Diagnosis

Eigenmannia sirius is diagnosed from all putative congeners with exception of Eigenmannia trilineata species-group (sensu [8]), by the presence of superior midlateral stripe (vs. absence). The new species differs from all species of the Eigenmannia trilineata species-group, except E. besouro, E. correntes, E. vicentespelaea, and E. waiwai, by the subterminal mouth (vs. terminal). It differs from E. besouro, E. correntes, E. vicentespelaea, and E. waiwai by relative depth of posterodorsal expansion on infraorbitals 1+2 corresponding to 70% length of infraorbitals 1+2 (Fig 2—vs. 40% in E. besouro, E. correntes and E. waiwai, and approximately equals total length of infraorbitals 1+2 in E. vicentespelaea), and in having 15 precaudal vertebrae (vs. 14 in E. besouro and E. correntes, 13–14 in E. vicentespelaea and 12–13 in E. waiwai). Eigenmannia sirius can be differentiated from E. besouro by the origin of the superior midlateral stripe at vertical between base of 23^rd to 31^st anal-fin ray (vs. origin at vertical between 5^th to 15^th anal-fin ray). The new species is also diagnosed from E. correntes by the eye diameter (17.2–23.8% H_L vs. 10.6–13.3% H_L), and by the mouth width (13.1–22.4% H_L vs. 23.5–26.0% H_L). Eigenmannia sirius is further distinguished from E. vicentespelaea and E. waiwai by having 15–24 premaxillary teeth (Fig 3A vs. 25–26 and 35–40, respectively) and 15–33 dentary teeth (Fig 3B vs. 38–45 and 37–38, respectively). It additionally differs from E. vicentespealea by having 9–12 scale rows above lateral line (vs. seven or eight) and the coronomeckelian bone corresponding to 20–25% of length of Meckel’s cartilage (Fig 3B vs. 45%). The new species can be distinguished from Archolaemus species by the eye completely covered by thin membrane (vs. a free orbital rim); from Distocyclus by the rounded snout in profile (vs. conical snout); from Japigny by the absence of alternating dark bands on flanks (vs. presence); and from Rhabdolichops by the region above the lateral line on the anterior portion of the body covered by scales (vs. absence of scales above the lateral line on the anterior portion of the body).

Description

Body shape and pigmentation in Figs 1, 4 and 5. Morphometric data for examined specimens in Table 1. Largest examined specimen 127.5 mm L_EA. Body elongate, distinctly compressed. Greatest body depth at vertical through distal margin of pectoral fin. Dorsal profile of body convex to straight. Ventral profile slightly convex. Caudal filament elongated, about a half of the length to end of anal-fin in adult specimens.

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Table 1. Morphometrics for examined specimens of Eigenmannia sirius, new species.

SD = Standart Deviaton.

https://doi.org/10.1371/journal.pone.0220287.t001

Head compressed, greatest width at opercular region and greatest depth at nape. Dorsal profile of head convex. Ventral profile of head slightly straight. Snout subconical in lateral view. Mouth subterminal with rictus extending posteriorly to vertical between nares. Anterior naris tube-like, closer to snout tip than to anterior margin of eye. Posterior naris rounded, without tube; near midpoint between anterior naris and anterior margin of eye. Eye small, circular, completely covered by thin membrane, on anterior one-half of H_L and dorsolaterally positioned. Gill opening limited to posterior margin of opercle and extending above and below pectoral-fin base. Gill rakers tiny and fleshy. Branchial membranes joined at isthmus.

Anus and urogenital papilla adjacent. Position of anus and urogenital papilla shifting through ontogeny from vertical through posterior margin of opercle to vertical through posterior margin of eye.

Scales small, cycloid, extending from immediately posterior of head to tip of caudal filament. Scales present on mid-dorsal region of body. Scales above lateral line at midbody nine (1), 10* (11), 11 (13) or 12 (1). Lateral-line scales to vertical through anal-fin terminus 92 (1), 94 (1), 96 (1), 97 (1), 100 (1), 103 (3), 104 (4), 105 (2), 106* (3), 107 (3), 110 (1), 111 (2) or 114 (1).

Pectoral-fin rays ii,13 (7), ii,14* (12) or ii,15 (8). Distal pectoral-fin margin straight and surpassing anal-fin origin. Total anal-fin rays 157 (1), 158 (1), 159 (1), 160 (1), 165 (2), 166 (1), 168 (2), 169 (2), 170 (1), 171 (1), 172 (1), 174* (2), 176 (5) or 183 (1). Anal-fin origin at vertical through posterior limit of pectoral-fin base. Distal margin of anal fin straight. First unbranched rays tiny; rays progressively increasing in size to first branched rays. Branched rays nearly equal length, except for posterior most rays that progressively decrease in size.

Color in alcohol

Ground coloration of body beige (Figs 1 and 4). Body densely covered by dark brown chromatophores gradually more spaced ventrally. Chromatophores more concentrated on perforated scales forming a narrow lateral line stripe. Superior midlateral stripe thick, one or one and half scales deep, tapering from vertical between base of 23^rd to 31^st anal-fin ray to posterior two-thirds of body. Second layer of pigmentation formed by multiple, small bars of dark chromatophores situated between musculature associated with anal-fin pterygiophores. Dark individual bars in combination form two stripe-like patterns. Inferior midlateral stripe approximately as wide as orbital diameter. Anal-fin base stripe approximately as wide as orbital diameter. Superior midlateral stripe and inferior midlateral stripe separated by weakly pigmented area, absent in juveniles (Fig 4). Head densely covered by dark brown chromatophores gradually more spaced ventrally. Pectoral and anal fins hyaline with scattered dark brown chromatophores overlying fin rays.

Color in life

Based on photographs taken in field (Fig 5). Overall body translucent, darker in mid-dorsal region of trunk; dorsal region of body yellowish brown in large specimen (95.9 mm L_EA) (Fig 5A) or greyish yellow in small specimen (79.0 mm L_EA) (Fig 5B). Dorsal region of head dark brown, ventral region lighter, with diffuse chromatophores. Opercle translucent in small specimens, exposing red color of gills; opercle light brown with a concentration of black chromatophores in large specimens. Silvery abdominal region due presence of iridophores covering external portion of peritoneal membrane, more evident in small than large specimens. Midlateral stripes equally located and similar to those found in fixed specimens. Pectoral and anal fins hyaline with scattered dark brown chromatophores overlying fin rays.

Distribution and habitat

Eigenmannia sirius is currently known only from rio Mutum, a tributary of the upper rio Juruena, rio Tapajós basin, Comodoro, Mato Grosso, Brazil (Fig 9). The type-locality is 502 m above sea level at the Chapada dos Parecis plateau. It is a clearwater river up to c. 3–6 m wide and 0.5–2.5 m deep, preserved riparian vegetation, swift current, and sand, pebbles and dead leaves on the bottom (Fig 10). Several types of microenvironment were sampled exhaustively, but E. sirius was captured only between root and subaquatic vegetation. Other species sampled syntopically were Aequidens cf. rondoni (Miranda Ribeiro), Erythrinus erythrinus (Bloch & Schneider), Hemigrammus skolioplatus Bertaco & Carvalho, Hyphessobrycon hexastichos Bertaco & Carvalho, H. melanostichos Carvalho & Bertaco, Hasemania nambiquara Bertaco & Malabarba, and Hoplerythrinus unitaeniatus (Spix & Agassiz) (for other species see [46]). No other Gymnotiformes were collected with E. sirius.

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Fig 9. Map of the upper portion of rio Tapajós basin, showing the locality of Eigenmannia sirius.

Star represents more than one lot.

https://doi.org/10.1371/journal.pone.0220287.g009

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Fig 10. Photograph of rio Mutum, type locality of Eigenmannia sirius, illustrating habitat.

https://doi.org/10.1371/journal.pone.0220287.g010

Conservation status

Eigenmannia sirius is currently known only from its type-locality, and it may have a very restricted distribution. However, considering that no imminent threats to the species were detected in the area of its occurrence, E. sirius would be classified as Least Concern (LC) according to the International Union for Conservation of Nature (IUCN) categories and criteria [47].

Myological analysis across Sternopygidae

Anatomical studies in Gymnotiformes follow the trend of efforts implemented in other Teleostei groups and focused on detailed descriptions of distinct osteological complexes (e.g., [15,32,48]), neuroanatomical structures or anatomical components associated with electrogenesis and electroreception (e.g., [37,38,49–51]). Recently, efforts were employed aiming to describe structures recently discovered in Gymnotiformes, like the caudal skeleton in Electrophorus Gill [52] and pseudotympanum [53]. Thus, studies of comparative anatomy in Gymnotiformes trends to explore traditional sources of information, resulting in a hiatus on several anatomical complexes that can be crucial in understanding the evolution of the group, such as myological tissues, which have rarely been explored [34,54–56]. As a result, characters derived from myology represents less than 0.2% of the entire universe explored of the morphological traits in the cladistic studies in Gymnotiformes (e.g., [18–21,42,57,58]). After a myological analysis, additional data derived from this poorly explored source of data led us to infer putative useful variations in a phylogenetic scenario.

The Sternopygidae genera presents the insertion of the adductor mandibulae, pars malaris occurring primary in the mesial face of the posterodorsal expansion of the infraorbital 1+2 (Fig 11). Towards its insertion, the mesialmost fibers of the pars malaris converge onto an endomaxilar ligament which, in turn, inserts in the mesial face of the posterodorsal portion of the maxillary bone. This configuration is an exclusive condition shared in all Sternopygidae genera, which differs from remaining Gymnotiformes families, and has been proposed as a synapomorphy for Sternopygidae, however, with superficial homology inferences on the sections of the adductor mandibulae (see Datovo & Vari, [28,29] for discussions on the homology of adductor mandibulae sections). For instance, previous studies [20,22] proposed the aforementioned condition as a homoplastic feature found in Rhamphichthys Müller & Troschel, however, in this genus, the pars malaris converge onto a wide endomaxilar ligament which, in turn, inserts onto the mesial face of the antorbital and maxillary bones (Fig 12) (per. obs.; [34]), without association with any infraorbital bone, thus non-homologous with those condition found in Sternopygidae genera. Additionality, the ventral fibers of Rhamphichthys converge in a ventral ligament, named herein as “accessory endomaxilar ligament”, which inserts only to the mesial face of the antorbital. The monophyly of Sternopygidae was questioned only by studies grounded in molecular data [18,22], with its monophyly well-corroborated in several studies from distinct source of data [15,16,19–21,40–43,49], including the aforementioned myological data source.

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Fig 11. Lateral view of adductor mandibulae, segmentum facialis (A) and medial view of adductor mandibulae, pars malaris (B) of Sternopygus astrabes, MZUSP 88795, 151.0 mm L_EA.

Am = adductor mandibulae; An = anguloarticular; Bm = buccopalatal membrane; De = dentary; El = endomaxilar ligament; Io = infraorbital; Mx = maxillary bone; Re = retroarticular. Endomaxilar ligament and buccopalatal membrane dissected. Scale: 3 mm.

https://doi.org/10.1371/journal.pone.0220287.g011

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Fig 12. Lateral view of adductor mandibulae, pars malaris of Rhamphichthys hahni, MZUSP 24736, 479.5 mm.

Am = adductor mandibulae; Ao = antorbital; Bm = buccopalatal membrane; El = endomaxilar ligament; Ela = endomaxilar ligament accessory; Mx = maxillary bone. Scale: 1 cm.

https://doi.org/10.1371/journal.pone.0220287.g012

All members of Eigenmanniinae exhibit the adductor mandibulae, pars stegalis originating from the anterior portion of hyomandibula, metapterygoid and quadrate, without attachment to the sphenotic bone. This condition differs that observed in most gymnotiform species, in which the pars stegalis originates from the sphenotic and suspensorial bones. In all Eigenmanniinae genera, except for Distocyclus, only the mid-posterior and posterior portion of the adductor arcus palatini is located medially to the pars stegalis (Fig 13A), resulting in a partial overlapping, which is distinct from the condition found in most Gymnotiformes, where the pars stegalis completely overlaps the adductor arcus palatini (Figs 13B and 14). In Distocyclus, the pars stegalis is located laterally only in the region near adductor arcus palatini insertion, without overlapping with the mid-posterior or posterior portion of the latter. Furthermore, the R-avn is located medially to the entire levator operculi (Figs 8 and 13A) in Eigenmanniinae, that contrasts with the condition found in all other Gymnotiformes analyzed herein where the R-Avn is located laterally to the levator operculi anterior and medially to the levator operculi posterior (Figs 13B and 14). Thus, grounded in the phylogenetic relationships hypothesized by [21], the following conditions are shared by Eigenmanniinae members: (1) absence of an association of pars stegalis with the sphenotic bone, (2) R-avn entirely medial to the levator operculi, and (3) partial overlapping between pars stegalis and adductor arcus palatini, with the loss of the overlapping between the last two muscles in D. conirostris (Eigenmann & Allen 1942).

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Fig 13. Lateral view of the dorsolateral head muscles of: A) Rhabdolichops troscheli, MZUSP 57704, 104.2 mm L_EA and B) Sternopygus macrurus, MPEG 22756, 245.8 mm L_EA.

Am = adductor mandibulae; Io = infraorbital; Mx = maxillary bone; Sc = supracleithrum + posttemporal canal; LO = Levator operculi; Rmt = ramus mandibularis trigeminus. Recurrent ramus of anteroventral part of anterior lateral line nerve in green. Scales: 3 mm (A) and 1 cm (B).

https://doi.org/10.1371/journal.pone.0220287.g013

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Fig 14. Lateral view of the dorsolateral head muscles of Gymnotus cylindricus, USNM 134701, 178.5 mm L_EA.

Am = adductor mandibulae; Io = infraorbital; LO = levator operculi; Mx = maxillary bone; R-avn = recurrent ramus of anteroventral part of anterior lateral line nerve; Rmt = ramus mandibularis trigeminus. Posterodorsal portion of LO, posterior accidentally removed. Scale: 4 mm.

https://doi.org/10.1371/journal.pone.0220287.g014

Within Eigenmanniinae, only Archolaemus, Distocyclus, Eigenmannia and Rhabdolichops exhibit the anterior margin of the levator arcus palatini insertion nearly straight in relation to the horizontal arm of the preopercle, resulting in an angle near 90° to the longitudinal axis of the head (Figs 8 and 13A). In contrast, the anterolateral margin of levator arcus palatini is arranged obliquely in relation to the longitudinal axis of the head, forming an angle near 45° with the longitudinal axis of the head in most gymnotiforms species, including Sternopygus and Japigny (Figs 13B and 14). Therefore, the condition found in Archolaemus, Distocyclus, Eigenmannia and Rhabdolichops could indicate an useful variation for the elucidation of a close relationship among these four genera within Eigenmanniinae, and due its absence in Japigny, could provide an additional evidence for the phylogenetically position of Japigny as a sister-group of all Eigenmaniinae genera [59], and not related to Eigenmannia species (see Remarks).

Taxonomic considerations

Although there is no agreement about the relationships within Eigenmanniinae, all genera have been demonstrated as monophyletic assemblages, except for Eigenmannia. In an effort to define Eigenmannia, Mago-Leccia [16] distinguished the genus among the remaining Sternopygidae by the presence of intermuscular bones at precaudal vertebrae 7 to 9 having a high branched structure, and presence of five branchiostegal rays, the two anteriormost narrow and the three remaining triangular in shape. However, despite their occurrence in E. sirius, the Mago-Leccia’s definition for Eigenmannia has not been recoveredin a phylogenetic scenario based on morphologial (e.g., [19,20]), molecular ([22]; in part) or model-based total evidence [21] data. Due to the doubtful status of Eigenmannia as a monophyletic genus, the inclusion of the E. sirius in Eigenmannia is provisionally justified by the presence in the former of the synapomorphies of Eigenmanniinae [15,19–21], and by the absence of those synapomorphies of the remaining Eigenmanniinae genera as discussed by previous studies [9,10,23,25,45]. In addition to our conservative position regarding the proposal of a new genus, we detail the differences between the species described herein and the current definition of all eigenmanniin genera (for external features see Diagnosis).

The new species does not fit in Japigny by the absence of single row teeth at the base of the upper oral valve (vs. presence); the presence of the contact of the lateral process of the second vertebrae with the parapophysis of the fourth vertebrae (vs. absence); and the retroarticular is included in the socket of the lower jaw with the quadrate (vs. retroarticular not included in the socket of the lower jaw with the quadrate) [59]. Eigenmannia sirius does not fit in the Archolaemus definition by the teeth of the first row immobile and firmly attached on the ventral surface of the premaxilla margins (vs. anterobasal margins of the teeth of the first tooth row attached to the dentigerous surface of the premaxilla) [59]. Eigenmannia sirius can be distinguished from the Distocyclus by the presence of endopterygoid teeth (vs. absence); dentary with 15–33 teeth arranged in two or three complete rows (vs. a single tooth row limited to the anterior portion of dentary) [7]. Further, the new species is distinguished from Rhabdolichops by the premaxilla approximately rectangular (vs. premaxilla trapezoidal and elongate); one prootic foramen (vs. two); the extrascapular independent from the neurocranium (vs. extrascapular fused with the neurocranium); and gill rakers short and not ossified (vs. gill rakers long and bony) [20,31].

The historical status of Eigenmannia as a non-monophyletic genus resulted in a classification based on species groups. The first author who advocates the classification grounded in species group was Alves-Gomes [22], which proposed the clade “Eigenmannia virescens species-group” composed by five undescribed species (Eigenmannia cf. virescens 1, 2, 3, 4 and 5; Alves-Gomes [22]). Few years later, Albert [20] did also not recover the Eigenmannia monophyly and proposed its classification in two species group: “Eigenmannia microstoma species-group” and “Eigenmannia virescens species-group”. In the same study, Albert [20] listed “Eigenmannia gr. macrops”, composed by E. macrops and an undescribed species (designated as “Eigenmannia sp. B”), however, without discussion about its relationships or classification.

According to Albert [20], the “Eigenmannia microstoma species-group” (proposed to allocate E. humboldtii, E. limbata, E. microstoma, E. nigra and an undescribed species from rio Paraiba) can be distinguished by the body depth in mature specimens with more than 11% T_L and total length over 350 mm in sexually mature individuals. The Eigenmannia virescens species-group sensu [20] (composed by E. virescens, E. trilineata and an undescribed species from Río Salí, Argentina) was proposed to distinguish species that share the presence of two or three longitudinal stripes on flanks, lateral valvula of cerebellum large, and anterior intermuscular bones highly branched. Despite the valuable contribution to the classification in Gymnotiformes in the early 2000’s, the classification of Eigenmannia presented in [20] failed to establish reliable data. Exemplifying, measurement taken as a proportion of total length in Gymnotiformes is not truthful given that fishes often suffer damages in caudal filament by predations, and posteriorly regeneration [60]. Additionally, even when considering only undamaged specimens, the body depth taken as proportion of L_EA are useful only in interspecific levels within Eigenmannia, being irrelevant to supra-specific taxa. Further, E. microstoma shows four dark longitudinal stripes in flanks [8,61] and anterior intermuscular bones highly branched (MCP 45216). The presence of stripes on flanks in E. microstoma is mentioned by Albert [20], however, the author did not explain the positioning of E. microstoma in its group, since the presence of stripes on flanks and the anterior intermuscular bones highly branched are synapomorphies for the Eigenmannia virescens species-group. In addition, E. virescens is characterized by a uniform color pattern, without midlateral stripes (see Peixoto et al. [8] for taxonomic discussion of E. virescens). Peixoto et al. [8] did not follow the classification proposed by Albert [20] because of these difficulties in recognizing the characters or the taxonomic compositions and proposed the so called “Eigenmannia trilineata species-group”, in which all species share the superior midlateral stripe.

In a recent contribution, Waltz & Albert [10] arbitrarily assigned E. meeki to the Eigenmannia trilineata species-group ([10]: 399; and Table 2 explicitly based on other contributions e.g., [8,23–25,45]). However, the color pattern of E. meeki is characterized by the lateral line stripe, inferior midlateral stripe and anal-fin base stripe, without the superior midlateral stripe [45], thus, lacking evidences for this proposition.

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Table 2. Taxonomic arrangement of species group in Eigenmannia.

https://doi.org/10.1371/journal.pone.0220287.t002

Moreover, Waltz & Albert [26] classified Eigenmannia into two groups, besides the E. trilineata species-group. The so called “Eigenmannia humboldtii species-group” sensu [26] was proposed as grouping species with: (1) larger body size (>45 cm T_L); (2) deep body at maturity (body depth >11% T_L); and a (3) darker body coloration in some specimens. Subsequently, Waltz & Albert [10] redefined the Eigenmannia humboldtii species-group by: (1) large adult body sizes—(>300 mm total length), (2) deep body shape—body depth greater than 18% of the length to the end of anal fin (L_EA), (3) broad and opaque body in life, and (4) absence of longitudinal stripes. In the latter contribution, the authors indicated E. humboldtii, E. limbata and E. nigra as members of the Eigenmannia humboldtii species-group. Herein, we corroborate the taxonomic composition of the Eigenmannia humboldtii species-group. However, through the discrepancies in the definition of this group, and in addition to the problematic reference to the measurements taken as proportion of T_L in Gymnotiformes, we redefine the Eigenmannia humboldtii species-group to accomplish species that share: (1) the presence of anal fin margin distinctly darkened and (2) absence of longitudinal stripes.

The second group proposed by Waltz & Albert [26] was the“Eigenmannia macrops species-group”, including only E. macrops, and according to authors, distinguished by: (1) body fairly laterally compressed, (2) translucent white/yellow coloration in life, (3) longitudinal stripes absent, (4) eye large (greater than ore qual to snout length), (5) long caudal filament (half of body length without head). Posteriorly, Waltz & Albert [10] exclude the character “body fairly laterally compressed” of its definition. Previously, the “Eigenmannia macrops species-group” had already been proposed by [20], and, not only it is composed by only one species, therefore lacking evidence for the proposition of a group of species, but the characters described are generalized features of the remaining Eigenmannia species, except for “eye large (greater than or equal to snout length)” and “(5) long caudal filament (half of body length without head)” which are putative diagnostic for E. macrops. Therefore, there is no evidence for the recognition of a species group designated as E. macrops until the detection of additional species putatively related to this species.

Consequently, the best resolution is a classification of Eigenmannia into two species groups. The striped species compose the E. trilineata species-group, in which all species share the superior midlateral stripe, and the species with anal-fin margin darkened and without longitudinal stripes on flanks constitute the Eigenmannia humboldtii species-group. In addition, we suggest the remaining species on hold for a formal taxonomic review or for having their phylogenetic relationships clarified (e.g. “Eigenmannia” goajira and E. meeki; see Table 2). Aiming the summarization of the taxonomic considerations synthetized herein, the first dichotomous key is provided for all Eigenmannia species (“Eigenmannia” goajira not included).

Key for Eigenmannia species (for specimens between 49.4 to 330.0 mm L_EA)

1. Longitudinal dark stripes on flanks absent………………………………………………2

1’. Longitudinal dark stripes on flanks present………………………………………………6

2. Distinctly darkened anal-fin margin (Eigenmannia humboldtii species-group)…………3

2’. Hyaline anal-fin margin…………………………………………………………………5

3. Dentary with 53–66 teeth arranged in three rows………………Eigenmannia humboldtii (restricted to the trans-Andean Río Atrato and Río Magdalena, northwestern Colombia).

3’. Dentary with 8–37 teeth arranged in one to three rows…………………………………4

4. Dentary with 8–18 teeth arranged in a single row; dorsal profile of head straight to concave………………………………………………………………Eigenmannia limbata (widespread in Amazon and Orinoco basins).

4’. Dentary with 27–37 teeth arranged in two or three rows; dorsal profile of head strongly convex……………………………………………………………………Eigenmannia nigra (widespread in Amazons, Araguaia-Tocantins, Essequibo and Orinoco basins).

5. Orbital diameter, 15.6–24.8% H_L; caudal filament length, 17.1–31.1% L_EA……………………………………………………………………………………Eigenmannia virescens (lower Paraná and de La Plata basins).

5’. Orbital diameter, 28.0–32.8% H_L; caudal filament lenght, 40.2–52.7% L_EA………………………………………………………………………………….Eigenmannia macrops (Potaro, Orinoco and Essequibo basins).

6. Superior midlateral dark stripe on flanks absent…………………………………………7

6’. Superior midlateral dark stripe on flanks present (Eigenmannia trilineata species-group)………………………………………………………………………………………8

7. Bony dorsolateral flange of dentary present which anchors numerous teeth along its extension; 38–42 teeth on premaxilla; 164–192 anal-fin rays…………Eigenmannia oradens (Río Ventuari, Río Orinoco basin).

7’. Bony dorsolateral flange of dentary absent; 30–35 teeth on premaxilla; 211–240 anal-fin rays……………………………………………………………….Eigenmannia meeki (Río Pucuro and Río Chucunaque, Río Tuíra basin).

8. Pectoral fin dusky or with conspicuous dark blotch; anal fin uniformly darkened………………………………………………………………………… Eigenmannia matintapereira (rio Uneiuxi and rio Urubaxi, rio Negro basin).

8’. Pectoral and anal fins hyaline………………………………………………………….9

9. Mouth subterminal…………………………………………………………………….10

9’. Mouth terminal……………………………………………………………………….14

10. Body depth at vertical through the tip of the longest pectoral-fin ray, 10.5–14.5% L_EA; seven or eight longitudinal series of scales above lateral line; length of the coronomeckelian bone equal to 45% of the length of Meckel’s cartilage………………………………………………………………………………. Eigenmannia vicentespelaea (caves of São Vicente I and II, rio Tocantins basin).

10’. Body depth at vertical through the tip of the longest pectoral-fin ray, 12–18.8% L_EA; seven to 12 longitudinal series of scales above lateral line; length of the coronomeckelian bone equal to 20–30% of the length of Meckel’s cartilage………………………………11

11. Origin of the superior midlateral stripe, at vertical between base of 5^th to 15^th anal-fin ray in adults……………………………………………………………Eigenmannia besouro (rio São Francisco basin).

11’. Origin of the superior midlateral stripe, at vertical between base of 23^th to 32^th anal-fin ray in adults……………………………………………………………………………… 12

12. Precaudal vertebrae, 12–13; 35–40 teeth on premaxillary; 37–38 teeth on dentary; orbital diameter, 22.6–28.8% H_L; nine or ten longitudinal series of scales above lateral line…………………………………………………………………. Eigenmannia waiwai (rio Mapuera and rio Trombetas, rio Trombetas basin).

12’. Precaudal vertebrae, 14–15; 15–24 on premaxillary teeth; 16–25 teeth on dentary; orbital diameter, 10.0–23.3% H_L; nine to 12 longitudinal series of scales above lateral line…………………………………………………………………………………………13

13. Relative depth of posterodorsal expansion on infraorbitals 1+2 corresponding to 70% length of infraorbitals 1+2; precaudal vertebrae, 15; orbital diameter, 17.2–23.8% H_L; mouth width, 13.1–22.4% H_L……………………………………. Eigenmannia sirius n.sp. (Rio Mutum, a tributary of rio Juruena, rio Tapajós basin).

13’. Relative depth of posterodorsal expansion on infraorbitals 1+2 corresponding to 40% length of infraorbitals 1+2; precaudal vertebrae, 14; orbital diameter, 10.6–13.3% H_L; mouth width, 23.5–26.0% H_L……………………………………. Eigenmannia correntes (rio Correntes, a tributary of rio Piquiri, rio Paraguai basin).

14. Suborbital depth, 28.3–46.6% H_L………………………………………………………15

14’. Suborbital depth, 18.2–28.9% H_L…………………………………………………….18

15. Relative depth of posterodorsal expansion on infraorbitals 1+2 corresponding to 60–100% length of infraorbitals 1+2; 11–16 teeth on premaxillary; 16–19 teeth on dentary……………………………………………………………………………………16

15’. Relative depth of posterodorsal expansion on infraorbitals 1+2 corresponding to 50% length of infraorbitals 1+2; 31–33 teeth on premaxillary; 31 teeth on dentary………………………………………………………………………………………Eigenmannia trilineata (lower rio Paraná and Río de La Plata basins).

16. Length of coronomeckelian bone 45% length of Meckel’s cartilage; 11–16 arranged in one or two rows on endopterygoid……………………………Eigemannia microstoma (rio São Francisco basin).

16’. Length of coronomeckelian bone 20–30% length of Meckel’s cartilage; 6–9 teeth arranged in one row on endopterygoid…………………………………………………….17

17. First basibranchial ossified; suborbital depth, 20.6–26.8% H_L……Eigemannia sayona (río Orinoco basin).

17’. First basibranchial unossified; suborbital depth, 28.3–35.8% H_L…………………….………………………………………………………………………Eigemannia loretana (río Pacaya and río Nanay basins).

18. Inferior midlateral stripe, one scale high; precaudal vertebrae, 11–12…………………………………………………………………………………………Eigenmannia desantanai (rio Cuiabá, rio Paraguai basin).

18’. Inferior midlateral stripe, two or three scale high; precaudal vertebrae, 13–15………19

19. Pectoral-fin rays, ii,11–12………………………………………………………………20

19’. Pectoral-fin rays, ii,13–15……………………………………………………………21

20. Orbital diameter, 15.4–19.4% H_L; 170–198 anal-fin rays; eight or nine endopterygoid teeth; 13–14 precaudal vertebrae…………………………………Eigenmannia muirapinima (Igarapé Santo Antônio and Lago Jará, rio Amazonas basin).

20’. Orbital diameter, 11.4–15.0% H_L; 151–170 anal-fin rays; five or six endopterygoid teeth; 15 precaudal vertebrae………………………………………Eigenmannia guairaca (upper rio Paraná basin).

21. Width mouth, 20.0–25.1% H_L; 11–13 teeth on premaxillary……Eigenmannia antonioi (rio Anapu, rio Amazonas).

21’. Width mouth, 10.8–19.0% H_L; 15–21 teeth on premaxillary………………………………………………………Eigenmannia pavulagem (Rio Capim, rio Guamá basin).