An ode to beauty: the discovery of one of the most spectacular tetras (Characiformes: Acestrorhamphidae) – a new species of Inpaichthys from the rio Juruena, Brazil

Fernando Cesar Paiva Dagosta¹

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Associate Editor: Fernando Carvalho

Section Editor: Bruno Melo

Editor-in-chief: José Birindelli

Abstract​

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Introduction​

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Characidae (sensu Mirande, 2019; Toledo-Piza et al., 2024), a megadiverse family of freshwater fishes native to the Americas and commonly referred to as tetras, displays especially high levels of species richness in the Neotropical region. With over 1,200 valid species, of which 400 have been described in the last two decades (Toledo-Piza et al.,2024), and a plethora of yet-to-be-described taxa, Characidae emerges as one of the most species-rich family among Neotropical freshwater fishes. Not only abundant in numbers, but also in the diversity of colors, body shapes and sizes, and evolutionary adaptations. In a recent study by Melo et al. (2024), the family Characidae was divided into three additional families: Spintherobolidae, Stevardiidae, and Acestrorhamphidae, based on ultraconserved elements. Despite the nomenclatural redefinition of Characidae in a more restricted sense, the clade formed by Characidae + Spintherobolidae + Stevardiidae + Acestrorhamphidae remains monophyletic and aligns with the circumscription of Characidae already present in Mirande (2019) and Toledo-Piza et al. (2024).

One of the most renowned genera among tetras is the Amazonian genus Inpaichthys Géry & Junk, 1977 which was described by Géry, Junk (1977) based on Inpaichthys kerri Géry & Junk, 1977. At present, the genus is assigned to the family Acestrorhamphidae sensu Melo et al. (2024). The species exhibits a distinctive life coloration, with a prominent longitudinal dark stripe traversing from the jaws to the caudal fin, complemented by a vivid blue dorsal region. Until recently, the genus was monotypic until Ferreira et al. (2024) described I. parauapiranga Ferreira, Ribeiro, Lima, Silva, Ferreira & Mirande, 2024 and also transferred Hasemania nambiquara Bertaco & Malabarba, 2007 to Inpaichthys. Thus, the genus had its diagnosis completely reformulated, and its richness was tripled (Ferreira et al., 2024; Fricke et al., 2025).

Recently, another remarkable species of Inpaichthys was discovered in the rio Juruena basin in Mato Grosso State. Specimens were first discovered by an ornamental fisherman who suspected the presence of a new species. He contacted me to check the identification and provided material for its scientific description. The aim of the present study is to describe the new species, including a brief discussion regarding its generic allocation and biogeographical insights.

Material and methods

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Counts and measurements follow Fink, Weitzman (1974), except for number of horizontal scale rows below lateral line, which are counted to the pelvic-fin insertion, not including the small scale at pelvic-fin insertion, and with the addition of head depth, measured at vertical through the base of supraoccipital process. Standard length (SL) is given in millimeters (mm) and all other measurements are expressed as percentage of SL or of head length (HL) for subunits of head. In the description, counts are followed by their frequency of occurrence in parentheses, and an asterisk indicates the counts of the holotype. Number of maxillary tooth cusps, small dentary teeth, supraneurals, branchiostegal rays, gill rakers, vertebrae, unbranched anal-fin rays, and procurrent caudal-fin rays were obtained only from cleared and stained (c&s) specimens prepared according to Taylor, Van Dyke (1985). Vertebrae of the Weberian apparatus are counted as four elements and the compound caudal centra (PU1+U1) as a single element. Circuli and radii counts were taken from scale row immediately above the lateral line and anterior to the dorsal fin. The sex of three specimens was confirmed by dissection and direct examination of the gonads. In the list of types and comparative material, catalog numbers are followed by the number of specimens in alcohol, their SL range, and if any, the number of c&s specimens and their respective SL range. In the type material, whenever DNA is mentioned, the specimens were fixed and preserved in absolute ethanol for genetic sampling. Institutional abbreviations follow Fricke, Eschmeyer (2025). Specimens of lot MUBIO 691 were neither measured nor counted due to their early juvenile developmental stage. Distribution data compiled for the biogeographic discussion were obtained directly from GBIF (2024), supplemented by an uncatalogued underscribed species of Astyanacinus Eigenmann, 1907 in the rio Jamanxim. Records falling outside the known distribution range of the taxa were suppressed. Astyanacinus was recently resurrected by Melo et al. (2024), and its definition here follows Dagosta (2011), encompassing Astyanax orthodus Eigenmann, 1907 and A. superbus Myers, 1942.

Results​

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Inpaichthys luizae,new species

urn:lsid:zoobank.org:act:D977A51A-B213-4804-8F70-66A7B1C2909C

(Figs. 1–4; 8; Tab. 1)

Holotype. MZUSP 130243, 32.1 mm SL, Brazil, Mato Grosso State, Juara municipality, tributary of the rio dos Peixes, near the confluence of the rio dos Peixes and rio Arinos, rio Tapajós basin, approx. 10°44’S 57°50’W, 11 Mar 2024, A. Bulgarelli.

Paratypes. Same data from holotype.MUBIO 500, 2, 25.5–26.3 mm SL c&s, 6, 25.5–38.2 mm SL, 1, 25.8 mm SL DNA; LBP 36835, 1, 26.9 mm SL DNA; ICTL 1570, 2, 24.2–26.4 mm SL. MUBIO 691, 8, 11.8–23.3 mm SL, 5 Feb 2025, A. Bulgarelli.

Diagnosis. Inpaichthys luizae can be easily distinguished from all congeners by the autapomorphic presence of a well-defined, relatively broad oblique dark ventrolateral stripe on body, extending approximately from end of pectoral fin rays to base of middle caudal-fin rays, and continuing straight onto the tip of the middle caudal-fin rays (vs. well-defined longitudinal stripe absent, or dark lateral stripe never oblique). Additionally, I. luizae can also be distinguished from I. nambiquara and I. parauapiranga by the absence of humeral blotch (vs. two in I. parauapiranga and one in I. nambiquara). It is further distinguished from I. kerri by 19–21 branched anal-fin rays (vs. 22–26 branched anal-fin rays).

Description. Morphometric data in Tab. 1. Greatest body depth at dorsal-fin origin. Dorsal profile slightly convex from tip of snout to dorsal-fin origin. Straight along dorsal-fin base and straight from that point to adipose-fin origin. Concave from terminus of adipose fin origin to origin of anteriormost dorsal procurrent caudal-fin ray. Ventral profile of head and body convex from tip of lower lip to anal-fin origin, slightly convex along anal-fin base (see Sexual dimorphism section), and concave from anal-fin end to origin of anteriormost ventral procurrent caudal-fin ray.

TABLE 1 | Morphometric data of the holotype and 12 paratypes of Inpaichthys luizae. Range includes the holotype. The high standard deviation observed in dorsal fin length is primarily attributed to damage sustained by many specimens, likely resulting from fin mutilation during live transport. SD = Standard deviation.

	Holotype	Range	Mean	SD
Standard length (mm)	30.2	23.3–38.2	28.7	–
Percentages of standard length
Depth at dorsal-fin origin	34.0	29.5–34.0	31.7	1.5
Snout to dorsal-fin origin	56.6	52.8–59.2	56.3	1.9
Snout to pectoral-fin origin	24.4	24.3–28.6	26.6	1.4
Snout to pelvic-fin origin	45.4	45.1–48.7	46.7	1.2
Snout to anal-fin origin	62.1	59.7–65.2	62.3	1.7
Caudal-peduncle depth	12.3	11.3–13.6	12.5	0.8
Caudal-peduncle length	12.2	8.8–12.2	10.2	1.2
Pectoral-fin length	20.8	17.8–22.4	20.4	1.3
Pelvic-fin length	14.2	12.2–15.6	14.0	1.0
Pelvic-fin origin to anal-fin origin	14.6	14.3–16.6	15.5	0.8
Dorsal-fin length	30.8	20.5–31.5	27.5	3.5
Dorsal-fin base length	13.9	12.6–14.3	13.7	0.5
Anal-fin length	14.6	12.9–18.8	16.4	1.8
Anal-fin base length	31.8	28.0–34.4	30.6	1.7
Eye to dorsal-fin origin	42.5	40.3–45.6	42.7	1.5
Dorsal-fin origin to caudal-fin base	54.2	47.5–54.2	50.7	2.1
Head depth	23.3	22.4–25.8	23.7	1.0
Head length	27.5	24.0–28.2	26.3	1.2
Percentages of head length
Horizontal eye diameter	36.5	34.4–39.3	36.8	1.6
Snout length	23.3	19.8–24.4	22.1	1.4
Interorbital width	35.5	32.7–36.0	33.7	1.1
Upper jaw length	39.0	39.0–44.2	41.6	1.5

Lower jaw slightly anterior to upper jaw, mouth slightly sub-superior. Second or third and fourth or fifth premaxillary teeth positioned slightly anteriorly, forming two roughly rows. Outer row with 2(10) or 3*(2) tricuspid teeth. Inner row with 5*(10) or 6(2) tricuspid teeth. Posterior tip of maxilla at vertical through posterior half of second infraorbital. Maxilla with 6(3), 7(8), or 8*(2) tricuspid teeth, anterior tooth larger and with more developed cusps than others. Dentary with 4(1) or 5*(12) larger tricuspid teeth followed by series of 9-10 diminute conical teeth. Central cusp in all teeth longer than lateral cusps. Four (2) branchiostegal rays; 3 on anterior ceratohyal, and 1 on posterior ceratohyal. Gill-rakers 9(2) in the lower (hypobranchial and ceratobranchial) and 5(2) in the upper branch (epibranchial) and one (2) on cartilage between ceratobranchial and epibranchial. Fourth infraorbital absent (2), sixth present but extremely reduced (2).

Cycloid scales, with 4–6 radii from focus to posterior border, and conspicuous circuli anteriorly. Lateral line incomplete and canals poorly developed, with 4*(2), 5(7), 6(2), or 7(2) perforated scales, and 29(4), 30(5), 31(2), 32*(1) or 33(1) total scales on longitudinal series. Longitudinal scale rows between dorsal-fin origin and lateral line 6*(5) or 7(8). Longitudinal scale rows between lateral line and pelvic-fin origin 4*(13). Scales along middorsal line between posterior tip of supraoccipital process and dorsal-fin origin 11(2) or 12*(11) and anteriorly irregular. Horizontal scale rows around caudal peduncle 14*(8) or 15(5). Base of anteriormost anal-fin rays covered by series of 4 to 6 scales. Caudal lobes scaled only at base, by scales roughly same size as rest of body. Supraneurals 6(1) or 7(1).

Dorsal-fin rays iii(2), 9*(13), first unbranched dorsal-fin ray reduced, visible only in c&s specimens. Base of last dorsal-fin ray posterior to anal fin origin. First dorsal-fin pterygiophore located after neural spine of 11^th vertebra (2). Pectoral-fin rays i*(13), 9*(8), 10(4) or 11(1). Pelvic-fin rays i*(13), 5(1) or 6*(12). Adipose fin with variable development: present (13)*, or absent (8). Anal fin without an anterior lobe, with iv(2), 19(3), 20*(5), or 21(5) branched rays. First anal-fin pterygiophore inserted behind haemal spine of 16^th vertebra (2). Males with bony hooks on anal-fin rays. Principal caudal-fin rays i,9,8,i(12) or i,8,8,i*(1); caudal fin forked, lobes somewhat pointed, of similar size. Dorsal procurrent caudal-fin rays 8(1) or 9(1); ventral procurrent caudal-fin rays 8(2). Total vertebrae 34(2): precaudal vertebrae 14(2) and caudal vertebrae 20(2).

Coloration in alcohol. Overall ground coloration of head and body beige (Fig. 1). Dorsal portion of head and dorsal midline of body darker. Snout, jaws and fifth and sixth infraorbitals with concentration of dark chromatophores, remaining infraorbitals and opercle region with scattered dark pigmentation. Central region of the opercle bone lacking pigmentation. Humeral blotch absent. Narrow subjacent longitudinal dark line extending along horizontal septum, from vertical through anal-fin origin to end of caudal peduncle. Broad oblique dark ventro-lateral stripe on body, extending approximately from end of pectoral fin rays to base of middle caudal-fin rays, and continuing straight onto the tip of the middle caudal-fin rays. Black chromatophores more densely concentrated in middle caudal-fin rays than in rest of ventrolateral stripe. Abdominal region below ventrolateral dark stripe with few scattered chromatophores. Sparse dark chromatophores above anal fin. Absence of delimited caudal-peduncle blotch. Adipose fin, when present, with dark chromatophores on anterior portion. Dorsal and anal fins with dark chromatophores on interadial membrane, not on lepidotrichia. Chromatophores darker at distal portion of anal fin, forming thin dark stripe at fin edge less intense in anteriormost anal-fin rays. Caudal fin with dark chromatophores on interadial membrane and lepidotrichia. Base of outer caudal fin rays, on both lobes, with lower pigment concentration, resulting in clearer areas above and below dark stripe. Pectorals and pelvics with dark chromatophores scattered along edge of lepidotrichia.

FIGURE 1| Inpaichthys luizae, holotype, MZUSP 130243, 30.2 mm SL, male, Brazil, Mato Grosso State, Juara municipality, tributary of the rio dos Peixes, affluent of the rio Juruena, rio Tapajós basin, approx. 10°44’S 57°50’W.

Coloration in life. Similar to color in alcohol except in the following details. Dorsal region olive green with blue iridescence (Figs. 2–3), abdominal region silvery. Midlateral surface with purplish coloration, mainly in males (see details in Sexual dimorphism section). Upper portion of eye orange to red, posterior region light blue. Opercular region with few guanine, exposing red branchial filaments inside branchial chamber, resulting in the cheek area with a rosy aspect. Gular region with orange pigmentation in some specimens. Bright orange stripe immediately above and contrasting oblique dark lateral stripe on body. All fins yellow to orange, less intense in pectoral fin. Distal portion of anal-fin rays darker. Base of outer caudal fin rays, on both lobes, usually distinctly orange.

FIGURE 2| Life coloration of the holotype, MZUSP 130243 (male, 30.2 mm SL).

FIGURE 3| Sexual dimorphism of Inpaichthys luizae: A. MUBIO 691 (male, 31.5 mm SL); B. MUBIO 691 (female, 38.2 mm SL); C. Unvouchered specimen (male); D. Unvouchered specimen (female). Photos C–D by André Bulgarelli.

Sexual dimorphism. Anal-fin rays of males more posteriorly inclined, appearing shorter than of females and resulting in a relatively convex distal margin (Figs. 3–4). Female anal-fin rays slightly more elongated than in males, resulting in distal margin straighter than in males. Male body slightly deeper and smaller. Males with more marked coloration, particularly in unpaired fins and in oblique dark lateral stripe. Hooks present on first to fifth branched anal-fin rays in males (Fig. 4). One pair of hooks per lepidotrichia segment, maximum four segments with hooks. Segment preceding the branching of the lepidotrichia is the most proximal with hooks.

FIGURE 4| Posterior half of the body of Inpaichthys luizae, highlighting the sexual dimorphism in the morphology of the anal fin: A. MUBIO 500 (male, 26.2 mm SL); B. MUBIO 500 (male, 25.5 mm SL, c&s), male, detail of hooks on rays; C. MUBIO 500 (female, 28.7 mm SL); D. MUBIO 500 (female, 26.3 mm SL, c&s).

Geographical distribution. Although the exact geographic coordinates remain unknown, the species occurs in tributaries of the rio dos Peixes basin, in the municipality of Juara, Mato Grosso State, Brazil (A. Bulgarelli, 2024, pers. comm.).

Etymology. The specific epithet luizae (n. f. gen. sg.) comes from Luíza. Inpaichthys luizae is named after my daughter Luíza Krauss Dagosta. A noun in a genitive case.

Conservation status. According to local fishermen who collected specimens, Inpaichthys luizae occurs in certain tributaries of the rio dos Peixes. These habitats are well-preserved and situated within privately owned lands, currently facing no immediate environmental threats (A. Bulgarelli, pers. comm.). The only potential concern for the species lies in its ornamental appeal, which is likely to be significantly heightened in the aquarium trade following the publication of its description. This could result in increased collection pressure in its natural habitat, potentially necessitating a future reassessment of its conservation status. At present, however, there is no substantial evidence of threats that would warrant a classification other than Least Concern. Therefore, Inpaichthys luizae should be listed as Least Concern (LC) under the categories and criteria of the International Union for Conservation of Nature (IUCN Standards and Petitions Subcommittee, 2024), pending the availability of additional data that may support a more comprehensive evaluation.

Discussion​

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MNew species lacking exact geographic coordinates: describe or not? A formal taxonomic description is essential for granting a species legal recognition, preventing unsustainable exploitation, and enabling effective conservation actions. Recognizing a species through valid scientific nomenclature is fundamental not only for its taxonomic legitimacy but also for allowing other biological research and management concerns. This is particularly crucial for species of commercial interest, such as Inpaichthys luizae, whose striking appearance and rarity make it valuable in the aquarium trade. Without a formal name, the species remains invisible to science, hindering ecological studies and precluding any assessment of its conservation status. A valid description not only enhances scientific recognition but also fosters public awareness and deepens understanding of the species’ natural populations.

Nevertheless, the process of describing species based on specimens lacking precise locality data raises significant taxonomic and conservation challenges. As highlighted by Marinho et al. (2016), several neotropical fish species have been described from individuals sourced via the aquarium trade. While this practice contributes to addressing the Linnean shortfall in neotropical ichthyology (Brown, Lomolino, 1998; Whittaker et al., 2005), it carries inherent risks related to the accuracy of distribution data and subsequent conservation assessments. For instance, some tetras described in the 2000s without precise locality data still lack published distribution records, such as Dectobrycon armeniacus Zarske & Géry, 2006, Hyphessobrycon clavatus Zarske, 2015, H. jackrobertsi Zarske, 2014, and H. paepkei Zarske, 2014 whose distributions remain entirely unknown since their original descriptions. Even more problematic is the case of Hyphessobrycon cyanotaenia Zarske & Géry, 2006 also described from aquarium material, whose type-locality was later proven to be completely incorrect, a mistake only rectified a decade later by Dagosta et al. (2016). A very similar situation occurred with Hyphessobrycon scholzei Ahl, 1937 also described from aquarium material, with its type-locality corrected only in Lima et al. (2025). Nevertheless, when used appropriately, aquarium specimens can be valuable for taxonomic purposes. For instance, Lima et al. (2025, fig. 1a) used individuals obtained through the aquarium trade to illustrate the coloration of the species, a practice also adopted by Marinho et al. (2016, fig. 2) and Marinho, Dagosta (2023, figs. 2C–D). These cases demonstrate that, even without precise provenance, such material can still contribute meaningfully to taxonomic research.

Marinho et al. (2016) emphasize that aquarium-derived material should only be used after comprehensive searches in ichthyological collections have failed to yield reliably localized specimens: “Therefore, an extensive examination of fish collections should be encouraged before naming a fish species from the aquarium trade in order to search for specimens from a reliable locality to be used in the description”. This is precisely the case with I. luizae, which, although lacking precise geographic coordinates, is not entirely of uncertain origin. Extensive surveys in the rio Juruena and neighboring basins have failed to locate specimens of I. luizae and no specimens of I. luizae have been found in Brazilian scientific collections, strongly suggesting that the species is narrowly distributed. Under these circumstances, relying on aquarium specimens obtained directly from local fishermen is currently a feasible approach for its formal description. Considering the species’ potential vulnerability, formally recognizing I. luizae is clearly preferable to leaving it in taxonomic obscurity. Doing so enables further biological research, informs conservation actions, and ultimately contributes to the sustainable management of its natural populations.

A particularly illustrative example of an even more challenging situation is Trichogenes beagle, described by de Pinna, Reis, and Britski (2020) without any assigned collection locality. At the time of its description, the complete absence of geographic information precluded any assessment of its conservation status. Only later in Santos et al. (2025) its distribution finally was identified, enabling a proper evaluation of its conservation needs. This case clearly demonstrates that formal taxonomic recognition, even under suboptimal circumstances, is a first step toward effective conservation.

While the practice of describing species without precise locality data should generally be avoided, given the taxonomic uncertainties and conservation challenges it entails, the case of I. luizae represents a justified exception. Unlike scenarios where no geographic information is available, in this case, the species’ occurrence is reliably associated with the rio dos Peixes, a tributary of the rio Juruena basin, and even the municipality of origin is known. This level of geographic certainty significantly mitigates the risks typically associated with descriptions based on aquarium-sourced material. Furthermore, it is expected that, in the near future, geo-referenced specimens will be collected, further refining the species’ distributional knowledge and strengthening its conservation framework.

Generic allocation. The generic allocation of the new species is based on the presence of synapomorphies for the genus proposed by Mirande (2019) and modified by Ferreira et al. (2024) (Fig. 5). Out of the eight synapomorphies listed by the authors, I. luizae presents seven, which are: nasal septum of mesethmoid formed by two parallel lamellae (9:1), absence of a dorsal expansion of rhinosphenoid projected medial to olfactory nerves (35:0), short supraoccipital spine covering dorsally only anterior axis of neural complex of Weberian apparatus (62:1), articulation between second and third infraorbitals anteroventrally angled (86:1), frontal and pterotic laterosensory canals forming an angle on pore receiving infraorbital canal (118:1), maxillary teeth extended approximately to half maxillary length (191:0), and presence of 24 or fewer branched anal-fin rays (420:0). The only synapomorphy of Inpaichthys absent in the species is the absence of the anguloarticular portion of the mandibular laterosensory canal (105:1).

FIGURE 5| Life coloration and hypotheses on the phylogenetic relationships among Inpaichthys species following Ferreira et al. (2024) with the addition of I. luizae. Characters and corresponding states follow Mirande (2019). The dashed line represents the hypothesized placement of I. luizae, inferred from the examination of genus-level synapomorphies as outlined by Ferreira et al. (2024), without the execution of a formal phylogenetic analysis.

Additionally, from nine of the characters listed by Ferreira et al. (2024) as useful for the diagnosis of the genus Inpaichthys, eight are present in I. luizae: two or more maxillary teeth with larger one, tri or pentacuspidate, ten or fewer gill rakers on first hypobranchial and ceratobranchial, interrupted lateral line, lack of parietal branch of supraorbital cephalic laterosensory canal, pore of the supraorbital cephalic laterosensory canal situated just anterior to dilatator fossa oriented dorsomedially, circuli absent on posterior field of scales, iii+9 dorsal-fin rays with the anterior one visible only in c&s specimens, and lack of pseudotympanum. Only the character-state ‘lack of secondary-sexual hooks on males’ differs in I. luizae, as the males of the species do have hooks (Fig. 4), being the first record of these structures in males of the genus.

The unequivocal phylogenetic placement of the species demands its inclusion in the character matrix of Ferreira et al. (2024). However, the species likely belongs to the clade I. kerri + I. parauapiranga, as it possesses all the three synapomorphies listed for that node: three cusps on anterior dentary teeth (201:0), metapterygoid foramen as a simple rounded opening (228:0), and first anal-fin pterygiophore located ventral to last dorsal-fin pterygiophores (416:1). Furthermore, I. luizae lacks any of the autapomorphies reported for I. nambiquara: third infraorbital not reaching the horizontal arm of preopercle in its anterior margin (88:1), presence of (a reduced, in this species) fourth infraorbital (90:0), three or fewer maxillary teeth (190:0), five or more cusps on anterior maxillary teeth (193:1), and the possession of four or fewer supraneurals (392:0).

Phylogenetic insights. The phylogenetic placement of the genus Inpaichthys remains somewhat controversial in the literature. Géry, Junk (1977:418) were the first to highlight the difficulty in establishing its evolutionary relationships, describing the genus as “difficult to position”. Four recent phylogenetic analyses of characids have proposed hypotheses regarding the placement of Inpaichthys (Fig. 6). Although the topologies proposed by Mirande (2019), Cucalón, Tan (2022), and Ferreira et al. (2024) are generally more congruent with one another and differ from that of Melo et al. (2024), a detailed comparison shows that the differences are not as substantial as they first appear.

FIGURE 6| Hypotheses on the phylogenetic relationships of Inpaichthys in the most recent publications: A. Mirande (2019), Cucalón, Tan (2022); B. Ferreira et al. (2024); C. Melo et al. (2024).

Melo et al. (2024) present the most comprehensive and up-to-date phylogenetic analysis of Characidae, incorporating an extensive dataset of newly generated molecular data based on ultraconserved elements (UCEs). In their study, Inpaichthys is recovered as sister to Bryconamericus orinocoensis Román-Valencia plus Hyphessobrycon sp. Jari, with this clade positioned at the base of Thayeriinae (sensu Melo et al., 2024). Thayeriinae, in turn, is the sister group to a clade composed of Rhoadsiinae and (Grundulinae + Acestrorhamphinae). While the relationship between Inpaichthys, B. orinocoensis, and Hyphessobrycon sp. Jari is strongly supported (100% bootstrap), at present, no morphological synapomorphies have been identified to support this arrangement, nor are there any morphological features considered relevant for discussion within this evolutionary scenario. Notably, B. orinocoensis exhibits a complete lateral line, a short anal fin, and pale body coloration marked by a broad silvery lateral stripe, whereas Hyphessobrycon sp. Jari represents an undescribed species.

In contrast, the other three studies consistently recover Inpaichthys as closely related to Nematobrycon Eigenmann, 1911, or to a clade including Nematobrycon along with the Rhoadsiini genera (Pseudochalceus Kner, 1863, Carlana Strand, 1928, and Rhoadsia Fowler, 1911) (Mirande, 2019; Cucalón, Tan, 2022; Ferreira et al., 2024). Although the precise phylogenetic affinities of Inpaichthys and its potential close relatives remain subject to debate (Fig. 6), several morphological features observed in I. luizae, in other Inpaichthys species, and in some of the aforementioned related taxa (e.g., fin coloration, bluish-purple iridescence on the lateral body, elongation of the posterior anal-fin rays, and absence of the adipose fin) are noteworthy and should not be overlooked at this point. Although the evolutionary significance of these traits requires further phylogenetic investigation, they would be interpreted as homoplasies if the hypothesis proposed by Melo et al. (2024) is ultimately supported over time. Accordingly, this brief discussion of these characters is intended to encourage further exploration of these questions in future studies, using as a framework the convergent hypotheses of Mirande (2019), Cucalón, Tan (2022), and Ferreira et al. (2024).

i. Yellowish to orangish fins

All species of Inpaichthys exhibit yellowish or orangish fins, particularly in males. This characteristic is less pronounced in I. kerri, particularly noticeable in its lightly pigmented pelvic fin (Fig. 5). Among Nematobrycon species, this condition varies; while present in N. palmeri Eigenmann, 1911, N. lacortei Weitzman & Fink, 1971 displays pinkish or reddish fins. In Rhoadsiini (sensu Mirande, 2019), a similar pattern of fin coloration is observed.

ii. Bluish-purple iridescence on lateral body

All species of Inpaichthys and Nematobrycon exhibit a bluish-purple iridescence along the lateral of their bodies. In I. nambiquara, this pattern is notably less intense, yet some specimens may exhibit such iridescence above the base of the anal fin (Fig. 7). Although bluish iridescence has independently evolved in various lineages of Characidae (sensu Mirande, 2019), such as in Stevardiinae (Boehlkea Géry, 1966, Cyanogaster Mattox Britz, Toledo-Piza & Marinho, 2013, Diapoma Cope, 1894, Glandulocauda Eigenmann, 1911, and Mimagoniates Regan, 1907), Heterocharacinae (Heterocharax Eigenmann, 1912), and Stethaprioninae (Paracheirodon Géry, 1960) and in some species of Hyphessobrycon (e.g., H. columbianus Zarske & Géry, 2002, H. margitae Zarske & Géry, 2016, and H. wadai Marinho, Dagosta, Camelier & Oyakawa, 2016), none of them have been proposed to be related to the Inpaichthys + Nematobrycon clade (cf. Mirande, 2019; Ferreira et al.,2024). Therefore, this coloration pattern could be interpreted as an additional morphological character supporting a close relationship between Inpaichthys and Nematobrycon.

FIGURE 7| Live specimen of Inpaichthys nambiquara, MZUSP uncatalogued, male. White arrows indicate bluish iridescence on lateral body and black arrow shows elongated posterior anal-fin rays reaching procurrent rays.

iii. Length of the posterior rays of the anal fin

Some species within the Inpaichthys + Nematobrycon + Rhoadsiini clade exhibit relatively elongated posterior anal-fin rays compared to other tetras, extending to the ventral procurrent rays and, in some cases, even reaching the principal caudal-fin rays of the ventral lobe. This pattern is not observed in I. kerri but is evident in well-developed individuals of I. nambiquara (see Fig. 7) present, albeit more subtly, in I. luizae, I. parauapiranga, and in other three undescribed species of the genus (FCPD, 2014, pers. obs.). Both species of Nematobrycon prominently display this trait. Among Rhoadsiini, the condition is either subtle or absent in Rhoadsia and Parastremma Eigenmann, 1912, whereas Carlana eigenmanni (Meek, 1912) and all four species of Pseudochalceus clearly exhibit it. Other lineages consistently recovered as closely related to this group, such as Hollandichthys Eigenmann, 1909 + Rachoviscus Myers, 1926 (cf. Mirande, 2019; Ferreira et al.,2024), also present this condition, rendering the evolutionary interpretation of this trait more complicated. Bertaco, Malabarba (2013) described the elongation of the posterior anal-fin rays in Hollandichthys as a synapomorphic modification for the genus. While Hollandichthys indeed exhibits an anterior constriction of the anal-fin rays, its homology with similar traits in other species remains uncertain, as relatively closely related groups exhibit conditions that, while morphologically similar, are not necessarily identical.

iv. Absence of adipose fin

Inpaichthys luizae is one of the few species of Characidae (sensu Mirande, 2019) exhibiting polymorphism in the presence of the adipose fin (Fig. 8). Dagosta et al. (2022) list eight tetra species that show variation in the presence or absence of this fin. Inpaichthys luizae falls into the “most specimens with an adipose fin” category, alongside four species of Hyphessobrycon. The compilation of Dagosta et al. (2022) highlights the rarity of this polymorphism, suggesting it may hold phylogenetic significance. Notably, while the absence of an adipose fin is generally uncommon among tetras, within the Inpaichthys + Nematobrycon + Rhoadsiini clade (cf. Mirande, 2019; Ferreira et al., 2024), it occurs in I. nambiquara, both Nematobrycon species, and as a polymorphic trait in I. luizae. Whether this pattern is merely coincidental or holds deeper significance remains uncertain and requires further investigation.

FIGURE 8| Juvenile specimen of Inpaichthys luizae (MUBIO 691, 23.3 mm SL) lacking adipose fin.

Biogeographic comments. Under the hypothesis proposed by Melo et al. (2024), little biogeographic information can be inferred, as Inpaichthys would be associated with other species from the crystalline shield (Bryconamericus orinocoensis and Hyphessobrycon sp. Jari). In contrast, the hypotheses of Mirande (2019), Cucalón, Tan (2022), and Ferreira et al. (2024), which suggest a close evolutionary relationship between Inpaichthys, Nematobrycon, and Rhoadsiini, may carry significant biogeographic implications. Both Nematobrycon and the Rhoadsiini lineages occur only in Andean region, whereas Inpaichthys is endemic to the uplands of the northwestern edge of the Brazilian shield, draining into the Amazon by the Aripuanã and Tapajós drainages. At least two other monophyletic lineages exhibit a similar allopatric pattern: Lebiasina and Astyanacinus. The genus Lebiasina Valenciennes, 1847 is predominantly diversified in the Andean slopes, with only a few exceptions such as Lebiasina yepezi Netto-Ferreira, Oyakawa, Zuanon & Nolasco, 2011, found in the upper rio Negro, rio Branco, and rio Orinoco in the Guiana Shield (Netto-Ferreira et al.,2011), and three Lebiasina species endemic to the rio Curuá basin in the Serra do Cachimbo, also on the northwestern edge of the Brazilian shield (Netto-Ferreira, 2012). Other case is Astyanacinus (sensu Dagosta, 2011), whose diversity is predominantly Andean but with records of Astyanacinus moori on the western edge of the Brazilian shield, drained by the headwaters of the rio Paraguay and a record in the northwestern edge of the Brazilian shield (a new species from the rio Jamanxim basin, FCPD, pers. obs.).

The three lineages are not closely related, but the biogeographic homology of their distribution pattern should be investigated using molecular approaches, whose dating may confirm whether they share a convergent spatial distribution driven by the same geomorphological events. These lineages conform to the pattern described by Lima, Ribeiro (2011) and Dagosta, de Pinna (2019), where species are restricted to the highlands surrounding the cis-Andean lowlands, predominantly around the Western Amazon. This pattern is intricately linked with the historical dynamics of the foreland basin and the uplift of the Andes and typically features rheophilic lineages adapted to swift-flowing, oxygen-rich waters.

The isolation of these records in southern Amazonia from their congeners in other uplands is remarkable (Fig. 9). Such occurrences lack plausible explanations for their presence other than being relictual lineages, remnants of a broader distribution that once extended continuously to the Andean foothills but subsequently became extinct in the lowland Amazon. These examples highlight the importance of shields as repositories of biodiversity, where lineages persist over geological time (Albert et al., 2011,2021). Clearly, the Brazilian shield region still requires more comprehensive sampling to thoroughly grasp distribution patterns. Consequently, isolated records may be attributed in part to the incomplete understanding of the region. Nevertheless, an extensive sampling of the ichthyofauna living in the Brazilian shield may reveal new lineages closely related to species associated with the highlands of western South America, particularly those from the Andean region.

FIGURE 9| Distributional maps of: A. Astyanacinus spp.; B. Lebiasina spp.; C. Inpaichthys spp. (purple), Nematobrycon spp. (pink), Parastremma spp. (yellow), Rhoadsia spp. (blue), type-locality of Inpaichthys luizae (pink star).

Comparative material examined. Material listed in Dagosta et al. (2016); Inpaichthys kerri, holotype, INPA 10408, 28 mm; Inpaichthys parauapiranga, paratypes, MUBIO 113, 5, 19.0–26.3 mm SL; ZUEC 17920, 4, 20.9–22.4 mm SL.

Acknowledgments​

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I acknowledge the valuable discussions with Flávio C. T. Lima (UNICAMP), Rafaela P. Ota, and Renata R. Ota (MUBIO-UFGD). Angela M. Zanata (UFBA), Juan M. Mirande (CONICET) and Katiane M. Ferreira (UFMT) reviewed an earlier version of this manuscript, and I am grateful for their insights. I am also grateful to André Bulgarelli for drawing my attention to the species and for kindly providing specimens for this study. I am also thankful to Michel Gianeti and Murilo Pastana (MZUSP), Cláudio Oliveira (LBP), and Fernando Carvalho (CITL) for curatorial assistance.

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Authors

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Fernando Cesar Paiva Dagosta¹

^[1]    Museu da Biodiversidade (MUBIO) and Laboratório de Biogeografia e Sistemática de Peixes (LABISPE), Universidade Federal da Grande Dourados, Rodovia Dourados/Itahum, km 12, 79804-970 Dourados, MS, Brazil. ferdagosta@gmail.com (corresponding author).

Authors’ Contribution

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Fernando Cesar Paiva Dagosta: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing-original draft, Writing-review and editing.

Ethical Statement​

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Specimens were collected under a professional license for the collection of ornamental fish issued by the Secretaria do Meio Ambiente do Estado de Mato Grosso (SEMA/MT), license n^o 698/2024–2025 and registered in the RGP/MPA (MTPA92778046100, 52813.100046/2018–06). Specimens were sent to me alive and, following acclimation and in vivo color pattern documentation, were euthanized using eugenol (clove oil) at a concentration of 200 mg/L.

Competing Interests

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The author declares no competing interests.

Data availability statement

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The data supporting the findings of this study are available from the author upon reasonable request.

Funding

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This study was supported by INCT – Peixes, funded by MCTIC/CNPq (proc. 405706/2022–7).

How to cite this article

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Dagosta FCP. An ode to beauty: the discovery of one of the most spectacular tetras (Characiformes: Acestrorhamphidae) – a new species of Inpaichthys from the rio Juruena, Brazil. Neotrop Ichthyol. 2025; 23(3):e250052. https://doi.org/10.1590/1982-0224-2025-0052

Copyright​

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This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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Creative Commons CC-BY 4.0

© 2025 The Authors.

Diversity and Distributions Published by SBI

Accepted July 3, 2025

Submitted March 28, 2025

Epub November 7, 2025