Taxonomic revision of the deep channel electric fish genus Sternarchella ( Teleostei : Gymnotiformes : Apteronotidae ) , with descriptions of two new species

This paper provides a taxonomic revision of the Neotropical electric fish genus Sternarchella, with redescriptions of seven valid species and descriptions of two new species. A maximum parsimony analysis of 76 morphological characters from seven ingroup and seven outgroup taxa recovered a non-monophyletic Sternarchella, in which a clade comprising two species with a ventral mouth (S. orinoco + S. sima) is the sister group to a clade comprising seven species that possess a terminal or superior mouth. Nested within this higher-diversity clade is the genus Magosternarchus (recognized herein as a junior synonym of Sternarchella) comprising M. duccis and M. raptor. The Magosternarchus clade forms a polytomy with S. orthos and S. schotti. Sternarchella calhamazon + a new species from the upper Río Madeira (sister species to S. calhamazon), and a new larger-bodied species from the central and upper Río Amazonas also form a clade. Sternarchella orthos is distributed in both the Amazon and Orinoco basins, where it exhibits considerable phenotypic diversity. Sternarchella orthos includes most specimens from the Amazon formerly assigned to the nominal species S. terminalis (recognized herein as a junior synonym of S. schotti).


Introduction
The genus Sternarchella Eigenmann (1905), known as bulldog knife-fishes in the aquarium trade, is a clade of apteronotid electric fishes with seven valid species currently recognized (Ferraris et al., 2017) distributed in the lowlands of the Amazon and Orinoco basins (Tab.1).Most of these species (S. calhamazon, S. duccis, S. orthos, S. raptor, S. schotti and S. sima) inhabit the deep (10-50 m) channels of the Amazon River and some of its larger tributaries, and one species (S. orinoco) is restricted to large rivers channels in the Orinoco basin.As with other gymnotiform fishes, Sternarchella species generate weak (mv) electric organ discharges (EODs) for use in navigation and communication (Crampton, Albert, 2006).Sternarchella species are notable in generating very high frequency EODs ranging from 772 to 2,180 cycles per second, making them the fastest known biological oscillators on Earth (Albert, Crampton, 2005).
Species of Sternarchella and other members of the clade Navajini exhibit a specialized suite of traits associated with life in deep and swiftly flowing river channels, including reduced eyes, pigmentation, and ossification (Albert, 2001).The genus exhibits one of the fastest rates of skull shape evolution among gymnotiforms (Evans et al., 2017a).Sternarchella also exhibits diversity in mouth position, presumably associated with trophic ecology, ranging from a sub-terminal gape in S. orinoco and S. sima, to a terminal gape in S. orthos and S. schotti, and a superior gape in S. calhamazon (Evans et al., 2017b).This diversity in mouth position may correlate with trophic habits, with sub terminalmouthed species feeding on benthic invertebrates, and terminal-and superior-mouthed species feeding primarily on fishes higher in the water column (K.M. Evans, pers. obs.).
The genus Sternarchella was erected in Eigenmann, Ward (1905) to include S. schotti, originally described as Sternarchus schotti by Steindachner (1868b).Since then, seven additional species of Sternarchella have been described that are currently recognized as valid (Lundberg et al., 2013;Ivanyisky, Albert, 2014) (Tab. 1).Sternarchella curvioperculata Godoy, 1968 described from the upper rio Paraná basin was excluded from Sternarchella by Lundberg et al. (2013), based on the presence of numerous small scales above the lateral line (13-14 versus 5-9 in Sternarchella).Based on visual inspection of the illustration and photograph of the specimen in the description (not available for physical examination), we determine that Sternarchella curvioperculata most closely resembles a member of the clade Apteronotus sensu stricto (Albert, 2001).
Here we describe two new species of Sternarchella (Tab.1), resolve the confusion between species boundaries in the S. schotti species complex, provide redescriptions for all valid species of Sternarchella and provide range extensions for two species of Sternarchella.We also provide an updated maximum-parsimony phylogeny based on 76 morphological characters for Sternarchella and Magosternarchus (Lundberg et al., 1996) and synonymize the genus Magosternarchus with Sternarchella.Furthermore, we provide a key of identification for all valid species of Sternarchella.

Materials and Methods
Taxon sampling.Ingroup sampling included all valid species of Sternarchellini (sensu Ivanyisky, Albert, 2014) (S1 -Available only as online supplementary file accessed with the online version of the article at http://www.scielo.br/ni).Museum acronyms are as presented in Ivanyisky, Albert (2014).The following outgroup Apteronotinae genera were used to polarize character states: Apteronotus, Compsaraia, Magosternarchus, Parapteronotus, Pariosternarchus, Porotergus and Sternarchogiton.Characters were coded from morphologically mature specimens as evidenced by the degree of ossification of the sphenoid region and total length relative to largest reported specimen.
Tab. 1. Summary of taxonomic data on valid species of Sternarchella.External Morphology.Morphometric measurements were collected using digital calipers following Ivanyisky, Albert (2014).Fourteen measurements were taken: LEA (length from the tip of the snout to end of anal fin), AFL (length from base of the anal fin to the end of the anal fin), HD1 (a measure of head depth through the nape), HD2 (A measure of head depth through the eye), HL (Distance from distal edge of opercle to the tip of the snout), PR (head length from the tip of the snout, to the front of the eye), PO (head length from the back of the eye, to the distal edge of the opercle), IO (distance between eyes from dorsal aspect), ED (diameter of the eye), HW (head width), MW (mouth width), PA (distance between the anus and the anal-fin origin), CPD (caudal peduncle depth), CPL (caudal peduncle length).Due to high levels of damage and regeneration in the tail region, body proportions taken as proportions of total length are unreliable; instead we used LEA.Geometric morphometric methods were used in addition to traditional morphometrics to capture shape features of the head, as part of an analysis of intraspecific variation in S. orthos between three populations: the western Amazon, the eastern Amazon and the Orinoco river basin.Photographs of 30 adult specimens were taken using a Nikon Coolpix digital camera with specimen orientations standardized.Photos were converted to .tpsfiles using tpsUtil (Rohlf, 2008), and 15 homologous landmarks were positioned on each photograph using 3 e160168 [3] tpsDig2 (Rohlf, 2006) (Fig. 1).The resulting tpsDig2 files were then imported into Morpho J (Klingenberg, 2011) and a Procrustes superimposition performed to remove the effects of size and scaling on specimens.A principal components analysis (PCA) was then performed to capture the primary axes of variance in the shape data.Osteology.Osteological information was collected after clearing and staining (C&S) specimens following Taylor, Van Dyke (1985), modified using xylene washes following Ivanyisky, Albert (2014).Specimens were dissected following Weitzman (1974) with modifications by Albert (2001).Osteological Illustrations were made using an Olympus SZX-12 stereomicroscope fitted with a camera lucida.Osteological information was also collected from digital radiographs using a Kevex MicroFocus X-ray Source and Varian PaxScan image receptor at the Academy of Natural Sciences in Philadelphia, and a Kodak DXS Pro digital X-ray system at the University of Central Florida.Meristic measurements consisted of anal-fin ray counts, anterior unbranched anal-fin ray counts, and pre-caudal vertebrae counts.
Phylogenetic methods.We coded 76 morphological characters to construct a phylogeny using Maximum Parsimony (MP) (S2 -Available only as online supplementary file accessed with the online version of the article at http://www.scielo.br/ni).Characters were selected on the basis of phylogenetic informativeness (Pimentel, Riggins, 1987).All osteological characters were collected from mature specimens as evidenced by the degree of ossification in the sphenoid, palatoquadrate and coracoid regions (Albert, 2001).Osteological nomenclature follows Albert (2001).A MP analysis was run in TNT (Goloboff et al., 2008) using a data matrix of 76 characters and 16 taxa.Seven apteronotid species were used as outgroup taxa to infer polarity, selected on the basis of results from earlier studies (see Ivanyisky, Albert, 2014).Polymorphic states were coded with "&".Parapteronotus hasemani was designated as the outgroup taxa for the parsimony analysis in TNT.A heuristic search was implemented using an implicit enumeration algorithm in TNT using 15,000 replicates.All multi-state characters were treated as unordered.Bremer Supports (Bremer, 1994) were run with 1000 replicates in TNT.Nodes with Bremer Supports lower than one were collapsed.
Electric Signals.Head-to-tail electric organ discharge (ht-EOD) waveforms were recorded from live specimens of Sternarchella rex by WGRC, within 12 hours of capture.Specimens were recorded in a loose nylon mesh envelope suspended in the center of an 88 x 37 cm insulated cooler filled to a depth of 34 cm.Single ht-EODS were taken from tank-end Ag/Ag-Cl or NiCr electrodes, using a custombuilt AC-coupled amplifier (DC -30 kHz) and digitized at 48 kHz to digital audio tape (and later redigitized to 96 kSs at 24-bit resolution using an Edirol UA5 analog-digital converter).Water temperature was standardized to 27.0 +/-0.1°C, and conductivity to 55 +/-1 μScm -1 .The EOD frequency of gymnotiforms is temperature dependent.Therefore, prior to placement in the mesh envelope, each fish was transferred from a holding tank to the cooler and allowed to move freely for a few minutes until the EOD fundamental frequency stabilized.EOD recordings are reported here with specimen numbers following the format year-month-day-sequential number.

Character descriptions.
A recent phylogenetic study of the Sternarchellini was conducted by using 70 morphological and osteological characters (Ivanyisky, Albert, 2014).This current study revises the previous character scheme (S3 -Available only as online supplementary file accessed with the online version of the article at http://www.scielo.br/ni).
Here we exclude character 61 (the shape of a process on the fourth epibranchial) because it was not phylogenetically informative, and add an additional nine characters, described here following the previous numbering scheme.Membership of clades A (S. sima clade), B (S. schotti clade), C (S. duccis clade), D (S. calhamazon clade), are summarized in Tab. 1, and described below under 'Clade Diagnoses'.
Skull.Character 67.Margin of parietal and supraoccipital.0: Margin of parietal and supraoccipital smooth lacking thorny ridges.1: Ridge or crown of thorny projections present at the border between the parietals and the supraoccipital that is continuous throughout the dorsal margin of the epioccipital and the pterotic (Fig. 2a).
This character is found in some of the larger predatory Sternarchella species (S. orthos and S. patriciae), whose diet was determined by gut-content analyses.However, this 4 e160168 [4] character is not directly linked to size as it is conspicuously absent in large-bodied congeners; i.e. S. orinoco, S. rex, S. schotti, and S. sima.It is likely that the thorny projections are sites for increased muscle attachment for jaw muscles possibly correlated to a more active, predatory lifestyle.Character 73.Posterior margin of dentary.0: Curves gradually to descending limb.1: Posterior margin of dentary deeply forked (Ivanyisky, Albert, 2014).
This character is shared by two species in the S. calhamazon clade (S. calhamazon and S. patriciae), and may be associated with the superior mouth and stout appearance of the dentary bone in these species.
Branchial basket.Character 56.Second basibranchial.0: Hour-glass shaped with most narrow portion at mid length.1: Fan-or rod-shaped, extending to long narrow end.2: Triangular with a short posterior portion (length of posterior rod-shaped portion less than half the width of the anteriorly positioned fan-shaped portion; Fig. 3).
The hour-glass shape of the second basibranchial is present in all species of the S. calhamazon clade.It was found that this condition is the ontogenetic precursor to the fan-shaped condition.Juvenile S. orthos were found to possess this condition later on ossifying the spaces on the side resulting ultimately in a fan-or rod shaped appearance seen in adult S. orthos and S. schotti.
Caudal peduncle.Character 68.Rows of bones in caudal peduncle.0: Single row of bones visible in caudal peduncle.1: Two parallel rows of bones visible in caudal peduncle.This character was originally described by Lundberg et al. (2013) in their description of S. calhamazon.This character is also observed in S. patriciae and S. rex.Character 70.0: Caudal peduncle depth shallow less than 30% HL. 1: Caudal peduncle deep greater than 30% HL (Lundberg et al., 2013).This character was also described in Lundberg et al. (2013) and was originally thought to be associated with the two rows of bones in the caudal peduncle (Character 68).However, in S. patriciae these two characters are not always linked.
Gymnotiformes often exhibit partially regenerated tails which confound measurements of characters in the caudal region.However, with large sample sizes and non-regenerated specimens, this character can be used to differentiate between species (Albert, Crampton, 2009).
Character 75.Caudal peduncle margin.0: Continuous membrane of tissue connecting anal-fin base and caudal peduncle.1: No apparent connective tissue between analfin base and caudal peduncle.
Displaced hemal spines.Character 69.0: One to two displaced hemal spines with a straight lower half of descending blades.1: Four to five displaced hemal spines laterally curved on lower half of descending blades (Fig. 4).This character was described by Lundberg et al. (2013) as an additional character to distinguish S. schotti from all other Sternarchella species.This character is associated with the elongate swim bladder found only in S. schotti.Eastern Amazon (EA), Western Amazon (WA), Orinoco (OR), Upper Madeira (UM).

Key to the adults of the genus
(*) One juvenile specimen of S. schotti was collected in the rio Meta in Colombia.

Synonymy of Magosternarchus.
We recovered a paraphyletic Sternarchella, with Magosternarchus nested inside it (Fig. 5).This topology was also recovered in Ivanyisky, Albert (2014) and Tagliacollo et al. (2016).A recent paper by Ferraris et al. (2017) recently used the name "Sternarchella" in reference to the Magosternarchus species citing the findings from Ivanyisky, Albert (2014) as grounds for synonomy.However, in the aforementioned manuscript, no formal synonomy was ever provided.Consequently, we herein recognize the genus Magosternarchus as a junior synonym of Sternarchella and we refer to M. duccis and M. raptor as S. duccis and S. raptor, respectively.

Synapomorphies of Sternarchella.
Members of the genus Sternarchella can be diagnosed from all other members of the family Apteronotidae by the following characters: -Premaxilla large, lateral margin of premaxilla longer than lateral margin of maxilla (Albert, 2001;fig. 6).-Ventral margin of maxillary blade curved evenly towards its distal tip (but see S. raptor) (Albert, 2001: figs. 7-8).-Ventral ethmoid large and robust with a large fan-shaped lateral process (Albert, 2001).-Dorsomedial portion of orbitosphenoids in contact (visible through anterior fontanel in dorsal view; Ivanyisky, Albert, 2014).-Opercle broad, width over half depth (Ivanyisky, Albert, 2014).Diversity of craniofacial shapes in Sternarchella.Among Sternarchella species, craniofacial shapes range from a rounded snout with a sub-terminal mouth in the S. sima clade, to a straight snout with a terminal mouth in the S. schotti clade (excluding the S. calhamazon clade), to a straight snout with a superior mouth in the S. calhamazon clade (Fig. 6).This diversity in craniofacial shapes is generally consistent with the trophic ecology of these fishes as estimated from gut content analysis (discussed in further detail in species descriptions and redescriptions).Mouth position is correlated with different feeding strategies in 7 e160168 [7] many fishes, including silver arowanas (Osteoglossum bicirrhosum), which use a superior mouth to launch aerial attacks on surface dwelling prey (Lowry et al., 2005), and sturgeons (Acipenser oxyrinchus) that use sub-terminal mouths to foraging on small-bodied epibenthic and infaunal aquatic animals (Johnson et al., 1997).This pattern of mouth position as an indicator of trophic ecology has also been extensively documented in cichlids (Montaña, Winemiller, 2013;Rüber, Adams, 2001).
Gut content analyses of indicates a rough association between craniofacial phenotypes and diet among Sternarchella species.Species in the S. sima clade with a sub-terminal mouth feed primarily on benthic animals such as aquatic crustaceans and arthropod larvae and fish remains have never been recovered in the guts of sub-terminal mouthed species (Evans, pers. obs.).The dietary range of species with a terminal or superior mouth species is much wider.Remains of small-bodied doradid catfishes have been found in guts of small-bodied specimens (100 mm LEA) of S. calhamazon, while other specimens of similar size from the same collection locality had only planktonic cladoceran larvae.All Sternarchella species with a terminal or superior mouth have robust oral and pharyngeal jaws and robust gill-rakers, allowing them to feed at several trophic levels.Sternarchella duccis and S. raptor also have robust oral and pharyngeal dentition and well developed gill rakers, and have been reported to specialize on consuming the tails of other electric fishes (Lundberg et al., 1996).In examining gut-contents of both species for this study we observed scales neatly stacked in the stomachs of both juveniles and adults in the absence of other fish tissue, suggesting facultative lepidophagous habits (Sazima, 1983).These preliminary findings suggest that these species exhibit a highly specialized piscivorous lifestyle, feeding primarily on external body parts of other fishes.Coloration in alcohol.Yellowish white color, with a darker brown mid-dorsum along the length of the body, with light striations overlying and demarcating the ribs at the body cavity.

Sexual dimorphism.
No sexual dimorphism found in 3 male and 7 female specimens.
Distribution and habitat.(Fig. 8).Specimens of Sternarchella patriciae were collected in the Madre de Dios in Peru by trawling the river bottom (Fig. 9).Individuals inhabit deep river channels (10-20 m) during the day and move to shallower water at night to feed near the river margin and over flooded beaches.Gut-content analysis indicates that these fishes feed primarily on larger-bodied (non-planktonic) aquatic insect larvae (e.g.Odonata and Ephemeroptera) and some juvenile fishes.and along ridge of epioccipital; Fig. 2a), a longer caudal peduncle (Fig. 10), more pre-caudal vertebrae (12-14 vs. 14-15), more teeth present on pharyngobranchial (14 or more vs. 12 or less), and an overall larger total body size (80% larger).These phenotypes all allow feeding on larger prey items like small fishes and large aquatic invertebrates.Sternarchella patriciae is allopatrically separated from S. calhamazon, and has to date only been found in the Madre de Dios basin in the upper Madeira basin in Peru.Genetic data are not yet available for S. patriciae.Genetic analyses of other fish taxa distributed in the upper Madeira and Lowland Amazon basins report varying levels of genetic differentiation, including difference described as within and between species (Albert, 2012;Albert, Reis, 2011;Farias et al., 2010;Ochoa et al., 2015;Torrente-Vilara et al., 2011).Coloration in alcohol.Yellowish white color, with a light brown mid-dorsum along the length of the body.Live specimens are pale white in color with a green sheen along the dorsum (Fig. 12a).Sexual dimorphism.Not known, insufficient number of male and female specimens to determine.

Sternarchella rex, new species
Distribution and habitat.(Fig. 8).The type series, from the vicinity of Tefé, Amazonas, Brazil, was collected only during the early rising water period of December-February (Fig. 13).Four of the six specimens in the type series were collected from a "paraná" channel located in whitewater 'várzea' inundation forest of the Mamirauá Reserve (paranás are narrow side channel of whitewater rivers that traverse adjacent floodplain).Sternarchella rex specimens were caught at depths of 2-4 m with seine nets deployed from the middle of the paraná channel to the edge.The substrate comprised mud and organic debris.
Seven additional non-type specimens of S. rex were collected from silt and fine sand beaches on the margins of the río Amazonas near the city of Iquitos, Loreto, Peru -all at depths of 3-10 m.These specimens were captured during the late part of the rising water period in December (Fig. 13).
The stomach contents of four specimens of S. rex in the type series from the Tefé region (one non-recorded specimen, and three which were recorded very soon after capture), and seven from the Iquitos region were examined.All stomachs contained unidentified fish scales, skin and fine bones.Aquatic arthropods were conspicuously absent.We noted that all specimens of S. rex had damage to the caudal fin and fin-base, as is common in wild caught gymnotiforms from riverine habitats.This was probably caused by predators.Electric Organ Discharges.The ht-EODs of five of the type series were recorded (Fig. 14).The ht-EOD comprises a wave-type waveform with two peaks in each cycle.A dominant biphasic component is followed by a secondary peak of positive polarity (Fig. 14, left).The waveform dips to near the zero voltage baseline between the two peaks (in some cases exhibiting a constant (flat) low voltage, only slightly positive to the baseline) and crosses the baseline twice during the main biphasic component.The fundamental frequency varied in recorded specimens from 945-1096 Hz, mean 1023 Hz, standard deviation 54 Hz).The power spectral density computed from a Fast Fourier Transform (Fig. 14, right) exhibits a harmonic distribution of energy, as is typical for wave-type gymnotiform ht-EODs (Crampton, Albert, 2006).In all but one specimen the peak (dominant) frequency of the power spectral density corresponds not to the fundamental frequency, but to the first harmonic.In one specimen (1999-02-03-05), the peak frequency corresponded to the second harmonic.Although none of these specimens were in full reproductive condition, two were sexed as male (fundamental frequency 945-1096 Hz) and one as female (fundamental frequency 1027 Hz), with no sexual dimorphism of EOD fundamental frequency (as is known for some apteronotids, Crampton, Albert, 2006).
Etymology.This species name rex from the Latin word for king, in reference its large body size and robust appearance.An adjective.
Remarks.This species is the largest known species of Sternarchella reaching a maximum adult body size of 412 mm LEA.Coloration in alcohol.Yellowish white color, with a light brown mid-dorsum along the length of the body.In life, this species is pale white with a pink hue (Fig. 12c).

Sexual dimorphism.
No sexual dimorphism found in 21 male and 15 female specimens.
Distribution and habitat.(Fig. 8).Distributed throughout the Amazon basin, where it inhabits deep river channels.
Sternarchella calhamazon is one of the most wide-spread and abundant apteronotid electric fish species in the Amazon basin (Lundberg et al., 2013).Gut-content analysis indicates that S. calhamazon is the only Sternarchella known to feed on planktonic organisms as a mature adult.Stacked scales of other fishes in the absence of other fish tissue were also recovered in the stomach contents of specimens examined.

Small body size of Sternarchella calhamazon.
Sternarchella calhamazon has the smallest body size among congeners, and exhibits several derived traits associated with small adult body size (max.145 mm LEA vs. max.291 mm LEA in S. orthos, and larger max.sizes in other congeners).These traits include lower pre-caudal vertebrae and anal-fin ray counts (Tab.4).Small body size may have arisen from paedomorphosis; i.e. truncation of the ancestral ontogeny (Alberch et al., 1979), including neurocranial shape with a shorter face (pre-orbital region, Fig. 2), and an hour-glass shaped second basibranchial bone (Fig. 3) (shared with S. patriciae).In S. orthos and S. schotti, the second basibranchial initially ossifies with an hour-glass shape in juveniles, and becomes more ossified during growth to become fan or cylinder-shape in adults.Furthermore, as compared with congeners, S. calhamazon exhibits a more lightly ossified neurocranium and bony elements of the branchial basket.Diagnosis.Sternarchella duccis can be diagnosed from all congeners by the presence of a highly superior mouth with the lower jaw projecting upwards beyond the upper jaw (vs.terminal in S. calhamazon, S. orthos, S. patriciae, S. raptor, S. rex, and S. schotti and sub-terminal in S. orinoco and S. sima) and a strongly concave dorsal margin of the frontal (vs.straight in S. calhamazon, S. orthos (western Amazon), S. patriciae, S. raptor, S. rex, and S. schotti and convex in S. orinoco and S. sima) (shared with Eastern Amazonian populations of S. orthos).
Description.Medium sized species of Sternarchella reaching an LEA of 266 mm.Pectoral fin size small, less than 80% HL.PA% Moderate, 37-59% HL.Head width narrow, distance between lateral margins 37-42% HL.Preorbital (snout) length moderate, 28-32% HL.Postorbital distance small, 62-71% HL.Eye diameter small, 6-7% HL.Interorbital distance small, 14-15% HL.Mouth wide, distance between ricti 21-25% HL.Body depth less than HL.Scales absent on posterolateral portion of body.Scales large in size with 5-8 present above lateral line at mid-body.Scales dorsal to lateral line rhomboid at midbody.Rictus extends to a vertical with mental symphysis, gape short, more than three times eye diameter.Oral aperture superior, lower jaw extends anteriorly to upper jaw.Body cavity long 16 pre-caudal vertebrae present.Proximal surface of first displaced hemal spine narrower then descending blade.One to two displaced hemal spines present.Swim bladder not extending posterior to body cavity.Anal-fin pterygiophore length equal to or shorter than hemal spines.Anal-fin proximal small, shorter than hemal spine.One row of intermuscular bones visible externally in caudal peduncle.Premaxilla large, lateral margin of premaxilla longer than lateral margin of maxilla.Premaxilla triangular in ventral view.Two rows of teeth present on premaxilla.Anterior hook of maxilla absent, anterior process broad and triangular with a continuous ventral margin with descending blade.Anterior process of maxilla extending as a shelf of bone less than onethird length of descending blade.Ventral margin of maxillary blade curves evenly towards its distal tip.Descending blade maxilla thin, evenly curved.Two rows of teeth present on dentary.Dentary longer than deep, oral margin of dentary longer than length of angular articular.Dorsal margin of dentary slightly concave in lateral view.Endopterygoid process oblique (greater than 90° to with dorsal surface of endopterygoid).Hyomandibula short, its width half its length.Dorsal margin of opercle e160168 [17] concave.Opercle broad, width over half depth.Anterior limb of cleithrum length greater than ascending limb length.Post-temporal fused with supracleithrum in mature specimens.Ventral ethmoid large and robust with a large fan shaped lateral process.Dorso-anterior portion of mesethmoid straight.Anterior tip of mesethmoid scyphate on dorsal surface.Posterior fontanel longer than anterior fontanel.Lateral ethmoid robust and large, may contact ventral portion on frontals, hour-glass shaped with most narrow portion at mid-length.Orbitosphenoid broad, well ossified in median nasal septum with ventral margin longer than dorsal margin.Dorso-medial portion of orbitosphenoids in contact (visible through anterior fontanel in dorsal view).Absence of ventral process of pterosphenoid, anterior ventral margin of pterosphenoid similar to posterior ventral margin of orbitosphenoids.Lateral process of parasphenoid small, lateral margins of parasphenoid not extending to a horizontal with trigeminal foramen.Parasphenoid ventral margin straight or slightly curved.Distance between parietal ridges narrow, just lateral to supraoccipital, parietal ridges are very large and pronounced.No thorny projections present at border of parietal and supraoccipital.Dorsal margin of supraoccipital crest extends beyond dorsal margin of parietals.Supraoccipital crest extends to a dorsal distal tip.Internal carotid foramen reduced.Ventral surface of basioccipital smooth.Anterior extension of infraorbital canal short.Supraorbital canal fused to frontal.Mandibular canal size small.Mandibular canal ossicles form long slender tubes.Supratemporal laterosensory canal curved at a sharp angle on surface of parietal, extending posteriorly onto epaxial surface of body, terminal canal pore oriented posteriorly, epidermis overlying supratemporal canal depigmented.Endopterygoid large, contacting frontal.Base of gill rakers contacting gill arch.Gill rakers long with ossified distal tips.Dorsal surface of basihyal flat; small ridge may be present posteriorly.Second basibranchial hourglass shaped with narrowest portion at mid-length.Third basibranchial unossified.Twelve or less teeth present on pharyngobranchial.Eight or more teeth present on sixth hypobranchial.Medial surface of fourth hypobranchial with a process or bridge extending to meet contralateral process on midline.Urohyal blade unossified.First hypohyal bell-or cylinder shaped.

Coloration in alcohol.
Yellowish white color, with a light brown mid-dorsum along the length of the body.

Sexual dimorphism.
No sexual dimorphism found in 2 male and 2 female specimens.
Distribution and habitat.(Fig. 17).Distributed throughout the Amazon basin, where it inhabits deep river channels, collected at low densities.Gut-content analysis indicates that S. duccis feeds entirely on the tails and scales of other electric fishes.Remarks.Sternarchella duccis possesses robust oral and pharyngeal dentition, which presumably aid in its predatory feeding habits.
Sexual dimorphism.Mature female specimens of S. orinoco have wider heads than male specimens of comparable sizes (Mago-Leccia, 1994).
Distribution and habitats.(Fig. 19).This species is known only from the Orinoco River basin, where it is inhabits deep river channels.It has been most commonly collected in the Amacuro estuary and Llanos floodplains.
Remarks.While superficially resembling S. sima, this species possesses several differences including tail length, gill raker condition (contacting gill arch vs. non-contacting in S. sima), a straight ventral profile, and skull width.
Females have a wider head and a deeper body (Mago-Leccia, 1994:99).Coloration in alcohol.Yellowish brown color, with striations demarcating the ribs at the body cavity.In life, this species is pale white in color with a pink hue (Fig. 12d).

Sexual dimorphism.
No sexual dimorphism found in 9 male and 11 female specimens.
Distribution and habitat.(Fig. 17).This species is widely distributed throughout the Amazon and Orinoco River basins.
Remarks.This species is highly variable throughout its range with narrower and longer individuals found in the Orinoco River basin, and deeper-bodied populations found in the Amazon basin (Fig. 21-22).This species is also highly variable in mouth position ranging from terminal in the Western Amazon to superior in the Eastern Amazon, as also observed by Lundberg et al. (2013).
In comparison to congeners, S. orthos exhibits a wide range of variation in head shape and body depth throughout its geographic range.Sternarchella orthos in the Eastern Amazon often possess a concave head and skull, resembling the head and skull shape of Sternarchella duccis (Lundberg et al., 1996).This morphology does not appear to be a secondary sexual phenotype of mature males or females (Lundberg et al., 2013;fig. 8).It is unclear if a concave head and skull is linked to trophic differences.In the Western Amazon, S. orthos exhibits a wider head, a straighter skull, deeper body, and larger body size than observed in populations from the Eastern Amazon.In the Orinoco basin, Sternarchella orthos exhibits a longer tail, a narrower head and a shallower body than populations from the Eastern and Western Amazon.No discrete osteological, or genetic differences are yet known between populations of S. orthos in the Amazon and Orinoco basins.
Diagnosis.Sternarchella raptor can be diagnosed from all congeners by the presence of a long snout, PR 35-38% HL (vs.24-32 %HL in all other Sternarchella species), eyes located near the dorsal margin of the head (vs.eyes located on lateral margin of head in S. calhamazon, S. orthos, S. orinoco, S. patriciae, S. rex, S. schotti, and S. sima) (shared with S. duccis), a maxilla with an enlarged anterior process greater than one-third the length of the descending blade (vs.reduced anterior process less than one-third the length of descending maxillary blade in all other Sternarchella species) and robust oral dentition usually visible externally in mature specimens (vs.obscured dentition in S. calhamazon, S. orthos, S. orinoco, S. patriciae, S. schotti, and S. sima) (shared with S. duccis and S. rex).
Description.Medium-sized species of Sternarchella, reaching 205 mm LEA.Pectoral fin size small, less than 80% HL.PA% moderate, 39-42% HL.Head width narrow, distance between lateral margins 38-41% HL.Preorbital (snout) length large, 35-38% HL.Postorbital distance small, 58-64% HL.Eye diameter small, 4-7% HL.Interorbital distance small, 10-12% HL.Mouth wide, distance between ricti 21-25% HL.Body depth less than HL.Scales absent on posterolateral portion of body.Scales large in size with 5-8 present above lateral line at mid-body.Scales dorsal to lateral line rhomboid at mid-body.Rictus extends to a vertical with mental symphysis, gape short, more than three times eye diameter.Oral aperture terminal, upper and lower jaws equal in length.Body cavity long, 15 precaudal vertebrae.Proximal surface of first displaced hemal spine narrower then descending blade.One to two displaced hemal spines.Swim bladder not extending posterior to body cavity.Anal-fin pterygiophore length equal to or shorter than hemal spines.Anal-fin proximal small, shorter than hemal spine.One row of intermuscular bones visible externally in caudal peduncle.Caudal peduncle narrow, 14-15% HL.Dark spot on caudal peduncle absent.No apparent connective tissue between anal-fin base and caudal peduncle.Caudal peduncle length short, less than HL.Premaxilla large, lateral margin of premaxilla longer than lateral margin of maxilla.Premaxilla triangular in ventral view.Two rows of teeth present on premaxilla.Anterior hook of maxilla absent, anterior process broad and triangular with a continuous ventral margin with descending blade.Anterior process of maxilla large and broad, extending more than one half length of descending blade in mature specimens.Ventral margin of maxillary blade strait to distal tip.Two rows of teeth present on dentary.Dentary longer than deep, oral margin of dentary longer than length of angular articular.Dorsal margin of dentary slightly concave in lateral view.Endopterygoid process oblique (greater than 90° with dorsal surface of endopterygoid).Hyomandibula short, its width half its length.Dorsal margin of opercle concave.Opercle broad, width over half depth.Anterior limb of cleithrum length greater than cleithrum ascending limb length.Post-temporal fused with supracleithrum in mature specimens.Ventral ethmoid large and robust with a large fan shaped lateral process.Dorso-anterior portion of mesethmoid strongly curved from anterior tip to frontal boundary.Anterior tip of mesethmoid scyphate on dorsal surface.Posterior fontanel longer than anterior fontanel.Lateral ethmoid very robust and large, may contact ventral portion on frontals, hour-glass shaped with most narrow portion at mid-length.Orbitosphenoid broad, well ossified in median nasal septum with ventral margin longer than dorsal margin.Dorso-medial portion of orbitosphenoids in contact (visible through anterior fontanel in dorsal view).Process originating from ventral portion of most anterior part of pterosphenoid present, sometimes contacting parasphenoid.Lateral process of parasphenoid small, lateral margins of parasphenoid not extending to a horizontal 24 e160168 [24] with trigeminal foramen.Parasphenoid ventral margin straight or slightly curved.Distance between parietal ridges narrow, just lateral to supraoccipital, parietal ridges are very large and pronounced.No thorny projections present at border of parietal and supraoccipital.Dorsal margin of supraoccipital crest extends beyond dorsal margin of parietals.Supraoccipital crest extends to a dorsal distal tip.Internal carotid foramen reduced.Ventral surface of basioccipital smooth.Anterior extension of infraorbital canal short.Supraorbital canal fused to frontal.Mandibular canal size small.Mandibular canal ossicles form long slender tubes.Supratemporal laterosensory canal curved at a sharp angle on surface of parietal, extending posterior onto epaxial surface of body, terminal canal pore oriented posteriorly, epidermis overlying supratemporal canal depigmented.Endopterygoid large, contacting frontal.Base of gill rakers contacting gill arch.Gill rakers long with ossified distal tips.Dorsal surface of basihyal flat, small ridge may be present posteriorly.Second basibranchial hour-glass shaped with most narrow portion at mid-length.Third basibranchial unossified.Twelve or less teeth present on pharyngobranchial.Eight or more teeth present on sixth hypobranchial.Medial surface of fourth hypobranchial with a process or bridge extending to meet contralateral process on midline.Urohyal blade unossified.First hypohyal bellor cylinder shaped.
Coloration in alcohol.Yellowish white color, with a light brown mid-dorsum along the length of the body.

Sexual dimorphism.
No sexual dimorphism found in three male and one female specimens.
Distribution and habitat.Distributed throughout the Amazon basin, where it inhabits deep river channels.(Fig. 19).Sternarchella raptor is wide-spread in the Amazon basin, however it is often collected in low densities.Gutcontent analysis indicates that S. raptor feeds on the scales and tails of other electric fishes (based on the presence of stacked scales, vertebrae and cartilaginous regenerated caudal filament rods).
Remarks.Sternarchella raptor possesses robust oral and pharyngeal dentition which presumably aid in its predatory feeding habits.This species also possesses the longest snout of any Sternarchella.Coloration in alcohol.Yellowish brown color, with a darker brown stripe confined to the dorsum running the length of the body, with striations demarcating the ribs at the body cavity (Fig. 12b).
Sexual dimorphism.Not known, insufficient mature male and female specimens to determine.

e160168[26]
Distribution and habitat.Distributed throughout the deep channels of the Amazon and Orinoco basins.Ivanyisky, Albert (2014) described S. schotti as being restricted to the Amazon basin (Fig. 17).However, a single juvenile specimen of S. schotti was found inhabiting the río Meta in Colombia, suggesting that S. schotti may have a wider range than originally hypothesized.
Remarks.Sternarchella schotti is readily differentiated from sympatric congeners by the presence of a swim-bladder that extends beyond the posterior margin of the body cavity, visible when held up to a strong light source.
Taxonomic status of Sternarchella terminalis.Sternarchella terminalis (Eigenmann & Allen, 1942) was originally described as Porotergus terminalis and reassigned to the genus Sternarchella by Mago-Leccia (1994).However, closer examination of the holotype of S. terminalis (IU or CAS 15994), collected near Iquitos, Peru, shows that it possesses the larger eye diameter and distinct configuration of displaced hemal spines that are diagnostic of S. schotti (Steindachner, 1868b;fig. 5).We therefore conclude that S. terminalis (Eigenmann & Allen) is a junior synonym of S. schotti (Steindachner, 1868b).A species with a similar external phenotype, also present in the Amazon and Orinoco basins, has been incorrectly referred to as S. terminalis in the Amazon previously (Ivanyisky, Albert, 2014;Lundberg et al., 2013), and was described as S. orthos Mago-Leccia (1994) from the Orinoco basin.Despite measurable morphometric variation among populations of this species in several parts of the Amazon and Orinoco basins, we were unable to discover diagnostic differences among these populations, and here treat the species as S. orthos with a broad geographic range and morphometric variation.A note on the taxonomic status of Sternarchus capanemae was published in Lundberg et al. (2013); here the authors recognize Sternarchus capanemae as a junior synonym of S. schotti.
Sexual dimorphism.Insufficient specimens are available to ascertain differences between males and females.However, Mago-Leccia (1994) reported that mature female specimens of S. sima have wider heads in the lateral dimension than males of comparable sizes.
Distribution and habitat.Known from the eastern portion of the lowland Amazon basin, with most specimens known from near its mouth (Fig. 19).
Remarks.Sternarchella sima superficially resembles, S. orinoco in possessing a sub-terminal mouth, three rows of teeth on the pre-maxilla and dentary, and ventral flexion of the mesethmoid region.However, S. sima is a smaller species, reaching 189 mm LEA (vs.221 mm in S. orinoco), with a slightly shorter face, pre-orbital length 18-28% HL (vs.26-32% HL in S. orinoco).Two specimens catalogued as S. sima from the western Amazon are not recognized in this species herein.The paratype of S. sima designated by Starks (1913) described the specimen as having 193 anal-fin rays, here we describe that specimen as possessing 191 anal-fin rays.

Fig. 5 .
Fig. 5. Phylogenetic tree of Sternarchella resulting from a maximum parsimony analysis of the 76 morphological characters in Appendix 1 (S1 -Available only as online supplementary file accessed with the online version of the article at http://www.scielo.br/ni).Bremer supports shown to the left of nodes.Clades labeled by letters and names as in text and Tab. 1.
Clade C (S. duccis clade): Comprised of S. duccis and S. raptor, and characterized by five synapomorphies.Rictus extends ventral to nasal capsule, gape more than three times eye diameter.Two rows of teeth present on premaxilla.Anterior process of maxilla large and broad, extending more than one half length of descending blade in mature specimens.Posterior fontanel longer than anterior fontanel.Twelve or less teeth present on pharyngobranchial.Clade D (S. calhamazon clade): Comprised of S. calhamazon, S. patriciae, and S. rex.This clade is characterized by five synapomorphies.PA% large, 59-61% HL.Mouth wide, distance between lateral ricti 20-27% HL.Body base color with a pale or whitish hue.Two rows of bones in caudal peduncle visible externally.Continuous membrane of tissue connecting anal-fin base and caudal peduncle.Clade E: Comprised of S. calhamazon and S. patriciae.This clade is characterized by six synapomorphies.Oral aperture superior, lower jaw extends anteriorly to upper jaw.Two, one, or no rows of teeth present on premaxilla.Lateral ethmoid large hour-glass shaped, most narrow portion at mid-length.Endopterygoid large, contacting frontal.Posterior margin of dentary deeply forked.Anal-fin proximal small, shorter than hemal spine.

Fig. 8 .
Fig. 8. Map of collection localities of species in the S. calhamazon species group.Sternarchella rex (triangles), S. patriciae (stars), and S. calhamazon (circles).Note: 1, the S. calhamazon species group is restricted to the Amazon basin; 2, the allopatric distribution of the sister species S. calhamazon and S. patriciae; and 3, S. calhamazon is represented at more sites than other members of the S. calhamazon species group.Base map of drainages provided by Conservation Science Program, World Wildlife Fund US; inset depicts elevations in gray shades.

Fig. 9 .
Fig. 9. Type locality of Sternarchella patriciae on the Río Madre de Dios, in front of Puerto Maldonado (Tambopata Department) near the confluence of the Tambopata River.Specimens were collected at about 10 m depth using a 12 foot shrimp trawl.Etymology.Patronym in honor of Patricia Evans, a prominent civil-rights activist and community leader in Philadelphia, Pennsylvania.Remarks.Sternarchella patriciae is a member of the S. calhamazon clade with which it shares six synapomorphies.Sternarchella patriciae also shares the highly predatory lifestyle of other Sternarchella species, and exhibits a peramorphic version of the S. calhamazon body plan.These relatively peramorphic characters are: more robust ossification of the skull among specimens of comparable size, more and larger muscle attachment sites on the skull (e.g.crown of thorns at border of parietal and supraoccipital

Fig. 10 .
Fig. 10.Biplot of length to end of anal-fin (LEA) vs. caudal peduncle depth (CPD) for eight species of Sternarchella showing the separation between S. calhamazon from white water, S. calhamazon from black water (BW), and seven other Sternarchella species in CPD.

Fig. 13 .
Fig.13.Amazon River margin habitat near type locality of Sternarchella rex in Tefé, Brazil.Specimens were collected using 50 x 6 m or 50 x 8 m beach seine nets operated in the mid-channel and edges of whitewater floodplain channels of the río Solimões-Japurá confluence, and on beaches of the río Japurá near its confluence with the río Solimões (Amazon).

Fig. 15 .
Fig. 15.Sternarchella calhamazon (USNM 373113).a. Lateral view of the body, b.Lateral view of the head, c.Lateral view of the caudal region showing two rows of intermuscular bones in the caudal peduncle and the continuous caudal peduncle membrane.Scale bars = 1 cm.

Fig. 17 .
Fig. 17.Map of collection localities of S. orthos (triangles), S. schotti (circles), and S. duccis (stars).Note these three species are broadly sympatric throughout the Eastern and Western Amazon, with S. orthos also present in the upper Madeira and Lower Orinoco, and S. schotti known from a single specimen in the Meta River, Orinoco basin.Note also, S. orthos is represented at more sites than S. schotti.

Fig. 18 .
Fig. 18.Sternarchella orinoco (USNM 228727).a. Lateral view of the body, b.Lateral view of the head, c.Lateral view of the caudal region showing a single row of intermuscular bones in the caudal peduncle and the discontinuous caudal peduncle membrane.Scale bars = 1 cm.
pterosphenoid, anterior ventral margin of pterosphenoid similar to posterior ventral margin of orbitosphenoids.Lateral process of parasphenoid small, lateral margins of parasphenoid not extending to a horizontal with trigeminal foramen.Parasphenoid ventral margin straight or slightly curved.Narrow, just lateral to supraoccipital, parietal ridges are very large and pronounced.No crown of thorny projections present at border of parietals and supraoccipital.Dorsal margin of supraoccipital crest exceed dorsal margin of parietals.Supraoccipital crest extends to a dorsal distal tip.Internal carotid foramen.Ventral surface of basioccipital smooth.Anterior extension of infraorbital canal short.Supraorbital canal fused to frontal.Mandibular canal size small.Mandibular canal ossicles form long slender tubes.Supratemporal laterosensory canal curved at a sharp angle on surface of parietal, extending posterior onto epaxial surface of body, terminal canal pore oriented posteriorly, epidermis overlying supratemporal canal depigmented.Base of gill rakers contacting gill arch.Gill rakers long with ossified distal tips.Dorsal surface of basihyal convex forming a robust ridge posteriorly.Second basibranchial triangular with short stem (length of descending rod less dorsal margin).Third basibranchial unossified.Fourteen or more teeth present on pharyngobranchial.Eight or more teeth present on sixth hypobranchial.Medial surface of fourth hypobranchial with a process or bridge extending to meet contralateral process on midline.Urohyal blade unossified.First hypohyal triangular, base longer than any other margin of the same bone.

Fig. 19 .FigFig. 20 .
Fig. 19.Map of collection localities for specimens in the S. sima group.Sternarchella sima (circles), S. orinoco (stars), and S. raptor (triangles).Note most specimens of the S. sima group have been collected near the mouths of the Amazon and Orinoco rivers.Sternarchella orthos Mago-Leccia, 1994 Fig. 20, Tab.7 present at border of parietals and supraoccipital, continuing to epioccipital.Dorsal margin of supraoccipital crest exceed dorsal margin of parietals.Supraoccipital crest extends to a dorsal distal tip.Internal carotid foramen reduced.Ventral surface of basioccipital smooth.Anterior extension of infraorbital canal shorter than width of canal pore, anterior canal pore of infraorbital near first infraorbital.Supraorbital canal fused to frontal.Mandibular canal size small.Mandibular canal ossicles form long slender tubes.Supratemporal laterosensory canal curved at a sharp angle on surface of parietal, extending posterior onto epaxial surface of body, terminal canal pore oriented posteriorly, epidermis overlying supratemporal canal depigmented.Endopterygoid process small, not contacting frontal.Endopterygoid process extends vertically at or near a 90º with dorsal surface of endopterygoid.Hyomandibula short, its width half its length.Dorsal margin of opercle concave.Opercle broad, width over half depth.Base of gill rakers contacting gill arch.Gill rakers long with ossified distal tips.Dorsal surface of basihyal convex forming a robust ridge posteriorly.Second basibranchial fan-shaped, extending to long narrow end.Third basibranchial unossified.Fourteen or more teeth present on pharyngobranchial.Eight or more teeth present on sixth hypobranchial.Medial surface of fourth hypobranchial with a process or bridge extending to meet contralateral process on midline.Urohyal blade unossified.Anterior limb of cleithrum length greater than cleithrum ascending limb length.Post-temporal fused with supracleithrum in mature specimens.First hypohyal bell-or cylinder shaped.

Fig. 21 .
Fig. 21.Results of geometric morphometric analysis of Sternarchella orthos from river basins of northern South America.a. PC1 corresponds to variation in position of mouth, anus, and posterior head margin.b.PC2 corresponds to variation in opercle width, and position of anus and analfin origin.

Fig. 22 .
Fig.22.Head-shape variation in Sternarchella orthos from three hydrogeographic regions of Greater Amazonia.a. Principal components analysis.Position of mouth, anus, and posterior head margin load heavily on PC1, and opercle width and position of anus and anal-fin on in PC2 (Fig.7).EA, Eastern Amazon (green); OR, Orinoco (red); WA, Western Amazon (blue), upper Madeira included in EA on both morphometric and hydrogeographic grounds.Note the partial separations among the three populations.b.Geographic distributions and representative phenotypes of S. orthos in the three regions.c.Significant correlation (p= 0.045) between morphometric and river distance among five locations (symbols in panel B).Morphometric distances from CVA of head shape data; river distances as thalweg measured in Google Earth.

Fig. 25 .
Fig. 25.Sternarchella sima paratype (AMNH 3864).a. Lateral view of the body, b.Lateral view of the head, c.Lateral view of the caudal region showing a single row of intermuscular bones in the caudal peduncle and a discontinuous caudal peduncle membrane.Scale bars = 1 cm.

Clade B (S. schotti clade): Comprised of Sternarchella
Ventral ethmoid large and robust with a large fan-shaped lateral process.Dorso-anterior portion of mesethmoid straight.Anterior tip of mesethmoid scyphate on dorsal surface.Anterior fontanel longer than posterior fontanel.Lateral ethmoid large, hour-glass shaped, most narrow portion at mid-length.Orbitosphenoid broad, well ossified in median nasal septum with ventral margin longer than dorsal margin.Dorso-medial portion of orbitosphenoids in contact (visible through anterior fontanel in dorsal view).Absence of ventral process of pterosphenoid, anterior ventral margin of pterosphenoid similar to posterior ventral margin of orbitosphenoids.Lateral process of parasphenoid small, lateral margins of parasphenoid not extending to a horizontal with trigeminal foramen.Parasphenoid ventral margin straight or slightly curved.Distance between parietal ridges narrow, just lateral to supraoccipital, parietal ridges very large and pronounced.Crown of thorny projections present at border of parietals and supraoccipital, continuing to epioccipital.
sima, S. schotti, S. rex, and S. raptor).Description.A medium-sized apteronotid species, reaching 208 mm LEA.Pectoral fin size small, less than 80% HL.PA% large, 54-72% HL.Head width narrow, distance between lateral margins 36-48% HL.Preorbital (snout) length moderate, 24-32% HL.Postorbital distance large, 60-68% HL.Eye diameter large, 6-11% HL.Interorbital distance small, 15-20% HL.Mouth wide, distance between ricti 18-25% HL.Body depth less than HL.Scales absent on posterolateral portion of body.Scales large in size with 5-8 present above lateral line at mid-body.Scales dorsal to lateral line rhomboid at mid-body.Rictus extends to a vertical with mental symphysis, gape very small, less than twice eye diameter.Oral aperture superior, lower jaw extends anteriorly to upper jaw.Body cavity long; 14-15 pre-caudal vertebrae present.Proximal surface of first displaced hemal spine narrower then descending blade.One to two displaced hemal spines.Swim bladder not extending posterior to body cavity.Anal-fin pterygiophore length equal to or shorter than hemal spines.Anal-fin proximal small, shorter than hemal spine.Two rows of bones in caudal peduncle visible externally.Caudal peduncle shallow, 19-30% HL.Dark spot on caudal peduncle absent.Continuous membrane of tissue connecting anal-fin base and caudal peduncle.Caudal peduncle length short, less than HL.Lateral margin of premaxilla longer than lateral margin of maxilla.Premaxilla triangular in ventral view.Two rows of teeth present on premaxilla.Anterior hook of maxilla absent, anterior process broad and triangular with a continuous ventral margin with descending blade.Anterior process of maxilla extending as a shelf of bone less than one-third length of descending blade.Ventral margin of maxillary blade curves evenly towards its distal tip.Descending blade maxilla thin, evenly curved.Two rows of teeth present on dentary.Dentary longer than deep, oral margin of dentary longer than length of angular articular.Dorsal margin of dentary slightly concave in lateral view.Posterior margin of dentary deeply forked.Endopterygoid large, contacting frontal.Endopterygoid process extends vertically at or near a 90º angle with dorsal surface of endopterygoid.Endopterygoid process slender with a narrow tip.Hyomandibula short, its width half its length.Dorsal margin of opercle concave.Opercle broad, width over half depth.Anterior limb of cleithrum length greater than cleithrum ascending limb length.Post-temporal fused with e160168[9]supracleithrum in mature specimens.Dorsal margin of supraoccipital crest extends beyond dorsal margin of parietals.Supraoccipital crest extends to a dorsal distal tip.Internal carotid foramen reduced.Ventral surface of basioccipital smooth.Anterior extension of infraorbital canal shorter than width of canal pore, anterior canal pore of infraorbital near first infraorbital.Supraorbital canal fused to frontal.Mandibular canal size small.Mandibular canal ossicles form long slender tubes.Supratemporal laterosensory canal curved at a sharp angle on surface of parietal, extending posterior onto epaxial surface of body, terminal canal pore oriented posteriorly, epidermis overlying supratemporal canal depigmented.Base of gill rakers contacting gill arch.Gill rakers long with ossified distal tips.Dorsal surface of basihyal convex forming a robust ridge posteriorly.Second basibranchial hour-glass shaped with most narrow portion at mid-length.Third basibranchial unossified.Fourteen or more teeth present on pharyngobranchial.Eight or more teeth present on sixth hypobranchial.Medial surface of fourth hypobranchial with a process or bridge extending to meet contralateral process on midline.Urohyal blade unossified.First hypohyal bell-or cylinder shaped.
Tab. 3. Morphometrics and meristic measurements forSternarchella rex.Ranges include holotype.Descending blade maxilla thin, evenly curved.Two rows of teeth present on dentary.Dentary longer than deep, oral margin of dentary longer than length of angular articular.Dorsal margin of dentary slightly concave in lateral view.Posterior margin of dentary curves gradually to descending limb.Endopterygoid process small, not contacting frontal.
Sternarchella calhamazon can be diagnosed from all congeners by the presence of a flat posterior dorsal surface of the basihyal (vs.ridge in all other Sternarchella species), the presence of twelve or fewer teeth on pharyngobranchial (vs.14 or more in all other Sternarchella species), the possession of a small body cavity usually with less than 14 pre-caudal vertebrae present (vs.14 or more in all other Sternarchella species), the absence of a crown of thorny projections present at border of parietals and supraoccipital (vs.present in S. patriciae and S. orthos) (shared with S. duccis,S.orinoco,S. rex, S. raptor, S. schotti, and S. sima), and the possession of a deep caudal peduncle, 27-42% HL (vs.15-21% HL in all other species of Sternarchella).Smallest known species of Sternarchella reaching an LEA of 169 mm.Pectoral fin size small, less than 80% HL.PA% large, 50-73% HL.Head width narrow, Post-temporal fused with supracleithrum in mature specimens.Ventral ethmoid large and robust with a large fan-shaped lateral process.Dorso-anterior portion of mesethmoid straight.Anterior tip of mesethmoid scyphate on dorsal surface.Anterior fontanel longer than posterior fontanel.Lateral ethmoid large hour-glass shaped, most narrow portion at mid-length.Orbitosphenoid broad, well ossified in median nasal septum with ventral margin longer than dorsal margin.Dorso-medial portion of orbitosphenoids in contact (visible through anterior fontanel in dorsal view).Absence of ventral process of pterosphenoid, anterior ventral margin of pterosphenoid similar to posterior ventral margin of orbitosphenoids.Lateral process of parasphenoid small, lateral margins of parasphenoid not extending to a horizontal with trigeminal foramen.Parasphenoid ventral margin straight or slightly curved.Distance between parietal ridges narrow, lateral to supraoccipital, parietal ridges are very large and pronounced.No thorny projections present at border of parietal and supraoccipital.Dorsal margin of supraoccipital crest exceed dorsal margin of parietals.Supraoccipital crest extends to a dorsal distal tip.Internal carotid foramen reduced.Ventral surface of basioccipital smooth.Anterior extension of infraorbital canal short.Supraorbital canal fused to frontal.Mandibular canal size small.Mandibular canal ossicles form long slender tubes.Supratemporal laterosensory canal curved at a sharp angle on surface of parietal, extending posterior onto epaxial surface of body, terminal canal pore oriented posteriorly, epidermis overlying supratemporal canal depigmented.Endopterygoid large, contacting frontal.Base of gill rakers contacting gill arch.Gill rakers long with ossified distal tips.Dorsal surface of basihyal flat; small ridge may be present posteriorly.Second basibranchial hour-glass shaped with most narrow portion at mid-length.Third basibranchial unossified.Twelve or less teeth present on pharyngobranchial.Eight or more teeth present on sixth hypobranchial.Medial surface of fourth hypobranchial with a process or bridge extending to meet contralateral process on midline.Urohyal blade unossified.First hypohyal bell-or cylinder shaped.
Tab. 4. Morphometrics and meristic measurements for Sternarchella calhamazon.distance between lateral margins 37-44% HL.Preorbital (snout) length moderate, 22-31% HL.Postorbital distance small, 62-70% HL.Eye diameter small, 6-9% HL.Interorbital distance small, 12-20% HL.Mouth wide, distance between ricti 17-23% HL.Body depth less than HL.Body translucent in living specimens, yellow or pink hue in living specimens.Scales absent on posterolateral portion of body.Scales large in size with 5-8 present above lateral line at mid-body.Scales dorsal to lateral line rhomboid at mid-body.Rictus extends to a vertical with mental symphysis, gape small, less than twice eye diameter.Oral aperture superior, lower jaw extends anteriorly to upper jaw.Body cavity short, 14 or fewer pre-caudal vertebrae present.Proximal surface of first displaced hemal spine narrower then descending blade.One to two displaced hemal spines.Swim bladder not extending posterior to body cavity.Anal-fin pterygiophore length equal to or shorter than hemal spines.Proximal anal-fin pterygiophores long, equal or longer than hemal spines.Two rows of bones visible externally in caudal peduncle.Caudal peduncle deep, 27-42% HL.Dark spot on caudal peduncle absent.Continuous membrane of tissue connecting anal-fin base and caudal peduncle.Caudal peduncle length short, less than HL.Premaxilla large, lateral margin of premaxilla longer than lateral margin of maxilla.Premaxilla triangular in ventral view.Two rows of teeth present on premaxilla.Anterior hook of maxilla absent, anterior process broad and triangular with a continuous ventral margin with descending blade.Anterior process of maxilla extending as a shelf of bone less than one-third length of descending blade.Ventral margin of maxillary blade curves evenly towards its distal tip.Descending blade maxilla thin, evenly curved.Two rows of teeth present on dentary.Dentary longer than deep, oral margin of dentary longer than length of angular articular.Dorsal margin of dentary slightly concave in lateral view.Endopterygoid process extends vertically at or near a 90º angle with dorsal surface of endopterygoid.Hyomandibula short, its width half its length.Dorsal margin of opercle concave.Opercle broad, width over half depth.Anterior limb of cleithrum length greater than ascending e160168[15] limb length.
(vs. terminal or superior in S. calhamazon, S. duccis, S.  orinoco, S. orthos, S. raptor, S. rex, and S. schotti)(shared with S. sima), the possession of a small interorbital distance, 17-21% HL (vs.19-25% HL in S. sima) (shared with S. calhamazon and S. patriciae) and the presence of a long tail (CPL length more than HL) (vs.23-93% HL in all other Sternarchella species).18 e160168[18] Taxonomic revision of the electric fish Sternarchella Medium to large size species reaching 256 mm LEA.Pectoral fin size small, less than 80% HL.PA% moderate, 46-49% HL.Head narrow, distance between lateral margins 36-57%.Preorbital (snout) length moderate, 28-32% HL.Postorbital distance small, 63-70% HL.Eye diameter small, 4-7% HL.Interorbital distance small, 12-18% HL.Mouth narrow, distance between ricti 15-41% HL.Body depth less than HL.Scales absent on posterolateral portion of body.Scales large in size with 5-8 present above lateral line at midbody.Scales dorsal to lateral line rhomboid at mid-body.Rictus extends to a vertical with mental symphysis, gape very small, less than twice eye diameter.Oral aperture terminal, upper and lower jaws equal in length.Body cavity long; 14-16 pre-caudal vertebrae present.Proximal surface of first displaced hemal spine narrower then descending blade.One to two displaced hemal spines.Swim bladder not extending posterior to body cavity.Anal-fin pterygiophore length equal to or shorter than hemal spines.Anal-fin proximal length shorter than hemal spine.One row of bones in caudal peduncle visible externally.Caudal peduncle shallow 8-22% HL.Dark spot on caudal peduncle absent.No apparent connective tissue between anal-fin base and caudal peduncle.Caudal peduncle length less than HL.Premaxilla large, lateral margin of premaxilla longer than lateral margin of maxilla.Premaxilla triangular in ventral view.Three rows of teeth present on premaxilla.Anterior hook of maxilla absent, anterior process of maxilla broad and triangular with a continuous ventral margin with descending blade.
duccis, S. patriciae, S. raptor, and S. rex and triangular in S. sima and S. orinoco) (shared with S. schotti) and the presence of a crown of thorny projections present at border between parietals and supraoccipital, continuing to epioccipital (vs.no crown present in S. calhamazon, S. duccis, S. orinoco, S. raptor, S. rex, S. schotti, and S. sima) (shared with S. patriciae).Description.Absence of ventral process of pterosphenoid, anterior ventral margin of pterosphenoid similar to posterior ventral margin of orbitosphenoids.Small, lateral margins of parasphenoid not extending to a horizontal with trigeminal foramen.Parasphenoid ventral margin straight or slightly curved.Narrow, just lateral to supraoccipital, parietal ridges are very large and pronounced.Crown Tab. 8. Morphometrics and meristic measurements for Sternarchella raptor.
Tefé, Toco Preto, 03°47.31'S64°59.91'W,23 October 1999.INPA 18262, Tab. 10.Morphometrics and meristic measurements for Sternarchella sima.Description.A medium-sized species, to 189 mm LEA.Pectoral fin size large, 80% HL or greater.PA% small, 24-36% HL.Head width narrow, distance between lateral margins 40-49% HL.Preorbital (snout) length small, 18-28% HL.Postorbital distance large, 64-75% HL.Eye diameter small, 6-9% HL.Interorbital distance large, 19-25% HL.Mouth narrow, distance between ricti 12-16% HL.Body depth equal or greater than HL.Scales absent on posterolateral portion of body.Scales large in size with 5-8 present above lateral line at mid-body.Scales dorsal to lateral line rhomboid at mid-body.Rictus extends to a vertical with mental symphysis, gape very small, less than twice eye diameter.Oral aperture sub-terminal, upper jaw extends anteriorly to lower jaw.Body cavity long, 15 precaudal vertebrae.Proximal surface of first displaced hemal spine narrower then descending blade.1-2displacedhemal spines.Swim bladder not extending posterior to body cavity.Anal-fin pterygiophore length equal to or shorter than hemal spines.Anal-fin proximal short, length shorter than hemal spine.One row of bones in caudal peduncle visible externally.Caudal peduncle shallow less than 30% HL Dark spot on caudal peduncle absent.No apparent connective tissue between anal-fin base and caudal peduncle.Caudal peduncle length short, less than HL.Premaxilla large, lateral margin of premaxilla longer than lateral margin of maxilla.Premaxilla square in ventral view.Four rows of teeth present on premaxilla.Anterior hook of maxilla absent, anterior process broad and triangular with a continuous ventral margin with descending blade.Anterior process of maxilla extending as a shelf of bone less than one-third length of descending blade.Ventral margin of maxillary blade curved evenly towards its distal tip.Descending blade maxilla thin, evenly curved.Three to four rows of teeth present on dentary.Dentary longer than deep, oral margin of dentary longer than length of angular articular.Dorsal margin of dentary slightly concave in lateral view.Posterior margin of dentary curved gradually to descending limb.Endopterygoid large, contacting frontal.Angle of endopterygoid process with dorsal margin of endopterygoid oblique (greater than 90° with dorsal surface of endopterygoid).Endopterygoid process slender with a narrow tip.Hyomandibula short, its width half its length.Dorsal margin of opercle concave.Opercle broad, width over half depth.Anterior limb of cleithrum length greater than cleithrum ascending limb length.Post-temporal fused with supracleithrum in mature specimens.Ventral ethmoid size large and robust with a large fan-shaped lateral process.Dorso-anterior portion of mesethmoid strongly curved from anterior tip to frontal boundary.Anterior tip of mesethmoid convex and rounded.Anterior fontanel longer than posterior fontanel.Lateral ethmoid large hour-glass shaped, narrowest portion at mid-length.Orbitosphenoid broad, well ossified in median nasal septum with ventral margin longer than dorsal margin.Dorso-medial portion of orbitosphenoids in contact (visible through anterior fontanel in dorsal view).Absence of ventral process of pterosphenoid, anterior ventral margin of pterosphenoid similar to posterior e160168[28]ventral margin of orbitosphenoids.Small, lateral margins of parasphenoid not extending to a horizontal with trigeminal foramen.Parasphenoid ventral margin straight or slightly curved.Distance between parietal ridges short, just lateral to supraoccipital, parietal ridges very large and pronounced.No crown of thorny projections present at border of parietals and supraoccipital.Dorsal margin of supraoccipital crest extends beyond dorsal margin of parietals.Supraoccipital crest extends to a dorsal distal tip.Internal carotid foramen reduced.Ventral surface of basioccipital smooth.Anterior extension of infraorbital canal shorter than width of canal pore, anterior canal pore of infraorbital near first infraorbital.Supraorbital canal fused to frontal.Mandibular canal size small.Mandibular canal ossicles form long slender tubes.Supratemporal laterosensory canal curved at a sharp angle on surface of parietal, extending posterior onto epaxial surface of body, terminal canal pore oriented posteriorly, epidermis overlying supratemporal canal depigmented.Base of gill rakers not contacting gill arch.Gill long, with ossified distal tips.Dorsal surface of basihyal convex, forming a robust ridge posteriorly.Second basibranchial triangular with short stem, length of descending rod less than dorsal margin.Third basibranchial unossified.Fourteen or more teeth present on pharyngobranchial.Eight or more teeth present on sixth hypobranchial.Medial surface of fourth hypobranchial with a process or bridge extending to meet contralateral process on midline.Urohyal blade unossified.First hypohyal triangular, base longer than any other margin of bone.