Description of a New Species of Hyphessobrycon (Characiformes: Acestrorhamphidae) from Rio Madeira, a Greatly Impacted Region of the Amazon Rainforest, Apuí, Brazil

HYPHESSOBRYCON is the most diverse genus within the acestrorhamphid family, comprising about 148 valid species (Marinho and Dagosta, 2024; Melo et al., 2024; Toledo-Piza et al., 2024). These species are spread across rivers, streams, and lakes from southern Mexico to the Río de La Plata basin, Argentina. Most species are found in the cis-Andean region, with more than half occurring in the Amazon basin (e.g., Ohara and Lima, 2015; Pastana and Ohara, 2016; Dagosta and de Pinna, 2019).

Traditionally, Hyphessobrycon has been characterized by having the premaxilla bearing two rows of teeth, an incomplete lateral line, and the caudal-fin lobes not covered by small scales (Durbin in Eigenmann, 1908, 1917). However, recent phylogenetic studies based on both phenotypic and genotypic data (e.g., Javonillo et al., 2010; Mirande, 2010; Oliveira et al., 2011; Mirande, 2018; Terán et al., 2020; Melo et al., 2024) have demonstrated that Hyphessobrycon is paraphyletic. As an example, Mirande (2018) included species of Hyphessobrycon together either with genera such as Astyanax and related taxa, with species belonging to Jupiaba, Pristella, and Bryconella, or as a sister group to Erythrocharax. Additionally, some species were considered to be part of Hemigrammus, Petitella, Paracheirodon, and Brittanichthys. A recent phylogenomic study corroborated the non-monophyletic nature of Hyphessobrycon and reallocated some species previously included in Hyphessobrycon to genera such as Hasemania, Psalidodon, and the resurrected Megalamphodus (Melo et al., 2024). In spite of the advances in phylogenetic studies of Hyphessobrycon, no morphological diagnosis of the genus was offered. Therefore, Hyphessobrycon is still diagnosed by a set of non-exclusive characters originally proposed by Durbin (in Eigenmann, 1908) and Eigenmann (1918) until a comprehensive and revisionary phylogenetic study of the genus is undertaken. Recent taxonomic approaches have proposed various groups of species within Hyphessobrycon (Géry, 1977; Weitzman and Palmer, 1997; Ingenito et al., 2013; Lima et al., 2014; Pastana and Ohara, 2016), which, although not extensively based on cladistic analyses, have been useful for understanding the rich diversity of the genus.

A recent expedition undertaken to the southeast region of the Amazonas State aiming at sampling in major tributaries of the Rio Madeira in Brazil has yielded an undescribed species of Hyphessobrycon. This new species is formally described herein, according to the traditional definition of the genus.

MATERIALS AND METHODS

Meristic and morphometric data follow Fink and Weitzman (1974) and Menezes and Weitzman (1990), with summarized modifications by De Lima et al. (2023), except by the number of horizontal scale rows below the lateral line, which is taken from the pelvic-fin origin (see Pastana and Ohara, 2016). Number of scale rows in the horizontal line between dorsal-fin origin and lateral line does not include scales of the median predorsal series, situated just anterior to first dorsal-fin ray. Counts and measurements were taken on the left side of specimens, except when structures were damaged. Measurements are presented as percent of standard length, except for subunits of the head, which are given as percent of head length. In the description, the frequency of each count is given in parentheses after the corresponding count, and counts corresponding to the holotype are indicated by an asterisk. Number of supraneurals, vertebrae, procurrent caudal-fin rays, unbranched dorsal- and anal-fin rays, branchiostegals, gill-rakers of the first branchial arch, tooth cusps, total number of dentary teeth, and the position of pterygiophores were taken from cleared-and-stained paratypes (CS), prepared according to the technique of Taylor and Van Dyke (1985). Vertebrae of the Weberian apparatus were counted as four elements (included in the vertebral counts), and the compound caudal centrum (preural 1 + ural 1) as a single element. Lateral-line nomenclature follows Pastana et al. (2020). Taxonomy and classification of Characiformes follows Melo et al. (2024). Institutional codes follow Sabaj (2020, 2023).

Hyphessobrycon mapinguari, new species

urn:lsid:zoobank.org:act:6685FDA5-DA0E-4211-A0C3-95FC0255BA3A

Figures 1, 2A, 3, 4, 5; Table 1

Holotype.—

MZUSP 130000, 34.8 mm SL, male, Brazil, Amazonas, Apuí, Rio dos Couros, tributary of Rio Acari, Rio Madeira basin, 7°12′27.22″S, 59°30′55.69″W, O. Oyakawa, W. Ohara, T. Teixeira, and M. Pastana, 10 October 2016.

Paratypes.—

All from Brazil, Amazonas, Apuí, Rio Madeira basin. MZUSP 122695, 13 + 2 CS, 21.5–32.2 mm SL, collected with holotype; INPA 61191, 10, 17.1–27.6 mm SL, MZUSP 130713, 6, 15.7–21.6 mm SL, same locality as holotype, M. Pastana, M. Gianeti, W. Ohara, and R. N. Mendes-Junior, 30 May 2024; INPA 61192, 22, 14.7–34.7 mm SL, MZUSP 130712, 36, 12.7–32.8 mm SL, small stream tributary of Rio dos Couros, itself a tributary of Rio Acari, 7°08′45.31″S, 59°33′43.45″W, M. Pastana, M. Gianeti, W. Ohara, and R. N. Mendes-Junior, 30 May 2024.

Diagnosis.—

Hyphessobrycon mapinguari can be diagnosed from all congeners, except H. bussingi, H. columbianus, H. daguae, H. diastatos, H. elachys, H. heliacus, H. jackrobertsi, H. loweae, H. negodagua, H. notidanos, H. paepkei, H. panamensis, H. peugeoti, H. platyodus, H. procyon, H. pyrrhonotus, and H. werneri by having an elongated dorsal fin in adult males (vs. dorsal fin not elongated). Hyphessobrycon mapinguari differs from H. bussingi, H. columbianus, H. daguae, and H. panamensis by lacking hooks on fins of mature male specimens (vs. presence of one large anterodorsally oriented bony hook on last unbranched and first branched anal-fin rays of mature males). It is distinguished from H. diastatos, H. elachys, H. heliacus, H. negodagua, H. peugeoti, and H. procyon by the absence of a dark caudal-peduncle blotch (vs. dark caudal-peduncle blotch present). Hyphessobrycon mapinguari can be diagnosed from H. notidanos by having 23–27 branched anal-fin rays (vs. 16–21 in H. notidanos), and from H. jackrobertsi, H. paepkei, H. pyrrhonotus, and H. werneri by having six to seven teeth in the inner series of the premaxilla (vs. 7–12 in H. pyrrhonotus and H. werneri; 8 in H. jackrobertsi; 7–8 in H. paepkei). Hyphessobrycon mapinguari is most similar to H. platyodus, being diagnosed from this species by having 6–7 longitudinal scale rows between dorsal-fin origin and lateral line (vs. 5), 14 circumpeduncular scales (vs. 11–12), and the inner row of premaxillary teeth tetra- or pentacuspid (vs. tetra- to octacuspid). The new species is further diagnosed from H. platyodus by details of live coloration, such as presence of red color at the dorsal portion of the iris not reaching the margin of the pupil (vs. red color on dorsal portion of iris reaching margin of pupil; Fig. 2), possession of a red to orange caudal-peduncle blotch (vs. caudal-peduncle blotch absent), presence of dark pigmentation on dorsal-fin interradial membrane (vs. dorsal-fin pigmentation uniformly red), and absence of chevron-shaped bars on the flank (vs. chevron-shaped bars present).

The recently resurrected genus Megalamphodus, currently containing 12 nominal taxa formerly allocated in Hyphessobrycon, also exhibit dorsal-fin elongation in adult males. The new species is diagnosed from most species of Megalamphodus by having six teeth in the inner series of the premaxilla (vs. 5 premaxillary teeth in the inner series of M. copelandi and M. khardinae; 7–12 in M. bentosi, M. epicharis, M. erythrostigma, M. rosaceus, and M. socolofi; and 9–10 in M. sweglesi). It differs from the remaining species of Megalamphodus by the absence of chalky-white pigmentation at the tip of the first three to six dorsal- and anal-fin rays (vs. dorsal- and anal-fin tips chalky-white in M. eques, M. haraldschultzi, and M. micropterus), and by having a single, vertically elongated dark humeral blotch with diffuse margins (vs. a dark, well-delimited humeral spot with regular margins in M. eques and M. megalopterus).

Description.—

Morphometric data in Table 1. Largest examined specimen 34.8 mm SL (holotype). Body compressed, moderately deep (Figs. 1, 2A, 3, 5). Greatest body depth at dorsal-fin origin. Dorsal profile of head convex from snout tip to vertical through nostrils, straight from that point to tip of supraoccipital spine. Dorsal profile of body convex from tip of supraoccipital spine to dorsal-fin origin, slightly convex along dorsal-fin base, straight from dorsal-fin terminus to adipose-fin origin, slightly concave from that point to anteriormost dorsal caudal-fin ray. Ventral profile of body convex from tip of lower jaw to pelvic-fin origin, approximately straight from pelvic-fin origin to anal-fin origin, straight along anal-fin base, concave between last anal-fin ray and anteriormost ventral caudal-fin procurrent ray.

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Table 1.Morphometry of Hyphessobrycon mapinguari. Min = minimum, Max = maximum, SD = standard deviation, n = number of specimens analyzed.

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Mouth terminal, posterior end of maxilla not reaching vertical through middle of orbit. Premaxillary teeth arranged in two rows (Fig. 4), outer row with 3(9) or 4*(7) tricuspid (16) teeth. Inner row with 6*(15) or 7(1) teeth, symphyseal and lateralmost teeth tetracuspid, remaining teeth pentacuspid (16). Maxilla with 3(7), 4*(6), or 5(2) tricuspid (largest maxillary teeth) to pentacuspid teeth (remaining maxillary teeth). Dentary teeth with 6*(7) or 7(9) large pentacuspid teeth followed by 7(2) conical to tricuspid teeth, gradually decreasing in size from symphyseal to lateralmost tooth.

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First gill-arch with 18 rakers (2), articulating with branchial elements as follows: hypobranchial with 1 raker, hypobranchial/ceratobranchial articulation with 1 raker, ceratobranchial with 8 rakers, ceratobranchial/epibranchial articulation with 1 raker, epibranchial with 6 rakers, epibranchial/pharyngobranchial articulation with 1 raker. Branchiostegals 4(2), 3 articulating with anterior ceratohyal, 1 with the posterior ceratohyal.

Scales cycloid, without circuli on exposed portion of scales, with two divergent radii reaching posterior margin of scale. Lateral line incompletely pored, with 5(1), 7(1), 8(8), 9(9), 10(2), or 11*(1) perforated scales. Longitudinal scale row, including perforated scales, with 31(1), 32(1), 33*(7), 34(4), or 35(1) scales. Longitudinal scale rows between lateral line and dorsal-fin origin 6*(15) or 7(1). Scale rows from lateral line to pelvic-fin origin 4*(16). Predorsal scales 9*(9), 10(4), or 11(3). Circumpeduncular scales 14*(15). Axillary scale present. Caudal-fin scales restricted to base of rays on upper and lower lobes of fin.

Laterosensory system well developed, tubules opening in individual pores (i.e., never forming subpores sensu Pastana et al., 2020). Supraorbital canal associated with nasal, frontal, and parietal. Epiphyseal and parietal branches of supraorbital canal present. Supraorbital connecting to infraorbital and otic canal at posterolateral margin of frontal. Infraorbital canal fully developed. Otic canal present, associated with anterior portion of pterotic. Postotic canal associated with pterotic, extrascapular, post-temporal, and supracleithrum. Supratemporal canal associated with extrascapula and posterior margin of parietal. Preopercular canal extending along entire extension of pre-opercular bone, continuous with mandibular canal. Mandibular canal associated with anguloarticular and dentary.

Pectoral-fin rays i,10(1) or i,11*(13), longest ray surpassing vertical through pelvic-fin insertion. Pelvic-fin rays i,7*(16), distal margin of pelvic fin nearly straight, reaching vertical through second branched anal-fin ray. Dorsal-fin rays ii,8(1) or ii,9*(15). First dorsal-fin pterygiophore inserted posteriorly to neural spine of eighth vertebra (2). Anal-fin rays iii,23(1), iii,24*(5), iii,25(6), iii,26(2), iv,25(1), or iv,27(1), first unbranched ray covered by skin, last unbranched and first branched anal-fin rays longest. First anal-fin pterygiophore inserted posteriorly to haemal spine of 15th vertebra (2). Caudal fin forked, with lobes equal in size, slightly pointed. Caudal-fin rays i,17, i*(16). Dorsal caudal-fin procurrent rays 10(1) or 11(1). Ventral caudal-fin procurrent rays 9(2). Adipose fin present. Supraneurals 4(2). Precaudal vertebrae 15(2). Caudal vertebrae 18(1) or 19(1). Area underneath humeral spot with hypaxial muscle thinner than surrounding areas, densely impregnated with adipose tissue, but not forming a pseudotympanum (i.e., muscular hiatus absent).

Coloration in alcohol.—

Ground body coloration cream (Figs. 1, 3). Dorsal portion of head dark brown, with high concentration of chromatophores. Lateral and ventral portion of head cream, except for a few large chromatophores present on infraorbitals, preopercle, and dorsal portion of opercle. Faint vertical black bar with diffuse borders across middle portion of eye. Body uniformly cream, with few small and sparse chromatophores. Humeral region with single vertically elongated dark blotch with diffuse margins, bordered anteriorly by unpigmented area. Humeral blotch composed of large chromatophores encompassing seven to eight scales vertically and three scales horizontally. Thin longitudinal line formed by dark pigmentation running along horizontal septum of body, starting on vertical immediately anterior to dorsal-fin insertion, extending to caudal peduncle. Dark caudal-peduncle spot absent. Caudal fin mostly dusky due to presence of scattered chromatophores along ray margins, and on inter-radial membranes. Dorsal-fin pigmentation sexually dimorphic. Dorsal fin of dimorphic males mostly dark, with concentration of black chromatophores at interradial membrane from 1st unbranched to 5th branched dorsal-fin rays. Non-dimorphic males, females, and juveniles with anterior half of dorsal fin dusky; distal portion of last unbranched and anteriormost branched, and entire area of second to fifth branched dorsal-fin rays and respective interradial membranes covered by black chromatophores. Males and females with pectoral, pelvic, and anal fins uniformly dusky, with dark chromatophores more concentrated along leading margin of anteriormost rays and interradial membrane. Adipose fin pale, with few scattered chromatophores.

Color in life.—

Based on photographs of freshly collected specimens (Figs. 2A, 5) and field observation. Lateral surface of head mostly silver due to guanine deposits, with iridescent golden yellow to green pigments concentrated on premaxilla, dorsal half of maxilla, and at dorsal half of opercle. Eye red dorsally and silver ventrally—red pigmentation restricted to dorsal region of pupil. One iridescent orange midlateral stripe starting immediately posterior to opercle and extending to dorsal lobe of caudal fin, located dorsal to thin dark longitudinal line running along horizontal septum of body. Scattered iridescent golden yellow pigmentation present anterior and posterior to humeral spot, and anterior half of flanks. Ventral portion of head and pectoral-fin base mostly white, with few scattered chromatophores. Lateral and ventral surfaces of body with scattered orange to red chromatophores. Dorsal fin mostly dark, except at proximal portion of unbranched and first three branched dorsal-fin rays, which exhibit diffuse orange pigmentation. Pectoral fin hyaline. Pelvic, anal, and caudal fins with scattered dark red chromatophores. Adipose fin translucent, with faint orange hue proximally and few scattered black chromatophores distally. Round, orange to red caudal-peduncle spot present at base of principal caudal-fin rays. Dimorphic males with ground coloration on top of head and dorsal portion of body orange to brown, and lateral and ventral portion of head and body white.

Females and immature specimens with similar, but less intense live color pattern. Ground coloration on top of head and dorsal portion of body faint yellow to green, partially translucent, with few scattered dark chromatophores. Abdominal region white to gray. Lateral and ventral surfaces of body with scattered dark red chromatophores. Dorsal fin with orange pigmentation at basal portion of unbranched and anteriormost three branched rays. Orange pigmentation with well-defined limits from subsequent dark pigmentation. Remaining fins with pigmentation similar to males.

Sexual dimorphism.—

Mature males of Hyphessobrycon mapinguari with anal-fin margin straight, without a discernible lobe along the fin. Females and immature specimens with a typical anal-fin lobe anteriorly, including the last unbranched and anteriormost six branched anal-fin rays. Hyphessobrycon mapinguari is sexually dichromatic (sensu Pastana et al., 2017). Ground coloration of sexually dimorphic males darker, with dorsal and lateral portions of body orange to brown (vs. overall ground coloration of females and juveniles faint yellow to green); sexually dimorphic males with ventral portion of body (abdominal region and anal-fin base) more intensely red-pigmented due to the presence of orange to red chromatophores, and a broader and more strongly pigmented caudal-peduncle spot (vs. flanks and ventral portion of bodies of females and juveniles not as densely pigmented, and caudal-peduncle spot not as wide); dorsal fin of sexually dimorphic males darker, with blurred dark pigmentation reaching base of all dorsal-fin rays (vs. base of unbranched and first three branched dorsal-fin rays of females and juveniles orange, black pigmentation not reaching base of anteriormost dorsal-fin rays).

Etymology.—

The specific epithet, “mapinguari,” alludes to the indigenous Brazilian folklore, particularly among tribes of the Amazon rainforest, in which Mapinguari is a mythic creature frequently regarded as a guardian of the forest, ensuring its natural balance and punishing those who damage the ecosystem. The legend of the Mapinguari can also be interpreted as a cautionary tale about the repercussions of unchecked exploitation of natural resources, highlighting the importance of environmental preservation. The specific epithet is a noun in apposition.

Geographical distribution.—

Hyphessobrycon mapinguari is known from Rio dos Couros and a small tributary draining to this river. Rio dos Couros drains to rio Acari, which is a major tributary of Rio Canumã, Rio Madeira basin, Apuí, Amazonas, Brazil (Fig. 6).

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Ecological notes.—

Hyphessobrycon mapinguari was found in two localities with little riparian vegetation and surrounded by pasture. The new species was captured in abundance in a slow-flowing, small stream with clear water with moderate concentration of marginal grasses (Fig. 7A). However, only a few specimens were captured in Rio dos Couros, which had little aquatic vegetation in its margins (Fig. 7B). Collection efforts in beach, leaf litter, and rocky areas with strong water flow failed to capture the species.

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Conservation status.—

Currently, Hyphessobrycon mapinguari is known only from the Rio dos Couros and one of its small, unnamed tributaries, both of which flow to the Rio Acari, Rio Madeira basin. Due to limited data on its geographic distribution, its conservation status remains uncertain. In the absence of more comprehensive information and a specific threat identified to the species, Hyphessobrycon mapinguari should be preliminarily classified as Least Concern (LC) according to the International Union for Conservation of Nature (IUCN) categories and criteria (IUCN Standards and Petitions Subcommittee, 2024).

DISCUSSION

The hypothesis that Hyphessobrycon constitutes an unnatural assemblage of fishes has long been recognized (e.g., Böhlke, 1955; Weitzman, 1977a, 1977b; Weitzman and Fink, 1983; Weitzman and Palmer, 1997). The polyphyletic nature of the genus has been supported by numerous phylogenetic analyses conducted over recent decades, utilizing both morphological and molecular data (Javonillo et al., 2010; Mirande, 2010; Ohara et al., 2017a, 2019; Melo et al., 2024). This non-monophyly, combined with the extensive diversity of the genus, renders Hyphessobrycon one of the most taxonomically challenging groups among Neotropical freshwater fishes. To address this taxonomic complexity, several species groups have been proposed within Hyphessobrycon. These groups are based on shared morphological traits and coloration patterns (e.g., Géry, 1977; Weitzman and Palmer, 1997; Ingenito et al., 2013; Lima et al., 2014; Pastana and Ohara, 2016). This approach remains in use mostly for empirical reasons, awaiting stability in the taxonomy and phylogeny of the genus.

Hyphessobrycon mapinguari exhibits morphological attributes that align with various groups within Hyphessobrycon, making its classification challenging. For example, this species shares several morphological characteristics with those in the so-called “Rosy-tetra” group, as hypothesized by Weitzman and Palmer (1997). Members of this proposed clade typically have a relatively deep and laterally compressed body, with adults reaching 40 to 60 mm standard length. Although the original composition of the group, as proposed by Weitzman and Palmer (1997: appendix 1), had long been known to be a non-monophyletic entity (e.g., Javonillo et al., 2010; Mirande, 2010), many of the species treated as rosy tetras by Weitzman and Palmer (1997) are resolved as a clade in analyses based both on morphological (Mirande, 2018; Terán et al., 2020) and molecular data (Javonillo et al., 2010; Melo et al., 2024). Some of these species currently comprise the resurrected genus Megalamphodus, allocated in the newly erected subfamily Megalamphodinae based on a broad phylogenomic analysis (Melo et al., 2024). At present, Megalamphodus comprises a clade of 12 acestrorhamphid species, morphologically diagnosed from other megalamphodines by the presence of a conspicuous black blotch on the dorsal fin (Melo et al., 2024). Numerous traits consistent in Megalamphodus can be observed in Hyphessobrycon mapinguari, including a relatively deep body, red to reddish-brown coloration, a red to orange coloration at the base of the anteriormost rays transitioning to a black blotch posteriorly, and a caudal peduncle lacking a dark spot but presenting an orange to red spot in that region. Furthermore, H. mapinguari shares with many megalamphodines a red dorsal portion of the eye, sexual dimorphism concerning the dorsal fin with elongation of the anteriormost rays in adult males, and expanded black pigmentation at the base of the dorsal fin (Fig. 5A; Weitzman and Palmer, 1997: figs. 1–4). Although we suspect that Hyphessobrycon mapinguari may fall within the subfamily Megalamphodinae, a generic allocation of the new species under the current classification of the Acestrorhamphidae would necessarily require molecular phylogenetic studies, as no morphological synapomorphies for megalamphodine genera were proposed by Melo et al. (2024).

Hyphessobrycon mapinguari possesses more than five teeth composing the inner row of the premaxilla. The number of teeth in the inner row of the premaxilla is a traditional morphological characteristic used to distinguish amongst genera of families formerly allocated under the term “Characidae,” i.e., Acestrorhamphidae, Stevardiidae, Spintherobolidae, and Characidae stricto sensu. For instance, Géry (1977) classified genera with four teeth in the inner premaxillary row under his “genus Hemibrycon and allied genera.” Conversely, Moenkhausia (Eigenmann, 1903, 1917) and Astyanax (Eigenmann, 1917, 1921) typically have a stable count of five teeth in the inner premaxillary row, a characteristic used to define these genera. While most species of Hyphessobrycon align with Moenkhausia or Astyanax in having five teeth in the inner premaxillary series, there is notable variation within the genus. For example, Hyphessobrycon compressus, the type species of the genus, has six to eight teeth in the inner premaxillary row (Carvalho and Malabarba, 2016), a count that overlaps that observed in H. mapinguari. Other species of Hyphessobrycon, such as H. caru, H. jackrobertsi, H. paepkei, H. pando, H. piorskii, H. pyrrhonotus, H. simulatus, and H. werneri, also exhibit more than five teeth in the inner series of the premaxilla, varying from 6–9 in H. piorskii (Guimarães et al., 2019) to 7–12 in H. pyrrhonotus and H. werneri (Weitzman and Palmer, 1997). Curiously, a high number of premaxillary teeth was one of the characters that led Weitzman and Palmer (1997) to allocate Megalamphodus epicharis within a putative sub-group of rosy tetras, the “bentosi-group” proposed by Géry (1977). Many of the taxa suggested by Weitzman and Palmer (1997) to be included in the “bentosi-group” are currently in Megalamphodus (e.g., M. bentosi, M. epicharis, M. erythrostigma, and M. socolofi), and share with the abovementioned species of Hyphessobrycon an increased number of teeth in the inner premaxillary series. Additional dental characters of Hyphessobrycon mapinguari present in some megalamphodines are a gradual decrease in dentary tooth size from the largest, symphyseal tooth to the lateralmost ones (Fig. 4; Hein, 2008: fig. 4B). This contrasts with many species of Hyphessobrycon, which typically have four or five significantly larger anteriormost dentary teeth followed by much smaller teeth (e.g., Lima and Gerhard, 2001: fig. 4; Moreira et al., 2002: fig. 2; Pastana and Ohara, 2016: fig. 2; Ohara et al., 2019: fig. 3). Despite the similarities mentioned above, an allocation of H. mapinguari within the Megalamphodinae based on dental characteristics would be premature. It is expected that a denser sampling within the Megalamphodinae, including other species of Hyphessobrycon morphologically similar to Megalamphodus (e.g., H. jackrobertsi, H. paepkei, or H. pando), may indicate some of the characters listed above as morphological diagnoses to groups or sub-groups within this clade.

The recently described Hyphessobrycon platyodus is the species that is morphologically most similar to H. mapinguari. It shares with this new species a yellow overall ground coloration, a dark vertical humeral spot with diffuse margins, absence of a dark caudal-peduncle spot, red pelvic and anal fins, anal fin with dark pigment on leading margin of anteriormost rays, lateral and ventral portion of body scattered with red chromatophores, dorsal portion of eye red, and eye crossed by a vertical black bar. In addition, both species also share a similar sexual dimorphism, by which dimorphic males exhibit the anal fin with a straight distal margin (vs. anal fin concave, with anteriormost anal-fin rays longer than others). Despite being a rosaceous to reddish species, the original description of Hyphessobrycon platyodus did not indicate resemblances between this taxon and species assigned to taxa currently allocated in the Megalamphodinae. According to Ohara et al. (2017b), H. platyodus mostly resembles species currently allocated in the Hyphessobrycon loweae-group (sensu Ingenito et al., 2013; Pastana and Ohara, 2016) based on an elongated dorsal fin in adult males, absence of bony hooks on fin rays, and a straight profile of the distal margin of the anal fin. The similarities shared between H. mapinguari and H. platyodus are so far restricted to morphological observations, and a definitive allocation of this species in one of the existing species groups within Hyphessobrycon, or at least a recognition of its possible allies, still awaits further investigations. Interestingly, Hyphessobrycon platyodus is a species described from the Rio Juma drainage—a tributary of the Rio Aripuanã in the Rio Madeira basin—and was described from an area less than 50 kilometers east of the type locality of H. mapinguari. Although Rio Juma and Rio Acari run parallel to one another, field surveys in Apuí did not locate H. platyodus in the Rio Acari drainage or H. mapinguari in the Rio Juma or its tributaries. This suggests that, despite their close geographical proximity and similar appearances, these species are confined to their respective drainages. Rio Juma flows into the Rio Aripuanã, which merges with the Rio Madeira at Novo Aripuanã town, while Rio Acari connects to the Rio Canumã and joins the Rio Madeira at Nova Olinda do Norte town, approximately 200 kilometers downstream from Novo Aripuanã.

The discovery of Hyphessobrycon mapinguari from Rio Acari, Rio Madeira basin, highlights the diversity of the headwaters in the area. Recent surveys undertaken along the Transamazonic Road, an unpaved highway crossing the southeastern portion of Amazonas State, more specifically in the Madeira-Tapajós interfluve, has yielded a number of new species of small-sized fishes with restricted occurrence, e.g., Cyphocharax orion, Hyphessobrycon platyodus, Hyphessobrycon procyon, Poecilocharax callipterus, and Poecilocharax rhizophilus (Pastana and Ohara, 2016; Ohara et al., 2017b, 2023; Melo and Faria, 2024). This segment of the Transamazonic Road provides direct access to headwaters of several tributaries of the Rio Madeira (e.g., Rio Marmelos, Rio Manicoré, Rio Aripuanã, Rio Acari, Rio Camaiu, and Rio Sucunduri), facilitating a deeper sampling of small streams before they connect to major river systems. In contrast, this same area has been greatly impacted by cattle ranching and logging, activities that affect the Amazon forest and small drainages and represent a direct thread for freshwater fish species. Research undertaken in the region indicates that Apuí, a municipality encompassing the type locality of all abovementioned species and located alongside the Transamazonic Road, is a deforestation hotspot in Brazil’s state of Amazonas. In only the 2013–2018 period, the city was responsible for a growth in area cleared corresponding to more than twice the rate for the rest of Brazilian Amazon (Carrero et al., 2020). A recent Deforestation Report published by the Amazon Institute of People and the Environment (IMAZON; Amorim et al., 2024) indicated that in June 2024 the Amazonas state was responsible for 35% (or 140 km²) of Brazil’s total deforestation. From the top five cities that most contributed to IMAZON’s index, four are located in Amazonas State and three of them (e.g., Apuí, Novo Aripuanã, and Lábrea) connect to each other and to other Brazilian states through the Transamazonic Road. Up to the present date, scientists are still working on the discovery and documentation of species in that area, and information on the ecology and distribution of many of these species is still unavailable. Nevertheless, the alarming rate of human occupation and deforestation affecting the south Amazonas State region represents an alert for the conservation status of the small-sized species occurring in the area, especially those of limited occurrence and/or of interest to aquarium hobbyists (e.g., Hyphessobrycon platyodus, Poecilocharax callipterus, or Poecilocharax rhizophilus).

MATERIAL EXAMINED

All from Brazil.

• Hyphessobrycon compressus: MZUSP 18422, 5, 22.2–26.1 mm SL.

• Hyphessobrycon diastatos: MZUSP 114026, 35 paratypes, 10.3–22.3 mm SL, 2 CS, 19.2–29.9 mm SL.

• Hyphessobrycon elachys: MZUSP 59415, 232, 10.2–17.9 mm SL, 9 CS.

• Hyphessobrycon hasemani: MZUSP 37652, 21, 12.5–28.6 mm SL.

• Hyphessobrycon heliacus: MZUSP 52891, holotype, 26.3 mm SL; MZUSP 52893, 15 paratypes, 22.9–27.6 mm SL; MZUSP 62943, 1 paratype, 19.5 mm SL.

• Hyphessobrycon kayabi: MZUSP 112222, 20 paratypes, 19.2–26.3 mm SL.

• Hyphessobrycon loweae: MZUSP 97435, 220, 11.2–26.6 mm SL; MZUSP 102816, 20, 2.1–34.1 mm SL.

• Hyphessobrycon moniliger: MZUSP 89117, 863, 17.3–30.5 mm SL, 5 CS.

• Hyphessobrycon myrmex: MZUSP 118672, 106 paratypes, 10.6–24.3 mm SL.

• Hyphessobrycon notidanos: MZUSP 89645, 15 paratypes, 12.6–25.7 mm SL.

• Hyphessobrycon platyodus: MZUSP 117607, 21 paratypes, 24.4–32.8 mm SL, 3 CS.

• Hyphessobrycon procyon: MZUSP 117601, 10 paratypes, 24.6–32.2 mm SL.

• Hyphessobrycon pulchripinnis: MZUSP 92682, 48, 18.3–26.7 mm SL.

• Hyphessobrycon takasei: MZUSP 29842, 32, 22.8–30.3 mm SL.

• Megalamphodus bentosi: MZUSP 29854, 47, 23.8–27.1 mm SL; MZUSP 100786, 21, 18.9–22.7 mm SL, 3 CS.

• Megalamphodus copelandi: MZUSP 76232, 24, 24.6–27.2 mm SL.

• Megalamphodus eques: MZUSP 90070, 111, 15.8–29.8 mm SL.

• Megalamphodus erythrostigma: MZUSP 77155, 3, 20.8–23.3 mm SL.

• Megalamphodus haraldschultzi: MZUSP 105356, 33, 16.3–24.9 mm SL.

• Megalamphodus megalopterus: MZUSP 115639, 24, 12.8–17.0 mm SL.

• Megalamphodus micropterus: MZUSP 47336, 8, 22.8–26.7 mm SL.

• Megalamphodus rosaceus: MZUSP 16900, 38, 22.2–29.4 mm SL.

• Megalamphodus socolofi: MZUSP 11700, 50, 22.6–28.9 mm SL.

• Megalamphodus sweglesi: MZUSP 75459, 36, 23.5–35.1 mm SL.

DATA ACCESSIBILITY

Unless an alternative copyright or statement noting that a figure is reprinted from a previous source is noted in a figure caption, the published images and illustrations in this article are licensed by the American Society of Ichthyologists and Herpetologists for use if the use includes a citation to the original source (American Society of Ichthyologists and Herpetologists, the DOI of the Ichthyology & Herpetology article, and any individual image credits listed in the figure caption) in accordance with the Creative Commons Attribution CC BY License. ZooBank publication urn:lsid:zoobank.org:pub:83ADCE69-5FA7-43B1-B5D8-4711C150873B.

AI STATEMENT

The authors declare that after the text was written, Grammarly (https://www.grammarly.com/) was used to check for clarity and grammar.