Archolaemus santosi, Vari & Santana & Wosiacki, 2012

Vari, Richard P., Santana, Carlos David De & Wosiacki, Wolmar B., 2012, South American electric knifefishes of the genus Archolaemus (Ostariophysi, Gymnotiformes): undetected diversity in a clade of rheophiles, Zoological Journal of the Linnean Society 165 (3), pp. 670-699 : 695-697

ARCHOLAEMUS SANTOSI SP. NOV.

( FIGS 10 , 11 ; TABLE 2)

Diagnosis: Archolaemus santosi is diagnosed from A. blax in the length of the gape (the rictus falling short of the vertical through the posterior border of the posterior naris versus extending beyond that line), the depth of the caudal filament (6.7–9.1% CL versus 5.0–6.5% CL), the form of the premaxilla (the anteroposterior length equal to the transverse width versus the anteroposterior length greater than the transverse width), and the length of the posterior ceratohyal (approximately the same length as the ventral hypohyal versus 1.5 times the length of the ventral hypohyal). Archolaemus santosi is diagnosed from A. ferreirai in the form of the premaxilla (the anteroposterior length equal to the transverse width versus the anteroposterior length greater than the transverse width) and the length of the coronomeckelian bone (less than 20% the length of Meckel’s cartilage versus 50% or more the length of Meckel’s cartilage). Archolaemus santosi is diagnosed from A. janeae by the head length to the rear of the opercle (15.0–16.3% LEA versus 13.5–14.7% LEA), the mouth width (7.0–10.3% HL versus 11.0–19.1% HL), the form of the premaxilla (the anteroposterior length equal to the transverse width versus the anteroposterior length greater than the transverse width), the distribution of the teeth on the dentary (dentition restricted to the anterior one-half or slightly more of the dorsal margin versus occupying most of the dorsal margin), and the length of the posterior ceratohyal (approximately the same length as the ventral hypohyal versus 1.5 times the length of the ventral hypohyal). Archolaemus santosi is diagnosed from A. luciae in the length of the gape (the rictus falling short of the vertical through the posterior border of the posterior nares versus extending beyond that line), the length of the caudal filament (14.4– 27.0% LEA versus 28.6–46.1% LEA), the depth of the caudal filament (6.7–9.1% CL versus 3.3–4.8% CL), the form of the premaxilla (the anteroposterior length equal to the transverse width versus the anteroposterior length greater than the transverse width), and the length of the coronomeckelian bone (less than 20% the length of Meckel’s cartilage versus 50% or more of the length of Meckel’s cartilage). Archolaemus santosi is diagnosed from A. orientalis by the pigmentation pattern (the presence of a narrow dark stripe along the lateral line and a broad band of dusky to dark pigmentation overlying the basal pterygiophores of the anal fin versus the lack of such dark pigmentation), the total number of pectoral-fin rays (19–21 versus 16), and the total number of anal-fin rays (204–221 versus 164–186).

Description: Morphometric data for the examined specimens of A. santosi are presented in Table 2.

Body elongate and laterally compressed. Greatest body depth located along abdominal cavity in specimens of all sizes. Body profile gently convex from rear of head for approximately anterior one-third of body, and then gradually angled posteroventrally along progressively tapering body and caudal filament. Ventral profile of body straight and posteroventrally aligned in juveniles and convex anteriorly along anterior half of abdominal cavity in larger individuals, and then gradually angled posterodorsally along base of anal fin and caudal filament. Anteriormost perforated lateralline scale located at vertical situated approximately at origin of pectoral fin. Lateral line continuous, extending from immediately posterior of vertical through origin of pectoral fin to end of caudal filament.

Head laterally compressed, widest at opercular region and deepest at nape. Head profile convex anteriorly from margin of upper lip in specimens of all sizes, then convex to varying degrees along antorbital region, with concavity more pronounced in larger specimens, and then straight to slightly convex from that point to rear of head in specimens of all sizes. Ventral profile straight and posteroventrally aligned. Snout subconical and anteroventrally directed, moderately elongate in smaller specimens to elongate in larger individuals. Eye small and located laterally on dorsal half of head. Orbital rim covered by membrane and attached to surface tissues of adjoining region of head in two specimens of approximately 73–104 mm TL, partially free on one side in an individual of 103 mm TL, and completely free in larger examined specimens. Anterior naris located at end of short tube and positioned short distance posterior of tip of snout, but distant from anterior margin of eye. Posterior naris elliptical, without tube, and located nearer to tip of snout than to anterior margin of eye in adults. Mouth distinctly inferior, with upper jaw clearly longer than, and overlapping, lower jaw. Gape short in adults, not extending posteriorly to vertical through anterior margin of posterior naris. Branchial opening moderately elongate, located along posterior margin of opercle and immediately anterior to anterior margin of pectoral-fin origin. Branchial membranes joined at isthmus and extending posteriorly to under pectoral-fin base. Anus situated proximate to elongate urogenital papilla, and both structures positioned approximately ventral of orbit. Position of anus and urogenital papilla shifting ontogenetically from distinctly posterior of vertical through posterior margin of orbit in juveniles, to along vertical through eye in largest specimens.

Scales small, cycloid, and present from immediately posterior of head to end of caudal filament. Scales along lateral line 132 to 147 (147) (N = 10). Scales above lateral line at midbody 14 to 17 (17) (N = 12). Scales absent on head.

Pectoral fin long, approximately two-thirds of head length, broad, and distally pointed, with lateral rays longer. Pectoral-fin rays between ii, 19 and ii, 21 (21) (N = 12). Anal-fin elongate, extending from vertical through base of pectoral fin along most of length of body. Fin convex anteriorly, and then straight for length of fin, but with rays becoming progressively shorter posteriorly. Anal fin with 26–35 unbranched rays (26) (N = 12) and 204–221 total fin rays (208) (N = 12). Caudal filament of moderate length.

Coloration in alcohol: Overall ground body coloration ranging from tan to brown. Head more lightly pigmented ventrally in specimens across all intensities of dark coloration, with dorsal two-thirds of head together with most of opercle distinctly darker in largest examined specimen. Narrow, dark stripe overlying lateral line for varying distances. Dark pigmentation along lateral line starts posterior of vertical through tip of pectoral fin in very small individuals, but extends anteriorly to beginning of lateral line in larger specimens. Basal anal-fin pterygiophores outlined on body surface by very narrow dark bars that cumulatively form dusky broad band running along ventral portion of body, with height of band gradually decreasing posteriorly. Surface bars along pterygiophore margins expand ontogenetically and coalesce to varying degrees to form solid dark band in larger specimens. Dark stripe along lateral line and dark ventral band separated by more lightly pigmented, broad midlateral stripe. Some specimens with ventral surface of the head tan, but most often with head dark over all surfaces. Pectoral fin hyaline overall, with scattered dark chromatophores. Anal fin hyaline.

Distribution: Archolaemus santosi is only known from the type locality at the Rio Jamari in Rondônia, Brazil ( Fig. 11 ).

Habitat: The area of the type locality of A. santosi was reported by Santos (1996: 249) as being rapids (= corredeira), indicative of the rapid water flow conditions at the site.

Remarks: Archolaemus santosi is only known from juveniles and mature females.

Etymology: The species name, Archolaemus santosi , is in honor of Geraldo Mendes dos Santos of the Instituto Nacional de Pesquisas da Amazônia, the collector of the type series, in recognition of his many contributions to our knowledge of the Amazonian fish fauna.

Material examined

Holotype: BRAZIL. Rondônia, Rio Jamari, above site of Usina Hidroelétrica Samuel (approximately 08°27′S, 063°30′W), station Polo 666, collected by G. M. Santos, 4 April 1985; INPA 20966 (197).

Paratypes: BRAZIL. Collected with holotype, INPA 36382 , 9 (73–212; 3 CS) ; MPEG 21686 , 1 (100) ; MZUSP 110185 , 1 (132) ; USNM 404357 , 1 (100).

Figures

Figure 10. Archolaemus santosi sp. nov., holotype, female, 197 mm total length, INPA 20966; Brazil, Rondônia, Rio Jamari, above site of future Usina Hidroelétrica Samuel (approximately 08°27′S, 063°30′W).

Figure 11. Map of middle portions of Rio Madeira basin and adjoining areas, showing the geographical distribution of Archolaemus santosi sp. nov. (the dot indicates the holotype locality and more than one lot of specimens).