded anteriorly, laterally, and medially by thin flap of skin. Posterior nostrils more closely positioned to each other than anterior ones.
All barbels wider at bases and gradually narrowing towards tips. Nasal barbel extending to eye, not surpassing its posterior margin. Origin of nasal barbel on posterolateral margin of integument flap of anterior nostril. Maxillary and rictal barbels extending along cheek to vertical through eye, not reaching interopercular patch of odontodes. Origin of maxillary and rictal b