Myloplus sauron Pereira, Ota, Machado, Collins, Ândrade, Garcia-Ayala, Jégu, Farias & Hrbek, 2024

Machado, Valéria N., Pereira, Victória D., Ota, Rafaela P., Collins, Rupert A., Ândrade, Marcelo, Garcia-Ayala, James R., Jégu, Michel, Farias, Izeni P. & Hrbek, Tomas, 2024, Integrative taxonomy of the black-barred disk pacus (Characiformes: Serrasalmidae), including the redescription of Myloplus schomburgkii and the description of two new species, Neotropical Ichthyology (e 230095) 22 (2), pp. 1-45 : 16-22

Myloplus sauron Pereira, Ota, Machado, Collins, Ândrade, Garcia-Ayala, Jégu, Farias & Hrbek , new species

urn:lsid:zoobank.org:act:ADA03DE2-2F3D-42B0-8860-67BEDC5B7A33

( Figs. 6–8 ; Tab. 4 )

Holotype: INPA 40824 , male, 166.39 mm SL, Brazil, Pará, Anapu municipality, rio Xingu , below the Tamaracá waterfall, entrance on the right bank of the river, parallel to BR-230, 03°07’48”S 51°36’50”W, 1 Oct 2013, M. H. Sabaj Pérez, L. M. Sousa, A. Gonçalves, N. K. Lujan, D. B. Fitzgerald & P. M. Ito.

Paratypes. All from Brazil: Pará: rio Xingu basin. Altamira: INPA 4151 , 1, 71.64 mm SL, ilha de Babaquara , rio Xingu , 03°18’14”S 52°12’37”W, 4 Oct 1990, L. H. R. Py-Daniel & J. Zuanon. INPA 30884 , 2, 56.56–67.06 mm SL, rio Iriri , close to its mouth in rio Xingu , 03°48’54”S 52°37’09”W, 15 Aug 2008, H. Lopez-Fernandez. INPA 31160 , 1, 106.63 mm SL, rio Iriri , down stream from mouth of rio Novo , 04°14’14”S 53°24’34”W, 22 Aug 2008, H. Lopez-Fernandez. MZUSP 105723 , 1, 110.48 mm SL, rio Xingu , 03°33’43.9”S 51°52’36.9”W, 6 Nov 2000, Eq. Ictiologia UFPA. MNRJ 35028 , 1, 158.7 mm SL, rio Curuá , northeast of Castelo dos Sonhos (22 km via BR plus 18 km via secondary road), 08°06’35”S 55°00’58” W, 30 Sep 2008, P. A. Buckup, J. Maldonado & C. Zawadzki. Anapú: INPA 40279 , 1 (x-ray), 109.97 mm SL, ( OR366896 ), rio Bacajá , 03°31’10”S 51°42’35”W, 15 Sep 2013, M. H. Sabaj. INPA 060148 , 1, 138.8 mm SL, rio Xingu , downstream of the Tamaracá waterfall, 03°07’48”S 51°36’50”W, 1 Oct 2013, M. H. Sabaj. MUBIO 109 , 1, 163.51 mm SL, rio Xingu , 3.5 km upstream from Praia do Caju , 03°24’29”S 51°43’03”W, 8 Nov 2014, M. H. Sabaj. Medicilândia: MZUSP 36827 , 2 of 4, 60.85–125.06 mm SL, rio Xingu at cachoeira do Espelho , 03°48’00”S 52°31’59.9”W, 23 Oct 1986, P. E. V. Vanzolini. Senador José Porfírio: INPA 47142 , 4 of 6, 38.6–62.77 mm SL, rio Bacajaí , tributaryof rio Xingu , 03°35’13”S 51°46’00”W, 9 Nov 2014, M. H. Sabaj. Uruará: INPA 31820 , 1, 110.8 mm SL, Maia community, rio Xingu , canal do Paletó , 03°31’35”S 51°45’04”W, 9 Nov 2008, L. H. R. Py-Daniel. Mato Grosso: Paranatinga: LBP 25971 , 1, 129.3 mm SL, rio Culuene , 13°50’48”S 53°15’39”W, 25 Jan 2018, N. Falusino Junior, N. Estevão & F. A. Machado.

Non-types. All from Brazil, Pará, rio Xingu basin. Altamira: INPA 4273 , 2, ilha de Kaituka , rio Xingu , 03°33’47”S 51°51’20”W, 8 Oct 1990, L. H. R. Py-Daniel & J. Zuanon. INPA 43638 , 1, 48.75 mm SL, rio Xingu, 03°33’18”S 52°21’24”W, 18 Mar 2014, I. M. Soares. INPA 47088 , 1, rio Xingu , 03°36’34”S 52°20’57”W, 4 Nov 2014, M. H. Sabaj. INPA 47284 , 1, 47.38 mm SL, rio Xingu , praia do Caju , 03°02’56”S 51°44’11”W, 7 Nov 2014, I. M. Soares. INPA 47568 , 6, ilha de Boa Esperança , rio Xingu , 03°33’44”S 52°21’22”W, 3 Nov 2014, M. H. Sabaj. INPA 47587 , 9, rio Xingu , praia Itapuama , 03°36’26”S 52°20’55”W, 3 Nov 2014, M. H. Sabaj. INPA 47793 , 5, rio Itatá , tributarity of rio Xingu , 03°37’15”S 51°49’15”W, 10 Nov 2014, M. H. Sabaj. Anapú: INPA 4076 , 1, 140.7 mm SL, rio Xingu , downstream from Volta Grande , 03°09’04”S 51°36’22”W, 28 Aug 2013, J. Zuanon. INPA 40363 , 1, rio Bacajaí , upstream to its mouth in rio Xingu , 03°35’30” S 51°45’56”W, 16 Sep 2013, M. H. Sabaj.

Diagnosis. Myloplus sauron can be readily distinguished from all congeners, except M. schomburgkii and M. aylan , by the presence of a vertical black bar on the flank commonly extending from the dorsal-fin base to the pelvic-fin distal end (vs. absence of any conspicuous mark on the flank in the rest of the congeners). The new species can be diagnosed from M. schomburgkii and M. aylan by having fewer branched dorsal-fin rays (17–19 vs. 20–25 in M. schomburgkii and 21–25 in M. aylan ), shorter dorsal-fin base (25.6–29.1% SL vs. 29.2–36.7% in M. schomburgkii and 31.1–37.1% in M. aylan ), greater dorsal-fin end to anal-fin end distance [27.8–32.0% SL (mean 29.7%) vs. 21.7–28.8% (mean 25.8%) in M. schomburgkii and 23.4–28.6% (mean 25.3%) in M. aylan ], longer adipose-fin base (7.1–9.7% SL (mean 8.4%) vs. 4.6–7.2% (mean 6.0%) in M. schomburgkii and 5.0–7.7% (mean 6.3%) in M. aylan ) and lower adipose-fin height (height 0.4–0.6 times in its base, vs. 0.5–1.0 in M. schomburgkii and 0.7–1.1 in M. aylan ). Additionally, the new species can be diagnosed from M. schomburgkii by presenting anterior ventral-keel spine at the vertical through pectoral-fin origin or anterior to it (vs. anterior spine of ventral-keel always inserted posterior to the vertical through pectoral-fin origin). Further, M. sauron differs from M. aylan by having 70–82 total of perforated scales on lateral line (vs. 82–95); fewer prepelvic (20–28 vs. 30–39) and total spines (29–40 vs. 38–55); 36–37 total vertebrae (vs. 40–41); serrae composed by short spines with wide bases (vs. long and with narrow bases); dorsal surface of parietal bone straight to slightly concave in lateral view (vs. markedly concave) and by abrupt anteroposterior decreasing of anal-fin rays length, forming narrow falcated anal-fin lobe in juveniles and females, restricted to the anterior half of the fin, not reaching the middle portion of anal-fin base length (see Sexual dimorphism) (vs. anteroposterior decreasing of anal-fin rays length almost uniform, forming broad lobe, occupying half of the anal-fin extension), with orange to reddish-orange pigmentation along its length, mostly concentrated on anterior rays (vs. conspicuous dark-red to black pigmentation on the entire anal fin).

Description. Morphometric data presented in Tab. 4 . Body compressed, overall shape with highest body depth at dorsal-fin origin. Predorsal length slightly longer than postdorsal length. Head rounded, eye at the center of the head. Snout short. Dorsal profile of head convex from mouth to horizontal through dorsal margin of the eye, and straight from this point to base of supraoocipital. Dorsal profile between supraoocipital base and dorsal-fin origin convex. Dorsal-fin base straight to slightly convex. Last dorsal-fin ray distal end not reaching adipose-fin origin when adpressed. Dorsal profile between dorsal-fin insertion and adipose-fin origin straight. Adipose fin longer than deep, with straight base. Dorsal and ventral profile of caudal peduncle concave. Ventral profile of head and body convex from lower lip to anal-fin origin. Anal-fin base straight to slightly convex.

Mouth terminal. Premaxillary teeth in two rows, outer row with 5*(50) molariform teeth, teeth 1–4 almost equal-sized, tooth 5 smaller, all with sharp, convex edges; inner row with 2*(50) equal-sized teeth with sharp, concave edges; in ventral view, contralateral outer rows forming a V-shaped arch with apex anteriorly pointed; contralateral inner rows forming straight line between the 3 rd teeth of outer series, space between rows forming triangular gap. Dentary with 5*(50) molariform teeth, teeth 1–3 substantially bigger than 4–5. Conical symphyseal tooth immediately behind tooth 1 of labial row. Maxilla edentulous.

Scales cycloid, small. Total of perforated scales on lateral line 70(1), 72(3), 73(2), 74(1), 75(6), 76(5), 77(5), 78(7), 79(3), 80(1), 81(1), or 82*(4). Dorsal-fin base covered by skin flap bearing one or two scale rows. Scale rows between dorsal-fin origin and lateral line 44(2), 46(2), 47(1), 48(4), 49(3), 50(6), 51(2), 52(3), 53(3), 54(3), 55*(2), 56(2), 57(2), or 59(2). Scale rows between lateral line and pelvic-fin origin 41(1), 42(2), 43(2), 44*(3), 45(5), 46(8), 47(2), 48(3), 49(3), 50(1), 51(4), 54(1), or 55(2). Adipose-fin base covered by five or six scale rows. Scales between adipose-fin origin and lateral line 18(1), 19(1), 21(4), 22(4), 23(5), 24*(14), 25(4), 26(1), or 27(3). Anal-fin base covered by five or six scales rows. Circumpeduncular scales 33(5), 34(5), 35(3), 36(6), 37*(3), 38(5), or 40(3).

Dorsal-fin origin slightly anterior to vertical through pelvic-fin origin. Dorsal-fin rays ii-iii, 17(6), 18*(32), or 19(11). Adipose fin rectangular. Pectoral-fin feather-shaped, anterior rays longest. Pectoral-fin rays i, 14(6), 15*(30), 16(5), or 17(3). Anterior pelvic-fin rays longest, not reaching vertical through last spines of serrae. Pelvic-fin rays i,7*(40). Last unbranched anal-fin ray most developed (longest and thicker). Anal-fin rays iii or iv, 30(1), 31*(6), 32(13), 33(15), 34(1), 35(4), or 36(1). Caudal-fin forked, with almost equal-sized lobes. Total gill rakers on first branchial arch 27(1), 28(6), 29*(12), 30(9), 31(7), or 32(2). Upper branch with 12(1), 13(3), 14*(16), 15(9), or 16(5) rakers; lower branch with 12(1), 13(4), 14*(21), or 15(11) rakers; 1*(38) at cartilage between cerato- and epibranchial.

Osteology. Dorsal profile of neurocranium convex from premaxillae to posterior margin of frontal bone, slightly concave to straight at parietal, convex from base to the tip of supraoccipital process. Lateral view of supraoccipital triangular. Supraneurals 4(1) or 5(9). Dorsal-fin pterygiophores 18(1), 19(1), or 20(8). First dorsal-fin pterygiophore inserted between neural spines of 9 th and 10 th (7) or 10 th and 11 th (3) vertebrae, more developed than remaining pterygiophores, with expanded anterior lamella, and bearing a forward-oriented Predorsal-spine. Predorsal spine somewhat similar to scythe, dorsal surface smooth; almost completely covered by skin. Anal-fin pterygiophores 32(1), 33(1), 34(4), 35(3), or 36(1) ( Fig. S2 ).

Total vertebrae 36(2) or 37(8); Weberian apparatus, 4(10); abdominal 14(1), 15(7), or 16(2) [pre-dorsal, 5(7) or 6(3); under dorsal-fin 9(2) or 10(8)]; caudal 17(3) or 18(7) [under dorsal-fin 3(4) or 4(6), posterior to dorsal-fin 13(1), 14(7), or 15(2)]. Anteriormost spine of ventral keel never reaching vertical through pectoral-fin origin. Short spines, with wide base and thin piercing tips. First prepelvic spines covered by skin. Postpelvic spines more developed than prepelvic spines. Total ventral keel spines 29(2), 32(4), 33(2), 34(6), 35*(10), 36(5), 37(5), 38(1), 39(1), or 40(1). Prepelvic spines 20(1), 21(3), 22(7), 23(10), 24*(7), 25(5), 26(3), or 28(1); unpaired post-pelvic spines 6(1), 7*(20), 8(11), or 9(4); and paired spines around anus 3(2), 4*(25), or 5(8).

Coloration in alcohol. Ground coloration light grayish brown dorsally, grading to light-yellow ventrally. Narrow, vertical, brown to dark-brown bar on middle of flanks, extending from region near dorsal-fin base to region near ventral-fin distal end, wider on medial portion, narrowing toward its distal ends ( Fig. 6 ). Inconspicuous plumbeous blot on humeral region above lateral line (most common in males). Head brownish gray antero-dorsally and light yellow postero-ventrally. Bright yellow sclera. Pectoral fins dark yellowish hyaline. Pelvic fins light yellow. Dorsal, anal and caudal fins overall yellowish hyaline. Caudal and adipose fins with diffuse brown pigmentation on basal portion. Anal and adipose fins presenting subtle dark line skirting its margins.

Coloration in life. Ground coloration silvery white. Iridescent bluish-green scales on dorsal portion of body, most concentrated above head. Subtle spot of melanophores on humeral region. Scattered patches of light orange-yellow pigmentation around head, covering markedly opercle, interopercle, branchiostegal membrane, and pectoral girdle. Dark vertical bar on flank similar to color in alcohol. Breeding females present a white area around the vertical dark bar. Vertical dark bar inconspicuous in dimorphic males (see Sexual dimorphism). Paired fins light yellowish hyaline. Dorsal and caudal fins overall yellowish orange, with diffuse dark pigmentation on interradial membranes from base to about two thirds of their length, resulting in an orange bar on their margins. Filaments extending dorsal-fin branched rays of breeding males, when present, dark brown. Anal-fin overall orange-yellow and somewhat hyaline, with subtle dark pigmentation along its length in adults; striking orange, with dark distal margins in juvenile ( Fig. 7 ). Adipose-fin orange-yellow and somewhat hyaline.

Sexual dimorphism. Mature males with two anal-fin lobes; first lobe at anterior rays, less developed; second lobe centered on 13 th or 14 th branched ray, about twice as long as first lobe. Females and juveniles present single falcate lobe, formed by remarkable prolongation of anterior rays. Mature males with long filaments extending dorsal-fin branched rays. Breeding males might present inconspicuous vertical black bar. Breeding females present well-marked bar surrounded by light area. Stiff hooks on distal-most lepidotrichia segment of anal-fin branched rays not observed ( Fig. 8 ).

Geographical distribution. Myloplus sauron is known only from the Xingu basin where it is widely distributed, also occurring in its main tributaries such as the Culuene, Iriri, and Bacajá rivers ( Fig. 4 ). However, this species has not yet been recorded below Volta Grande do Xingu rapids.

Ecological notes. Myloplus sauron is a rheophilic species only known from the Xingu River basin. The species feeds most on plant material being categorized as herbivore ( Andrade et al., 2019).

Etymology. The specific name sauron alludes to the Eye of Sauron, from J. R. R. Tolkien’s “The Lord of the Rings”. The elliptical body of Myloplus sauron , marked with a vertical, black bar tapering toward both ends, resembles the famous vertical-pupilled eye from the novel. A noun in apposition.

Conservation status. Myloplus sauron is a rheophilic species, restricted to the Xingu River basin and, together with its congener M. schomburgkii , is commercially exploited to a limited extent by fishing for the ornamental trade ( Prang, 2007; Isaac et al., 2015). Like most rheophilic fish in this basin, M. sauron may be seriously threatened by changes in its habitat caused by alterations in the course of the Xingu River after the construction of the Belo Monte hydroelectric plant, as the flow of the river changed in some stretches ( Fitzgerald et al., 2017, 2018). However, the species has a wide distribution within this basin, also occurring in tributaries less affected by the Belo Monte dam, such as the Iriri and Culuene rivers. Although some threats are detected in its range, M. sauron can be categorized as Least Concern (LC) according to IUCN categories and criteria (IUCN Standards and Petitions Subcommittee, 2022).

Figures

FIGURE 2 | Myloplus aylan, INPA 60150, holotype, male, 213.01 mm SL, Brazil, Roraima, Caracaraí municipality, rio Baraúna, lago do Bento.

FIGURE 4 | Geographic distribution of Myloplus schomburgkii (blue circles; the pink star at rio Negro basin represents the type-locality), M. sauron (red squares; the yellow star at rio Xingu basin represents the type-locality) and M. aylan (purple triangles; the black star at rio Branco basin represents the type-locality).

FIGURE 6 | Myloplus sauron, INPA 40824, holotype, male, 132.8 mm SL, Brazil, Pará, Anapu municipality, rio Xingu, downstream from the Tamaracá waterfall.

FIGURE 7 | Myloplus sauron, color pattern in life. Young specimen from Xingu River basin, specimen not preserved.

FIGURE 8 | Myloplus sauron, coloration immediately after capture. A. Male. Notice the faint vertical mark, common during breeding period. B. Mature female. Xingu River basin. Photo by M. H. Sabaj.