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(Figs 1–3)

Type species: Phycocharax rasbora, new species, by monotypy and original description.

Diagnosis: Phycocharax can be diagnosed from the remaining characid genera by the combination of the following character-states, none of them unique: 1) presence of single row of relatively compressed premaxillary teeth (Fig 2); 2) large teeth on premaxillary, and dentary with four to nine cusps (Fig 2); 3) pseudotympanum absent; 4) incomplete lateral line with 7–13 pored scales; 5) anal fin dimorphic, males with distal margin approximately straight, and decreasing gently posteriorly (Figs 1a and 3a), while females present slightly concave margin of anal fin with anteriormost branched rays distinctly longer than remaining rays (Figs 1b and 3b); 6) presence of horizontally-elongated somewhat triangular blotch extending from vertical through dorsal-fin terminus to caudal peduncle end (Figs 1 and 3).

Etymology: From the Greek phykos, meaning “algae”, in allusion to the feeding habit of the new taxon owing to the dominance of this resource in its stomach contents, plus charax, meaning “pointed stake” or “palisade of pointed sticks”, the first generic name in Characidae. Gender masculine.

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(Figs 1–3)

Holotype: MZUSP 119843, 29.1 mm SL, Brazil, Mato Grosso, Guarantã do Norte, Igarapé do Arnaldo, small stream of the rio Braço Norte, a tributary of rio Peixoto de Azevedo, rio Teles Pires drainage, rio Tapajós basin, 9°37’13”S 54°57’38”W, 9 Feb 2014, W. M. Ohara & J. Bilce.

Paratypes: All from Brazil, Mato Grosso, Guarantã do Norte at rio Braço Norte as data of holotype. INPA 52858, 25, 17.1–30.1 mm SL; MCP 49957, 20, 15.8–29.7 mm SL; MNRJ 45956, 18, 20.2–30.5 mm SL; MZUSP 115341, 115 (8 c&s), 15.0–32.1 mm SL, collected with holotype. CI-FML 7164, 4 (2 c&s), 25.8–29.0 mm SL; MZUSP 115344, 42, 17.1–30.5 mm SL; ZUEC 10440, 10 (2 c&s), 17.5‒28.6 mm SL, PCH Braço Norte IV reservoir, 9°37’51”S 54°58’38”W, 28 April 2012, J. Bilce & R. Rosa. MZUSP 115313, 3, 26.4–32.1 mm SL, immediately downstream of the PCH Braço Norte IV, 9°41’22”S 54°57’56”W, 9 Feb 2014, W. M. Ohara & J. Bilce. MZUSP 115343, 20, 27.9–33.2 mm SL; ZUEC 12775, 3, 32.3–34.1 mm SL, rio Braço Norte, Braço Norte IV reservoir, 9°37’48”S 54°58’15”W, 28 April 2012, J. Bilce et al. ANSP 200243, 10, 20.3–29.2 mm SL; CAS 241425, 10, 18.5–31.0 mm SL; CI-FML 7165, 10, 21.5–29.2 mm SL; MPEG 33928, 10, 23.4–29.0 mm SL; ZUEC 12776, 20, 14.3–30.8 mm SL; MZUSP 119405, 309, 11.6–32.1 mm SL, same locality as holotype; O. T. Oyakawa, W. M. Ohara & M. Pastana, 4 Aug 2015.

Diagnosis: Same as for the genus.

Description: Morphometric data presented in Table 1. Small sized characid, largest examined specimen with 34.1 mm SL. Body compressed, moderately short and deep (Fig 1a) to horizontally elongated (Fig 3a). Greatest body depth situated slightly anterior to vertical through dorsal-fin origin. Dorsal profile of head convex from upper lip anterior tip to vertical through anterior nostril; slightly concave or straight from that point to tip of supraoccipital spine. Dorsal profile of body convex from supraoccipital spine tip to base of last dorsal-fin ray, approximately straight or convex from that point to adipose-fin insertion and slightly concave between adipose-fin insertion and origin of anteriormost dorsal procurrent caudal-fin ray. Ventral profile of head and body convex from anterior dentary tip to anal-fin origin. Ventral profile of caudal peduncle slightly concave. Pseudotympanum absent.

Jaws of equal size, mouth terminal. Posterior terminus of maxilla reaching vertical through anterior margin of pupil. Maxilla approximately at 45° angle relative to longitudinal axis of body. Anterior and posterior nostrils separated only by skin fold, posterior opening crescent-shaped. Frontals meeting each other only through the epiphyseal bar. Rhinosphenoid present with developed dorsal process. Relatively long sphenotic spine, reaching dorsal margin of hyomandibula. Frontal fontanel triangular and parietal fontanel large. Infraorbital series with five or six elements. Anterior region of third infraorbital not reaching preopercle.

Single row of premaxillary teeth, 4(9) or 5*(37) slightly compressed teeth with four to nine cusps. Symphyseal tooth narrower distally than subsequent two teeth and slightly asymmetric. Maxilla with 0(2), 1(8), 2*(33) or 3(1) compressed teeth on anterodorsal margin of bone, with three to seven cusps (Fig 2); anteriormost tooth usually largest. Maxilla suddenly expanded ventrally to maxillary teeth. Dentary with 4(2) or 5(6) slightly compressed teeth with four to nine cusps, followed by 3(2), 4(2), 5(2) or 8(2) relatively smaller conical or tricuspid teeth. Central cusp of all teeth more developed than remaining lateral cusps. Premaxillary teeth with cusp edges slightly curved outward, and dentary teeth cusp edges curved inward.

Scales cycloid, moderately large, circuli restricted to anterior field and five to twelve slightly divergent radii extending to posterior margin of scales. Incomplete lateral line slightly deflected downward with 7(6), 8*(11), 9(9), 10(4) 11(3), 12(1) or 13(2) perforated scales. Lateral series with 30(1), 31(8), 32*(15), 33(9) or 34(1) scales including pored scales. Horizontal scale rows between dorsal-fin origin and lateral line 5*(38). Horizontal scale rows between lateral line and pelvic-fin origin 3(7) or 4*(30). Scales in median series between tip of supraoccipital spine and dorsal-fin origin 6(1), 9(10) or 10*(20). Circumpeduncular scale rows 12(1), 13(4) or 14*(31). Anteriormost anal-fin rays base with single row of 2(9), 3*(18) or 4(6) scales. Caudal fin with scales only basally.

Dorsal-fin rays ii, 8(1) or 9*(42), not including small ossification anterior to first dorsal fin unbranched ray present in all examined specimens. Dorsal-fin origin slightly ahead of midbody. First dorsal-fin pterygiophore located behind neural spine of 10th(3) or 11th(5) vertebrae. Adipose fin present. Anal-fin rays iv(7) or v(1), 16(24), 17(20) or 18*(2), anteriormost rays slightly longer, subsequent rays decreasing posteriorly in size; distal margin of anal fin slightly concave in females or straight in males (see “Sexual Dimorphism”, below). Anteriormost anal-fin pterygiophore inserted posterior to haemal spine of 16th(2), 17th(5) or 18th(1) vertebrae. Pectoral-fin rays i, 9(1), 10(24), 11*(16) or 12(4). Pectoral-fin tip typically not reaching pelvic-fin insertion. Pelvic-fin rays i, 7*(46). Pelvic-fin tip almost reaching anal-fin insertion. Caudal fin with i, 9*(39) rays on the upper and 8*, i (39) rays on lower lobe. Caudal fin forked with lobes similar in size, and rounded tips. Ten (5) or 11 (3) dorsal procurrent caudal-fin rays, and 7(1), 8(3) or 9(3) ventral procurrent caudal-fin rays.

Total vertebrae 32(4), 33(3) or 34(1). Precaudal vertebrae 16(6) or 17(2); caudal vertebrae 16(4) or 17(4). Supraneurals 4(1) or 5(7) with narrow bony lamellae on upper portion. Branchiostegal rays 4(8). First gill arch with 1(1), 2(6) or 3(1) gill rakers on hypobranchial, 8(4) or 9(4) on ceratobranchial, 1(8) on cartilage between ceratobranchial and epibranchial, and 6(7) or 7(1) on epibranchial. One row of gill rakers on first ceratobranchial; two rows on remaining ones.

Color in alcohol: Overall body color beige to light brown, darker dorsally (Fig 1). Snout, top of head, anterior portion of maxilla, and dentary tip dark; numerous small dark chromatophores scattered on infraorbital series, opercle, and gular area, mainly in males (females lack dark chromatophores on gular area and infraorbitals). Scales on upper portion of body presenting conspicuous reticulated pattern formed by dark pigmentation concentrated along scale borders, gradually fading to lower flanks, typically more intense in males (Fig 1a) than in females (Fig 1b). Two vertically elongated and narrow humeral blotches, the first lying at level of third and fourth pored lateral-line scales, extending vertically over two or three vertical scales rows above lateral line and over one or two below it. Second blotch lying at level of sixth and seventh pored lateral line, extending vertically over two scales rows above lateral line. Humeral blotches usually conspicuous, excepting in specimens smaller than 20.8 mm SL.

Posterior region of flanks with conspicuous horizontally-elongated blotch, approximately triangular, of variable size, and extending posteriorly from vertical through dorsal-fin terminus to caudal peduncle end; its intensity ranging from blurred to sharply defined. One series of 9–20 longitudinal, anteriorly directed V-shaped marks of variable intensity along horizontal septum, more discernible in alcohol-preserved specimens. Dorsal and anal fins with numerous large dark chromatophores concentrated along interradial membranes, giving overall dark coloration to these fins, more intense in males (Fig 1a) than in females (Fig 1b). Pelvic, caudal, and adipose fins also with dark chromatophores, but in little amount in females. Pectoral fin hyaline in both sexes.

Color in life: Males noticeably more colored than females (Fig 3). Dorsal portion of body light beige. Humeral blotches and large dark triangular blotch located on posterior portion of the body conspicuous. Upper portion of eye red to shiny red in males (Fig 3a) and pale red in females (Fig 3b). Dorsal, adipose, caudal, and anal fins pale yellow with interradial membranes more intensely pigmented by dark chromatophores in males than females. Pectoral and pelvic fins hyaline. Head and anterior portion of body of males exhibiting red pigmentation irregularly scattered (Fig 3a). Red pigmentation in males situated anterior to vertical through anal-fin origin and more concentrated in anteroventral portion of body and head. Head and body of females predominantly silver yellowish.

Sexual dimorphism: Males notably more brightly colored than females (Fig 3: see “Color in life”) with conspicuous concentration of dark chromatophores on body and fins (Fig 1: see “Color in alcohol”). In addition, males show anal-fin base slightly convex and distal margin approximately straight with rays decreasing posteriorly in size, whereas females present anal-fin base straight and distal margin clearly concave, with anteriormost rays distinctly longer than remaining rays, forming a discrete anterior lobe (Fig 3). Profile along anal-fin base approximately straight (in females) or slightly convex (in males). Males apparently reach larger size than females (largest specimens examined, male 34.1 mm SL vs. female 31.4 mm SL). Secondary sexual characters related to bony-fin hooks on fins were not found and are certainly absent given the sexual maturity of many examined specimens (see “Ecological notes”). Gill filaments lack fusion.

Ecological notes: Phycocharax rasbora was collected primarily in dammed portions of the rio Braço Norte. Contrasting from other tributaries of the rio Tapajós basin, which are primarily clearwaters rivers, the rio Braço Norte is a blackwater tributary of the rio Teles Pires. In the site of occurrence of P. rasbora was build a small hydroelectric dam (PCH Braço Norte IV), and apparently the reservoir condition promoted the proliferation of algae. As consequence of the damming, the species thrived and can be considered as highly abundant at the dammed portion of the rio Braço Norte. In addition, P. rasbora seems to be uncommon in the lotic stretch of rio Braço Norte downstream of the dam, where only three specimens were collected. The type locality of the P. rasbora, the igarapé do Arnaldo, is a small tributary which was flooded during the damming of river, presenting muddy substrate with a great amount of decomposed organic matter with undergrowth marginal vegetation formed by grasses (Fig 4). In this area, Phycocharax rasbora was the most abundant species with 77% of all specimens collected. The section of the rio Braço Norte downstream the dam present sandy bottom with pebbles and rocks. Phycocharax rasbora occurs syntopically at Braço Norte reservoir with other Characiformes species, such as Cyphocharax cf. spilurus (Curimatidae), Leporinus friderici (Anostomidae), Serrapinus aff. micropterus, Astyanax aff. bimaculatus (Characidae), Hoplerythrinus unitaeniatus (Erythrinidae), Gymnotiformes, as Gymnotus diamantinensis (Gymnotidae), and Cichliformes, i.e., Aequidens sp. and Apistogramma sp. (Cichlidae). Although little abundant downstream the dam, P. rasbora was found co-occurring with the following Characiformes: Leporinus desmotes (Anostomidae), Hemiodus quadrimaculatus (Hemiodontidae), Astyanax aff. bimaculatus, Bryconops sp., Jupiaba polylepis, Jupiaba paranatinga, Moenkhausia hasemani, Serrapinnus aff. micropterus (Characidae), and with a single species of Siluriformes, Hypostomus sp. (Loricariidae).

Phycocharax rasbora was collected during the rainy seasons (April 2012; February 2014) and a dry season (August 2015). Males and females presented gonads well developed in both seasons. Oocytes with different sizes are present in the gonads, suggesting that females present multiple spawning, as well as several other small characids (e.g. [19,20]). Stomach contents of ten examined specimens contained mainly algae and vegetal matter.

Distribution: Phycocharax rasbora is so far known only from its type locality at upper rio Braço Norte, a right-bank tributary of the rio Peixoto de Azevedo, part of rio Teles Pires drainage, rio Tapajós basin (Fig 5). The rio Braço Norte drains the Serra do Cachimbo at northern Mato Grosso State, Brazilian Amazon.

Etymology: From the Bengali word “rasbora”, the common name of the fish Rasbora rasbora (Hamilton). Rasbora is a generic name encompassed a great radiation of small cyprinids from southeastern Asia, including the species currently allocated in the genus Trigonostigma [21], which possess a dark triangular blotch on body sides very reminiscent in shape and position similarly as found in the new species. Gender masculine. A noun in apposition.

Phylogenetic analysis: The analysis gave relatively divergent hypotheses, especially by instabilities of some taxa whose relationships are not in the scope of this paper. However, in a broad range of values of K between 5.9 and 23.5, most parsimonious trees ranging from 2750 to 2869 steps (CI = 0.138–0.144, RI = 0.652–0.669), results are stable concerning the new taxon. In that range of parameters, Phycocharax rasbora is recovered as the sister species of [Paracheirodon axelrodi (Schultz) + Hyphessobrycon elachys Weitzman] (Fig 6). Although far from conclusive, the clade recovered by P. rasbora disclosed positive GC value, suggesting a well-supported relationship. In most analysis the clade composed of Phycocharax rasbora, Paracheirodon axelrodi and Hyphessobrycon elachys is sister group of clade formed by Hyphessobrycon loweae + H. vanzolinii. It is worth noting that this latter clade is relatively little supported compared with its sister clade (i.e. P. rasbora, P. axelrodi and H. elachys). The single synapomorphy found for the Phycocharax clade is the abrupt expansion of the maxilla ventrally to the maxillary teeth (character 107, state 1). The section of the phylogenetic tree where Phycocharax is included, with clade supports and obtained synapomorphies provided (Fig 6). The complete phylogenetic hypothesis including clade supports is available in S1 Fig.