Prochilodus britskii Castro 1993

Dorsal spines (total): 3; Dorsal soft rays (total): 9 - 10; Analspines: 3; Analsoft rays: 8; Vertebrae: 40 - 41

Prochilodus britskii Castro, 1993

Prochilodus bristkii Castro, 1993:57, fig. 1 [type locality: Brazil, Mato Grosso, Rio Apiacá (tributary of Rio Armos, Rio Tapajós basin) N of city of Juará, upriver from a tall (∼10°36′S, 58°04′W)]—Oyakawa, 1996:492 [holotype ind paratype depository]

DIAGNOSIS.—Prochilodus bristkii is distinguished from all of i's congeners by its relatively shallow caudal-peduncle height (8.8% to 9.4% of SL versus 9.7% or greater in all other Prochilodus species), It is further distinguished from P. brevis, P. lacustris. P. mariue, P. nigricans, and P. rubrotaeniatus by the lack of dark, irregular, wavy, bar-like patterns on the caudal-fin lobes that are present in those five species. Prochilodus britskii may be separated from other Prochilodus species that lack dark caudal-fin pigmentation by the presence of 6 or 7 teeth in the inner tooth row of each side of the lower jaw versus 8 to 18 teeth in that series among most congeners with hyaline fins, with the exception of P. vimboides, which has 6 to 13 teeth in that series. Prochilodus bristkii differs from P. vimboides in various features, most distinctly in the number of scales along the lateral line (41 to 44 versus 34 to 39, respectively). Prochilodus britskii demonstrates two osteological features (the secondary tube that extends from the ventral border of the primary laterosensory canal in the first infraorbital and the single tubular ossification located ventral of the quadrate and anterior of the preopercle) that may represent autapomorphies for the species. We qualify that statement because of the distinctly larger size of the single cleared and stained specimen of P. britskii relative to the cleared and stained specimens of other members of the genus.

DESCRIPTION.—Morphometric and meristic data for Prochilodus bristkii presented in Table 7. Body relatively elongate, subcylindrical, greatest body depth at dorsal-fin origin. Caudal peduncle notably shallow. Dorsal profile of head slightly concave to straight. Dorsal profile of body slightly convex predorsally. Body profile slightly posteroventrally slanted along dorsal-fin base; slightly convex from posterior of dorsal- fin base to adipose-fin origin, and slightly concave along caudal peduncle. Predorsal portion of body with very slightly developed median ridge. Postdorsal portion of body transversely rounded. Ventral profile of body gently convex from tip of lower jaw to termination of anal-fin base, slightly concave along caudal peduncle. Prepelvic region moderately flattened transversely proximate to pelvic-fin insertion. Slight mid-ventral keel present between pelvic-fin insertion and anus.

Head pointed in profile. Mouth terminal. Snout length exceeding horizontal eye diameter. Nostrils of each side of head close together, anterior nares circular, posterior nares crescent shaped. Adipose eyelid present but scarcely developed; most pronounced anteriorly, but with greater portion of eye uncovered. Fleshy lips forming oral disk when protruded.

Functional teeth in two rows in each jaw. All teeth movably implanted in flesh that overlies jaws. All teeth of similar size, with exposed portion spoon shaped in frontal view, except when worn down. Inner tooth series in each jaw with 10 to 13 [13] teeth on left side of upper jaw and 6 or 7 [7] teeth on left side of lower jaw. Outer row of teeth in each jaw with approximately 73 teeth on each side of upper jaw and approximately 60 teeth on each side of lower jaw in holotype. Upper and lower lips bordered by numerous small, globular, fleshy papillae.

Scales spinoid. Scales along dorsal midline between posterior termination of dorsal fin and adipose-fin origin comparable in form to those of adjoining regions of body, without spatulate membranous process along posterior margin of each scale in that series. Lateral line with 41 to 44 (43% of specimens with 41, 29% with 42) [44] pored scales; 6 or 7 (71% of specimens with 7) [7] horizontal rows of scales from dorsal-fin origin to lateral line; 6 or 7 (57% of specimens with 7) [7] horizontal rows of scales from pelvic-fin insertion to lateral line; 5 or 6 (71% of specimens with 5) [6] horizontal rows of scales from anal-fin origin to lateral line; 13 or 14 (71% of specimens 14) [14] median predorsal scales; 13 to 15 [14] median scales between posterior of dorsal-fin base and adipose-fin origin; 13 or 14 [14] horizontal scale rows around caudal peduncle.

Dorsal fin preceded by small, anteroventrally bifurcate, procumbent spine. Dorsal-fin rays (including procumbent spine) iii,9 or 10 (iii,9 rare) [iii,10]; anal-fin rays iii,8 [iii,8]; pectoral-fin rays i, 13 or 14 (i,14 most frequent) [i,13]; pelvic-fin rays i,8 [i,8]; principal caudal-fin rays 10/9 [10/9].

Vertebrae 40 or 41 (40 most frequent).

Dorsal fin truncate distally; posterior unbranched and anterior branched rays longest and subequal. Dorsal-fin origin located nearer to tip of snout than to caudal-fin base. Longest length of adipose fin approximately equal to, or slightly longer than, horizontal width of orbit. Origin of adipose fin located along vertical that passes just anterior of posterior termination of anal-fin base. Pectoral fin distally pointed. Tip of adpressed pectoral fin reaching approximately two-thirds of distance between pectoral-fin and pelvic-fin insertions. Pelvic fin falcate. Pelvic-fin origin located along vertical that passes through midpoint of dorsal-fin base. Tip of adpressed pelvic fin reaching approximately four-fifths of distance to anus. Axillary scale present, pointed, its length about one-third of pelvic-fin length. Posterior unbranched and anterior branched rays of anal fin longest and subequal. Caudal fin forked.

COLOR IN ALCOHOL.—Ground coloration silvery yellow to silvery brown on dorsal half of body and head. Approximately seven very diffuse vertical bands on sides of body between head and caudal fin; bands formed by chromatophore fields without distinct limits. Lateral surface of body lacks wavy, dark, horizontal stripes present in many congeneric species. Field of black or brown chromatophores forming irregularly shaped spot on dorsal one-half of opercle.

Dorsal fin with irregularly distributed, diftiise, and barely visible small dark spots. Adipose fin with central region dusky and dorsal margin black. Pectoral, pelvic, and anal fins mostly hyaline, with distal portions somewhat dusky. Iris silvery yellow, with dorsal and ventral portions dusky.

COLORATION IN LIFE.—Shortly after its collection, the type series of Prochilodus britskii demonstrated strong reddish yellow coloration on the pelvic, anal, and caudal fins (Heraldo A. Britski, MZUSP, pers. comm., 1987).

DISTRIBUTION.—Prochilodus britskii is only known from the type locality, the Rio Apiaca, a tributary of Rio Arinos, which is, in turn, a tributary of Rio Juruena of the upper Rio Tapajóos system, Mato Grosso, Brazil (Figure 30, square). The species may have a more extensive distribution in that river basin, possibly occurring in the upper Rio Arinos basin at Fazenda Ferve- douro (14°13′S, 56°01′W), a locality where it, or a similar species, has been observed living at the base of rapids on that river (H.R. Mendes, Universidade de São Paulo; Ribeiráo Preto, pers. comm., 2001). Even if that reported range extension is confirmed, the known distribution of P. britskii still would be the most restrictive of any Prochilodus species.

COMPARISONS.—As noted in the “Diagnosis,” the relative caudal-peduncle depth serves to unequivocally distinguish P. britskii from its congeners. Prochilodus britskii also differs from all congeners in the overall form of its head and body.

MATERIAL EXAMINED.—7 specimens (7, 195.5–238.5 mm SL).

BRAZIL. Mato Grosso: Rio Apiaca, tributary of Rio Arinos, Rio Tapajós basin, N of Juara, upriver from a water fall (∼10°36′S, 58°04′W), MZUSP 38856–38861, 6 (195.5–238.5, paratypes of Prochilodus britskii; MZUSP 38856 (cleared and counterstained for cartilage and bone) [R]; MZUSP 41519, 1 (221.5 mm SL, holotype of Prochilodus britskii) [R].