Scleronema minutum (Boulenger, 1891)

Ferrer, Juliano & Malabarba, Luiz R., 2020, Systematic revision of the Neotropical catfish genus Scleronema (Siluriformes: Trichomycteridae), with descriptions of six new species from Pampa grasslands, Neotropical Ichthyology (e 190081) 18 (2), pp. 1-81 : 40-49

Scleronema minutum (Boulenger, 1891)

( Figs. 5D , 16–18 , Tabs. 3, 8)

Trichomycterus minutus Boulenger, 1891: 235 (original description, type locality: San Lorenzo district), plate 26; fig. 3 (drawing of one syntype from lateral view and drawing of the head and pectoral fins of one syntype from dorsal view). —Ihering, 1893: 110 (original description). —Burgess, 1989: 322 (listed). —Nion et al., 2002: 15 (listed). — Eschmeyer, 1998: 2123 (listed in catalog). —Nion et al., 2016: 15 (listed).

Pygidium angustirostris Devincenzi , in Devincenzi , Teague , 1942 new synonym 30– 31 (original description, type locality: Cañada de las Piedras), plate 4, fig. 3 (drawing of a type from lateral view and head detail from dorsal view). — Olazarri et al., 1970: 4 (type catalog of the MNHM). —Eschmeyer, 1998: 2123 (listed in catalog). —Loureiro et al., 2016: 412 (notes on types).

Pygidium minutum —Ihering, 1898: 172–173 (notes on original description). — Eigenmann, 1910: 399 (listed). —Eigenmann, 1918: 340, fig. 20 (drawings of original description and original description with few modifications). —Gosline, 1945: 61 (listed).

Scleronema angustirostris —Tchernavin, 1944: 238 (brief redescription). —Costa, Bockmann, 1993: 44 (comparative material). —López et al., 2003: 38 (listed). —de Pinna, Wosiacki, 2003: 278 (listed). —Menni, 2004: 82, 96 (listed). —Liotta, 2005: 623 (distribution in Argentina)

Scleronema operculatum [non Eigenmann, 1917] —Castello et al., 1978: 125–127 (brief description), figs. 2–3 (drawings of a specimen from dorsal, lateral and ventral views and detail of the opercle and interopercle). —López et al., 2003: 38 (listed). —López et al., 2005: 326, 336 (listed). —Menni, 2004: 82 (listed). —Liotta, 2005: 280 (distribution in Argentina). —Arias et al., 2013: 13 (listed).

Scleronema angustirostre —Ferraris, 2007: 413 (listed). —Datovo, Bockmann, 2010: 198, 203, 219, 220; fig. 31, 221; fig. 32, 234, 235, 236 (phylogenetic relationships, data matrix, phylogenetic tree). —Serra et al., 2014: 73 (photo, notes on distribution, habitat, and morphological traits). —Litz, Koerber, 2014: 22 (listed). —Paullier et al., 2019: 69, 74; fig. 6b (listed). —Ferrer, 2016: 122–131; figs. 47–50 (phylogenetic relationships, taxonomy)

Scleronema cf. angustirostre —Ochoa et al., 2017a: 75, 76; fig. 2, 77; fig. 3, 79 (phylogenetic relationships, phylogenetic tree).

Scleronema minutum —Tchernavin, 1944: 236–237 (redescription). —Fowler, 1954: 37–38, fig. 639 (listed, drawing of one syntype from lateral view). —Burgess, 1989: 313 (drawing of Fowler, 1954). —Malabarba, 1989: 145 (listed). —Casciotta, Almirón, 1996: 51–53 (record to Argentina). —de Pinna, 1998: 300 (notes on vertebrae). — Wosiacki, 2002: 259; fig. 2, 260; fig. 3, 261; fig. 4, 262; fig. 5, 263; fig. 6 (phylogenetic relationships). —de Pinna, Wosiacki, 2003: 278 (listed). —López et al., 2003: 38 (listed). —Menni, 2004: 82, 96 (listed). —López et al., 2005: 336 (listed). —Liotta, 2005: 279 (distribution in Argentina). —Ferraris, 2007: 413 (listed). —Wosiacki, de Pinna, 2007:69 (listed). —Arias et al., 2013: 13 (listed). —Fernández et al., 2015: 14 (catalog of the MLP). —Bertaco et al., 2016: 421 (listed). —Ferrer, 2016: 111–121; figs. 47–50 (phylogenetic relationships, taxonomy). —Ochoa et al., 2017a: 75, 76; fig. 2, 77; fig. 3, 79 (phylogenetic relationships). —Ochoa et al., 2017b: 558, fig. 5 (phylogenetic tree). —Carvalho, 2017: 14 (diet, ecomorphology, and reproduction). —Ochoa et al., 2020: fig. 3 (phylogenetic relationships).

Trichomycterus angustirostris —Nion et al., 2002: 15 (listed). —Nion et al., 2016: 15 (listed).

Lectotype by present designation. BMNH 1891.3 .16.84, 32.4 mm SL, Brazil, Rio Grande do Sul, São Lourenço do Sul , stream tributary of laguna dos Patos, ca 31º21’55”S 51º58’42”W, H. Ihering ( Fig. 16 ).

Paralectotypes. BMNH 1891.3.16.85–86, 2, 16.6–18.7 mm SL, Brazil, Rio Grande do Sul, São Lourenço do Sul , stream tributary of laguna dos Patos, ca 31º21’55”S 51º58’42”W, H. Ihering .

Diagnosis. Scleronema minutum is distinguished from S. macanuda and S. operculatum by the following external characters: maxillary barbel longer than half-length of the head (vs. shorter than half-length of the head); tips of pectoral-fin rays not extending beyond the interadial membrane (vs. extending beyond the interadial membrane), skin flap in the posterior margin of the opercle rounded and short (vs. skin flap pointed and long); fleshy flap at the base of the maxillary barbel located posteriorly, thin, restricted to the maxillary 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 minutum differs from S. ibirapuita , S. milonga , and S. teiniagua by having the pore s3 of the supraorbital line of the laterosensory system (vs. pores s3 absent). Scleronema minutum is distinguished from S. mate and S. guapa by having the rounded brown blotches at the midlateral line larger than opercle (vs. rounded brown blotches as large as or smaller than opercle in S. mate and absent in S. guapa ).

Description. Based on specimens ranging from 14.5 to 53.9 mm SL; 7 c&s (2 dissected) and 4 xr (including lectotype and two paralectotypes). Morphometric data for types and non-types in Tab. 8.

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, 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. 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 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 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 = 2), 6/7(n = 1) or 7(n = 69; including lectotype and two paralectotypes) principal rays; first one always unbranched

Lectotype of Paralectotypes of Syntypes of P. Non-types T. minutus T. minutus angustirostris BMNH BMNH BMNH MNRJ 3605 Min Max Mean SD N 1891.3.16.84 1891.3.16.85-86 1944.6.20.1 Standard length (mm) 32.4 18.7 16.6 34.5 32.4 22.3 51.7 39.3 - Percent of standard length Total length 113.9 119.7 114 115.9 121.3 117.8 1.3 60 Head length 17.7 22.2 20.7 21 19.3 19.6 22.7 21.2 0.7 60 Predorsal length 55.1 56.8 57.6 58.6 57.4 55.2 61.2 58.2 1.5 60 Prepelvic length 52.5 51.6 55.5 55.2 54.0 50.0 57.1 53.0 1.4 60 Preanal length 67 65.9 68 69.4 70.5 64.0 72.2 68.9 1.5 60 Scapular girdle width 14 15.6 16.6 16.3 15.4 13.9 17.5 15.5 0.9 60 Trunk length 38.1 30.9 34.9 36.3 35.5 31.6 40.3 35.0 1.7 60 Pectoral-fin length 17.1 18 18.1 18.8 15.2 14.3 17.5 15.8 0.6 60 Pelvic-fin length 9.2 11.1 11.7 11.1 13.5 12.4 0.5 60 Distance between pelvic-fin base and anus 9.8 10.8 9.7 9.4 7.8 11.5 9.6 0.8 60 Caudal peduncule length 21.5 23.6 21.4 25.6 23.8 21.1 25.4 23.3 1.1 60 Caudal peduncule depth 8.8 9.9 8.8 10.2 9.4 7.4 13.3 10.7 1.0 60 Body depth 12 14.4 14 15.3 12.7 11.3 18.2 14.3 1.6 60 Body width 7.6 8.1 9.1 12.9 6.0 11.1 8.5 1.0 60 Length of dorsal-fin base 12.9 17.1 14.2 11.1 5.3 11.3 15.0 13.2 0.9 60 Length of anal-fin base 8.5 8.1 6.7 5.7 16.7 6.1 8.4 7.3 0.5 60 Percent of head length Head depth 47.5 46.1 50.6 49.7 36.4 52.3 44.6 3.4 60 Nasal barbel length 28.3 26.9 25.8 26.4 39.4 32.2 3.3 59 Maxillary barbel length 62.2 44 41.4 47.2 32.4 58.9 45.2 5.9 57 Rictal barbel length 39 41.1 38.5 40.6 27.0 52.3 39.7 4.7 60 Snout length 41.8 33.2 29.5 38.4 41.0 32.6 43.5 38.3 2.4 60 Interorbital length 16.9 13.7 16.2 16.3 17.5 11.6 44.0 15.5 4.2 60 Mouth width 40.9 31 33.6 36.3 40.7 28.4 46.5 38.2 4.0 60 Eye diameter 20.2 17.5 20.3 15.2 15.5 11.6 37.0 15.3 3.2 60 Distance between snout tip to posterior nare 20.7 25.0 14.9 27.4 22.9 2.5 60 Intranarial length 8.5 6.5 7.6 7.4 4.8 29.2 7.7 3.1 60 Anterior internarial width 13.9 13 12.3 16.0 8.5 17.1 12.8 2.2 60 Posterior internarial width 11.1 13.9 11.3 10.4 8.3 20.1 11.9 1.9 59 Supraorbital pore s6 distance 16.5 17.5 12.7 12.3 6.3 15.8 10.4 1.8 58

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 visible. Pelvic fin with distal margin convex when expanded, 5(n = 71; including lectotype and one paralectotype) 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, 9(n = 36),

10(n = 33; including one paralectotype) or 11(n = 1) rays; first two rays unbranched. Dorsal fin with 2(n = 3) or 3(n = 3) 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 = 69; including one paralectotype) rays (one anomalous specimen with 4 rays); first two rays unbranched. Anal fin with 2(n = 6) procurrent rays. 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 = 2), 12(n = 69; including lectotype and two paralectotypes) 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 10(n = 1), 11(n = 1), 12(n = 2) or 14(n = 2) procurrent rays dorsally and 8(n = 2), 9(n = 2) or 10(n = 2) procurrent rays ventrally. Procurrent rays of dorsal, anal, and caudal fins rarely visible.

Osteology. Premaxilla with 19–25(n = 5) arranged in two or three rows. Dentary with 23–24(n = 1) arranged in one to three rows. Opercle with 10–15(n = 7) odontodes and interopercle with 13–23(n = 7) odontodes. Hyoid arch with 6(n = 6) branchiostegal rays. Free vertebrae 34(n = 1), 35(n = 3) or 36(n = 7; including paralectotype and two paralectotypes); abdominal vertebrae 2(n = 1) or 3(n = 6). Ribs 10(n = 3), 11(n = 5; including one paralectotype) or 12(n = 3; including one lectotype and one paralectotype). First complete haemal arch in 3 th (n = 1) or 4 th (n = 6) free vertebra, first haemal spine in 12 th (n = 5) or 13 th (n = 2) free vertebra. Dorsal fin with 8(n = 1), 9(n = 3) or 10(n = 4) pterygiophores; first one inserted anteriorly to neural spine of 13 th (n = 1) or 14 th (n = 6; including lectotype) or 16 th (n = 4; including two paralectotypes) vertebra. Anal fin with 6(n = 8; including lectotype) pterygiophores; first one inserted anteriorly to haemal spine of 18 th (n = 1), 19 th (n = 4) or 20 th (n = 6; including lectotype and two paralectotypes) vertebra.

Laterosensory system. Data for 291 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 with pores s3 and s6 ( Fig. 5D ). 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 each other. Otic, posotic and scapular canals present with preoperculo-mandibular and pterotic branches short with one pore each (po1 and po2, respectively). Trunk canal short usually with two pores.

Coloration in alcohol. Lateral surface of body with midlateral line of 5–9 (rarely 4 or 10) rounded brown blotches larger than opercle over light yellow background; blotches of some individuals becoming fade or absent towards caudal peduncle ( Figs. 16–20 ). Lateral surface of body with additional smaller brown blotches. Some smaller individuals (<28.0 mm SL) with lateral blotches in contact, almost forming continuous stripe (UFRGS 8622 and UFRGS 13665). Dorsal surface of body with 5–9 rectangular brown blotches extending ventrally to laterodorsal surface of body; some individuals with blotches of dorsal surface discontinuous (UFRGS 19647). Ventral surface of body light yellow with few brown blotches in caudal peduncle. Dorsal and laterodorsal surfaces of head with numerous brown rounded 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-, pelvic-, and anal-fin rays weakly brown or completely hyalines. Dorsal- and caudal-fin rays brown. Caudal fin with vertical light brown stripe basally.

Coloration in life. Coloration in life similar to that of specimens preserved in ethyl alcohol, but more intense ( Fig. 18 ).

Geographical distribution. Scleronema minutum occurs in the laguna dos Patos system, lower rio Uruguay basin and lower rio Paraná basin. In the former, the species is widely distributed in rivers and streams that drain directly to laguna dos Patos ( Brazil)

and lagoa Mirim ( Brazil and Uruguay). The species is known from only one stream of the right bank of the lower rio Uruguay, the arroyo Yuquerí Chico ( Argentina), and widespread in following drainages of its other side: rio Negro ( Brazil and Uruguay), rio

Ibicuí ( Brazil), río Dayman ( Uruguay), río Queguay ( Uruguay), and río San Salvador

( Uruguay). In the lower rio Paraná basin, the species has few records to two tributaries of its left bank, the arroyo Feliciano and río Guayquiraro, Argentina ( Fig. 4 ). Other two possible records of S. minutum for Argentina (MLP 10557; arroyo Mármol, tributary of río Uruguay, and MLP 10649; río Gualeguay, tributary of río Paraná) could not be accessed and examined.

Ecological notes. Scleronema minutum inhabits rivers and streams usually with sand or gravel-bottoms. The species is found syntopically with S. macanuda in the laguna dos Patos system and rio Negro basin and syntopically with S. operculatum in the headwaters of the rio Ibicuí. The stomachs of eight specimens were analyzed and seven had immature aquatic Diptera ( Chironomidae ), Ephemeroptera, and Plecoptera. Individuals of S. minutum larger than 29.0 mm SL are capable to spawn and considered adults by Carvalho (2017). Its diet is composed mainly by “organic matter indeterminate” and immature, autochthonous and benthic insects ( Chironomidae, Ephemeroptera and Trichoptera ) in adults and Chironomidae in immatures (Carvalho, 2017).

Conservation status. Scleronema minutum is frequent and abundant in the lower rio Uruguay and Laguna dos Patos system, also occuring in the lower rio Paraná. No specific threats were detected and the species can be classified as Least Concern (LC) according to IUCN criteria (IUCN, 2019).

Remarks on the type locality of Scleronema minutum . Boulenger (1891) quoted the type locality of Scleronema minutum as San Lorenzo District, in the state of Rio Grande do Sul, Brazil. Such a locality was erroneously interpreted by de Pinna, Wosiacki (2003) as “São Lourenço das Missões = 28°30’S 54°40’W ”, also in Rio Grande do Sul State, Brazil, in the rio Uruguay drainage. Such mistake has been later repeated in subquently published systematic fish catalogs (Ferraris, 2007; Wosiacki, de Pinna, 2007).

The actual type locality, however, has long been correctly referred by Ihering (1898: 172, 173), who collected those fishes and sent them to Boulenger. Ihering mentions the type locality of S. minutum as “arroios da colônia de S. Lourenço ” (= creeks of the colony of S. Lourenço). The German colony called San Lorenzo by Boulenger (1891) or S. Lourenço by Ihering (1898) is currently the city of São Lourenço do Sul (31º21’55”S 51º58’42”W), which encompass two streams tributaries of the laguna dos Patos, the arroio São Lourenço and the arroio Cahará, possibly the localities where the type specimens were collected ( Fig. 4 ) .

Malabarba (1989) discussed the origin of several fish specimens collected in Rio Grande do Sul by Herman von Ihering that were sent to the collections of the British Museum (Natural History) and Indiana University (now at California Academy of Sciences), referring to Ihering (1898) who clearly stated that all fishes he collected come from laguna dos Patos drainage, and not from the rio Uruguay drainage: “It [the Rio Grande do Sul State] is divided, considering the freshwater fauna, into two perfectly distinct regions, that of the Uruguay River and that of the Lagoa dos Patos, one and the other comprising the respective slopes. Regarding the first, nothing is known about the tributaries and slopes of the Uruguay River born in the State of Rio Grande do Sul. (...) Only the works of Eigenmann bring some fish from Rio Grande do Sul that were not collected by Hensel, nor by me and that evidently belong to the collection made in Uruguayana [this is a reference to the collection made by Brazilian Emperor Dom Pedro II and donated to the Thayer expedition]. (...) In abstraction from them there remain fifty-nine species which were signaled by Hensel, by myself and others in the latter region [the laguna dos Patos drainage].”

Finally, it is worthy mentioning that the locality given by de Pinna, Wosiacki (2003) is not included in the area of distribution of Scleronema minutum . Instead, it is whitin the distribution range of a distinct species, S. milonga .

Remarks on type specimens of Trichomycterus minutus . The three syntypes of T. minutus (BMNH 1891.3.16.84–86) are small in comparison with the maximum SL of the species (16.3–32.4 vs. 55.5 mm SL). The largest and better-preserved specimen among the three syntypes of T. minutus (BMNH 1891.3.16.84) is designated herein as lectotype according to the Article 74 of the ICZN (1999) ( Fig. 16 ) and because S. minutum occurs sympatrically with two congeners, S. macanuda and S. operculatum . Rounded blotches in the dorsal and lateral surfaces of body are visible in the lectotype herein designated, but not clearly visible in the two paralectotypes (BMNH 1891.3.16.85–86). The lectotype is also uncharacterized by the broken fin rays (more evident in the dorsal and anal fins) and the cutted nasal barbels. However, the fleshy flap in the maxillary barbel, the skin flap posterior to opercle, and the membrane in the dorsal margin of caudal peduncle are easily visible in all type specimens. The lectotype has 5–6 odontodes in the opercle and 9–12 in the interopercle, but at least three odontodes are lost in both bones.

Remarks on type specimens and type locality of Scleronema angustirostre . The holotype and three paratypes of Pygidium angustirostris housed at MHNM (Montevideo) are lost (Olazarri et al., 1970; de Pinna, Wosiacki, 2003; Ferraris, 2007; Loureiro et al., 2016). The paratype sent to London (BMNH 1944.6.20.1; 34.5 mm SL) has the fins damaged (with rays broken and dyed of blue); the skin of the lower jaw was removed, possibly to observe the teeth, which are conical and directed inward; and odontodes dyed of blue and some missing ( Fig. 19 ). However, the coloration of body is not faded completely and it is possible to observe the rounded blotches in the dorsal and lateral surfaces of body. Other important traits observed in the paratype are the papillae on lips, the posterior flap in the opercle, the thin membrane above caudal peduncle, and the pattern of pores in the laterosensory system compatible with that of S. minutum . The state of preservation of the two MNRJ 3605 paratypes of P. angustirostris (19.9–32.4 mm SL) is quite worse ( Fig. 20 ) and few information could be obtained from them, such as in the laterosensory system. Coloration of specimens faded completely, barbels are broken, fins rays are broken or bent, and the skin over the head in the smallest specimen is damaged.

Devincenzi, Teague (1942) described the type locality of S. angustirostre , Cañada de las Piedras, in detail including photos and a map with the itinerary of its collector, Mr. Teague. The Cañada de las Piedras, also called by these authors as “ Cañada de las Piedras Blancas”, is a stream tributary to the right bank of the río Queguay in Paysandú, lower rio Uruguay ( Fig. 4 ), near to a railroad bridge (ca 32º05’09”S 57º53’13”W) .

Material examined. 899 specimens. Argentina, lower río Parana: MACN 7130, 1, 27.5 mm SL, Corrientes, río Guayquiraró. MACN 7131, 1, 26.9 mm SL, Entre Ríos, arroyo Feliciano. MACN 7132, 3, 30.4–35.3 mm SL, Entre Ríos, puddles 1 km from arroyo Feliciano. Brazil, Rio Grande do Sul State, laguna dos Patos system: BMNH 1891.3.16.84, 32.4 mm SL, lectotype of Trichomycterus minutus Boulenger, 1891 , stream tributary of laguna dos Patos, São Lourenço do Sul. BMNH 1891.3.16.85–86, 2, 16.6– 18.7 mm SL, paralectotypes of Trichomycterus minutus Boulenger, 1891 , collected with lectotype. FMNH 82728, 2, 12.5–13.5 mm SL, Santa Maria, rio Vacacaí-Mirim. MCN

14792*, 1, 39.0 mm SL, Mariana Pimentel, arroio Ribeiro Pequeno. MCN 16523, 6, 15.3–17.6 mm SL, São Jerônimo, arroio dos Cachorros. MCN 19007, 3, 25.1–44.4 mm SL, Sentinela do Sul, irrigation water channel draining to arroio Velhaco. MCP 11169, 12 (4 c&s), 17.7–36.5 mm SL, Jaguarão, rio do Telho in the mouth with rio Jaguarão. MCP 17260, 3, 24.6–29.1 mm SL, Caçapava do Sul, arroio Santa Bárbara. MCP 17341, 2, 34.0– 35.5 mm SL, Minas do Leão, arroio Taquara. MCP 17506*, 1, 33.3 mm SL, São Lourenço do Sul, arroio São Lourenço. MCP 17509*, 8, 27.1–43.9 mm SL, Pelotas, arroio dos Porcos, tributary of rio Cadeia. MCP 17512*, 39, 16.4–37.3 mm SL, Vila Nova do Sul, arroio Cambaí, tributary of rio Vacacaí. MCP 17514*, 1 (xr), 39.8 mm SL, São Lourenço do Sul, arroio São Lourenço. MCP 19108, 1, 48.2 mm SL, Barra do Ribeiro, arroio do Ribeiro. MCP 25761*, 4, 47.5–51.5 mm SL, Santana da Boa Vista, unnamed stream tributary of arroio das Neves. MCP 25911*, 6, 18.5–45.1 mm SL, Lavras do Sul, unnamed stream tributary of rio Camaquã Chico. MCP 25927, 5, 19.6– 45.9 mm SL, Lavras do Sul, arroio da Mantiqueira. MCP 34758, 1, 50.7 mm SL, Pedro Osório, arroio Mata Olho. MCP 36805, 1, 29.7 mm SL, Viamão, stream at Praia da Pedreira, Parque Estadual de Itapuã Conservation Unit. MCP 36808, 10, 27.0– 45.3 mm SL, Pedro Osório, arroio Mata Olho. MCP 36810*, 8, 31.6–44.9 mm SL, Piratini, arroio Piratini. MCP 41668, 2, 20.8–21.7 mm SL, Arroio Grande, unnamed stream tributary of arroio Basílio. MCP 45808*, 5, 28.7–49.0 mm SL, Pelotas, arroio Corrientes. MPEG 34067, 5, 30.9–45.7 mm SL, Jaguarão, unnamed stream tributary of arroio Telho Chico. UFRGS 3968, 1, 28.4 mm SL, Tapes, arroio Velhaco. UFRGS 7617, 11, 23.0– 44.4 mm SL, Herval, unnamed stream tributary of arroio Grande. UFRGS 7639*, 31 (2 c&s), 18.5–43.6 mm SL, Herval, unnamed stream tributary of rio Jaguarão Chico. UFRGS 7640, 4, 20.2–36.9 mm SL, Pinheiro Machado, arroio dos Pires. UFRGS 7641, 2, 39.5–45.0 mm SL, Herval, sanga Arroito, tributary of rio Jaguarão. UFRGS 8626, 6, 24.0– 38.1 mm SL, Pedro Osório, rio Piratini. UFRGS 10625, 12, 21.3–46.1 mm SL, Candiota, rio Jaguarão. UFRGS 10841*, 1, 25.3 mm SL, Encruzilhada do Sul, unnamed stream tributary of arroio da Caneleira. UFRGS 13612, 15, 23.6–36.5 mm SL, Turuçu, arroio Corrientes. UFRGS 13665, 10, 24.1–42.6 mm SL, Pinheiro Machado, unnamed stream tributary of rio Piratini. UFRGS 13709, 19, 27.2–39.1 mm SL, Pinheiro Machado, unnamed stream tributary of arroio Boici. UFRGS 13962, 10, 22.0– 44.2 mm SL, Bagé, arroio Camaquã Chico. UFRGS 14097*, 2 (1 c&s), 32.6–33.1 mm SL, Pelotas, arroio Basílio. UFRGS 14100*, 2 (1 c&s), 37.9–39.1 mm SL, Pelotas, unnamed stream tributary of rio Piratinizinho. UFRGS 14105*, 1, 40.7 mm SL, Pelotas, unnamed stream tributary of arroio Basílio. UFRGS 17281, 1, 45.0 mm SL, Pinheiro Machado, unnamed stream tributary of arroio Banhado Grande. UFRGS 17282, 1, 38.1 mm SL, Pinheiro Machado, unnamed stream tributary of Banhado Grande. UFRGS 19384, 16, 33.7–53.9 mm SL, arroio Telho Chico. UFRGS 19388, 45, 21.5–53.4 mm SL, Jaguarão, unnamed stream tributary of arroio Telho Chico. UFRGS 21333, 9, 28.5–43.0 mm SL, Jaguarão, unnamed stream tributary of rio Jaguarão. UFRGS 22100, 14, 15.8–41.1 mm SL, Santana da Boa Vista, arroio das Neves, tributary of rio Camaquã. Rio Ibicuí basin, lower rio Uruguay. MCP 14139, 2 (1 c&s), 24.6–35.6 mm SL, Dom Pedrito, arroio Santa Maria Chico. MCP 54171, 3, 22.4–34.5 mm SL, São Gabriel, arroio Jaguari. MCP 43919, 38, 16.1–46.8 mm SL, Lavras do Sul, arroio Jaguari. MCP 43925, 31, 18.6–46.6 mm SL, São Gabriel, arroio Jaguari. MCP 46740, 1, 31.9 mm SL, Dom Pedrito, rio Jaguari. UFRGS 8622*, 15, 21.2–47.1 mm SL, Bagé, rio Santa Maria. UFRGS 8623*, 7, 30.4–36.5 mm SL, Bagé, arroio Santa Maria Chico. UFRGS 21925*, 1, 40.2 mm SL, arroio Santa Maria Chico. UFRGS 9380*, 1, 45.0 mm SL, Lavras do Sul, arroio Taquarembó. UFRGS 9381, 3, 25.4–29.9 mm SL, Lavras do Sul, arroio Taquarembó. UFRGS 9382, 2, 30.0– 35.6 mm SL, Lavras do Sul, arroio Taquarembó. UFRGS 9383, 1, 34.1 mm SL, Lavras do Sul, arroio Taquarembó. UFRGS 9384, 2, 31.1–37.2 mm SL, Lavras do Sul, arroio Taquarembó. UFRGS 9385*, 1, 37.1 mm SL, Lavras do Sul, arroio Taquarembó. UFRGS 11761*, 14, 21.5–36.3 mm SL, Dom Pedrito, arroio Taquarembó. UFRGS 11762, 2, 20.8–28.6 mm SL, Dom Pedrito, arroio Taquarembó. UFRGS 19647, 12, 25.0– 52.3 mm SL, Santana do Livramento, arroio Porteirinha. UFRGS 19648, 4, 24.0– 48.3 mm SL, Santana do Livramento, Sanga do Cerro Verde. UFRGS 19650, 1, 38.7 mm SL, Lavras do Sul, arroio Santo Antônio. UFRGS 19651, 28, 21.9–46.3 mm SL, Lavras do Sul, stream tributary of rio Santo Antônio. UFRGS 19652, 18, 17.2– 46.4 mm SL, Lavras do Sul, arroio Santo Antônio. Rio Negro basin, lower rio Uruguay. MCP 35127, 8, 22.9–46.9 mm SL, Bagé, arroio Santa Tecla. UFRGS 20740, 2, 33.5–37.4 mm SL, Bagé, rio Negro. MCP 22874, 1, 21.3 mm SL, Bagé, rio Piraizinho. UFRGS 8625*, 2, 30.8–43.5 mm SL, Bagé, arroio Piraí. UFRGS 20741, 36, 30.9–45.1 mm SL, Bagé, Sanga do Acampamento, stream tributarty of rio Piraí. UFRGS 20742, 28, 23.1–50.0 mm SL, Bagé, unnamed stream tributaty to arroio Quebrado. UFRGS 21636, 59 (10 c&s), 14.5–41.5 mm SL, Bagé, rio Piraí. UFRGS 22128, 38, 16.6–39.4 mm SL, arroio Igrejinha. UMMZ 225384, 3, 30.4–35.6 mm SL, Bagé, rio Piraizinho. Uruguay, laguna de los Patos system. FMNH 125691, 2, 34.8–35.3 mm SL, Lavalleja, arroyo Polanco, río Cebollatí basin. ZVC-P 1933, 4, 23.7–39.7 mm SL, Lavalleja, arroyo de los Chanchos, río Cebollatí basin. ZVC-P 3377, 2, 46.7–48.8 mm SL, Maldonado, arroyo Aiguá, río Cebollatí basin. ZVC-P 5799, 2, 35.4–49.7 mm SL, Maldonado, arroyo Calera, río Cebollatí basin. ZVC-P, 6590*, 3, 1.73–36.4 mm SL, Treinta y Tres, río Tacuarí at Paso del Dragón. ZVC-P 6740, 1, 35.6 mm SL, Cerro Largo, arroyo de la Mina at Paso del Duraznero, río Yaguarón basin. ZVC-P 6782, 9, 18.8–45.3 mm SL, Lavalleja, río Cebollatí at Paso del Rey. ZVC-P 6819, 3, 38,9–42,3 mm SL, Lavalleja, arroyo de los Chanchos, bacia do río Cebollatí. ZVC-P 6910*, 2, 38.3–39.7 mm SL, Treinta y Tres, río Olimar, río Cebollatí basin. ZVC-P 7177, 3, 33.0– 41.4 mm SL, Treinta y Tres, río Olimar at Paso de las Piedras, río Cebollatí basin. ZVC-P 7178, 2, 32.3–42.8 mm SL, Treinta y Tres, arroyo de las Averias at Paso del Aguila, río Cebollatí basin. ZVC-P 7180*, 1, 39.5 mm SL, Lavalleja, río Cebollatí at Paso del Rey. ZVC-P 8571, 4, 19.0– 39.9 mm SL, Lavalleja, arroyo de los Chanchos, río Cebollatí basin. ZVC-P 12765, 2, 34.3–38.4 mm SL, Maldonado, arroyo Valdivia, río Cebollatí basin. Río Dayman basin, lower río Uruguay. ZVC-P 2847, 3, 25.0– 26.4 mm SL, Paysandú, río Dayman at Paso de las Piedras. ZVC-P 7516, 1, 48.0 mm SL, Salto, río Dayman. Río Queguay basin, lower río Uruguay. MNRJ 3605, 2 (xr), 19.9–32.4 mm SL, paratypes of Pygidium angustirostris , Paysandú, Cañada de las Piedras Blancas, tributary of río Queguay. BMNH 1944.6.20.1*, 1 (xr), 34.5 mm SL, paratype of Pygidium angustirostris , Paysandú, Cañada de las Piedras Blancas, tributary of río Queguay. MCP 17513*, 3, 30.2–34.6 mm SL, Cañada del Pantano, tributary of río Queguay. MHNM 3475, 2, 32.6–42.9 mm SL, Paysandú, Cañada de las Piedras Blancas, tributary of río Queguay. MZUSP 3428*, 1, 39.0 mm SL, Cañada de las Piedras, tributary of río Queguay. ZVC-P 1058, 1, 50.1 mm SL, Paysandú, Cañada de las Piedras Blancas, tributary of río Queguay. ZVC-P 7480, 3, 26.9–38.2 mm SL, Paysandú, río Queguay Chico. ZVC-P 11606, 5, 29.6–37.0 mm SL, Paysandú, río Queguay Grande. ZVC-P 12491, 14 (1 c&s), 21.2– 46.2 mm SL, Paysandú, Cañada de las Piedras Blancas, tributary of río Queguay. Río Negro basin, lower río Uruguay. MZUSP 81018, 1, 47.0 mm SL, Tacuarembó, Paso Lambaré. UFRGS 7238, 19, 20.1–47.0 mm SL, Rivera, arroyo Corrales. UFRGS 7244*, 2, 36.9–40.0 mm SL, Rivera, arroyo Yaguarí. UFRGS 7333*, 3, 31.6–42.3 mm SL, Rivera, arroyo Batovi. UFRGS 7367*, 2, 47.3–49.7 mm SL, Durazno, río Yí. UFRGS 13798, 1, 25.9 mm SL, Rivera, arroyo Batovi. UFRGS 14534, 2, 28.0 mm SL, Rivera, arroyo Corrales. ZVC-P 1139, 1, 22.7 mm SL, Rivera, arroyo Cuñapirú. ZVC-P 1148, 1, 31.4 mm SL, Rivera, arroyo Cuñapirú at Paso de la Calera. ZVC-P 1964, 1, 33.2 mm SL, Tacuarembó, arroyo Yaguarí at Paso del Sauce. ZVC-P 2607, 1, 25.8 mm SL, Durazno, arroyo Córdobes at Paso del Gordo. ZVC-P 2815, 5, 29.5– 45.6 mm SL, Rivera, arroyo Cuñapiru. ZVC-P 3414, 4, 34.3–43.6 mm SL, Durazno, arroyo Córdobes, Paso de la Cruz. ZVC-P 5181*, 9, 29.4–46.8 mm SL, Tacuarembó, arroyo Pororo. ZVC-P 7338, 1, 55.5 mm SL, Tacuarembó, arroyo Carpintería. ZVC-P 7404, 3, 30.6–35.3 mm SL, Durazno, río Negro. ZVC-P 7532, 8, 25.6–41.3 mm SL, Flores, arroyo Grande. ZVC-P 8568, 9, 27.9–47.8 mm SL, Durazno, río Yí at Paso San Borja. ZVC-P 8575, 2, 27.2–29.3 mm SL, Tacuarembó, arroyo Yaguarí at Paso del Sauce. ZVC-P 8577, 2, 32.9–34.9 mm SL, Tacuarembó, arroyo Yaguarí, Paso del Sauce. ZVC-P 11125, 3, 26.9–32.5 mm SL, Tacuarembó, río Negro. ZVC-P 11159, 1, 33.0 mm SL, Cerro Largo, arroyo Quebracho. ZVC-P 11287, 3, 27.1–39.1 mm SL, Cerro Largo, arroyo Tupambaé at Paso del Sauce. ZVC-P 11442, 2, 42.4– 42.6 mm SL, Florida, arroyo Mansavillagra. ZVC-P 11528, 1, 47.1 mm SL, Florida, arroyo del Monzón. ZVC-P 13640, 1, 32.3 mm SL, Durazno, río Yí, Paso Villasboas. ZVC-P 13641, 1, 45.4 mm SL, Soriano, arroyo del Monzón at Paso de los Carros. Río San Salvador basin, lower rio Uruguay. ZVC-P 3580, 3, 27.6–42.1 mm SL, Soriano, río San Salvador. ZVC-P 3584, 2, 23.1–23.8 mm SL, Soriano, río San Salvador. ZVC-P 7541*, 17 (2 c&s), 39.6–51.7 mm SL, Soriano, río San Salvador at Paso de las Tamberas. ZVC-P 12594, 1, 49.6 mm SL, Soriano, río San Salvador.

Figures

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.

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.

FIGURE 16 | Scleronema minutum, lectotype of Trichomycterus minutus (BMNH 1891.3.16.84; 32.4 mm SL), Brazil, Rio Grande do Sul, São Lourenço do Sul, stream tributary to laguna dos Patos. Photograph taken by Sandra Raredon.

FIGURE 17 | Scleronema minutum (MCP 17514; 39.8 mm SL), Brazil, Rio Grande do Sul, São Lourenço do Sul, arroio São Lourenço, laguna dos Patos system.

FIGURE 18 | Specimens in life of Scleronema minutum (A. UFRGS 22100; 37.4 mm SL, Santana da Boa Vista, arroio das Neves, B. UFRGS 22128; 39.4 mm SL, arroio Igrejinha), Brazil, Rio Grande do Sul.

FIGURE 19 | Scleronema minutum, paratype of Pygidium angustirostris (BMNH 1944.6.20.1; 34.5 mm SL), Uruguay, Paysandú, cañada de las Piedras Blancas, tributary to río Queguay, lower rio Uruguay. Photograph taken by Sandra Raredon.

FIGURE 20 | Scleronema minutum, paratype of Pygidium angustirostris (MNRJ 3605; 32.4 mm SL), Uruguay, Paysandú, cañada de las Piedras Blancas, tributary to río Queguay, lower rio Uruguay.