Sternarchella rex, Evans & Crampton & Albert, 2017

Evans, Kory M., Crampton, William G. R. & Albert, James S., 2017, Taxonomic revision of the deep channel electric fish genus Sternarchella (Teleostei: Gymnotiformes: Apteronotidae), with descriptions of two new species, Neotropical Ichthyology 15 (2), No. e 160168, pp. 1-30 : 10-15

Sternarchella rex , new species

Figs. 11-12a, Tab. 3

urn:lsid:zoobank.org:act:67F5A55F-A093-4512-861E-0B53A27CECCB

Sternarchella n. sp. A. - Ivanyisky, Albert, 2014:569, fig. 3E [ Brazil, rio Amazonas near Tefé]. - Crampton, Albert, 2006: 386: [ Brazil, rio Amazonas near Tefé]. - Crampton, 2007:316, fig. 11.9 3E [ Brazil, rio Amazonas near Tefé]. - Crampton, 2011:165-189 [ Brazil, rio Amazonas near Tefé].

Holotype. MCP 49422, 405 mm LEA, Brazil, Tefé, Mamirauá Lake System, Paraná Maiana station A, 03º06’44”S 64°47’32”W, 28 Jan 1999, W. G. R. Crampton . Paratypes. INPA 18149 , 1, 279 mm LEA. Brazil, Tefé, Mamirauá Lake System, Paraná Maiana station A, 03º06’44”S 64°47’32”W, 28 Jan 1999 , W. G. R. Crampton. INPA 18150 , 1, 373 mm LEA, Brazil, Tefé, Mamirauá Lake System, Paraná Maiana station C, 03º04’10”S 64°47’52”W, 5 Feb 1999 , W. G. R. Crampton. MCP 49423, 2 (1 C/S), 365 mm LEA, Brazil, Tefé, Mamirauá Lake System, Paraná Maiana station A, 03º06’44”S 64°47’32”W, 28 Jan 1999 . ZUEC 12337 , 1, 305 mm LEA, Brazil, Tefé, rio Japurá at mouth of Lago Caxinguba , 03º06’17”S 64º45’84”W, 3 Feb 1999 , W. G. R. Crampton. ZUEC 12338 , 1, 313 mm LEA, Brazil, Tefé, rio Japurá, West bank at Boca do Lago Mamirauá , 03º07’13”S 64º47’30”W, 8 Dec 1999 , W. G. R. Crampton.

Non-types. ANSP 200294 , 1, 357 mm LEA, Peru, Iquitos, Loreto, Río Amazonas , 1 Jan 2015 , M. J. Bernt. MUSM 54500 , 6, 321-382 mm LEA, Peru, Iquitos, Loreto, Isla Milagro beach, East bank río Amazonas , upstream of Iquitos , 7.64 km and 21.86° from Plaza de Armas ( Iquitos town center), 03°43’28”S 73°12’31”W, 17 Dec 2015 . UF 238215, 1 , 271 LEA, Peru, Iquitos, Loreto, Isla Milagro beach, East bank río Amazonas , upstream of Iquitos , 7.64 km and 21.86° from Plaza de Armas ( Iquitos town center), 03°43’28”S 073°12’31”W, 19 Dec 2015 .

Diagnosis. Sternarchella rex can be diagnosed from all congeners by the following combination of characters: a wide head, HW 46-64% HL (vs. 36-45% HL in all other Sternarchella ), a large interorbital distance, IO 18-27% HL (vs.11-22% HL in S. calhamazon S. duccis , S. orinoco , S. orthos , S. patriciae , S. raptor , and S. schotti ) (shared with S. sima ), a deep body, depth greater than or equal to HL (vs. body depth less than HL in S. calhamazon , S. duccis , S. orinoco , S. orthos , S. patriciae , S. raptor , and S. schotti ) (shared with S. sima ), an ossified third basibranchial bone (vs. unossified in S. calhamazon , S. duccis , S. orinoco , S. orthos , S. patriciae , S. raptor , and S. sima ) (shared with S. schotti ) and anal-fin pterygiophores that are longer than hemal spines (vs. short anal-fin pterygiophores in S. duccis , S. orinoco , S. orthos , S. patriciae , S. raptor ) (shared with S. calhamazon , S. patriciae , and S. schotti ).

Description. Largest known species of Sternarchella reaching an LEA of 405 mm. Pectoral fin size small, less than 80% HL. PA% large, 44-66% HL. Head wide, distance between lateral margins 46-64% HL. Preorbital (snout) length moderate, 30-35% HL. Postorbital distance large, 64-73% HL. Eye diameter small, 5-7% HL. Interorbital distance large, 18-27% HL. Mouth wide, distance between ricti 22-32% HL. Body depth equal or greater than HL. Body pale white with pinkish and metallic green sheen 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 midbody. 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, 15-16 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 longer than hemal spines. Proximal anal-fin pterygiophores long, equal or longer than hemal spines. Two rows of bones in caudal peduncle visible externally. Dark spot on caudal peduncle absent. Continuous membrane of tissue connecting anal-fin base and caudal peduncle. 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 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 curves gradually to descending limb. Endopterygoid process small, not contacting frontal. Endopterygoid process extends vertically at or near a 90º angle with dorsal surface of endopterygoid. Endopterygoid process oblique (greater than 90º). 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 straight. Anterior tip of mesethmoid scyphate on dorsal surface. Anterior fontanel longer than posterior fontanel. Lateral ethmoid robust, may contact ventral portion on frontals. 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. Dorsal margin of supraoccipital crest extends beyond dorsal margin of parietals. Supraoccipital crest extends to a dorsal distal tip. Internal carotid foramen reduced, less than half the size of prootic 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 hour-glass shaped with most narrow portion at mid-length. Third basibranchial ossified. 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.

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.

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.

Sternarchella calhamazon Lundberg, Cox Fernandes & Campos-Da-Paz, 2013

Fig. 15, Tab. 4

Sternarchella calhamazon Lundberg et al., 2013:159 , figs. 1, 2a, b [type locality: Pará, Brazil, rio Madeira , 35 km above confluence with rio Amazonas. Collected with 3 m bottom trawl in channel 14-16 m deep, 400 m off linear beach and bank, 3°35’44.2”S 58°57’45.8”W].

Sternarchella n. sp. B. - Crampton, Albert, 2006:386 [ Brazil, rio Amazonas near Tefé]. - Crampton, 2007:316: fig. 11.6 3E [ Brazil, rio Amazonas near Tefé]. - Crampton, 2011:165-189 [ Brazil, rio Amazonas near Tefé].

Sternarchella sp. - Crampton, Cella-Ribeiro, 2013:274-275, [ Brazil, rio Madeira].

Diagnosis. 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 ).

Description. 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, 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 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. 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.

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.

Material examined. Holotype. INPA 37898 , 162.8 mm TL male, Brazil, Amazonas State, rio Madeira, 35 km above confluence with rio Amazonas , collected with 3 m bottom trawl in channel 14-16 m deep, 400 m off linear beach and bank, 3°35’44.2”S 58°57’45.8”W, 6 Aug 1996, Zanata et al. Field no. AMZ-96- 139. Non-types. Sternarchella calhamazon: USNM 373113, 8 (1 C&S), 106-139 mm LEA, Brazil, rio Madeira , USNM 375362 , 6 , 91-125 mm LEA, Brazil, rio Içá 9 km below Bretania. USNM 373093 , 3 , 87-138 mm LEA, Brazil, rio Amazonas 11.5 km below Novo Oriente. IDSM 496 , 1, 132 mm TL, Brazil, rio Japurá , near Boca do Lago Mamirauá , 03º07.02’S 64º46.91’W, 19 Jan 1999, W. G. R. Crampton. INPA 15796 , 11, 109-171 mm TL, Brazil, Mamirauá Lake System , Paraná Maiana station A, 03º06.74’S 64°47.53’W, 2 Feb 1999. INPA 18151 , 1, 138 mm TL, Brazil, rio Japurá , West bank, between Boca do Lago Mamirauá and Boca do Paraná do Jaquiri , 03º07.58’S 64º47.30’W, 9 Feb 1999. INPA 18152 , 5, 110-146 mm TL, Brazil, rio Solimões , South bank of Ilha do Jaquiri , 03º09.51’S 64º48.76’W, 9 Dec 1999. MCP 49414, 1 , 79 mm TL, Brazil, Mamirauá Lake System , Lago Promessa , 03º04’23”S 64°46’52”W, 19 May 1998. MCP 49415, 9, 106-158 mm TL, Brazil, Mamirauá Lake System , Paraná Maiana station A, 03º06’44”S 64°47’32”W, 2 Feb 1999. MCP 49416, 1, 139 mm TL, Brazil, rio Japurá , West bank at Boca do Lago Mamirauá , 03°07’36”S 64°46’15’W, 4 Feb 1999. MCP 49417, 3, 125-138 mm TL, Brazil, rio Solimões , South bank of Ilha do Jaquiri , 03°09’31”S 64°48’46”W, 9 Dec 1999. MCP 49418, 7 , 89-140 mm TL, Brazil, Mamirauá Lake System , Paraná Maiana station B, 03º04’50”S 64°47’18”W, 11 Jan 2000. MCP 49419, 1 , 96 mm TL, Brazil, Mamirauá Lake System , Paraná Maiana station A, 03º06’44”S 64°47’32”W, 23 Jan 2000. MCP 49420, 5, 111-140 mm TL, Brazil, rio Japurá-Solimões confluence, Praia Caborini , 03º09’08”S 64º47’04”W, 19 Feb 2001. MCP 49421, 1, 135 mm TL, Brazil, rio Solimões near Alvaraes , 03º13’06”S 64º47’01”W, 27 Jan 2001. ANSP 200258 , 123 , (2 C&S), 62-170 mm LEA, Peru, Río Amazonas , braid off right bank of main channel ca. 6 km northeast (downstream) of mouth of Río Nanay 03°39.621 S 073°12.278 W, 21 Sep 2015.