Systematic revision of the Neotropical catfish genus Scleronema (Siluriformes: Trichomycteridae), with descriptions of six new species from Pampa grassland

Juliano Ferrer¹ and Luiz R. Malabarba¹

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Abstract​

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Introduction​

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Trichomycteridae is a monophyletic group of freshwater catfishes endemic to the Neotropical region which is classified into eight subfamilies: Copionodontinae, Glanapteryginae, Sarcoglanidinae, Stegophilinae, Trichomycterinae, Trichogeninae, Tridentinae, and Vandelliinae (de Pinna, Wosiacki, 2003). Trichomycterids are easily recognized by their highly modified opercular system, involving the interopercular and opercular bones equipped with odontodes (de Pinna, 1998; Adriens et al., 2010), and comprise the second richest family of Siluriformes with 328 species (Fricke et al., 2020). The subfamily Trichomycterinae contains the majority of these species, being composed of nine genera: Bullockia Arratia, Chang, Menu-Marque, Rojas, 1978, CambevaKatz, Barbosa, Mattos, Costa, 2018, Eremophilus Humboldt, 1805, Hatcheria Eigenmann, 1909, Ituglanis Costa, Bockmann, 1993, Rhizosomichthys Miles, 1943, Scleronema Eigenmann, 1917, Silvinichthys Arratia, 1998, and Trichomycterus Valenciennes, 1832.

Though trichomycterines are usually known to preferentially occur in mountain rivers and rapids, species of Scleronema (Tab. 1) occupy areas of low altitudes in the subtropical region of the La Plata basin (Paraná, Paraguay, and Uruguay rivers) and coastal drainages from Southern Brazil and Uruguay. Their distributions encompass almost the entire area of the South American “Campos” – called “Pampa biome” in Brazil according to IBGE (2004, ²⁰¹⁹) – an ecological region composed mainly of natural grasslands and herbs with sparse shrubs and tree formations generally in the banks of rivers and streams (Pallarés et al., 2005). Among the Brazilian biomes, the Pampa has the lowest percentage of legally protected areas and, according to ICMBio (2016), having had the fewest number of issued research permits.

TABLE 1 | Species of Scleronema with authors, name-bearing types and type localities.

Scleronema has long been defined by the large base of the maxillary barbel and by the presence of a skin flap in the posterior margin of the opercle, two unusual characters proposed by Eigenmann (1917) and corroborated by subsequent authors (e.g., Eigenmann, 1918; Tchernavin, 1944; de Pinna, 1989, ¹⁹⁹⁸; Arratia, 1990). Hitherto, some phylogenetic analyses using morphological (Wosiacki, 2002) and molecular data (Ochoa et al., 2017a; Katz et al., 2018) recovered the monophyly of Scleronema.

Currently, the genus Scleronema encompasses three sand-dwelling species: S. angustirostre (Devincenzi, 1942), S. minutum (Boulenger, 1891), and S. operculatumEigenmann, 1917. However, the diversity of the genus seems to be larger in view of the number of undescribed taxa listed for the genus in fish catalogs (de Pinna, Wosiacki, 2003; Becker et al., 2013; Bertaco et al., 2016). Even identifications of Scleronema species given in papers focused on higher-level relationships within Trichomycteridae (Wosiacki, 2002; Ochoa et al., 2017a; Katz et al., 2018), as well as some species recorded in local fish inventories (e.g., Burns et al., 2015; Corrêa et al., 2015) are uncertain due to the lack of a taxonomic review of the genus. To add some uncertainty to species definition and boundaries, information on type localities and vouchers for the three species of Scleronema given in checklists of trichomycterids or siluriforms (i.e., de Pinna, Wosiacki, 2003; Ferraris, 2007; Wosiacki, de Pinna, 2007) are inaccurate, therefore requiring revisions.

In this paper, we provide a systematic revision of the genus Scleronema, advanced by the first author during his Doctoral Thesis (Ferrer, 2016), based on the examination of all available type specimens and on a dense sampling of new specimens from the entire geographical distribution of the genus, as well as other trichomycterids.

Material and methods

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Specimens examined are housed at the following fish collections: AMNH, American Museum of Natural History, New York, U.S.A.; ANSP, Academy of Natural Sciences of Drexel University, Philadelphia, U.S.A.; BMNH, Natural History Museum, London, U.K.; FML, Fundación Miguel Lillo, Tucumán, Argentina; FMNH, Field Museum of Natural History, Chicago, U.S.A.; LIRP, Laboratório de Ictiologia de Ribeirão Preto, Faculdade de Filosofia, Letras e Ciências Humanas, Universidade de São Paulo, São Paulo, Brazil; MACN, Museo Argentino de Ciencias Naturales Bernardino Rivadavia, Buenos Aires, Argentina; MCP, Museu de Ciências e Tecnologia, Pontifícia Universidade Católica do Rio Grande do Sul, Porto Alegre, Brazil; MHNM, Museo Nacional de Historia Natural y Antropología, Montevideo, Uruguay; MLP, Museu de La Plata, Instituto de Limnologia, La Plata, Argentina; MPEG, Museu Paraense Emílio Goeldi, Belém, Brazil; MNRJ, Museu Nacional, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil; MZUSP, Museu de Zoologia, Universidade de São Paulo, São Paulo, Brazil; UFRGS, Departamento de Zoologia, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil; UMMZ, University of Michigan Museum of Zoology, Ann Arbor, U.S.A.; UNICTIO, Laboratório de Ictiologia, Universidade do Vale do Rio dos Sinos, São Leopoldo, Brazil; USNM, National Museum of Natural History, Smithsonian Institution, Washington D.C., U.S.A.; ZVC-P, Departamento de Zoologia Vertebrados, Universidad de Montevideo, Montevideo, Uruguay.

Specimens identified by “c&s” were cleared and counter-stained for bone and cartilage according to protocol of Taylor, Van Dyke (1985); some of these were dissected on the right side of the head. Radiographs (“xr”) were taken of some specimens and additional ones were obtained in the Database of the All Catfish Species Inventory (Morris et al., 2006). Osteological illustrations were prepared based on photographs of c&s specimens in a stereomicroscope with a Nikon AZ100M camera attached.

Morphometrics. Measurements were taken point-to-point using a digital caliper to the nearest 0.1 mm and presented as percentages of standard length (SL) or head length (HL) for its subunits. Measurements follow Tchernavin (1944) for barbel length; Bockmann, Sazima (2004) for total length, body width, distance between snout tip and posterior nare, intranarial length, anterior internarial width, and posterior internarial; Wosiacki, de Pinna (2008) for length and depth of the caudal peduncle and for supraorbital pore s6 distance; and Ferrer, Malabarba (2011) for scapular girdle width. Remaining measurements follow Costa (1992).

Counts. Vertebral count excludes those in the Weberian complex; the compound caudal centrum was counted as a single element. Abdominal vertebrae are those without haemal arch and haemal spine, sensude Pinna (1998) explained by Datovo, Bockmann (2010: 237). Counts of fin rays include the unbranched ones plus branched ones. Number of branchiostegal rays, odontodes, pterygiophores, ribs, teeth, vertebrae and procurrent rays (those anterior to unbranched rays of dorsal, anal, and caudal fins) were counted only in c&s and xr specimens. Rays of paired fins and branchiostegal rays were counted in both sides of specimens and, if variable, the counts of each side separated by a slash.

Nomenclature. Nomenclature and homologies of laterosensory canal system and associated pores follow Rizzato, Bichuette (2016). Nomenclature for caudal skeleton morphology follows Lundberg, Baskin (1969) and for remaining bones follows, whenever possible, de Pinna (1989) with few variations discussed by Arratia (1998), Bockmann, Sazima (2004) and Bockmann et al. (2004) who named the “lacrimal” as “antorbital” and the “supraoccipital” as “parieto-supraoccipital”. Nomenclature for bones not cited in de Pinna (1989) follows Bockmann, Sazima (2004).

Species accounts. All obtainable type specimens were measured and radiographed. Asterisk indicates lots with specimens measured for species descriptions. Number of specimens with each count given in parentheses, also noting the count observed in the holotypes, paratypes, lectotypes, or paralectotypes. The synonym lists were based on the analysis of the vouchers cited in the literature whenever possible. Alternatively, species identity was confirmed by photographs or illustrations provided in publications or through exclusive geographical distribution of some taxa. External characters have preference in the diagnosis rather than internal osteological traits observed only in c&s or xr specimens. If necessary, doubtful and incomplete geographical coordinates were corrected or estimated according to the available information and use of the Google Earth software. Type localities of S. angustirostre, S. minutum and S. operculatum are clarified following the recommendation 76A of the International Code of Zoological Nomenclature (ICZN, 1999), as well as a lectotype is designated for Trichomycterus minutus according to its Article 74.

Results​

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Scleronema Eigenmann, 1917

Scleronema Eigenmann, 1917: 691. -Eigenmann, 1918: 260, 269 (distribution notes); 277 (relationships); 278 (diagnosis in key); plate 36 (distribution map). –Tchernavin, 1944: 234, 235, 272 (review). –Myers, 1944: 597 (diagnosis in key). –Gosline, 1945: 55 (listed). –Fowler, 1954: 37 (listed). –Myers, Weitzman, 1966: 278, 284, 285 (phylogenetic relationships). –Ringuelet et al., 1967: 351 (diagnosis in key). –Baskin, 1973: 79, 98 (listed, notes on mouth width). –Castello et al., 1978: 124-125 (listed, diagnosis). –Burgess, 1989: 307, 313, 321 (listed, diagnosis in key). –Arratia, 1990: 400 (diagnosis). –de Pinna, 1989: 9, 24, 25, 28, 29; fig. 21, 30, 31, 35 (putative apomorphic characters, phylogenetic relationships, phylogenetic tree). -de Pinna, 1992: 213 (notes on dorsal-fin position). –Costa, Bockmann, 1993: 45 (phylogenetic relationships). –Arratia, Huaquin, 1995: 21; fig. 9b, 25, 27, 28, 29, 30 (notes on laterosensory canal system; drawing of the neurocranium and laterosensory canal system from dorsal view). –Eschmeyer, 1998: 2123 (listed). –Arratia, 1998: 366, 367; fig. 14e, 368 (phylogenetic relationships, drawing of the neurocranium and laterosensory canal system from dorsal view). -de Pinna, 1998: 299; fig. 10 (phylogenetic relationships, synapomorphies). –Nion et al., 2002: 15 (listed). –Wosiacki, 2002: 227, 324; fig. 87 (synapomorphies, phylogenetic relationships). -de Pinna, Wosiacki, 2003: 278 (listed). –Ferraris, 2007: 413 (listed). –Datovo, Bockmann, 2010: 233; fig. 37, 235-238 (phylogenetic tree, phylogenetic relationships). –Adriaens et al., 2010: 349, 357; fig. 10 (notes on length of the interopercular odontodes and body elongation). –DoNascimiento, 2015: 938, 941, 943, 944, 951 (notes on osteological characters). -Fernández et al., 2015: 6 (distribution in Argentina). –Nion et al., 2016: 15 (listed). –Ferrer, 2016: 61-62; figs. 47-50 (synapomorphies, phylogenetic relationships). –Ochoa et al., 2017a: 74-79 (phylogenetic relationships). –Katz et al., 2018: 560-564 (phylogenetic relationships). –Ochoa et al., 2020: fig. 3 (phylogenetic relationships).

Type species. Scleronema operculatumEigenmann, 1917 (by original designation).

Diagnosis. Scleronema is phylogenetically diagnosed by four synapomorphies: 1) fleshy flap at the base of the maxillary barbel (Fig. 1) vs. fleshy flap absent or extending beyond the base of the maxillary barbel; 2) skin flap in the posterior margin of the opercle (Fig. 1) vs. skin flap absent or covering practically the entire opercle; 3) articulation between the autopalatine and the vomer ventrally located, with the medial margins of the autopalatines very close to each other (Fig. 2) vs. articulation between the autopalatine and the vomer situated laterally, with the medial margins of the autopalatines not close to each other; and 4) autopalatine with an interrupted or not interrupted ossified arch-shaped process on its dorsal surface forming a canal (Fig. 3) vs. arch-shaped process absent.

Identification key. The geographical distribution of the species in the key are given according to the names and limits of the freshwater ecoregions of Abell et al. (2008) and illustrated in the Fig. 4. The two species groups mentioned in key (Scleronema minutum species group and S. operculatum species group) are further commented in the Discussion section.

FIGURE 1 | Lateral view of A. Scleronema macanuda, new species, paratype (ZVC-P 9374; 74.5 mm SL) and B. S. milonga, new species, holotype (MCP 54165; 37.8 mm SL) showing the thick fleshy flap in the base of the maxillary barbel (red arrows), the skin flap in the posterior margin of the opercle (black arrows), the dorsal membrane in the caudal peduncle (white arrows), and the vertical black bar in the caudal fin (blue arrow).

FIGURE 2 | Ventral view of the autopalatine (au) and vomer (vo) of Scleronema operculatum (UFRGS 19654) showing the articulation ventrally located between these two bones (black arrow). Scale bar = 1 mm.

FIGURE 3 | Dorsal view of the autopalatine (au) and maxilla (ma) of Scleronema minutum (ZVC-P 12491) showing the arch-shaped process on its dorsal surface (black arrow). Scale bar = 2 mm.

FIGURE 4 | Geographical distribution of the genus Scleronema. Circles and squares indicate species of the S. minutum group and S. operculatum group, respectively. Some symbols represent more than one collection locality. AR = Argentina; BR = Brazil; PY = Paraguay; UY = Uruguay.

Identification key to the species of Scleronema

1a. Body compressed; maxillary barbel with thinner portion shorter than wider one;
fleshy flap at the base of the maxillary barbel located anteriorly, thick, prolonged up to the snout and with distal margin straight;
skin flap posterior to opercle pointed and long; caudal fin with a vertical black bar distally (Fig. 1A) ……………….. 2 (Scleronema operculatum species group)

1b. Body roughly cylindrical; maxillary barbel with thinner portion longer than wider one;
fleshy flap at the base of the maxillary barbel located posteriorly, thin, restricted to the maxilla and with distal margin rounded;
skin flap posterior to opercle rounded and short; caudal fin lacking a vertical black bar (Fig. 1B) ……………….. 3 (Scleronema minutum species group)

2a. Lateral surface of body with a midlateral series of 6-9 rounded black blotches larger than opercle (Figs. 1A, 9, 11);
tip of maxillary barbel extending between anterior and posterior margins of interopercle ……………….. Scleronema macanuda (Laguna dos Patos and lower rio Uruguay)

2b. Lateral surface of body with a midlateral series of 10-14 rounded black blotches smaller than or equivalent in size to opercle (Figs. 21, 22, 25A);
tip of maxillary barbel never surpassing anterior margin of interopercle ……………….. Scleronema operculatum (lower rio Uruguay)

3a. Lateral surface of body with diffuse brown spots or rounded black blotches as large as or smaller than opercle ……………….. 4

3b. Lateral surface of body with rounded black blotches larger than opercle ……………….. 5

4a. Lateral surface of body with diffuse, scattered brown spots (Figs. 6, 7);
infraorbital line of the laterosensory system with pores i10 and i11 and no additional pores associated ……………….. Scleronema guapa (lower rio Uruguay)

4b. Lateral surface of body with rounded black blotches (Figs. 12, 13A);
infraorbital line of the laterosensory system with pores i10 and i11 and additional pores associated (Fig. 5A) ……………….. Scleronema mate (Laguna dos Patos)

5a. Pore i10 of the sphenotic canal of the laterosensory system absent; 8 pterygiophores in the dorsal fin ……………….. Scleronema teiniagua (lower rio Uruguay)

5b. Pore i10 of the sphenotic canal of the laterosensory system present (Fig. 5B-D); 9-10 pterygiophores in the dorsal fin ……………….. 6

6a. Pore s6 of the frontal canal of the laterosystem canal absent (Fig. 5B) ……………….. Scleronema ibirapuita (lower rio Uruguay)

6b. Pore s6 of the frontal canal of the laterosystem canal present (Fig. 5C-D) ……………….. 7

7a. Pore s3 of the frontal canal of the laterosensory system absent (Fig. 5C);
dorsal-fin origin located at vertical through origin of pelvic fin or slightly posterior ……………….. Scleronema milonga (lower rio Uruguay)

7b. Pore s3 of the frontal canal of the laterosensory system present (Fig. 5D);
dorsal-fin origin located at vertical through half-length of pelvic fin ……………….. Scleronema minutum (Laguna dos Patos, lower rio Uruguay and lower rio Paraná)

FIGURE 5 | Dorsal view of the head of A. Scleronema mate, new species, paratype (UFRGS 17418; 34.9 mm SL), B. S. ibirapuita, new species, paratype (UFRGS 27402; 49.4 mm SL), C. S. milonga, new species, paratype (LIRP 16775; 35.3 mm SL), and D. S. minutum (UFRGS 19651; 46.4 mm SL) showing the pores of the laterosensory system. “Ap” = additional pore.

Scleronema guapa, new species

urn:lsid:zoobank.org:act:78F28718-7E96-401E-A8C5-AC73A4555E3E

(Figs. 6-7; Tabs. 2-3)

Scleronema sp. n. 4 –Bertaco et al., 2016: 421 (listed). –Ferrer, 2016: 85-89; figs. 47-50 (phylogenetic relationships, taxonomy).

Holotype. UFRGS 23500, 36.4 mm SL, Brazil, Rio Grande do Sul State, Rosário do Sul, sanga Santo Antônio, tributary of rio Ibicuí da Armada, rio Ibicuí basin, lower rio Uruguay, 30°17’41”S 54°59’17”W, 8 Sep 2013, A. Duarte, J. Ferrer, L. R. Malabarba, M. Loureiro & M. Volcan.

Paratypes. 63 especimens from Brazil, Rio Grande do Sul State, rio Ibicuí basin, lower rio Uruguay: LIRP 16770, 5, 28.4-32.6 mm SL, collected with holotype. LIRP 16769, 5, 33.0-35.3 mm SL, Alegrete, arroio Jacaquá, 29°51’39”S 55°20’56”W, 1 May 2007, B. Klotzel, L. E. Lanés & M. Volcan. MCN 20231, 5, 29.7-36.9 mm SL, Alegrete, arroio Jacaquá, 29°51’39”S 55°20’56”W, 1 May 2007, B. Klotzel, L. E. Lanés & M. Volcan. MCP 25224*, 3, 26.7-31.7 mm SL, São Francisco de Assis, arroio Taquari, 29°23’46”S 55°08’52”W, 27 Set 1999, J. F. P. Silva, R. E. Reis & V. A. Bertaco. UFRGS 14034*, 4, 29.5-37.6 mm SL, São Gabriel, sanga do Areal, tributary of rio Santa Maria, 8 Oct 2007, 30°09’50”S 54°43’58”W, J. F. P. Silva. UFRGS 18087*, 27 (1 c&s), 16.2-33.6 mm SL, collected with holotype. UFRGS 19307*, 1, 29.2 mm SL, Alegrete, unnamed stream tributary of arroio São João, 29°46’07”S 55°23’53”W, 28 Oct 2013, C. Hartmann, L. Poldigaiski, M. Dalmolin, R. B. Dala-Corte & T. Guimarães. UFRGS 19649, 2, 31.6-40.0 mm SL, Santana do Livramento, arroio Capivara, tributary of sanga da Divisa, 30°59’42”S 55°24’12”W, 14 May 2014, C. Hartmann, M. Dalmolin, R. B. Dala-Corte & T. Guimarães. UFRGS 19655*, 10 (1 c&s), 29.8-42.3 mm SL, Alegrete, arroio Jacaquá, 29°51’39”S 55°20’56”W, 1 May 2007, B. Klotzel, L. E. Lanés & M. Volcan.

Diagnosis. Scleronema guapa is distinguished from all congeners by the lateral surface of body with diffuse, scattered brown spots, sometimes grouped forming irregular and small marks at midlateral line (vs. lateral surface of body with a midlateral line of black or brown rounded blotches). Scleronema guapa is further distinguished from S. macanuda and S. operculatum by the maxillary barbel longer than half-length of the head (vs. smaller than half-length of the head); the tips of pectoral-fin rays not extending beyond the interadial membrane (vs. extending beyond the interadial membrane), the skin flap in the posterior margin of the opercle rounded and short (vs. skin flap pointed and long); the fleshy flap at the base of the maxillary barbel located posteriorly, thin, restricted to the maxilla and distal margin rounded (vs. fleshy flap located anteriorly, thick, prolonged up to the snout and with distal margin straight); and by the caudal fin uniformly brown (vs. caudal fin with a transversal black bar distally). Scleronema guapa is further distinguished from S. ibirapuita, S. milonga, and S. teiniagua by having the pore s3 of the supraorbital line of the laterosensory system (vs. pore s3 absent).

Description. Based on specimens ranging from 16.2 to 42.3 mm SL; 2 c&s (one dissected). Morphometric data for 20 type specimens in Tab. 2.

TABLE 2 | Morphometric data of Scleronema guapa, new species (data of holotype included in the range). N = number of specimens; SD = standard deviation.

External morphology. Greatest height and width of body in half-length of trunk. Body elongate, trunk roughly cylindrical gradually compressed towards to caudal fin. Dorsal profile of trunk convex and ventral profile straight to slightly convex. Dorsal and ventral profiles of caudal peduncle straight. Dorsal margin of caudal peduncle with thin membrane, resembling adipose fin. Head depressed and wide, usually trapezoid-shaped from dorsal view, wider posteriorly; square-shaped in specimens with muscles of cheeks well developed. Dorsal and ventral profiles of head straight to slightly convex. Anterior snout profile usually rounded from dorsal view (holotype with anterior snout profile straight; Fig. 6). Nostrils of equivalent size, smaller than eye diameter. Anterior nostril surrounded by fleshy flap of integument, posterolaterally continuous with nasal barbel. Posterior nostril surrounded anterolaterally by thin flap of integument. Eyes rounded, dorsally oriented but also visible from lateral view; located behind posterior nostrils; orbital rim not free; eyes covered by thin and transparent skin.

FIGURE 6 | Scleronema guapa just after the fixation in formalin, holotype (UFRGS 23500; 36.4 mm SL), Brazil, Rio Grande do Sul State, Rosário do Sul, sanga Santo Antônio, tributary to rio Ibicuí da Armada, lower rio Uruguay.

FIGURE 7 | Scleronema guapa just after the fixation in formalin, paratype (UFRGS 18087, 32.0 mm SL), Brazil, Rio Grande do Sul, Rosário do Sul, sanga Santo Antônio, tributary to rio Ibicuí da Armada, lower rio Uruguay.

Barbels with large bases and tapering gradually towards tips. Nasal barbel long; emerging from posterolateral edge of anterior nostril extending between anterior and posterior margins of eye. Maxillary barbel long; emerging from edge of upper lip and extending between anterior and posterior margins of interopercle. Basal portion of maxillary barbel wide with thin fleshy flap dorsally and distal margin rounded. Maxillary barbel with thinner portion longer in length than wider one. Rictal barbel emerging from lateral lobe of lower lip and slightly shorter than maxillary barbel. Mouth subterminal with edges posteriorly oriented. Upper lip wider than lower lip. Lower lip with round fleshy lobes in corners. Ventral surface of lower lip with small papillae. Gill openings not constricted united with isthmus anteriorly forming free fold. Opercular patch of odontodes rounded, inserted in posterior region of head visible from dorsal and lateral views. Posterior margin of opercle with distinct skin flap short and rounded. Interopercular patch of odontodes elongate inserted on posteroventral region of head visible from lateral and ventral views. Odontodes of opercle and interopercle barely visible, completely involved by flesh.

Pectoral fin with distal margin convex when expanded, 6(n = 5), 6/7(n = 9) or 7(n = 48; including holotype) rays; first ray unbranched and not prolonged as filament; fourth and fifth longest. Pectoral-fin insertion posterior to branchial aperture usually covered by branchial membrane anteriorly. Some specimens with intumescence above anterior portion of pectoral fin and axillary pore not visible. Pelvic fin with distal margin convex when expanded, 4(n = 1), 4/5(n = 2) or 5(n = 59; including holotype) rays; first ray unbranched. Pelvic-fin origin located at half-length of SL extending between urogenital papilla and anal-fin anterior insertion; tangentially inserted with inner margins separated by large interspace. Urogenital papilla located between last third of pelvic fins.

Dorsal fin with distal margin straight to slightly convex when expanded, 7(n = 2), 8(n = 5), 9(n = 46; including holotype), or 10(n = 9) rays; usually first two rays unbranched. Dorsal-fin origin located at vertical through half-length of pelvic fin. Anal fin with distal margin slightly convex when fin expanded, 5(n = 2) or 6(n = 60; including holotype); usually first two rays unbranched. Anal-fin origin located at vertical through last third of dorsal-fin base. Caudal fin with distal margin straight and corners slightly rounded, 11(n = 5), 12(n = 57; including holotype) rays; most-external rays of dorsal and ventral plates of caudal fin always unbranched and smaller than branched rays. Branched rays of caudal fin splitting up to twice. Caudal fin with 9(n = 1) procurrent rays dorsally and 8(n = 1) procurrent rays ventrally. Procurrent rays of dorsal, anal, and caudal fins rarely visible.

Osteology. Premaxilla with 14-19(n = 2) teeth arranged in two rows. Dentary with 28-32(n = 1) teeth arranged in one to three rows. Opercle with 9-14(n = 2) odontodes and interopercle with 10-14(n = 2) odontodes. Hyoid arch with 6(n = 1) or 6/7(n = 1) branchiostegal rays. Free vertebrae 35(n = 1) or 36(n = 1); abdominal vertebrae 3(n = 1). Ribs 11(n = 2). First complete haemal arch in 4^th(n = 1) free vertebra, first haemal spine in 12^th(n = 1) free vertebra. Dorsal fin with 9(n = 2) pterygiophores; first one inserted anteriorly to neural spine of 16^th(n = 2) vertebra. Anal fin with 6(n = 2) pterygiophores; first one inserted anteriorly to haemal spine of 20^th(n = 2) vertebra.

Laterosensory system. Data for 61 specimens summarized in Tab. 3. Canals of laterosensory system with simple (non-dendritic) tubes and external pores. Supraorbital line with nasal canal usually absent and frontal canal usually with pores s3 and s6. Infraorbital line with antorbital segment invariably absent and sphenotic canal usually with pores i10 and i11. Posterior segment of frontal, sphenotic and otic canals fused to each other. Otic, posotic and scapular canals present with preoperculo-mandibular and pterotic branches short and usually with one pore each (po1 and po2, respectively). Trunk canal short usually with two pores.

Coloration in alcohol. Lateral surface of body with brown spots over light yellow background (Fig. 6). Occasionally, smaller specimens with spots grouped forming small blotches in midlateral line of trunk (Fig. 7). Dorsal surface of body usually with brown spots irregularly distributed or with vermicular brown marks extending ventrally to dorsolateral surface of trunk over light yellow background. Ventral surface of body light yellow with few brown blotches in caudal peduncle. Dorsal and laterodorsal surfaces of head with small brown spots over light yellow background. Anterior portion of opercle black. Ventral surface of body light yellow. Barbels uniformly yellow or intercalated with brown areas. Pectoral and dorsal fins hyaline or with rays of anterior portion faintly brown. Pelvic and anal fins hyaline. Caudal fin with rays faintly brown and distal margin hyaline. Caudal fin with vertical light brown stripe basally (Figs. 6-7).

TABLE 3 | Distribution of pores of the laterosensory system for the species of Scleronema. Asterisks indicate the pattern present in the type specimens of Scleronema minutum and S. operculatum and the holotype for other species. Additional pores are present in the infraorbital line or posteriorly to the pore s6 of the supraorbital line.

Coloration in life. Coloration in life similar to that of specimens preserved in ethyl alcohol, but more intense (Figs. 6-7).

Geographical distribution. Scleronema guapa is endemic to the rio Ibicuí basin, a tributary to the left bank of rio Uruguay, State of Rio Grande do Sul, southern Brazil (Fig. 4).

Ecological notes. Scleronema guapa inhabits rivers and streams, with fine sand-bottoms. The species is usually collected syntopically with S. operculatum.

Etymology. The species epithet “guapa” is a regional adjective used to describe a beautiful person, an allusion to the beauty of the new species.

Conservation status. Scleronema guapa has an Extent of Occurrence (EOO) less than 5,000 km², but no specific threats were detected and the species can be classified as LC according to IUCN criteria (IUCN, 2019).

Additional material examined. 55 specimens from Brazil, rio Ibicuí basin, lower rio Uruguay: MCP 17436, 6, 24.6-29.5 mm SL, São Francisco de Assis, rio Jaguari. MCP 48738, 1 (c&s), not measured (specimen with axial skeleton and fins damaged), São Francisco de Assis, rio Jaguari. MCP 26866, 1, 38.8 mm SL, Rosário do Sul, arroio do Salso, tributary of rio Ibicuí da Armada. MCP 25192, 14, 20.0-21.7 mm SL, São Francisco de Assis, rio Inhacunda. MCP 27505, 1, 23.2 mm SL, São Francisco de Assis, arroio Taquari, tributary of rio Miracatu. MCP 54170, 19, 17.6-27.0 mm SL, São Francisco de Assis, unnamed stream tributary of rio Inhacundá. MCP 54169, 13, 17,3-26,3 mm SL, Jaguari, arroio do Tigre, tributary of rio Jaguari.

Scleronema ibirapuita, new species

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(Figs. 5b, 8; Tabs. 3-4)

Scleronema sp. –Bertaco, Azevedo, 2013: 969 (listed).

Scleronema sp. n. 2 –Bertaco et al., 2016: 421 (listed). –Ferrer, 2016: 90-94; figs. 47-50 (phylogenetic relationships, taxonomy).

Holotype. MCN 19470, 39.5 mm SL, Brazil, Rio Grande do Sul State, Santana do Livramento, “Área de Proteção Ambiental Ibirapuitã” Conservation Unit, rio Ibirapuitã Chico, tributary of rio Ibirapuitã, rio Ibicuí basin, lower rio Uruguay, 30º33’29”S 55º31’03”W, 28 Aug 2012, C. L. Castilho, M. A. Azevedo & V. A. Bertaco.

Paratypes. 45 specimens from Brazil, Rio Grande do Sul State, rio Ibirapuitã basin, rio Ibicuí basin, lower rio Uruguay: MCP 11171*, 6 (1 c&s), 28.3-39.4 mm SL, Rosário do Sul, unnamed stream tributary of rio Ibirapuitã, 30º11’S 55º39’W, 13 Nov 1986, C. Lucena, L. Bergmann & P. Azevedo. MCN 2632*, 33 (2 c&s), 19.4-37.7 mm SL, Santana do Livramento, rio Ibirapuitã Chico, 24 Jul 1975, M. E. F. Beurmann, M. I. Vieira & P. C. Braun. MCN 19545*, 2, 35.2-42.5 mm SL, Santana do Livramento, rio Ibirapuitã at Passo do Cerrito, “Área de Proteção Ambiental Ibirapuitã” Conservation Unit, 30º37’37”S 55º40’57”W, 30 Aug 2012, C. L. Castilho, M. A. Azevedo & V. A. Bertaco. UFRGS 17386, 3, 26.7-35.8 mm SL, Santana do Livramento, rio Ibirapuitã at Passo do Cerrito, “Área de Proteção Ambiental Ibirapuitã” Conservation Unit, 30º37’37”S 55º40’57”W, 30 Aug 2012, C. L. Castilho, M. A. Azevedo & V. A. Bertaco. UFRGS 27402*, 1, 39.4 mm SL, Santana do Livramento, arroio Passo das Pedras, “Área de Proteção Ambiental Ibirapuitã” Convervation Unit, 30º32’38”S 55º25’58”W, 29 Aug 2012, C. L. Castilho, M. A. Azevedo & V. A. Bertaco.

Diagnosis. Scleronema ibirapuita is distinguished from all congeners with the exception of S. teiniagua by the absence of the pores s1, s2, s3, and s6 of the supraorbital line of the laterosensory system (vs. presence of, at least, the pore s6). Scleronema ibirapuita differs from S. teiniagua by having the pore i10 of the infraorbital line of the laterosensory system (vs. pore i10 absent) and nine pterygiophores in the dorsal fin (vs. eight).

Description. Based on specimens ranging from 19.4 to 42.5 mm SL; 3 c&s (one dissected). Morphometric data for 18 type specimens in Tab. 4.

TABLE 4 | Morphometric data of Scleronema ibirapuita, new species (data of holotype included in the range). N = number of specimens; SD = standard deviation.

External morphology. Greatest height and width of body in half-length of trunk or in dorsal-fin origin. Body elongate, trunk roughly cylindrical gradually compressed towards to caudal fin. Dorsal and ventral profiles of trunk straight to slightly convex. Dorsal and ventral profiles of caudal peduncle straight. Dorsal margin of caudal peduncle with thin membrane, resembling adipose fin. Head depressed and wide, trapezoid-shaped from dorsal view, wider posteriorly. Dorsal and ventral profiles of head straight. Anterior snout profile usually rounded from dorsal view. Nostrils of equivalent size, smaller than eye diameter. Anterior nostril surrounded by fleshy flap of integument, posterolaterally continuous with nasal barbel. Posterior nostril surrounded anterolaterally by thin flap of integument. Eyes rounded, dorsally oriented but also visible from lateral view; located behind posterior nostrils; orbital rim not free; eyes covered by thin and transparent skin.

Barbels with large bases and tapering gradually towards tips. Nasal barbel long; emerging from posterolateral edge of anterior nostril briefly surpassing posterior margin of eye. Maxillary barbel long; emerging from edge of upper lip and extending up to posterior margin of interopercle or briefly surpassing. Basal portion of maxillary barbel wide with thin skin fold dorsally and distal margin rounded. Maxillary barbel with thinner portion longer in length than wider one. Rictal barbel emerging from lateral lobe of lower lip and slightly shorter than maxillary barbel. Mouth subterminal with edges posteriorly oriented. Upper lip wider than lower lip. Lower lip with round fleshy lobes in corners. Ventral surface of lips with small papillae. Gill openings not constricted united with isthmus anteriorly forming free fold. Opercular patch of odontodes rounded, inserted in posterior region of head visible from dorsal and lateral views. Posterior margin of opercle with distinct skin flap short and rounded. Interopercular patch of odontodes elongate inserted on posteroventral region of head visible from lateral and ventral views. Odontodes of opercle and interopercle barely visible, completely involved by flesh.

Pectoral fin with distal margin convex when expanded, 6(n = 1), 6/7(n = 3), or 7(n = 39; including holotype) rays; first one always unbranched and not prolonged as filament; fourth and fifth longest. Pectoral-fin insertion posterior to branchial aperture usually covered by branchial membrane anteriorly. Some specimens with intumescence above anterior portion of pectoral fin and axillary pore not visible. Pelvic fin with distal margin convex when expanded, 4/5(n = 2) or 5(n = 41; including holotype) rays; first one always unbranched. Pelvic-fin origin located at half-length of SL extending between urogenital papilla and anal-fin anterior insertion; tangentially inserted with inner margins separated by large interspace. Urogenital papilla located between last third of pelvic fins.

Dorsal fin with distal margin straight to slightly convex when expanded, 8(n = 2), 9(n = 39; including holotype), or 10(n = 2) rays; first two or three rays unbranched. Dorsal fin with 2(n = 2) procurrent rays. Dorsal-fin origin located at vertical through half-length of pelvic fin. Anal fin with distal margin slightly convex when expanded, 6(n = 43; including holotype) rays; usually first two rays unbranched. Anal fin with 2(n = 2) procurrent rays. Anal-fin origin located at vertical through last third to posterior edge of dorsal-fin base. Caudal fin with distal margin straight and corners slightly rounded, 11(n = 2) or 12(n = 41; including holotype) rays; most-external rays of dorsal and ventral plates of caudal fin always unbranched and smaller than branched rays. Branched rays of caudal fin splitting up to twice. Procurrent rays of dorsal, anal and caudal fins rarely visible. Caudal fin with 12(n = 1) or 13(n = 1) procurrent rays dorsally and 9(n = 1) or 10(n = 1) procurrent rays ventrally.

Osteology. Premaxilla with 26-28(n = 1) teeth arranged in three rows. Dentary with 30-32(n = 1) teeth. Opercle with 12-13(n = 2) odontodes and interopercle with 16-18(n = 2) odontodes. Hyoid arch with 6(n = 2) branchiostegal rays. Free vertebrae 33(n = 1) or 35(n = 2); abdominal vertebrae 3(n = 1). Ribs 12(n = 3). First complete haemal arch in 4^th(n = 1) free vertebra, first haemal spine in 12^th(n = 1) free vertebra. Dorsal fin with 9(n = 3) pterygiophores; first one inserted anteriorly to neural spine of 14^th(n = 2) or 15^th(n = 1) vertebra. Anal fin with 6(n = 3) pterygiophores; first one inserted anteriorly to haemal spine of 18^th(n = 1) or 20^th(n = 2) vertebra.

Laterosensory system. Data for 43 specimens summarized in Tab. 3. Canals of laterosensory system with simple (non-dendritic) tubes and external pores. Supraorbital line with nasal canal invariably absent and frontal canal usually absent (Fig. 5B) (one of 43 specimens with pore s6). Infraorbital line with antorbital segment invariably absent and sphenotic canal with pores i10 and i11. Posterior segment of frontal, sphenotic and otic canals fused to each other. Otic, posotic and scapular canals present with preoperculo-mandibular and pterotic branches short usually with one pore associated each (po1 and po2, respectively). Trunk canal short with two pores.

Coloration in alcohol. Lateral surface of body with midlateral line of 5-8 round brown blotches larger than opercle over light yellow background (Fig. 8); blotches of some individuals becoming fade or absent towards caudal peduncle. Dorsal surface of body with 5-6 rectangular brown blotches extending ventrally to laterodorsal surface of body. Ventral surface of body light yellow with few brown blotches in caudal peduncle. Dorsal surface of head almost entirely black. Laterodorsal surface of head with numerous brown round blotches over light yellow background. Anterior portion of opercle black. Ventral surface of head light yellow with few small brown blotches in lower lip, sometimes forming thin stripe. Barbels uniformly yellow or intercalated with brown areas. Pectoral and anal fins with rays faintly brown and distal margins hyaline. Pelvic fin hyaline. Dorsal and caudal fins with vertical light brown stripe basally, rays faintly brown, and distal margins hyaline (Fig. 8).

FIGURE 8 | Scleronema ibirapuita, new species, holotype (MCN 19470; 39.5 mm SL), Brazil, Rio Grande do Sul, Santana do Livramento, “Área de Proteção Ambiental (APA) Ibirapuitã”, rio Ibirapuitã Chico, rio Ibicuí drainage, lower rio Uruguay.

Geographical distribution. Scleronema ibirapuita occurs in the rio Ibirapuitã basin, a tributary to the left bank of rio Ibicuí (Brazil), and in the río Arapey (Uruguay; see remarks); lower rio Uruguay basin (Fig. 4).

Ecological notes. Scleronema ibirapuita inhabits rivers and streams with sand- or gravel-bottoms. The species has not been collected with congeners. The stomach of one specimen had immature aquatic insects and unidentified plant remains.

Etymology. The species epithet “ibirapuita” is given in reference to the Conservation Unit “Área de Proteção Ambiental Ibirapuitã”, where the new species can be found and that includes its type locality. A noun in apposition.

Conservation status. Scleronema ibirapuita has an Extent of Occurrence (EOO) of less than 5,000 km², but no specific threats were detected to the species. In addition, S. ibirapuita is widespread in streams and rivers draining a Federal Conservation Unit (Área de Proteção Ambiental Ibirapuitã). Thus, the species can be classified as Least Concern (LC) according to IUCN criteria (IUCN, 2019).

Remarks. Scleronema ibirapuita has only one record for the río Arapey basin collected in 1972, Uruguay (ZVC-P 5123; five specimens). Although these specimens have the diagnostic pattern of the laterosensory system for the species, they are listed herein as non-type specimens.

Additional material examined. ZVC-P 5123, 5, 26.1-32.2 mm SL, Uruguay, Salto, río Arapey Grande near Termas, lower río Uruguay.

Scleronema macanuda, new species

urn:lsid:zoobank.org:act:6931E764-6A53-4014-848A-C794FAB57E00

(Figs. 1a, 9, 10a, 11; Tabs. 3, 5)

Scleronema operculatum [nonEigenmann, 1917] –Vaz-Ferreira, Soriano, 1960: 6-11 (brief description), figs. 1, 3, 4, 5 (drawings of a specimen from lateral view and detail of head).

Scleronema minutum [nonBoulenger, 1891] –Carvalho et al., 2012: 32 (listed), 45; fig. 17M (photo in lateral view), 46 (diagnosis in key).

Scleronema sp. –Ferrer, Malabarba, 2011: 66 (material examined). –DoNascimiento, 2012: 329 (material examined; phylogenetic relationships).

Scleronema sp. n. 5 –Bertaco et al., 2016: 421 (listed). –Ferrer, 2016: 77-84; figs. 47-50 (phylogenetic relationships, taxonomy).

Scleronema aff. operculatum –Carvalho, 2017: 21 (diet, ecomorphology and reproduction).

Holotype. MCN 20230, 71.8 mm SL, Brazil, Rio Grande do Sul State, Sentinela do Sul, irrigation water channel draining to arroio Velhaco, laguna dos Patos system, 30º43’13”S 51º39’28”W, 15 Jul 2010, M. A. Azevedo & T. V. Aguzzoli.

FIGURE 9 | Scleronema macanuda, new species, holotype (MCN 20230; 71.8 mm SL), Brazil, Rio Grande do Sul Sentinela do Sul, irrigation water channel draining to arroio Velhaco, laguna dos Patos system.

Paratypes. 229 specimens. Brazil, Rio Grande do Sul State, laguna dos Patos system: LIRP 16771, 2, 68.8-79.9 mm SL, Jaguarão, arroio Telha Chico, 32°13’52”W 53°26’13”W, L. Poldigaiski, M. Camana, R. B. Dala-Corte, T. Guimarães & V. Bastazini, 11 Jan 2014. MCN 12660*, 7 (1 c&s), 15.8-52.3 mm SL, São Lourenço do Sul, arroio Evaristo, 31°11’22”S 52°11’39”W, 14 Jun 1996, K. M. Grosser, M. R. da Costa & S. C. Freitas. MCN 12670, 13, 31.0-41.8 mm SL, São Lourenço do Sul, arroio Evaristo, 31°09’42”S 52°10’05”W, 14 Jun 1996, K. M. Grosser, M. R. da Costa & S. C. Freitas. MCN 14811*, 1, 47.1 mm SL, Mariana Pimentel, arroio Ribeiro Pequeno, 31 Jul 1997, L. F. Guterrez, P. Guterrez & W. R. Koch. MCP 17257*, 3 (1 c&s), 31.0-41.8 mm SL, São Sepé, rio São Sepé, rio Vacacaí basin, 30º11’06”S 53º33’35”W, A. R. Cardoso, A. Ramires & J. F. Pezzi, 24 Jun 1994. MCP 23034, 1, 46.2 mm SL, São Sepé, rio São Sepé, rio Vacacaí basin, 30º11’06”S 53º33’35”W, A. R. Cardoso, A. Ramires & J. F. Pezzi, 24 Jun 1994. MCP 54168*, 8 (1 c&s), 25.1-67.2 mm SL, Viamão, stream at Praia da Pedreira, Parque Estadual de Itapuã Conservation Unit, 30°21’30”S 51°02’48”W, 23 Jun 1999, C. A. Lucena, C. Porto, J. P. Silva & V. A. Bertaco. MCP 54167*, 29 (2 c&s), 24.6-84.0 mm SL, Pedro Osório, arroio Mata Olho, rio Piratini basin, 31º54’56”S 53º00’17”W, 20 Nov 1999, C. A. Lucena, E. Pereira, V. Bertaco & Z. M. Lucena. UFRGS 3955*, 5, 39.7-49.9 mm SL, Barra do Ribeiro, arroio Ribeirinho, 30°21’15”S 51°25’43”W, 22 Set 1986, Malabarba et al. UFRGS 7615*, 7, 23.3-43.5 mm SL, Encruzilhada do Sul, unnamed stream tributarty of rio Camaquã, 30°53’60”S 52°32’19”W, 15 Jul 2005, A. Schaan, G. Neves, J. Anza, J. Ferrer & J. Giora. UFRGS 7616*, 1, 41.2 mm SL, Canguçu, rio Camaquã, 16 Jul 2005, 30°56’24”S 52°38’45”W, A. Schaan, G. Neves, J. Anza, J. Ferrer & J. Giora. UFRGS 7618*, 6 (1 c&s), 25.4-44.5 mm SL, Canguçu, unnamed stream tributary of rio Camaquã, 30°57’49”S 52°39’26”W, 16 Jul 2005, A. Schaan, G. Neves, J. Anza, J. Ferrer & J. Giora. UFRGS 8770*, 10 (1 c&s), 29.2-49.6 mm SL, Rio Pardo, unnamed stream, rio Pardo basin, 12 Jul 2006, J. Anza. UFRGS 8972*, 4, 25.6-48.7 mm SL, Encruzilhada do Sul, arroio Abranjo, rio Camaquã basin, 30°53’60”S 52°32’19”W, 1 Dez 2006, J. Anza & R. Hirano. UFRGS 12484, 5, 31.0-41.8 mm SL, Camaquã, arroio Velhaco, 30°45’02”S 51°38’09”W, 26 Mar 2010, J. Ferrer & J. Wingert. UFRGS 12580*, 4 (2 c&s), 30.4-41.7 mm SL, Camaquã, arroio Velhaco, 30°45’02”S 51°38’09”W, 26 Mar 2010, J. Ferrer & J. Wingert. UFRGS 13099*, 5 (1 c&s), 41.7-53.3 mm SL, Eldorado do Sul, arroio Calombos, rio Jacuí basin, 30°06’02”S 51°41’41”W, 30 Abr 2010, J. Giora & J. Ferrer. UFRGS 14966, 4, 39.3-64.0 mm SL, Eldorado do Sul, arroio Calombos, rio Jacuí basin, 30°06’02”S 51°41’41”W, 6 May 2011, C. K. Fukakusa. UFRGS 17417, 8, 25.6-45.5 mm SL, Eldorado do Sul, arroio Calombos, rio Jacuí basin, 30°06’02”S 51°41’41”W, 17 Mar 2013, J. Ferrer. UFRGS 19304, 8, 37.3-81.9 mm SL, Santana da Boa Vista, arroio das Neves, 30°51’17”S 53°13’38”W, 13 Dez 2013, C. Hartmann, L. Poldigaiski, M. Dalmolin, R. B. Dala-Corte & T. Guimarães. UFRGS 19322 14, 39.8-77.9 mm SL, Jaguarão, unnamed stream tributary of arroio Telha Chico, 32º14’28”S 53º27’17”W, L. Poldigaiski, M. Camana, R. B. Dala-Corte, T. Guimarães & V. Bastazini, 12 Jan 2014. UFRGS 19387, 4, 41.5-75.6 mm SL, Jaguarão, unnamed stream tributary of arroio Quilombo, 32°15’09”S 53°23’44”W, L. Poldigaiski, M. Camana, R. B. Dala-Corte, T. Guimarães & V. Bastazini, 13 Jan 2014. Rio Negro basin, lower rio Uruguay: LIRP 16772, 10, 36.6-59.0 mm SL, Bagé, rio Piraí, 31º28’31”S 54º24’34”W, 17 Mar 2016, J. Chuctaya, J. Ferrer, L. R. Malabarba & M. C. Malabarba. MPEG 34068, 5, 32.0-71.5 mm SL, Bagé, rio Piraí, 31º28’31”S 54º24’34”W, 17 Mar 2016, J. Chuctaya, J. Ferrer, L. R. Malabarba & M. C. Malabarba. UFRGS 20739, 2, 38.5-38.7 mm SL, Bagé, arroio do Acampamento, tributary of rio Piraí, 31°15’03”S 54°21’08”W, 13 Mar 2015, B. Collares, B. Meneses, L. de Fries & T. Guimarães. UFRGS 21635, 40 (10 c&s), 22.2-75.6 mm SL, Bagé, rio Piraí, 31º28’31”S 54º24’34”W, 17 Mar 2016, J. Chuctaya, J. Ferrer, L. R. Malabarba & M. C. Malabarba. Uruguay, laguna Merín basin, laguna de los Patos system: ZVC-P 14526*, 2, 47.1-76.8 mm SL, Treinta y Tres, río Tacuarí at Paso del Dragón, 32°45’51”S 53°43’10”W, 24 Fev 2001, F. Scasso, F. Teixeira, M. Loureiro & N. Marchand. ZVC-P 14525*, 6, 23.4-70.7 mm SL, Treinta y Tres, río Olimar, 33°15’27”S 54°23’06”W, 21 Fev 2001, F. Scasso, F. Teixeira, M. Loureiro & N. Marchand. ZVC-P 14524*, 1, 73.2 mm SL, Lavalleja, río Cebollatí at Paso del Rey, 33°44’19”S 54°53’03”W, 2 Fev 2001, F. Scasso, F. Teixeira, M. Loureiro & N. Marchand. ZVC-P 8951*, 2, 51.4-93.6 mm SL, Treinta y Tres, río Tacuarí at Paso del Dragón, 32°45’51”S 53°43’10”W, 21 Fev 2001, F. Scasso, F. Teixeira, M. Loureiro & N. Marchand. ZVC-P 13639, 1, 50.9 mm SL, Treinta y Tres, río Tacuarí at Paso del Dragón, 32°45’51”S 53°43’10”W, 6 Dez 2013, A. Duarte, D. Hernández, E. Burress, M. Loureiro & S. Serra. Río Negro basin, lower río Uruguay: UFRGS 21923*, 1, 49.5 mm SL, Rivera, arroyo Batovi, 31°06’58”S 55°24’56”W, 27 May 2005, F. Cantera, L. R. Malabarba, P. Lehmann & V. Bertaco. ZVC-P 7531*, 5, 37.5-55.1 mm SL, Flores, arroyo Grande, 33°14’56”S 57°15’44”W, 20 Nov 2006, A. D’Anatro, F. Teixeira de Mello, I. González, M. Loureiro & S. Oviedo. ZVC-P 9374*, 1, 74.5 mm SL, Durazno, río Yi at Paso San Borja, 33°23’50”S 56°24’12”W, 23 Ago 2005, I. González-Bergonzoni. ZVC-P 14523*, 5, 50.8-70.3 mm SL, Durazno, río Yi at Paso San Borja, 33°23’50”S 56°24’12”W, 23 Nov 2005, A. D’Anatro, F. Teixeira de Mello, I. González, M. Loureiro & S. Oviedo.

Diagnosis. Scleronema macanuda is distinguished from all congeners with the exception of S. operculatum by the following external characters: maxillary barbel smaller than half-length of the head (vs. larger than half-length of the head); tips of the pectoral-fin rays extending beyond the interadial membrane (vs. not extending beyond the interadial membrane), skin flap in the posterior margin of the opercle pointed and long (vs. skin flap round an short); fleshy flap at the base of the maxillary barbel located anteriorly, thick, prolonged up to the snout and with distal margin straight (vs. fleshy flap located posteriorly, thin, restricted to the maxilla and with distal margin rounded); and the caudal fin with a transversal black bar distally (vs. caudal fin with black bar absent). Scleronema macanuda differs from S. operculatum by having a midlateral line of 6-9 rounded black blotches larger than opercle (vs. midlateral line of 10-14 rounded black blotches as large as or smaller than opercle); tip of nasal barbel usually extending beyond anterior margin of eye (vs. tip of nasal barbel never reaching anterior margin of eye), tip of maxillary barbel extending between anterior and posterior margins of interopercle (vs. tip of maxillary barbel never surpassing anterior margin of interopercle).

Description. Based on types ranging from 15.8 to 93.6 mm SL; 11 c&s (3 dissected). Morphometric data for types and non-types in Tab. 5.

TABLE 5 | Morphometric data of Scleronema macanuda, new species (data of holotype included in the range). N = number of specimens; SD = standard deviation.