Melanosternarchus, Bernt & Crampton & Orfinger & Albert, 2018

Bernt, Maxwell J., Crampton, William G. R., Orfinger, Alexander B. & Albert, James S., 2018, Melanosternarchus amaru, a new genus and species of electric ghost knifefish (Gymnotiformes: Apteronotidae) from the Amazon Basin, Zootaxa 4378 (4), pp. 451-479 : 453-457

Melanosternarchus , new genus

( Figs. 1–10 ; Table 1.)

Type species. Melanosternarchus amaru , new species, by monotypy and original designation.

Diagnosis. Melanosternarchus is diagnosed from all other apteronotid genera within the Navajini by bones 1– 4 of the infraorbital laterosensory canal present as tubes fused to expanded bony plates (versus only tubular-shaped bones in other genera), and overlapping contact between infraorbital bones 3 and 4 (versus these bones not contacting one another in other genera), see Fig. 3B . Melanosternarchus can be further distinguished from all other apteronotid genera by the following combination of readily-observable characters: 1) gape long, rictus located at or behind a vertical with the eye (vs. gape short, rictus located anterior to vertical with eye), 2) scales absent over entire dorsum (vs. scales completely or partly covering dorsum), 3) one or no teeth on the premaxilla (vs two more teeth on the premaxilla), and 4) dark brown body coloration, without a pale middorsal stripe (vs. pink, light-brown, black, or uneven body colorations, sometimes with a pale middorsal stripe).

Although Melanosternarchus is diagnosed from all known species within the Navajini by its platelike infraorbitals, this character can only be readily observed in cleared and stained specimens. We therefore provide additional characters by which it can be separated from all other Navajini genera. Melanosternarchus can be distinguished from the Apteronotus bonapartii group ( A. apurensis , A. bonapartii , and A. ellisi ) by the following four characters: 1) one or no teeth on the premaxilla vs. 4 or more; 2) scales absent along entire dorsum from nape to dorsal organ, vs. large scales present along dorsal midline posterior to nape 3); dentary teeth forming a cluster of two short, irregular rows on the posterior region of the dentary, tapering into a single row anteriorly, vs. all dentary teeth arranged in two rows; 4) dorsal margin of lateral fenestra composed of orbitosphenoid and pterosphenoid, with the margin of these bones protruding ventrally into the fenestra vs. margin of orbitosphenoid and pterosphenoid anterior to lateral fenestra (i.e. dorsal margin of fenestra composed only of pterosphenoid). This condition is illustrated for A. apurensis in Albert (2001: fig. 14A) , and for A. bonapartii in Hilton & Cox Fernandes (2006 : fig. 4, 2017: fig. 4).

Melanosternarchus can be distinguished from all species of Compsaraia by four characters: 1) the absence of an L-shaped (longer longitudinally than vertically) supratemporal lateral-line canal; 2) uniform brown coloration on the head and snout vs. pale patch of low melanophore density on snout in a narrow band, anterior to the eyes and lateral to nares; 3) four mandibular canal ossicles, all ventral to the lower jaw, vs. five ossicles present, with the posterior two ossicles ventral to the preopercle; 4) dark pigmentation present on pectoral and anal fins, vs. pigmentation absent from pectoral and anal fins.

Melanosternarchus can be distinguished from Pariosternarchus by five characters: 1) gape extending to or beyond vertical with the eye, vs. gape not reaching vertical with eye; 2) head not expanded ventrolaterally, vs. head expanded ventrolaterally (a condition associated with expanded mandibular canal-ossicles); 3) one or no teeth present on premaxilla vs. 6 or more teeth on premaxilla; 4) teeth forming a cluster of two short, irregular rows on the posterior region of the dentary, tapering into a single row anteriorly, vs. teeth arranged in two rows on the dentary; 5) urohyal without posterior ossified blade, vs. posterior ossified blade present.

Melanosternarchus can be distinguished from all species of Sternarchogiton except for S. porcinum Eigenmann & Allen by gape extending to or beyond vertical with the eye vs. gape not reaching vertical with eye, and from S. porcinum by brown pigmentation on head and sides, vs. pink to pale straw color on head and sides.

Melanosternarchus can be distinguished from all Sternarchella by 1) gape extending to or beyond vertical with the eye, vs. gape not exceeding vertical with posterior naris and 2) one or no teeth on premaxilla vs. two to four rows of teeth on premaxilla.

Although there is not currently a diagnosis for Porotergus, Melanosternarchus can be distinguished from each species in the genus. It can be differentiated from P. gimbeli Ellis by one or no teeth on the premaxilla vs. 6–8 teeth on the premaxilla; from P. duende de Santana & Crampton by gape extending to or beyond vertical with eye; and from P. gymnotus Ellis by 1) dentary teeth forming a cluster of two short, irregular rows on the posterior region of the dentary, tapering into a single row anteriorly, vs. teeth arranged in two rows on the dentary and 2) one or no teeth on premaxilla vs. two or more teeth on the premaxilla.

Etymology. Melano from the Greek melas, meaning black in reference to dark pigmentation and presence in blackwater rivers, and sternarchus, a name commonly used in apteronotid taxonomy, from the Greek sternon (chest) and archos (rectum), referring to the anterior position of the anus.

Figures

FIGURE 1. Lateral view of Melanosternarchus amaru, holotype MUSM 59405, 230 mm TL.

FIGURE 2. Lateral view of two living specimens of Melanosternarchus amaru, ANSP 200459.

FIGURE 3. Detail of the head of living Melanosternarchus amaru, ANSP 200459 (A) and cephalic lateral line bones of Melanosternarchus amaru, LSUMZ 20734, 237 mm TL (B). Abbreviations: ant, antorbital; io, infraorbital; na, nasal; otc, otic canal; mco, mandibular canal ossicles; pa, parietal canal; prc, preopercular canal; soc, supraorbital canal; stc, supratemporal canal.

FIGURE 4. CT scan of Melanosternarchus amaru, FMNH 128388, 210 mm TL in dorsal (A), ventral (B), and lateral (C) views.

FIGURE 5. Illustrations of the neurocranium of Melanosternarchus amaru (ANSP 200459, 187 mm TL, male) in dorsal (A), ventral (B), and lateral (C) view. Cartilages shown in black. Abbreviations: bao, basioccipital; epo, epioccipital; exo, exoccipital; fro, frontal; let, lateral ethmoid; met, mesethmoid; obs, orbitosphenoid; par, parietal; pas, parasphenoid; pro, prootic; pto, pterotic; pts, pterosphenoid; soc, supraoccipital; spo, sphenotic; vet, ventral ethmoid; vom,vomer.

FIGURE 6. Suspensorium, opercular series, and maxilla of Melanosternarchus amaru (ANSP200459) in lateral (A) and medial (B, image reflected) view. Thin, unstained hard tissue associated with opercular bones shown in light gray. Abbreviations: ang, anguloarticular; ap, autopalatine cartilage; cm, coronomeckelian; d, dentary; enp, endopterygoid; h, hyomandibula; iop, interopercle; mx, maxilla; mco, mandibular canal ossicles; m, Meckel’s cartilage; mpt, metapterygoid; op, opercle; pop, preopercle; q, quadrate; rar, retroarticular; sop, subopercle; sym, symplectic.

FIGURE 7. Premaxillae of three specimens of Melanosternarchus amaru, ANSP 200459, 187 mm TL—shown in dorsal and ventral views and bearing a single tooth (A) and MUSM 59453, 214 mm TL, and C) MUSM 59453, 147 mm TL (B). Scale bar = 2.5 mm.

FIGURE 8. Ventral hyoid arch in lateral view (A) with branchiostegal 4 removed from normal position in order to show branchiostegal 3, and urohyal (B) in dorsal (left) and ventral views. Abbreviations:br, branchiostegal; cha, anterior ceratohyal; chp, posterior ceratohyal; hhd, dorsal hypohyal; hhv, ventral hypohyal.

FIGURE 9. Gill arches of Melanosternarchus amaru (ANSP 20045). Ventral arches (A) shown in dorsal view and dorsal arches (B) in ventral view. Abbreviations: bb, basibranchial; bh, basihyal, cb, ceratobranchial; eb, epibranchial; gr, gill raker; hb, hypobranchial; iph, infrapharyngobranchial; uptp, upper pharyngeal toothplate.

FIGURE 10. Pectoral girdle of Melanosternarchus amaru (ANSP 20045) in lateral (A), and medial (B) views. Abbreviations: cl, cleithrum; co, coracoid; pcl, postcleithrum; pt, posttemporal; ptg, propterygium; ra, radial; sc, scapula; scl, supracleithrum.