Scleronema operculatum Eigenmann, 1917

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 : 53-61

Scleronema operculatum Eigenmann, 1917

( Figs. 10B , 21–24 , 25A–B ; Tabs. 3, 9)

Scleronema operculatum Eigenmann, 1917: 691–692 (original description, type species by original designation, type locality: Cacequy , Uruguay basin). — Eigenmann, 1918: 381; plate 44; fig. 1 (original description, drawing of the holotype from lateral view), 282; fig. 2d–e (drawing of the neurocranium, suspensorium, opercular apparatus, and inferior jaw from dorsal view of one paratype). — Henn, 1928: 81 (listed, type catalog of the Carnegie Museum). — Tchernavin, 1944: 235–236 (redescription based on original description). — Gosline, 1945: 55 (listed). — Fowler, 1954: 38, fig. 640 (listed, distribution, drawing of a type from lateral view). — Ibarra, Stewart, 1987: 78 (type catalog of the FMNH). — Burgess, 1989: 321 (listed). — Arratia, 1990: 399, fig. 2d (drawing of a specimen from lateral view). — de Pinna, 1998: 300 (observation on vertebrae). — Eschmeyer, 1998: 1243 (listed in catalog). — Wosiacki, 2002: 259; fig. 2, 260; fig. 3, 261; fig. 4, 262; fig. 5, 263; fig. 6, 291; fig. 38, 309; fig. 69, 312, fig. 73, 323; fig. 86 (phylogenetic relationships, drawings of characters). — de Pinna, Wosiacki, 2003: 278 (listed). — Ferraris, 2007: 413 (listed). — Wosiacki, de Pinna, 2007: 69 (listed). — Bertaco et al., 2016: 421 (listed). — Ferrer, 2016: 66–76; figs. 47–50 (phylogenetic relationships, taxonomy).

Scleronema minutum [non Boulenger, 1891] —de Pinna, 1989: 4, fig. 22a, 23a (comparative material, drawings of the maxilla, premaxilla, and anterior portion of the mesethmoid from dorsal view).

Diagnosis. Scleronema operculatum is distinguished from all congeners with the exception of S. macanuda by the following external characters: maxillary barbel shorter than half-length of the head (vs. longer 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 rounded and 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 by the caudal fin with a transversal black bar distally (vs. caudal fin uniformly brown). Scleronema operculatum differs from S. macanuda by having a midlateral line of 10–14 rounded black blotches as large as or smaller than opercle (vs. midlateral line of 6–9 rounded black blotches larger than opercle); tip of nasal barbel never reaching anterior margin of eye (vs. tip of nasal barbel usually extending beyond anterior margin of eye), and tip of maxillary barbel never surpassing anterior margin of interopercle (vs. tip of maxillary barbel extending between anterior and posterior margins of interopercle).

Description. Based on specimens ranging from 22.4 to 72.8 mm SL; 4 c&s (2 dissected) and 3 xr (holotype and two paratypes). Morphometric data for types and non-types in Tab 9.

External morphology. Greatest height of body in trunk and greatest width of body in anterior portion of trunk. Body elongate and compressed. Dorsal and ventral profiles of trunk straight to slightly convex. Dorsal and ventral profiles of caudal peduncle straight to slightly convex up to anteriormost procurrent ray insertion. Dorsal margin of caudal peduncle with thin membrane, resembling adipose fin. Head depressed and wide, trapezoidal from dorsal view, wider posteriorly. Dorsal and ventral profiles of head straight. Anterior snout profile 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 short; emerging from posterolateral edge of anterior nostril usually surpassing posterior nostril and never reaching anterior margin of eye. Maxillary barbel short; emerging from edge of upper lip and extending near to or reaching anterior margin of interopercle. Basal portion of maxillary barbel wide with thick fleshy flap dorsally and distal margin straight. Maxillary barbel with thinner portion smaller 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. Lower lip and corners of upper 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, thin and pointed; some specimens (as the holotype) with groove in skin flap. 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, 7 principal rays (n = 51; including holotype and three paratypes); 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. Rays of pectoral fin extending slightly beyond interadial membrane. Some specimens with wilted skin above anterior portion of pectoral fin (including holotype and one paratype) or with intumescence, axillary pore not visible. Pelvic fin with distal margin convex when expanded, 5(n = 50; including holotype and three paratypes) or rarely 6(n = 1) 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 when expanded, 11(n = 28; including holotype and one paratype), 12(n = 20; including one paratype) or 13(n = 3; including one paratype) rays; usually first two or three rays unbranched. Eight small specimens (<33.4 mm) of 51 analyzed without branched rays in dorsal fin. Dorsal fin with 2(n = 5; including holotype and two paratypes) or 3(n = 2) procurrent rays. Dorsal-fin origin located at vertical through first half of pelvic fin. Anal fin with distal margin slightly convex when expanded, 6(n = 29; including one paratype) or 7(n = 22; including holotype and two paratypes) rays; usually first one or two rays unbranched. Eight small specimens (<33.4 mm) of 51 analyzed without branched rays in anal fin. Anal fin with 1(n = 1; paratype) or 2(n = 6; including holotype and one paratype) 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, 12(n = 51; including holotype and three paratypes) 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 12(n = 2) or 13(n = 5; including holotype and two paratypes) procurrent rays dorsally and 8(n = 1), 9(n = 1; holotype), 10(n = 1) or 11(n = 4; including two paratypes) procurrent rays ventrally. Procurrent rays of dorsal, anal, and caudal fins rarely visible.

Osteology. Premaxilla with 25–42(n = 2) arranged in three to five rows. Dentary with 46–88(n = 2) teeth arranged in one to five rows. Opercle with 8–13(n = 4) odontodes and interopercle with 10–15(n = 4) odontodes. Hyoid arch with 6(n = 4) branchiostegal rays. Free vertebrae 35(n = 2) or 36(n = 5; including holotype and two paratypes); abdominal vertebrae 3(n = 4). Ribs 8(n = 1; paratype), 9(n = 3; including one paratype) or 10(n = 3; including holotype). First complete haemal arch in 4 th (n = 4) free vertebra, first haemal spine in 7 th (n = 3) or 9 th (n = 1) free vertebra. Dorsal fin with 10(n = 2; including one paratype), 11(n = 4; including holotype and one paratype) or 12(n = 1) pterygiophores; first one inserted anteriorly to neural spine of 15 th (n = 3; including one paratype) or 16 th (n = 4; including holotype and one paratype) vertebra. Anal fin with 6(n = 7; including holotype and two paratypes) pterygiophores; first one inserted anteriorly to haemal spine of 20 th (n = 5; including two paratypes) or 21 st (n = 2; including holotype) 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 presence of nasal canals variable: with two paired canals (right and left), only one canal present or both canals absent ( Fig. 10B ). Nasal canal, if present, interrupted (not connecting with frontal canal) with pores s1 and s2. Frontal canal usually with pores s3 and 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 each other. Otic, posotic and scapular canals present with preoperculo-mandibular and pterotic branches short usually with one pore each (po1 and po2, respectively). Trunk canal short usually with two pores.

Coloration in alcohol. Lateral surface of body with two longitudinal lines (midlateral and laterodorsal) of 10–14 black rounded blotches (as large as or smaller than opercle) over light yellow background; blotches in trunk of some individuals become fade or absent towards caudal peduncle ( Figs. 21–22 ). Some large individuals with smaller blotches among larger ones. Longitudinal line of laterodorsal blotches visible from dorsal view. Ventral surface of body uniformly light yellow. Dorsal and lateral surfaces of head and nape area with numerous black rounded blotches or uniformly black. Ventral surface of head light yellow. Dorsal surface of maxillary and rictal barbels weakly black and ventral surface light yellow. Nasal barbel weakly black. Opercle with anterior, dorsal and ventral margins black; posterior fold light yellow. Pectoral, pelvic, dorsal and anal fins hyaline; one large individual (UFRGS 19654; 70.1 mm SL) with black bar in distal margin of anal fin. Caudal fin hyaline with transversal stripe basally and transversal black bar distally, which could extend up to half-length of fin in larger individuals.

Coloration in life. Coloration in life similar to that of specimens preserved in ethyl alcohol, but more intense ( Fig. 22 ).

Geographical distribution. Scleronema operculatum 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 operculatum inhabits rivers and streams usually with fine sand-bottoms. Scleronema guapa and S. minutum may be collected in the same microhabitat of S. operculatum . The stomachs of three specimens were analyzed and two had immature aquatic Diptera ( Chironomidae ), Odonata and grains of sand.

Conservation status. Scleronema operculatum has an Extent of Occurrence (EOO) less than 20,000 km 2 being found in specific habitats. However, no threats were detected and the species can be classified as Least Concern (LC) according to IUCN criteria (IUCN, 2019).

Remarks on type specimens and type locality of Scleronema operculatum . John Haseman collected the holotype and three paratypes of Scleronema operculatum in February 1909 in an expedition to South America. Haseman returned to USA one year later and the fishes were stored in the Carnegie Museum, Pittsburgh (Haseman, Eigenmann, 1911). Eigenmann described S. operculatum in 1917 and the FMNH purchased all fishes from the former Carnegie Museum collection in 1951 (Ibarra, Stewart, 1987) and, currently, the type specimens of S. operculatum are catalogued under the numbers FMNH 58080 (holotype; ex CM 7077) and FMNH 58520 (paratypes; ex CM 7539).

The coloration of the type specimens of S. operculatum faded and only one paratype has the black round blotches in the lateral surface of body ( FMNH 58520 ; 58.6 mm SL) ( Fig. 23 ). The caudal-fin rays of the holotype are slightly bent in their insertion and mid-length ( Fig. 24 ). Additionally, the skin of the right maxilla in the holotype was ruptured making the bone exposed (probably by Eigenmann to analyze internally the structure of the conspicuous maxilla). Paratypes present faint wrapping marks in their bodies ( Fig. 23 ). One of paratypes ( FMNH 58520 ; 58.6 mm SL) has a longitudinal slit ventrally from abdomen extending to the lateral surface of trunk. The head of another paratype ( FMNH 58520 ; not measured), osteologically studied by Eigenmann, was cut off and dissected dorsally making the bones exposed, which were drew in Eigenmann (1918; p. 282, fig. 2d–e; reproduced herein in the Fig. 25 ). In this same paratype, the anal-fin rays are broken .

External synapomorphies of the genus could be observed in the type specimens of S. operculatum , such as the skin folds in the maxillary barbel and posterior to opercle. The membrane on the dorsal margin of caudal peduncle is present in all types. Diagnostic characters for the Scleronema operculatum species group (see discussion) are also observed in the types, such as the maxillary barbel shorter than half-length of the head. Additionally, all types present the tip of nasal barbel not reaching the anterior margin of eye, a diagnostic character for S. operculatum .

It is possible to count the principal rays of fins (see description). Through the skin is visible 3–5 branchiostegal rays and were counted 5–7 and 6–11 odontodes in the opercular and interopercular, respectively; with posterior ones longer and curved. However, these counts should be interpreted with caution considering the losses of some odontodes along the time and those completely covering by skin.

The type locality, based on notes accompanying the type material, “Cacequy, Feb 2 09, 2383” [sic], fits with John Haseman´s itinerary of expedition (cf. Haseman, Eigenmann, 1911: 308), and can be specified as Brazil, Rio Grande do Sul State, Cacequi, rio Cacequi , one mile from railroad station, rio Ibicuí basin, lower rio Uruguay, [ca 29°53’58”S 54°50’24”W] .

Remarks on drawings of Eigenmann (1918). Even though Eigenmann (1917) did not provide illustrations of Scleronema operculatum in his original description, one year later drawings of the holotype from lateral view and of the neurocranium, suspensorium,

opercular apparatus, and lower jaw from dorsal view of one paratype were include in a revision of Trichomycteridae ( Fig. 25 ; reproduced from Eigenmann, 1918: 381; plate

44, fig. 1 and 282; fig. 2d–e). However, some details of those figures do not correspond with the redescription above and deserve comments. The distal margins of the dorsal and caudal fins are straight in all types and non-types analyzed in opposition with the convex profile exhibited in the drawing of the holotype in Eigenmann (1918).

Similarly, the anterior margin of the mesethmoid Eigenmann (1918) is convex whereas the c&s specimens observed have this margin straight to slightly concave. Lastly, the posterior process of the autopalatine in S. operculatum is long, overlapping almost the entire area of the metapterygoid, distinctly longer than the illustration in Eigenmann

(1918). However, some characters diagnostic for S. operculatum or synapomorphic for the genus can be recognized in the drawings: the lateral surface with 14 round black blotches smaller than opercle, the long and pointed skin flap posterior to opercle and the conspicuous ventral flap in the ventral margin of the opercle.

Material examined. 54 specimens from Brazil, rio Ibicuí basin, lower rio Uruguay: FMNH 58080 , 65.9 mm SL, holotype of Scleronema operculatum Eigenmann, 1917 , Cacequi, rio Cacequi. FMNH 58520 , 3 , 53.3–58.6 mm SL (one specimen not measured), paratypes of Scleronema operculatum Eigenmann, 1917 , collected with holotype. MCP 9315 *, 2 (1 c&s), 38.2–41.8 mm SL, São Francisco de Assis , rio Jaguari. MCP 27457 *, 4 (2 c&s), 24.2–41.3 mm SL, São Francisco de Assis , stream tributary of rio Inhacundá. MCP 27543 , 5 , 24.0– 28.6 mm SL, Jaguari , arroio do Tigre , tributary of rio Jaguari. MCP 43904 *, 4, 22.5–34.2 mm SL, São Gabriel , arroio Jaguari , tributary of rio Santa Maria. UFRGS 5292 *, 3, 28.6–33.4 mm SL, Rosário do Sul , arroio do Salso. UFRGS 5349 *, 1, 30.7 mm SL, Santana do Livramento , rio Ibicuí da Faxina. UFRGS 8624 *, 1, 51.4 mm SL, Bagé , arroio Santa Maria Chico. UFRGS 18086 *, 18 (1 c&s), 25.5–42.3 mm SL, Rosário do Sul , arroio Santo Antônio , tributary of rio Ibicuí da Armada. UFRGS 19308 , 2 , 29.9–45.6 mm SL, Alegrete , unnamed stream tributary of arroio São João. UFRGS 19309 , 1 , 38.5 mm SL, Alegrete , arroio São João. UFRGS 19646 , 4 , 41.9–57.7 mm SL, Santana do Livramento , unnamed stream tributary of arroio do Salso. UFRGS 19654 *, 3 (1 c&s), 54.3–72.8 mm SL, Alegrete, arroio Jacaquá .

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 10 | Dorsal view of the head of A. Scleronema macanuda, new species, paratype (UFRGS 21635; 75.6 mm SL) and B. S. operculatum (UFRGS 19646; 56.1 mm SL) showing the pores of the laterosensory system.

FIGURE 21 | Scleronema operculatum (UFRGS 18086; 40.0 mm SL), Brazil, Rio Grande do Sul, Rosário do Sul, sanga Santo Antônio, tributary to rio Ibicuí da Armada, lower rio Uruguay.

FIGURE 22 | Specimen in life of Scleronema operculatum (UFRGS 18086; 42.3 mm SL), Brazil, Rio Grande do Sul, Rosário do Sul, sanga Santo Antônio, tributary to rio Ibicuí da Armada, lower rio Uruguay.

FIGURE 23 | Scleronema operculatum, paratype (FMNH 58520; 58.6 mm SL), Brazil, Rio Grande do Sul, Cacequi, rio Cacequi, tributary to rio Santa Maria, rio Ibicuí basin, lower rio Uruguay. Photograph taken by Sandra Raredon.

FIGURE 24 | Scleronema operculatum, holotype (FMNH 58080; 65.9 mm SL), Brazil, Rio Grande do Sul, Cacequi, rio Cacequi, tributary to rio Santa Maria, rio Ibicuí basin, lower rio Uruguay. Photograph taken by Sandra Raredon.

FIGURE 25 | Original drawings published by Eigenmann (1918): A. lateral view of the holotype of Scleronema operculatum, FMNH 58080, (plate 44; fig. 1); B. neurocranium, suspensorium, opercular apparatus, and inferior jaw from dorsal view of one paratype of S. operculatum, FMNH 58520, (p. 282; fig. 2d–e); C. part of the distribution map (modified from original drawing) showing the occurrence area of Scleronema (dashed square) at that time (plate 36).