Pimelodella humeralis Slobodian, Akama & Dutra, 2017

Slobodian, Veronica, Akama, Alberto & Dutra, Guilherme Moreira, 2017, A new species of Pimelodella (Siluriformes: Heptapteridae) from the Guiana Shield, Brazil, Zootaxa 4338 (1), pp. 85-100 : 86-91

Pimelodella humeralis Slobodian, Akama & Dutra new species

Figures 1–3 , Table 1

Pimelodella sp.— Aleixo et al., 2011:20 [in list of species].

Holotype. MPEG 34994, 77.4 mm SL, xr, Brazil, Pará, Almeirim, Rio Ipitinga, Rio Jari basin, left bank tributary of the Rio Amazonas, 0°49'38.3"N 53°55'29.6"W, 28 Oct 2008, T. M. S. Freitas.

Paratypes. MPEG 15712, 3, 67.3-70.4 mm SL, xr (1, 77.4 mm SL); MZUSP 119908, 1, 73.0 mm SL, xr; USNM 432550, 1, 50.3 mm SL, xr, same data as the holotype. ANSP 203219, 2, 71.8-72.5 mm SL, xr; MPEG 15744, 5, 71.7-77.5 mm SL, xr (1, 77.5 mm SL), 1 c&s (71.0 mm SL); MZUSP 121686, 1, 68.3 mm SL, 1 c&s (76.1mm SL); USNM 432551, 1, 77.7 mm SL, Brazil, Pará, Almeirim, Rio Ipitinga, Rio Jari basin, left bank tributary of the Rio Amazonas, 0°49'0.4"N 53°55'43.6"W, 29 Oct 2008, T. M. S. Freitas.

Diagnosis. Pimelodella humeralis differs from all congeners by having a dark anteriorly oblique blotch in the humeral region ( Fig. 1 ), its anterior limit slightly ahead of vertical through the spinelet, and its posterior limit at vertical through the base of second branched dorsal-fin ray. Moreover, P. humeralis differs from most congeners (except P. breviceps , P. cristata , P. dorseyi , P. gracilis , P. ophthalmica , P. parnahybae , P. steindachneri , P. taenioptera , P wesselii and P. witmeri ) by having 47 to 49 total vertebrae (vs. 37 to 44 total vertebrae). Pimelodella humeralis differs from P. breviceps , P. cristata , P. dorseyi , P. gracilis , P. ophthalmica , P. parnahybae , P.

steindachneri , P. taenioptera and P. wesselii by having 8–11 retrorse triangular, broad-based and strongly inclined dentations confined to basal three-fourths of pectoral-fin spine posterior margin (vs. 11–25 retrorse, narrow-base dentations along almost entire pectoral-fin spine posterior margin in P. breviceps , P. cristata , P. dorseyi , P. gracilis , P. ophthalmica , P. parhanybae , P. steindachneri , P. taenioptera and P. wesselii ; condition unknown in P. witmeri ), and also by the presence (vs. absence) of unpigmented region dorsally and ventrally adjacent to the dark midlateral stripe. The new species is further distinguished from P. breviceps by maxillary barbels reaching at least the caudalfin insertion (vs. maxillary barbels not surpassing vertical through middle rays of anal fin); from P. steindachneri , P. taenioptera and P. wesselii by longest ray of dorsal-fin 18.7–22.5% SL (vs. 23.1–28.0% SL); from P. dorseyi and P. gracilis by the relatively longer dorsal-fin spine (spine 13.6–24.7% vs. 25.0–50.0% shorter than first dorsal-fin ray total length); from P. breviceps and P. taenioptera by having caudal-fin lobes subequal (vs. dorsal lobe distinctly longer); from P. gracilis by having dark midlateral stripe from pseudotympanum to the caudal-fin insertion narrow (vs. wide), and dorsal region of body lacking dark stripes (vs. paired diffuse dark stripes evident from posterior region of head to at least adipose-fin insertion); from P. ophthalmica P. parnahybae , P. steindachneri , P. wesselii and P. witmeri by the having dorsal fin pigmentation composed of hyaline stripe near base, followed distally by a dark stripe, another hyaline stripe and distalmost two-fifths dark (vs. dorsal fin dark near base, followed distally by a hyaline stripe and rest of fin darker; condition uncertain in P. breviceps , P. cristata , P. gracilis and P. taenioptera ).

Description. Measurements in Table 1. Body of moderate height, depth at dorsal-fin origin five times or more in standard length, and compressed, body width at dorsal-fin origin not exceeding half of head length ( Fig. 1 ). Greatest body depth at dorsal-fin origin. Dorsal profile straight from snout to dorsal-fin origin; slightly concave from that point to adipose-fin origin. Profile slightly convex along adipose fin base, and concave along caudal peduncle. Ventral profile of body slightly convex from snout to posterior limit of branchiostegal membrane, convex between branchiostegal membrane and pelvic-fin origin, slightly concave from the latter to anal-fin origin, and broadly concave from that point to caudal insertion.

Pseudotympanum large, oval, dorsal to posterior process of cleithrum and reaching 6th vertebrae (6). Posterior process of cleithrum triangular, its dorsal border straight to slightly concave. Axillary pore present at pectoral-fin base. Anus and urogenital papilla adjacent. Urogenital papilla short, triangular and tubular.

Head deep, depth at supraoccipital-process base half of head length. Mouth subterminal. Eye slightly elliptical, its greatest diameter along horizontal axis, placed dorsolaterally. Limits of eye well defined by a free orbital rim; margin distinctly invaginated, especially along anterior and dorsal portions. Bony interorbital distance smaller than eye diameter. Anterior naris tubular; posterior naris enclosed anteromedially by shallow flap. Premaxilla and dentary each with five or six rows of small villiform teeth. Barbels thin, and slightly depressed, elliptical in crosssection. Maxillary barbel finishing between verticals through anterior and posterior limits of caudal-fin, respectively. Outer mental barbel, when stretched parallel to main body axis, finishing between verticals through almost posterior limit of adpressed pectoral-fin and pelvic fins origin. Inner mental barbels, when lying stretched parallel to main body axis, finishing between verticals through pectoral-fin origin and posterior limit of pectoral-fin base, respectively. Branchiostegal membranes almost entirely free, united to isthmus only at medial apex and not joined to each other anteriorly. Branchiostegal rays 6 (8). Supraoccipital process relatively large, reaching anterior nuchal plate; shape subrectangular, distal third slightly tapered, sometimes with a discrete constriction near base. Dorsal lamina of Weberian complex vertebrae reaching supraoccipital process near its anteriormost part, or anterior half extension, but never along its entire length.

Dorsal fin triangular, distal margin convex, depressed tip finishing between verticals through second-fifth and last fourth of adpressed pelvic-fin, respectively. Dorsal-fin rays I,6 (17) plus anteriormost spinelet. Distance between terminus of dorsal-fin base and adipose-fin origin about a third of dorsal-fin base. Anteriormost dorsal-fin pterygiophore inserted posterior to neural spine of vertebra 4 (8); posteriormost dorsal-fin pterygiophore located ahead of neural (or pseudoneural) spine of vertebrae 11* (5) or 12 (1). Spinelet large with wide base and rounded distal tip. Unbranched dorsal-fin ray ( Fig. 2 ) mostly ossified as a spine; rigid part usually relatively long (approximately a fourth or less shorter than the first dorsal-fin ray total length); distal half of anterior margin with serrae. Posterior margin of dorsal-fin spine with serrae along distal third, and small, straight to retrorse, dentations along second-third of its length.

Pectoral-fin rays I,8 (6)– I,9* (11), pectoral fin triangular with concave distal border. First pectoral-fin ray curved with proximal portion rigid, forming a spine ( Fig. 3 ), and short distal tip flexible and distinctly segmented. Anterior margin of pectoral-fin spine with minute, smooth-sided, straight dentations along its basal half (except for a smooth anterior ridge near the spine base), and serrae along its distal half; posterior margin with 8–11 retrorse dentations along its basal three-quarters (10 dentations in holotype).

Pelvic-fin rays i,5 (17), extended pelvic fin triangular with straight distal border. Pelvic-fin origin at or slightly posterior to vertical through terminus of dorsal-fin base. Tip of adpressed pelvic fin surpassing adipose-fin origin, but never exceeding the first quarter of its base. First unbranched ray distinctly shorter than first and second branched rays, the latter two roughly the same size; remaining rays progressively shorter.

Anal-fin rays iv,10 (1), v,10* (9), vi,10 (5), v,11 (1) or vi,11 (11); distal border of extended anal fin convex. Two or three anteriormost anal-fin rays vestigial, unsegmented, embedded in thick skin fold. Anal-fin origin surpassing first quarter of adipose-fin base, but never exceeding its half; anal-fin adpressed terminus between verticals through 85 and 90% of adipose-fin base, respectively. Tip of anteriormost anal-fin pterygiophore inserted posterior to hemal spine of vertebra 22 (1), 23 (4) or 24* (3). Tip of posteriormost anal-fin pterygiophore inserted ahead of hemal spine of vertebra 31 (1), 32* (2) or 33 (5).

Adipose fin very long, twice in SL, forming ascending elevated curve in lateral profile with deepest point approximately at midlength. Adipose fin emerging gradually, its posterior limit as a rounded, free lobe. Adipose-fin origin at vertical through vertebral centra 14 (1), 16* (4) or 17 (3); adipose-fin terminus at vertical through vertebral centra 43 (8).

Caudal fin deeply forked, lobes subequal. Caudal peduncle length usually more than three times its depth or slightly longer. Dorsal lobe with 7* (16), rarely 8 (1) branched, 1 (17) unbranched principal, and 15 (1) to 20 (1) (holotype 17) procurrent fin-rays. Ventral lobe with 8* (16), rarely 9 (1) branched, 1 (17) unbranched principal, and 13 (1) to 25 (1) (holotype 20) procurrent fin-rays. Parahypural not fused to Hypural 1. Hypurals 1 and 2 completely co-ossified into single ventral caudal plate. Hypurals 3 and 4 completely fused to each other. Hypural 5 not fused to hypural 3+4. Single rod-like autogenous epural. Hypurapophysis and secondary hypurapophysis fused, forming a horizontal shelf (hypurapophysis type C of Lundberg & Baskin, 1969). Median caudal-fin rays not articulated directly to caudal plate. Seven (8) rays articulated to dorsal caudal-fin plate (5 on hypurals 3+4 and 2 on hypural 5) and 7 (4) or 8* (4) rays articulated to ventral caudal-fin plate (5 or 6 on hypurals 1+2 and 2 on parahypural). Total vertebrae 47 (1) to 49 (2), usually 48* (5). Ribs 9 (9).

Head laterosensory canals with simple (unbranched) tubes usually ending in single pore. Supraorbital canal continuous and connected to infraorbital branch anteriorly (forming complex s2+ i2 pore), and posteriorly to the otic branch. Supraorbital laterosensory canal with at least five branches: s1, s2, s3, s6 (epiphyseal branch), and s8 (parietal branch). Contralateral epiphyseal branches fused to each other medially, with single opening; a small canal towards posterior midline of head (13).

Holotype Min. Max. n x SD

Total length (mm) 93.5 60.3 95.8 17 87.1

Standard length (mm) 77.4 50.3 77.7 17 71.4

Percentages of SL

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Holotype Min. Max. n x SD Length of second left pelvic-fin ray (first branched) 11.9 11.3 14.8 16 13.0 1.0 Length of third left pelvic-fin ray (second branched) 12.8 11.2 14.0 17 12.9 0.8 Anal-fin base 13.4 11.6 14.4 17 13.0 0.9 Preanal length 65.1 63.2 66.7 17 65.0 0.9 Distance between snout tip and terminus of anal-fin base 77.4 76.5 79.9 17 78.0 0.9 Distance between snout tip and anal-fin distal end 20.7 17.8 21.5 16 20.2 0.9 Adipose-fin length 42.4 41.8 45.3 17 43.2 0.9 Preadipose length 49.1 49.1 53.2 17 50.7 1.2 Distance between snout tip and terminus of adipose-fin base 91.7 90.4 93.1 17 91.5 0.6 Adipose-fin depth 5.1 4.5 6.5 17 5.4 0.5 Caudal-peduncle length 22.5 19.4 23.8 17 22.4 0.9 Caudal-peduncle depth 7.3 6.7 7.7 17 7.3 0.3 Snout-anus distance 49.2 47.4 52.1 17 49.8 1.1 Snout-urogenital papilla distance 50.9 50.2 53.4 17 51.7 0.8 Anus-urogenital papilla distance 1.3 1.1 1.9 17 1.6 0.3 Dorsal lobe of caudal fin length 23.9 20.0 26.6 17 23.7 1.8 Ventral lobe of caudal fin length 24.9 21.9 26.3 17 24.5 1.2

Percentages of HL

Head depth 48.2 47.1 52.5 17 49.6 1.5 Head width 47.5 43.2 51.0 17 48.2 1.9 Eye diameter (left) 21.9 20.5 25.9 17 23.7 1.5 Fleshy interorbital 22.3 22.3 25.1 17 23.8 0.8 Bony interorbital 17.2 14.3 17.2 17 15.7 0.9 Mouth gape 24.8 24.7 29.9 17 26.4 1.4 Snout length (left) 35.9 32.2 37.4 17 35.1 1.4 Distance between snout tip and posterior nare (left side) 18.6 17.7 20.3 17 18.9 0.7 Anterior internarial width 12.5 12.5 15.2 17 14.0 0.8 Posterior internarial width 12.4 12.1 14.8 17 13.2 0.7 Intranarial length (left side) 14.3 11.9 15.7 17 13.7 1.1 Coloration in alcohol. Background body coloration yellowish ( Fig. 1 ). Ventral region of head and body lighter. Conspicuously dark humeral blotch, anterior limit slightly ahead of vertical through the spinelet and posterior limit at vertical through base of second branched dorsal-fin ray. Dark midlateral stripe well delimited, narrow, extending from region posterior to humeral blotch to caudal-fin insertion. Regions dorsally and ventrally adjacent to midlateral stripe unpigmented. Dorsal-fin base, especially near distal radials region, heavily pigmented. Dorsal fin darkly pigmented except for two hyaline stripes: one near its base and the other almost at its half. Dorsal surface of maxillary barbel darker than ventral surface.

Distribution and habitat. Pimelodella humeralis is only known from the Rio Ipitinga, a major right bank tributary of the Rio Jari which drains the eastern Guiana Shield into the lower Amazon ( Fig. 4 ). The Ipitinga headwaters are located in the Colinas do Amapá geomorphological formation (approx. 300 m elevation), where it has many rapids along the main channel. The specimens were collected along a sandy beach in the middle stretch of the river ( Fig. 5 ). Water parameters measured during the collection event were: 26.1–26.7ºC; pH 4.25–4.46; conductivity 76-77 mS.

Etymology. The specific epithet humeralis refers to the conspicuous dark blotch in the humeral area, a feature not observed in any other nominal species of Pimelodella . An adjective.

Figures

FIGURE 1. Pimelodella humeralis, holotype, MPEG 34994, 77.4 mm SL. Brazil, Pará, Rio Jari basin, Rio Ipitinga. Left lateral (A) and dorsal (B) views.

FIGURE 2. Left lateral view of dorsal-fin spine in Pimelodella humeralis, paratype, MZUSP 121686, 76.1 mm SL, total length of spine 11.3 mm.

FIGURE 3. Ventral view of left pectoral-fin spine in Pimelodella humeralis, holotype, MPEG 34994, 77.4 mm SL, total length of spine 12.0 mm.

FIGURE 4. Distribution of Pimelodella humeralis in Rio Ipitinga (1), northern South America; black circle represents more than one lot and locality.

FIGURE 5. Sand beach in Rio Ipitinga where Pimelodella humeralis was collected.