Centromochlus akwe Coelho, Chamon, and Sarmento-Soares 2021

(Figure 1, Table 1)

Description. Morphometric data presented in Table 1. Small-sized species, largest known specimen 61.0 mm SL (MNRJ 51961). Compact body. Anteriorly depressed head. Dorsal profile of head longer than broader. In lateral view, ventral profile of head and body approximately straight, profile slightly convex to slightly concave between anal fin and caudal fin. Greatest body width at pectoral-fin origin. Greatest body depth at dorsal-fin origin. Head covered with thick skin (making obscuring outline of cranial roof bones); eye ventrolateral; small terminal mouth, with rictus well-developed; snout margin rounded in dorsal and lateral views; anterior nostril tubular, located at anterior border of snout; posterior nostril limited anteriorly by skin flap; transverse distance between anterior nostrils less than that between posterior ones. Maxillary barbel elongate, reaching approximately vertical through dorsal-fin origin; Very short inner and outer mental barbels, not reaching the ventral edge of eye; bases of outer and inner barbels side by side, equidistant between inner and outer. Posterior process of cleithrum moderately long exceeding vertical through origin of dorsal fin. Dorsal fin I, 5 dorsal-fin spine shorter than first branched ray; serrations on anterior and posterior faces of dorsalfin spine well-developed, anterior face with 19 – 25 serrations, posterior face with 18 – 20 serrations; first branched ray elongated, subsequent branched rays gradually decreasing in size; dorsal fin with truncated convex margin (n = 19). Adipose fin small 3.7 – 7.0 % SL (n = 19), with free posterior margin. Pectoral fin I, 4; pectoral-fin spine large, tip reaching pelvic-fin origin, when adpressed, anterior margin with 31 – 23 antrorse serrations, posterior margin with 18 – 26 retrorse serrations; serrations of anterior and posterior margins larger towards distal tip; pectoral-fin spine longer than subsequent rays; pectoral fin margin truncated (n = 19). Pelvic fin i, 6; origin in the posterior half of the body; first branched ray longest, subsequent rays progressively smaller; distal margin of pelvic-fin approximately round. Anal fin iii, 7 (n = 19); Anal-fin origin posterior to posterior margin of pelvic fin, its origin beyond posterior third of the body; last unbranched-ray and first branched-ray elongate; distal margin rounded. Caudal fin i, 7 – 8, i; forked, with rounded lobes; dorsal and ventral lobes of equal size; 16 upper and 14 lower procurrent rays (n = 19). Anterior margin of cranium (Fig. 2) with mesethmoid wide and short; premaxillae with synchondral articulation to each other; anterior cranial fontanel narrow and ovoid, with two openings delimitated by mesethmoid and frontals; posterior cranial fontanel completely closed. Nasal ossified, short and tubular situated between mesethmoid cornua and lateral ethmoid. Lateral ethmoid not forming the dorsal surface of cephalic shield. Autopalatine rod-like, oriented almost parallel to longitudinal axis of body; maxilla slightly elongated; vomer arrow-shaped with short anterolateral processes. Jaws of equal size; premaxilla and dentary slender, each with two or three rows of conical teeth. Anterior nuchal plate absent; middle nuchal plate wide and with deeply concave lateral margins; posterior nuchal plate short, projected laterally, with rounded tips. Epiotic process small, not visible in dorsal view. Hyomandibula slightly more elongated than broad and forward projected, connected to quadrate by an interdigitated suture and cartilaginous tissue. Quadrate trapezoidal in shape and anteriorly connected to hyomandibula and posteriorly connected to metapterygoid by suture and cartilage. Metapterygoid trapezoidal in shape, connected to quadrate by suture; preopercle ventrally elongated and situated dorsally to quadrate and hyomandibular; preopercular canal exiting on anterior portion of pterotic. Opercle laminate and triangular (Figure 3). Hyoid arch with urohyal moderate in size, dorsal and ventral hypohyal associated to urohyal and relatively with the same size; anterior ceratohyal well developed, posterior ceratohyal smaller than anterior one; branchiostegal rays associated to hyoid arch, six branchiostegal rays, four slender rays associated with anterior ceratohyal, two flattened rays with posterior ceratohyal (Figure 4 A). Branchial (gill) arches with basibranchial 2 elongated anteriorly, slightly separated from basibranchial 3; basibranchial 3 shorter, forming osseous rod; basibranchial 4 large, flattened and cartilaginous; basibranchial 2 bordered laterally by cartilaginous head of hypobranchial 1; basibranchial 3 between cartilaginous head of hypobranchial 2 and hypobranchial 3; basibranchial 4 bordered laterally by cartilaginous head of ceratobranchial 4 and caudally by cartilaginous head of ceratobranchial 5. Hypobranchial 1 hourglass-like, with anterior edge slender and posterior edge triangular; hypobranchial 2 mainly ossified, trapezoidal; hypobranchial 4 absent. Five ceratobranchials, all supporting single row of rakers; fifth ceratobranchial expanded posteromedially to support lower pharyngeal toothplate bearing short conical teeth. Four epibranchials, all supporting single row of few rakers, close to articulation with ceratobranchials. Epibranchials 1 and 2 rod-like; epibranchial 3 with posterior uncinate process; epibranchial 4 with laminar extension. Pharyngobranchials 1 and 2 absent; pharyngobranchial accessory cartilage somewhat ellipsoid placed between anteromedial cartilaginous tips of epibranchials 1 and 2; pharyngobranchial 3 elongate, ossified, with expanded posterior border; pharyngobranchial 4 ossified, shaped as a half-circle. Upper pharyngeal tooth plate bearing conical teeth, supported by pharyngobranchial 3 and 4, and also epibranchials 3 and 4 (Figure 4 B). Eleven ribs ribs adhered to vertebrae 6 added to vertebrae 6 – 17, becoming progressively smaller and progressively smaller later. Total number of vertebrae 33 (n = 5), observed in cleared and stained specimens (c & s) and from radiographs. Color in alcohol. Head and trunk countershaded. Dorsal surface of head with dark dumbbell-shaped blotch from anterior naris to midlength of middle nuchal plate, with clear area above hyomandibula. Posterior portion of middle nuchal plate and posterior nuchal plate clear (tan). Posttemporal-supracleithrum and posterior process of cleithrum also clear, tan. Opercular series with dark pigment (colored as lateral face of trunk). Specimens from Javaés River with dorsal surface of trunk with vermiculated dark blotches. This pattern is continuous on the lateral surface above the lateral line, and on the lateral face of the caudal peduncle. Lateral face of trunk below lateral line and in front of caudal peduncle with irregularly-shaped and erratically-distributed dark blotches in some specimens. Specimens from Tocantins River (Figure 5) with vermiculation restricted to dorsum and around nuchal shield, and lateral face of trunk, including caudal peduncle with uniformly distributed melanophores, which become sparser towards ventrum. Ventral portion of head anterior to pectoral girdle and ventral portion of trunk posterior to pelvic fins marbled with dark blotches in most specimens (paler with uniformly distributed melanophores in others). Ventral surface of head and trunk from anterior margin of pectoral girdle to pelvic-fin bases completely pale. Dorsal, pectoral and caudal fins with dark transverse bands, sometimes inconspicuous in specimens preserved longer. Pelvic, anal and adipose fins hyaline. Caudal fin with background covered with large dark and rounded blotches that reduce in size from the base to distal edge. Color in life. Color in life almost dark gray with vermiculated pattern described in alcoholic specimens (Figure 6). Sexual Dimorphism. In nuptial males, anal-fin rays joined together forming single structure with rigid triangular shape with elongated distal tip retrorse spines structure (distal edge of anal fin thinner than the proximal edge) and pointed to posterior region of the body. Female and immature specimens have anal-fin rays separate, obliquely oriented, with distal tip rounded and with the almost the same width as the proximal border (Figure 7).

Diagnosis. The new species is diagnosed among Centromochlinae by having a vermiculated color pattern on the dorsum (vs. dorsum uniform in all Centromochlinae, except Tatia brunnea, T. dunni, and T. meridionalis). The new species is distinguished from these three aforementioned species by having eye ventrolaterally displaced on head in a way that almost the entire eye is visible in ventral view (vs. eye displaced dorsolaterally and not visible in ventral view). The new species is included in Centromochlus by sharing all the derived features for the genus, such as the ventrolateral position of eye socket; a sphenotic notched for the exit of infraorbital canal; and posterior serrations along pectoral-fin spine numerous (mentioned by Sarmento-Soares & Martins-Pinheiro, 2020: 127 to diagnose Centromochlus). The new species is diagnosed from congeners by having the pectoral-fin spine with dark bars, alternating with light bars (vs. pectoral-fin spine with light and uniform color in all Centromochlus). It is further distinguished from its congeners (except C. carolae) by the ventral surface of head moderate to largely pigmented (vs. ventral surface of head unpigmented in C. heckelii, C. existimatus, C. orca, C. musaicus, C. schultzi or with few scattered dark chromatophores in C. macracanthus and C. melanoleucus). It is distinguished from C. carolae by the presence of vermiculated color pattern or scattered chromatophores on lateral surface of the body that extends to the caudal peduncle (vs. lateral surface of the body with distinct demarcation between dark and light areas continuous posteriorly onto the caudal peduncle whereas the dark pigmentation extends nearly to the ventral midline in C. carolae), by lacking dark large and rounded blotches over a pale background on head and trunk (vs. present in C. schultzi); and by the smaller length of the dorsal-fin spine (18.1 – 27.1 % of SL vs. more than 27 % of SL in C. macracanthus, C. heckelii, C. existimatus), and pectoral-fin spine (24.2 – 32.2 % of SL vs. more than 33 % of SL in C. macracanthus, C. heckelii, C. existimatus).

Remarks. Almost all the specimens from the middle Tocantins River in UNT collections were previously misidentified as Centromochlus cf. punctatus (Tatia punctata). A comparison with Tatia punctata (Mees 1974), a species described from Guiana shield rivers, revealed a similar color pattern. The new species is promptly distinguished from T. punctata by having two openings in the cranial fontanel (vs. single opening in T. punctata) and by the last branched ray of the modified anal fin of nuptial males, slightly shorter than penultimate ray in the new species (vs. last ray rudimentary in T. punctata).

Distribution. Centromochlus akwe is known from Tocantins-Araguaia River basin. It was reported from the upper and middle stretches of the Tocantins River; and in median and lower portions of the Araguaia River basin, at Javaés River (Ilha do Bananal) and at Araguaia River near the confluence to the Tocantins River (Figure 8). Ecological notes. Some specimens of Centromochlus from the Javaés River (Araguaia system) were collected in recent expeditions. The new species was found hidden in crevices within laterititic bedrock substrate covered by alluvial sediment, typical of the median portion of the Araguaia River basin (Figure 9). The specimens were manually collected and sometimes it was necessary to break the rocks to remove specimens that were hidden. The laterite is a geomorphological formation that originates from the weathering of lateritic crusts that cover the geological units, and it is common at the middle and lower rio Araguaia stretches (Latrubesse & Stevaux, 2002). In this type of environment were collected several species of Siluriformes that occur collected syntopically with the new species: such as Centromochlus schultzi, Tatia intermedia, Auchenipterichthys longimanus (Auchenipteridae) Rhinodoras boehlkei, Platydoras armatulus (Doradidae), Rhamdia quelen (Pimelodidae), Leporacanthicus galaxias, Peckoltia vittata, P. sabaji, Parancistrus aurantiacus, Pseudacanthicus sp., and Spectracanthicus javae (Loricariidae) (Chamon et. al., 2018). The sampling effort to collect the specimens were made in the twilight and daytime, with most specimens collected during the morning. At periods of capture, the collected specimens were in lethargic condition.

Etymology. The specific name is in honor to the Akwê (Xerente self-denomination) indigenous people. The Akwê people were previously distributed throughout the middle and upper Tocantins River basin. Since the colonization of the indigenous territories in the so-called “ Capitania de Goiás ” (in the 18 th century), the Akwê-Xerente and other ethnic groups have been losing their territory and being decimated. Since the 19 th century, the Akwê- Xerente have resisted conflicts with squatters and farmers, leading to a drastic reduction of their vast territory, now restricted to the city of Tocantínia, north of Palmas City (Instituto Socioambiental, ISA). Conservation status. Centromochlus akwe is known from the Javaés River, Araguaia system, and for the upper and middle stretches of the Tocantins drainage. In the Araguaia system, there is no major threats to the species. Although C. akwe was abundant at the sampling points, in both middle and upper stretches of the Tocantins River, there are at least two dams (UHEs Lajeado and Peixe Angical) that may have affected the new species. According to GeoCAT analysis its Extent of Occurrence (EOO) is 19,111; 132 km 2, what suggests that the species could be Vulnerable (VU), however, there was no more capture effort since 2010, thus we suggest that C. akwe should be categorized as deficient data (DD) according to the International Union for Conservation of Nature categories and criteria (IUCN, 2020).

Holotype. MNRJ 51961, 61.0 mm SL, Javaés River, Ilha do Bananal, Pedral da Sambaíba, Pium, Tocantins, Brasil. 10 ° 00 ’ 01 ” S, 50 ° 01 ’ 29 ” W, 16 Sep 2017, Chamon, C. C. et al. Paratypes. All from Brazil. Tocantins State, Tocantins-Araguaia River basin basin. UNT 10879, 2, 39.1 – 47.5 mm SL, rio Araguaia, Ananás, 6 ° 7 ’ 12 ” S, 48 ° 18 ’ 3 ” W, 8 Dec 2009, Marques, E. E. et al. UNT 5846, 1, 44.8 mm SL, rio Areias, Porto Nacional, 10 ° 50 ’ 30 ” S, 48 ° 23 ’ 35 ’’ W, 12 Dec 2000, Marques, E. E. et al. UNT 12676, 50, 42.2 – 68.3 mm SL, rio Crixás, Brejinho de Nazaré, 11 ° 31 ’ 11 ” S, 48 ° 34 ’ 21 ” W, 4 Nov 2010, Marques, E. E. et al. UNT 14211, 1, 71.3 mm SL, rio Tocantins, Porto Nacional, 10 ° 43 ’ 15. ” S, 48 ° 25 ’ 14 ” W, 3 Feb 2011, Marques, E. E. et al. UNT 14255, 20, 17.5 – 47.5 mm SL, rio Santo Antônio, Sucupira, 11 ° 57 ’ 48 ” S, 49 o 00 ’ 13 ” W, 10 May 2010, Marques, E. E. et al. UNT 15983, 1, 52.1 mm SL, rio Palma, Arraias, 12 ° 21 ’ 44 ” S, 47 ° 5 ’ 59 ” W, 24 Jan 2009, Aloisio, G (Consultoria CTE Engenharia). UNT 17391, 77, 3. 79 – 61.0 mm SL, collected with the holotype. UNT 17716, 1, 66.9 mm SL, rio Tocantins, Porto Nacional, 10 ° 43 ’ 15 ” S, 48 ° 25 ’ 14 ” W, 17 Aug 2007, Marques, E. E. et al.