Astyanax nobre, Dagosta & Marinho, 2022

Dagosta, Fernando C. P. & Marinho, Manoela M. F., 2022, New small-sized species of Astyanax (Characiformes: Characidae) from the upper rio Paraguai basin, Brazil, with discussion on its generic allocation, Neotropical Ichthyology (e 210127) 20 (1), pp. 1-18 : 3-7

Astyanax nobre , new species

urn:lsid:zoobank.org:act:60528808-774C-4C73-91C1-D7AD3C45D435

( Figs. 1–4 ; Tab. 1 )

Astyanax sp. —Marinho et al., 2021: tab. 2 (listed as having intraspecific variation in the lateral line trunk morphology).

Holotype. MZUSP 125905 , 25.6 mm SL, Brazil, Mato Groso State, Municipality of Nobres, rio Salobra at Recanto Ecológico Lagoa Azul , Bom Jardim district , tributary of rio Cuiabá , rio Paraguai basin, 14º35’43.2”S 55º58’09.6”W, F. C. P. Dagosta & W. M. Ohara, 18 Nov 2014.

Paratypes. All from Brazil, Mato Grosso State, Municipality of Nobres , Lagoa Azul, rio Paraguai basin : MZUSP 110406 , 11 , 23.8 –30.0 mm SL, I. K. Ribeiro, 23 July 2011 ; MZUSP 118302 , 154 , 11.9 –27.0 mm SL, 3 c&s ; UFPB 12087 , 10 , 15.4–23.8 mm SL, same data as holotype .

Diagnosis. Astyanax nobre can be distinguished from its congeners, except A. joaovitori Oliveira, Pavanelli & Bertaco, 2017 and A. scintillans Myers, 1928 , by the presence of a well-defined, dark midlateral stripe on body extending from opercle to middle caudal-fin rays (vs. longitudinal stripe absent; stripe starting posterior the anterior humeral and never connected to it; stripe starting immediately posterior to the humeral blotch; narrower stripe starting at posterior margin of the opercle, becoming blurred posteriorly and not reaching the caudal fin). The new species can be clearly distinguished from A. scintillans by the presence of a vertical humeral blotch (vs. absence). Astyanax nobre can be distinguished from A. joaovitori by having the dark midlateral stripe ending at the base of the middle caudal-fin rays (vs. reaching the distal tip of the middle caudal-fin rays) and three scale series below the lateral line (vs. five or six). Another remarkable difference between these species is the body size. Astyanax nobre has mature individuals at about 25 mm SL and reaching up to 30 mm SL whereas A. joaovitori reaches a much larger body size, with up to 77 mm SL (see Oliveira et al., 2017). Astyanax nobre fits within the A. scabripinnis species complex sensu Bertaco, Lucena (2006). According to the authors, the group is characterized by species with body deepest and heaviest in area close to middle of pectoral fins, head heavy, snout short and abrupt by tapering, body depth smaller than 41% of SL (mean 30–33% of SL), reduced number of branched anal-fin rays (13–21, usually 17–18, rarely 22 or 23), presence of one or two humeral spots, and a dark, midlateral, body stripe extending to the tip of the middle caudal-fin rays. Except for the midlateral dark stripe that does not reach the tip of the middle caudal-fin rays, all characteristics are found in Astyanax nobre . According to Oliveira et al. (2017), it is impossible to infer about the dark midlateral stripe of A. scabripinnis (Jenyns, 1842) due to the loss of coloration in the holotype. Astyanax nobre differs from the holotype of A. scabripinnis by having 32–36 lateral line scales (vs. 38).

Description. Morphometric data presented in Tab. 1 . Body compressed, moderately elongate. Greatest body depth at dorsal-fin origin. Dorsal profile of head convex from upper lip to vertical through posterior nostril; slightly convex from that point to tip of supraoccipital spine. Dorsal profile of body slightly convex along predorsal region, straight to slightly convex along dorsal-fin base, slightly convex from terminus of dorsal-fin base to adipose-fin origin, and slightly concave from that point to origin of anteriormost dorsal procurrent caudal-fin ray. Ventral profile of head and body convex from tip of lower lip to pelvic-fin origin, straight to slightly concave between latter point to anal-fin origin, straight to slightly convex along anal-fin base, and slightly concave from that point to origin of anteriormost ventral procurrent caudal-fin ray.

Upper and lower jaws equal lenght, mouth terminal. Upper lip slightly anteriorly positioned than lower lip in some specimens. Premaxillary teeth in two distinct rows. Outer row with 2(1), 3*(27), or 4(2) tricuspid teeth. Inner row with 5*(30) teeth with 7–9 cusps ( Fig. 2 ). Posterior tip of maxilla at vertical through posterior half of second infraorbital. Maxilla with 1(4) or 2*(26) tri- to pentacuspid teeth. Dentary with 4*(30) larger penta- to heptacuspid teeth, followed by one small tricuspid tooth and a series of five to eight diminute conical teeth. Central median cusp in all teeth longer than lateral cusps. Branchiostegal rays four (3). First gill arch with two (3) gill rakers on hypobranchial, seven (1) or eight (2) rakers on ceratobranchial, one (3) raker on intermediate cartilage, and four (1) or five (2) rakers on epibranchial.

Lateral line straight to slightly curved ventrally, with total of 32(1), 33*(11), 34(12), 35(5), or 36(1) scales. Scales on lateral line series variably perforated. Five specimens with completely perforated lateral line, with 33*(4) or 34(1) perforated scales; 20 specimens with incomplete lateral line, with 31(4), 32(11), or 33(5) perforated scales followed by 1(5), 2(12), or 3(3) non-perforated ones, and five specimens with discontinuous lateral line, with perforated scales interspersed with non-perforated ones of variable pattern. Longitudinal scale rows between dorsal-fin origin and lateral line 5*(30). Longitudinal scale rows between lateral line and pelvic-fin origin 3*(30). Scales along middorsal line between posterior tip of supraoccipital process and dorsal-fin origin 9*(3), 10(21), or 11(6). Horizontal scale rows around caudal peduncle 14*(30). Base of anteriormost anal-fin rays covered by a series of 3 or 4 scales. Caudal fin not scaled.

Supraneurals five (3). Dorsal-fin rays ii*(30), 9*(29) or 8+i(1). First dorsal-fin pterygiophore inserted posterior to neural spine of nineth (3) vertebra. Base of last dorsal-fin ray at vertical through base of first branched anal-fin ray. Pectoral-fin rays i*(30), 9(2), 10*(20), or 11(8). Pelvic-fin rays i*(30), 7*(30). Adipose fin present. Analfin, with iv(2) or v(1) 18(4), 19*(16), 20(9), or 21(1) rays. First anal-fin pterygiophore inserted posterior to haemal spine of 16 th (3) vertebra. Principal caudal-fin rays i,9,8,i*(30); caudal fin forked, lobes somewhat rounded and small, of similar size. Dorsal procurrent caudal-fin rays 11(1) or 12(2); ventral procurrent caudal-fin rays 9(1) or 10(2). Total vertebrae 34(3): precaudal vertebrae 15(2) or 16(1) and caudal vertebrae 18(1) or 19(2).

Coloration in alcohol. Overall ground coloration pale ( Fig. 1 ). Some specimens retaining guanine on opercular and gular areas. Dorsal portion of head and body dark. Dark chromatophores concentrated at jaws and opercle. Dark chromatophores scattered at infraorbital series. Humeral blotch present. Humeral blotch roughly rectangular, vertically oriented, extending vertically five scale rows and encompassing two scales horizontally. Conspicuous dark midlateral stripe on body, extending from upper half of opercule to base of middle caudal-fin rays. Dark midlateral stripe expanded at caudal-peduncle region forming a roughly oval caudal-peduncle blotch ending onto base of middle caudal-fin rays. Scattered dark chromatophores above anal fin, especially along margins of myosepta. Abdominal region with sparse chromatophores. Dorsal-fin rays and anteriormost anal-fin rays with dark chromatophores along edge of lepidotrichias. Pectoral and pelvic fins and distal portion of dorsal and anal fins with sparse pigmentation on interadial membranes. Adipose fin with concentration of dark chromatophores in its proximal and middle portions, distal margin hyaline.

Coloration in life. Dorsal portion of body light tan, ventral portion with silvery hue ( Figs. 3–4 ). Dorsal and middorsal area with dark pigmentation. Infraorbitals, preopercle, and opercle silvery. Jaws and gular area light yellow to silver. Dorsal portion of eye orange to red; ventral portion silvery. Greenish iridescent pigmentation bordering above and below the longitudinal dark stripe, less evident in region posterior to adipose fin. Paired fins hyaline. Adipose, base of dorsal and of caudal fin orange. Anal fin slightly orange anteriorly, posterior portion nearly hyaline.

Sexual dimorphism. Males with distal margin of anal fin nearly straight ( Figs. 1 , 3A , 4A ) (vs. anal fin falcate in females, distal margin anteriorly concave, Figs. 3B , 4B ). Dorsal and caudal fins shorter and rounder in males ( Figs. 1 , 3A , 4A ) than females ( Figs. 3B , 4B ). Unpaired fins coloration more intense in males ( Figs. 3A , 4A ). No bony hooks were found.

Geographical distribution. Astyanax nobre is so far known only from the rio Salobra, tributary of rio Cuiabá, rio Paraguai basin ( Fig. 5 ).

Ecological notes. Astyanax nobre was collected in clear water river, with moderate water flow, over bottoms typically composed of rock and sand ( Fig. 6 ). Vegetation includes areas with dense aquatic macrophytes and well preserved riparian forest. The species is one of the most abundant fish species in the locality. Individuals are very used to the human presence; frequently approaching the swimmers to nibble skin from the legs and arms.

Etymology. The specific name nobre refers to the municipality of Nobres (Mato Grosso State, Brazil) where the species occurs. Additionally, “nobre ” means noble in Portuguese, in allusion to the beauty of the type locality and of being a noteworthy species of Astyanax . A noun in apposition.

Conservation status. Astyanax nobre is endemic to Brazil and is a restricted-range species. The new species is known by only the type locality, but its EOO (Extent of occurrence) is likely underestimated since the region is poorly sampled. The species is so far only known from a tourist site, but with apparently low environmental impact since only snorkeling is allowed. The headwaters of the rio Cuiabá are relatively well protected, with part of it inside of the Parque Estadual Águas do Cuiabá. Therefore, given that no significant threats were identified for this species, it is assessed as Least Concern ( LC) according to the International Union for Conservation of Nature ( IUCN) categories and criteria ( IUCN Standards and Petitions Subcommittee, 2019).

Figures

FIGURE 1 | Holotype of Astyanax nobre, MZUSP 125905, 25.6 mm SL, Brazil, Mato Grosso State, Municipality of Nobres, rio Paraguai basin, upper rio Salobra drainage.

FIGURE 2 | Lateral view of right side, premaxillary, maxillary, and dentary of Astyanax nobre, MZUSP 118302, paratype. Scale bar = 0.5 mm.

FIGURE 3 | Live coloration of Astyanax nobre, paratypes, MZUSP 118302: A. Male; B. Female. Specimens not measured. Photo by W. Ohara.

FIGURE 4 | Underwater photographs of wild specimens of Astyanax nobre at its type locality Recanto Ecológico Lagoa Azul, Bom Jardim district, Municipality of Nobres, Mato Grosso State, rio Salobra drainage, rio Paraguai basin: A. Pair of males; B. Pair of females. Details about differences between sexes can be found in Sexual dimorphism section. Photo by M. Melo.

FIGURE 5 | Distribution map of Astyanax nobre in the rio Salobra, upper rio Paraguai basin, Brazil. Brown star (type locality).

FIGURE 6 | Type locality of Astyanax nobre, rio Salobra at Lago Azul, tributary of rio Cuiabá, rio Paraguai basin, 14º35’43.2”S 55º58’09.6”W, Nobres, Mato Grosso State, Brazil. Photo by M. A. Junghans.