Mylossoma unimaculatum ( Steindachner, 1908 ) Mateussi & Oliveira & Pavanelli, 2018

Mateussi, Nadayca T. B., Oliveira, Claudio & Pavanelli, Carla S., 2018, Taxonomic revision of the Cis-Andean species of Mylossoma Eigenmann & Kennedy, 1903 (Teleostei: Characiformes: Serrasalmidae), Zootaxa 4387 (2), pp. 275-309 : 299-304

Mylossoma unimaculatum ( Steindachner, 1908) comb. nov.

( Figs. 20–23 ; Table 5)

Metynnis unimaculatus Steindachner, 1908: 326 [original description; type-locality: "Lagune am Rio Medonho, einem Nebenfluβ des Parnahyba nördich von Sa. Filomena"].— Zarske & Géry, 1999: 188 [cited].

Mylossoma duriventre (non Cuvier, 1818). — Santos et al., 2004: 65 [lower rio Tocantins].— Lucinda et al. 2007: 78 [Lajeado Reservoir, rio Tocantins, Tocantins]— Mérona et al. 2010: 106 [Lower rio Tocantins, fisheries].— Ferreira et al. 2011: 280 [Cantão State Park, rio Araguaia].

Diagnosis: Mylossoma unimaculatum differs from M. aureum by presenting the last abdominal spine reaching the anal-fin origin or almost (vs. last abdominal spine clearly separated from anal-fin origin), 37 vertebrae (vs. 38–39) and a conspicuous black blotch on opercle (vs. black blotch absent or inconspicuous). It differs from M. albiscopum by presenting 28 to 32 branched rays on the anal fin (vs. 31 to 38), 34 to 40 circumpeduncular scales (vs. 28 to 36) and 95 to 100 perforated scales in the lateral line (vs. 74 to 99). It differs from M. duriventre by presenting 34 to 40 circumpeduncular scales (vs. 30 to 34), 95 to 110 perforated scales on lateral line (vs. 74 to 97) and 37 vertebrae (vs. 35–36).

Description: Morphometric data presented in Table 5. Body deep, compressed laterally. Dorsal profile concave at posterior region of head and convex between head and dorsal fin. Ventral profile convex; slightly concave at isthmus and strongly convex from this point to end of anal fin. Highest body depth on vertical line passing through dorsal- and pelvic-fin origins. Caudal peduncle deeper than longer.

Eyes lateral, at middle of head; upper margin of eyes below longitudinal axis of lateral-line origin. Frontal and parietal fontanel broadly expanded laterally. Snout short and rounded on lateral view. Nostrils dorsolateraly positioned, above longitudinal axis through upper margin of eye, between tip of snout and orbits.

Mouth terminal to slightly upturned, at same level of orbits. Premaxilla projected forward, with inner premaxillary row over or surpassing dentary teeth. Premaxillary teeth molariform with robust base; outer portion of anteriormost three teeth of outer row and posterior portion of remaining teeth pointed. Inner premaxillary row with two teeth, separated from outer row. Outer premaxillary row with five teeth. Dentary with four or five modified tricuspid, robust, molariform teeth; symphyseal dentary teeth present, with robust base and large conical cusp, behind main row ( Fig. 2 ). Maxilla edentulous.

First gill arch with elongated and conical gill rakers; epibranchial gill rakers 12(4), 13(3) or 14(13); one (21) gill raker at cartilage between epi- and ceratobranchial; ceratobranchial gill rakers 15(2), 16(6), 17(3), 18(8) or 19(2).

Body completely covered by small cycloid scales. Accessory scales covering intersections of major scales, mainly at anterior portion of body. Lateral line complete, with 95(2), 96(1), 97(1), 98(5), 99(1), 100(2), 102(1), 103(1), 104(2), 105(3), 106(1), 107(3), 108(1) or 110(2) perforated scales, extending into caudal fin. Scales above lateral line 56(2), 59(1), 60(3), 61(1), 62(4), 63(3), 64(3), 65(1), 66(1), 67(1), 68(1) or 72(1). Scales below lateral line 46(2), 51(1), 53(2), 54(3), 55(4), 56(2), 57(1), 58(1), 63(2) or 64(1). Circumpeduncular series of scales 34(2), 36(14), 38(9) or 40(3). Ventral keel with well-developed spines forming high serra. Prepelvic spines 29(1), 30(2), 31(3), 32(10), 33(3), 34(2), 35(4), 36(3), 37(2) or 38(1). Postpelvic spines 12(4), 13(4), 14(1), 15(12) or 16(8). Anal spines 6(7), 7(13), 8(8), 9(3) or 10(1).

Dorsal fin not preceded by spine, its origin equidistant from tip of snout and end of hypural plate. First dorsalfin ray much shorter than second ray; in some specimens covered by skin, in such cases discernible only if dissected or x-rayed. Branched dorsal-fin rays gradually decreasing in size; dorsal-fin rays 12(1), 13(2), 14(6), 15(21) or 16(1). Adipose fin short, entirely covered by small scales. Pectoral fins falcate, with i(32) + 13(4), 14(4), 15(16), 16(7) or 17(1). Pelvic fin small, with i(31) + 5(3) or 6(28) branched rays. Anal fin long, with convex edge, median and posterior rays longer than anterior ones, not lobed; sheath of scales covering at least one-third of analfin length; anal-fin rays 28(3), 29(9), 30(13), 31(4) or 32(2). Caudal fin bifurcated, lobes of similar size, with 16(1), 17(25), 18(5) or 19(2) rays. Vertebrae 37(11). Supraneurals 4(5) or 5(2).

Color in alcohol. General body color light brown, darker dorsally ( Fig. 21 ). Fins pale yellow; often with bases of caudal and anal fins and distal margin of anal fin dark. Conspicuous black or brown blotch on opercle. Juvenile specimens presenting several vertical narrow, brown bands on flanks and a round, pale brown, inconspicuous blotch below dorsal fin, sometimes absent ( Fig. 22 ).

Color in life. General body color silvery, darker dorsally; orange to reddish tint on head and around the eyes. Presence of a black blotch on opercle. Anal fin with yellow and orange pigmentation, and a black band at distal margin; remaining fins hyaline ( Fig. 23 ).

Geographic distribution. Mylossoma unimaculatum is restricted to the rio Tocantins-Araguaia system, Amazon basin, Brazil ( Fig. 3 ).

Ecological notes. According to Santos et al. (2004), Mylossoma unimaculatum inhabits rivers margins and lagoons, feeding primarily on leafs, fruits, seeds and invertebrates, and presents total spawning during the flood season. Given its migratory behavior, Mylossoma unimaculatum was one of the most affected species after the establishment of the Tucuruí reservoir, since the river impoundment prevents the upstream reproductive migration and also limits the recolonization of downstream stretches ( Mérona et al. 2010).

Molecular data. DNA barcoding showed that the genetic distance between Mylossoma unimaculatum ( M. duriventre "group 4" in Mateussi et al. 2016) and the remaining analyzed species ranged from 0.014 to 0.091, being more similar genetically to M. duriventre ( M. duriventre "group 3" in Mateussi et al. 2016).

Remarks. Mylossoma unimaculatum was previously identified from the Tocantins-Araguaia basin as Mylossoma duriventre (e.g. Santos et al. 2004).In the original description, Steindachner (1908) indicated a lake at rio Medonho, a tributary to the rio Parnaíba (an independent river system flowing at the northeastern Brazilian States of Maranhão and Piauí), as the type-locality. However, this is the only record of Mylossoma specimens for this river basin. Ramos et al. (2014) have undertaken an extensive survey of the fish fauna from the rio Parnaíba basin, and listed the genus Mylossoma as occurring in this basin based solely in the literature. During the " Brasilien Expedition" directed by F. Steindachner in 1903, when the type specimens of Metynnis unimaculatus were collected, not only the rio Parnaíba but also the lower rio Tocantins was sampled, including fish markets at the rio Tocantins ( Böhme 1996), where Mylossoma unimaculatum is relatively common. We thus consider the typelocality given by Steindachner (1908) as very likely a mistake and we suggest that the type-locality of Metynnis unimaculatus to be more likely somewhere at the lower rio Tocantins basin.

Material examined. Type material. NMW 56451, 3, 75.9–78.1 mm SL, lake at rio Medonho, Santa Filomena , Brazil [type-locality probably incorrect, see above]. Syntypes of Metynnis unimaculatus . Non-type material. Brazil. Rio Tocantins-Araguaia basin. Goiás: INPA 46074 , 5 (1 sk), 156.7–184.0 mm SL, Aragarças , rio Araguaia , 15°53'25"S 52°14'25"W, 0 4 May 2014. R. R. Oliveira & J. L. Oliveira . MCP 17282, 1, 184.8 mm SL; MCP 17283, 1 (rd), 233.3 mm SL; MCP 17284, 1 (rd), 201.5 mm SL; MCP 17285, 1 , 256.0 mm SL, Luís Alves, floodplain lakes of the rio Araguaia , 13°14'00"S 50°34'59"W, 21 Apr 1994 . MCP 44841, 1 (rd), 226.4 mm SL, Nova Crixás, rio do Peixe (trib. rio Araguaia ), c. 14°11'S, 50°23'W, 13 Jun 2008 . NUP 13023, 3, 80.3–85.7 mm SL, São Miguel do Araguaia, lago Piratinga, rio Araguaia basin, 13°04'10"S 50°35'06"W, 0 4 Dec 2011, Nupélia team.

NUP 13043, 1, 84.1 mm SL, Nova Crixás, rio Araguaia , 13°21'53"S 50°37'46"W , 0 2 Nov 2011, Nupélia team. NUP 13051, 1, 90.0 mm SL, São Miguel do Araguaia, rio Crixás–Açu (trib. rio Araguaia ), 13°20'33"S 50°36'41"W , 0 2 Nov 2011, Nupélia team. NUP 13087, 2, 71.8–104.0 mm SL, Nova Crixás, rio Crixás-Açu (trib. rio Araguaia ), 13°21'42.20"S 50°36'22.50"W , 0 2 Nov 2011, Nupélia. Mato Grosso: LBP 1823, 2, 177.9– 188.1 mm SL, Barra do Garças, rio Araguaia, 15°32'S 52°12'W , 27 Jul 2003, C. Martins et al. LBP 8854, 1, 59.5 mm SL, Cocalinho, Lagoa da Montaria, rio Araguaia , 13°22'36.1"S 50°40'08.4"W , 27 Sep 2009, R. Devidé et al. . LBP 15286, 1, 75.4 mm SL, Cocalinho, lagoa da Boca Franca, rio Araguaia , 13°48'58"S 51°10'46"W , 0 6 Sep 2008, J. A. Senhorini et al. LBP 18462, 5 (rd), 86.8–112.8 mm SL, Cocalinho, rio Araguaia , 13°18'37.3"S 50°36'47.6"W , 29 Sep 2009, R. Devidé et al. . MZUSP 20419 , 2 (rd), 199.6–214.8 mm SL, Cocalinho, lago Rico, rio Araguaia , 14°22'00"S 51°00'00"W , EMGOPA. NUP 12745, 1, 72.1 mm SL, Cocalinho, lago Goiaba, rio Araguaia , 12°50'54"S 52°32'02"W , 0 5 Dec 2011, Nupélia team. NUP 13030, 1, 72.2 mm SL, Cocalinho, lago Varal, rio Araguaia , 13°00'52"S 50°36'08"W , 0 3 Nov 2011, Nupélia team. Pará: UNT 11125, 1 (rd), 108.4 mm SL, Marabá, rio Taurizinho , 05°22'33"S 49°00'53"W , 17 Dec 2009, Neamb team. Tocantins: INPA 20489 , 1 , 176.2 mm SL, Caseara, lago das Ariranhas, rio Araguaia , 09°20'S, 50°00'W , 23 Feb 2000, INPA Ictiofauna team.

Figures

FIGURE 2. Mylossoma albiscopum premaxillary (pmx) and dentary (den), INPA 45467, Careiro da Várzea, lagos do Catalão, Amazonas, Brazil.

FIGURE 21. Mylossoma unimaculatum, LBP 1823, 188 mm SL, rio Araguaia, Barra do Garças, Mato Grosso, Brazil.

FIGURE 22. Mylossoma unimaculatum, NUP 13030, 76 mm SL, juvenile, Lago Varal, tributary to the rio Araguaia, Goiás, Brazil.

FIGURE 23. Mylossoma unimaculatum, rio Araguaia, Brazil, specimen freshly collected, not preserved. Photo by Marcos Prado.

FIGURE20.Mylossoma unimaculatum, syntypeof Metynnisunimaculatus, NMW56451, 75 mm SL, lake at rioMedonho, rio Parnaíba basin, Brazil (type-locality incorrect, see text). Photo by Anja Palandacic.

FIGURE3. Geographic distributionof the four Cis-Andean Mylossomaspecies.One dot may represent more than one locality. Red circles: M. albiscopum; yellow triangles: M. aureum; purple squares: M. duriventre; orange diamonds: M. unimaculatum. Mylossoma aureum is also recorded for the Río Orinoco basin (see text).