Chasmocranus brachynema, Gomes & Schubart, 1958

Deprá, Gabriel de Carvalho & Slobodian, Veronica, 2024, Redescription of ‘ Chasmocranus’ brachynema (Heptapteridae: Heptapterini), Neotropical Ichthyology (e 230091) 22 (1) : -

‘Chasmocranus’ brachynema Gomes & Schubart, 1958

( Figs. 1–3; Tab. 1)

Chasmocranus brachynema Gomes, Schubart, 1958:413–16, figs. 1–3 (original description; type-locality: rio Mogi-Guaçu, cachoeira de Emas, Piraçununga, São Paulo, Brazil; holotype: EEBP 617) . — Schubart, 1964:12, 18 ( rio Mogi-Guaçu basin, collection of a new specimen EEBP 629 , list of species). —Bockmann, Guazzelli, 2003:411 (catalog, type information, distribution). —MMA, 2004: anexo 1 (list of endangered species). —Langeani et al., 2007:187 (upper rio Paraná basin, list of species). —Meschiatti, Arcifa, 2009:136, 140 (Mogi-Guaçu basin, list of species). —Oyakawa et al., 2009:353, 381, 642 (list of endangered species of State of São Paulo, distribution). —Oyakawa, Menezes, 2011:25 (upper rio Paraná basin, list of species). — Azevedo-Santos et al., 2023:541, 549, 551, fig. 8, 11 (list of species, whereabouts of holotype, photography of holotype EEBP 617 , photography of topotype EEBP 629 ) .

Heptapterus brachynema . —Mees, 1974:180 (comparison with Heptapterus lopezi [= ‘ Chasmocranus ’ lopezae]). —Mees, 1987:455 (list of Heptapterus species, anal-fin rays number, adipose-fin morphology).

Chasmocranus brachynemus . —Ferraris Jr., 2007:182 (catalog, type information, distribution). —MMA, 2014: anexo 1 (list of endangered species). —Akama et al., 2018:228–230 (endangered species, distribution, threats).

“ Chasmocranus ” brachynema . —Bockmann, Slobodian, 2018:250 ( Heptapteridae genus D composition).

‘ Chasmocranus ’ brachynema . —Deprá et al., 2022:330 (Mogi-Guaçu basin, comparison with Heptapterus genus, comparison with Chasmocranus , sensu stricto, examined material EEBP 629).

Diagnosis. ‘ Chasmocranus ’ brachynema is placed in Heptapterini by having the dorsal- and pectoral-fin spines stiffened only basally and without serrations and by having a minute supraoccipital process, far apart from the dorsal-fin insertion. ‘ Chasmocranus ’ brachynema is distinguished from all other Heptapterini, except Acentronichthys Eigenmann & Eigenmann, 1889 , Chasmocranus bleekeri (Boeseman, 1953) (sensu Deprá et al., 2022), Heptapterus , ‘ H. ’ multiradiatus Iheringi, 1907 , ‘ H. ’ stewarti Haseman, 1911 , ‘ H.’ sympterygium Buckup, 1988 , and Nemuroglanis Eigenmann & Eigenmann, 1889 , by the presence of an adipose fin extensively fused with the caudal fin. The very shallowly bifurcate caudal fin distinguishes ‘ Chasmocranus ’ brachynema from Acentronichthys (deeply bifurcate), Heptapterus (ellipsoid, obliquely truncate, falcate or lanceolate, but never bifurcate), ‘ H.’ multiradiatus (ellipsoid), ‘ H.’ stewarti (ellipsoid), ‘ H. ’ sympterygium (ellipsoid), and Nemuroglanis (deeply bifurcate or lanceolate). The anal-fin insertion posterior to a vertical through the adipose-fin insertion distinguishes ‘ C. ’ brachynema from all Chasmocranus species, including C. bleekeri (sensu Deprá et al., 2022). In addition, ‘ C. ’ brachynema is distinguished from all Heptapterus species by having a longer posterior extension of the mouth rim, with the rictus reaching a vertical line between the posterior nostril and the eye vs. shorter, with the rictus barely reaching a vertical line through the posterior nostril; and the premaxillary tooth plate with a very long posterolateral extension ( Fig. 4A) vs. no posterolateral extension or a small one ( Fig. 4B).

Description. Morphometric data in Tab. 1. Dorsal profile convex from premaxillary symphysis to posterior nostril, straight to eye. Dorsally positioned eye with strongly convex profile. Dorsal profile straight from eye to end of supraoccipital, slightly convex to dorsal-fin insertion, slightly concave to adipose-fin insertion, straight along adipose-fin base. Caudal-fin base slightly convex. Ventral head profile and ventral abdominal profile slightly convex. Ventral profile from pelvic-fin to anal-fin insertion approximately straight. Anal-fin base straight, slightly ascending. Caudal peduncle profile straight. In dorsal view, mouth rim convex. Lateral profile of head convex due to well-developed adductor mandibulae muscle. Lateral profile of body straight to slightly convex along abdomen, then tapering to caudal-fin base. Abdominal region depressed, distinctly broader than deep; in cross-section, something between elliptic and rectangular. Cross section at dorsal-fin base approximately as broad as deep, between round and square. Body compressed from adipose-fin base to caudal fin, cross-section distinctly deeper than broad.

Head much depressed, flat dorsally and ventrally, rounded laterally. Mouth slightly prognathous. Mouth rictus fleshy, folding ventrally, with large sub-labial groove ventral to it. Lips double, divided by deep labial slit. Lips with numerous small papillae. Tubular anterior nostril far apart from mouth rim. Deep skin fold surrounding entire posterior nostril, but with deep posterior notch. Maxillary barbel groove extending from base of barbel to vertical through pupil; in dorsal view, rims of contralateral groove diverging posteriorly. Very subtle depression between posterior nostril and eye. Elongate depression marking anterior cranial fontanel. Bulging eyes, covered with thick skin, with no free rim, almost completely dorsal. Base of inner mental barbel anterior to outer mental barbel, and posterior to vertical through base of maxillary barbel. Maxillary barbel reaching anterior margin of first pectoral-fin ray. Shallow cleithral skin fold immediately posterior to branchial aperture, posterior terminus medial to base of first pectoral-fin ray.

Dorsal fin triangular, distal margin convex, not reaching adipose fin when adpressed. Dorsal fin with i,6*(5) rays (first ray rigid only at basal half). Distance between dorsal-fin terminus and adipose-fin origin larger than dorsal-fin base. Anteriormost dorsal-fin pterygiophore inserted posterior to neural spine of vertebra 6 (1), posteriormost dorsal-fin pterygiophore inserted anterior to neural (or pseudoneural) spine of vertebra 14 (1).

Pectoral fin with i,7,i(1), i,7,ii*(1), i,8(2), i,8,ii(1) rays on left side and i,7,i(2), i,8(2), i,8,i*(1) on right side (total pectoral-fin rays 9–11). Pectoral-fin triangular, distal margin convex. Large axillary pore dorsally to pectoral-fin base. First pectoral-fin unbranched, rigid only at its basal half, without ornamentations.

Pelvic fin with i,5*(4), i,6(1) rays on left side and i,5*(5) on right side. Expanded pelvic fin with distal margin convex, slightly pointed at middle. Pelvic-fin insertion between verticals through second (first branched) (1), third (second branched) (1) to fourth (third branched) (1*) dorsal-fin ray.

Anal fin with iii,7(1), iv,6(2), iv,6,i(1), iv,7*(1) rays (total rays 10–11). Distal margin of expanded anal fin round. Anal-fin origin slightly posterior to adipose-fin origin; anal-fin terminus distinctly anterior to adipose-fin posterior limit. Anteriormost anal-fin pterygiophore inserted posterior to haemal spine of vertebra 23(1), posteriormost anal-fin pterygiophore inserted anterior to haemal spine of vertebrae 30(1).

Adipose fin originating slightly anteriorly to vertical through anal-fin insertion. Adipose fin long, forming a pronounced ascending curve in lateral profile, emerging gradually, with deepest point between its half and last third, descending gradually towards posterior region. Posterior limit continuous (i.e., connected) with anteriormost procurrent ray of caudal fin dorsal lobe.

Caudal fin with two rounded lobes, not forked, dorsal lobe slightly longer. Longest dorsal-lobe ray fourth (counting from the diastema between dorsal and ventral hypural plates). Longest ventral-lobe ray third (counting from diastema). Caudal-fin rays xii,6,6,ix*(1), xii,6,6,xii(1),?,6,6,?(1). Eight (1) rays articulated with dorsal caudal-fin plate, six (1) rays articulated with ventral caudal-fin plate.

Premaxillary tooth plate with very long posterolateral extension; length of lateral margin at least twice as long as symphyseal margin; about nine rows of conical teeth. External gill rakers on first arch 0+4(1), 1+3(2), 1+4*(1), 1+5(1) on right side, 0+3(1), 1+3(1), 1+4*(2) on left side. Branchiostegal rays 8*(5) on both sides. Vertebrae 42(1). Ribs 11(1).

Cephalic laterosensory system. Based on three specimens (EEBP 617, holotype; EEBP 629; and NUP 22699; Fig. 5). Cephalic laterosensory pores as in Rhamdella cainguae Bockmann, Miquelarena (2008) , except by ( Fig. 5): s2+i2 closer to anterior nostril (vs. at about the middle of the distance between anterior and posterior nostrils); s4 absent from both sides (vs. present); s6+s6 situated at transversal line across posterior limit of eye (vs. across middle of eye); pm5 anteromedial to rictus (vs. posterior to it). Pore s3 absent from left side of one specimen (NUP 22699); po3 protruding from skin as short tube in holotype; ll1 and ll2 close together or not, protruding or not from skin as short tube; pm6–9 variably developed; pm7 absent from right side of one specimen (NUP 22699); i 4 in one specimen (EEBP 629) displaced anteriorly, close to posterior nostril.

Color in alcohol. Based mainly on NUP 22699 (other specimens faded) ( Fig.

2). Ground color light yellowish-brown, grading to beige on ventral side of head

and abdominal region. Transition slightly more abrupt on head. Pre-orbital stripe,

interorbital bar, and dorsal bar ( DB)1 diffuse, grey. DB2–DB5 brown. Laterodorsal

stripe between DB2 and DB3. DB2 immediately posterior to transverse bar through

base of last pectoral-fin ray. DB3 immediately anterior to dorsal-fin origin. DB4 at

posterior half of dorsal-fin base. DB5 closer to adipose fin than to dorsal fin. DB6–DB8,

caudal spot, humeral mark and midlateral stripe absent. Fin rays greyish brown. Fin

membranes mostly hyaline, except for slight yellowish-brown tint at the base. Dorsal

surface of maxillary barbel light brown; remaining barbels light beige.

Geographical distribution. ‘ Chasmocranus ’ brachynema is known from the main

stream of the rio Mogi-Guaçu at Pirassununga, State of São Paulo; from the main stream

of the rio Paraná at Jupiá, between states of São Paulo and Mato Grosso do Sul; and from

the córrego Piava, a tributary of the rio Ivaí, State of Paraná ( Fig. 6).

Ecological notes. At the localities in the Mogi-Guaçu where ‘ C.’ brachynema was collected (EEBP 617 and 629), the river is 100 m wide and comprises a series of rapids with a rocky bottom ( Figs. 7A, B). Jupiá is the name given to a stretch of the rio Paraná, next to the town of Três Lagoas, State of Mato Grosso do Sul, in which the rocky river channel was narrow, and the flow strong, forming a whirlpool that was famous among ancient navigators for being capable of swallowing whole canoes. This place was submerged in 1974 during the filling of the reservoir of the Engenheiro Souza Dias hydroelectric power plant, thus after the collection of MZUSP 22511. The córrego Piava ( Fig. 7C) is about 2 m wide, circa 0.8–1.0 m deep, with sandy bottom including small pebbles and grassy margins in the collection site of NUP 22699. The waterbody has been subject to a high degree of siltation due to the anthropized landscape in which it is inserted, mostly pasturelands.

Conservation status. With our additional locality from córrego Piava, ‘ Chasmocranus ’ brachynema Extension of Occurrence ( EOO) and Area of Occupation ( AOO) are 45,143 km 2 and 4,868 km 2, respectively. We opted not to include the Jupiá rapids in our calculations, since they have been completely altered after collecting of the ‘ C. ’ brachynema specimens. Therefore, even with this new site, the species would still be considered under threat of extinction following the IUCN criterion B, as previously assessed by Akama et al. (2018). Furthermore, the imminent reactivation of a hydropower plant in Cachoeira de Emas, and the continuous decline in the habitat quality due to anthropization in the Upper Paraná region (Akama et al., 2018), also contribute to maintaining ‘ C. ’ brachynema as an Endangered ( EN) species.

Remarks. The specimens identified by Pereira et al. (2013) and Thereza, Langeani

(2019) as ‘ Chasmocranus ’ brachynema belong to Heptapterus sp. 1 , a putatively new species

similar to Heptapterus longicauda (Borodin, 1927) . Thereza, Langeani (2019) examined

specimens from two different river basins, viz. Mogi-Guaçu ( LIRP 10970) and Tietê

( DZSJRP 7973). While the photographed specimen is from LIRP 10970, we assume

the morphological data was taken from both lots. However, their data does not match

the original description of ‘ C. ’ brachynema , as the adipose-fin base length was contained

about three times in SL (vs. 3.5 times in SL in the original description). As shown in

Fig. 8, that proportion matches smaller specimens of Heptapterus sp. 1 instead. So does

the shape of the caudal fin, which is obliquely truncate in the young and lanceolate in

adults ( Fig. 9; vs. with two rounded lobes in ‘ C. ’ brachynema ); the shape of the mouth, in

which the rictus reaches the vertical through posterior nostril’s anterior rim (vs. between

posterior nostril and eye); and anterior nostril reaching or almost reaching the snout

rim (vs. far from reaching it). Thereza, Langeani (2019) also recorded specimens they

identified as ‘ Imparfinis ’ borodini (a name that currently is a synonym of Heptapterus

longicauda; see Deprá et al., 2023) from several drainages in the Upper Paraná ecoregion.

We analyzed two of these lots, viz. DZSJRP 20527 and 20532, which also belong to

Heptapterus sp. 1 ( Fig. 9).

Thereza, Langeani (2019), in their identification key, distinguished their ‘ Chasmocranus ’ brachynema from their ‘ Imparfinis ’ borodini based on the degree of body elongation, adipose-fin base length and caudal peduncle depth. Despite that, our data show that those supposed differences are satisfactorily explained by allometry rather than by interspecific variation ( Fig. 8). On the other hand, allometric morphometric characters help distinguish between Heptapterus sp. 1 and H. longicauda , especially dorsal caudal-fin lobe length, which is extremely high in the latter ( Figs. 8–10). In sum, the only reliable previously published records of ‘ C.’ brachynema are the holotype and the topotype examined herein. With the addition of MZUSP 22511 and NUP 22699, only five preserved specimens of ‘ C.’ brachynema are known to us.

Material examined. All from Brazil. ‘ Chasmocranus ’ brachynema : All from the upper rio Paraná basin. EEBP 617 , holotype, 128.3 mm SL, State of São Paulo, municipality of Pirassununga, rio Mogi-Guaçu at Cachoeira de Emas , tributary to the rio Grande , 21°56’40”S 47°21’52”W , 24 Dec 1956. EEBP 629 , 1 , 74.9 mm SL, 1 km downstream of Cachoeira de Emas, 28 Sep 1952. MZUSP 22511 , 2 , State of Mato Grosso do Sul, municipality of Três Lagoas , rio Paraná, ca. 20°46’S 51°37’W , Zoology Department Expedition, 15 Sep 1962. NUP 22699 , 1 , 93.0 mm SL, State of Paraná, municipality of Umuarama, córrego Piava , tributary to the lower rio Ivaí basin, 23°40’42”S 53°15’47”W , W. J. da Graça, 31 Dec 2011. Heptapterus longicauda : All from upper rio Paraná basin. AMNH 8639 , holotype (photograph), 105 mm SL, State of São Paulo, municipality of Franca, rio Grande , 20°35’38”S 47°25’27”W , E. Garbe, 1910. NUP 5221 , 6 (5, 32.8–64.6 mm SL), State of Goiás, municipality of Caldas Novas, rio Corumbá , tributary to the rio Paranaíba , 17°43’37”S 48°32’54”W . NUP 6088 , 1 , 74.2 mm SL, State of Goiás, municipality of Corumbaíba, Gameleira Stream , tributary to the rio Corumbá , 17°59’49”S 48°29’46”W . NUP 14882 , 3 , 44.5–85.9 mm SL, collected with NUP 22699 . NUP 17591 , 1 , 28.0 mm SL, State of Mato Grosso do Sul, municipality of Carapó, Araponga Stream , tributary to the rio Amambaí , 22°50’39”S 54°49’28”W , Y. Suárez. Heptapterus sp. 1 : All from the upper rio Paraná basin. DZSJRP 7973 , 15 (5, 31.0– 108.4 mm SL), State of São Paulo, municipality of Analândia , unnamed tributary to rio Corumbataí ( rio Piracicaba , rio Tietê basin), 22°09’59”S 47°37’38”W , P. Gerhard, 15 Sep 2006. DZSJRP 20527 (10, none measured), State of São Paulo, municipality of Itirapina, Cachoeira Stream , tributary to rio Passa Cinco , 22°21’42”S 47°53’04”W , G. Brejão, 7 Sep 2006. DZSJRP 20532 , 9 (2, 106.4– 152.6 mm SL), State of São Paulo, municipality of Itirapina, Anzol Stream , tributary to rio Passa Cinco ( rio Piracicaba , rio Tietê basin), 22°21’48”S 47°53’30”W , G. Brejão, 7 Sep 2006. LBP 6414 , 4 (1, photograph), State of Paraná, municipality of Campo Mourão, rio Mourão ( rio Ivaí River ), 22°04’22”S 52°17’29”W , R. Devidé. LIRP 10970 , 1 (photograph), 65.7 mm SL, State of São Paulo, municipality of São Simão, rio Mogi Mirim , tributary to the rio Mogi-Guaçu , 21°35’25”S 47°57’13”W .