Megalonema orixanthum, Lundberg & Dahdul, 2008

Lundberg, John G. & Dahdul, Wasila M., 2008, Two new cis-Andean species of the South American catfish genus Megalonema allied to trans-Andean Megalonema xanthum, with description of a new subgenus (Siluriformes: Pimelodidae), Neotropical Ichthyology 6 (3), pp. 439-454 : 450-453

Megalonema orixanthum , new species Fig. 7

Holotype. ANSP 187449 (ex ANSP 148143), 100.3 mm SL, Colombia , Meta State, río Metica, ca. 3 km SE of Hacienda Mozambique, 3 ° 57 ' N, 73°02' W, 24 Mar 1975, J. Böhlke, et al.

Paratypes. All records within the Orinoco drainage basin. Colombia, Meta State: ANSP 148143, 23, 90-114 mm SL (8, 90- 114 mm SL), FMNH 117836, 2, 95-97 mm SL, IAvH-P 11021, 2, 93-96 mm SL, MBUCV V-35380, 2, 90-97 mm SL, MZUSP 99748, 2, 92-96 mm SL, USNM 393560, 2, 85-95 mm SL, all collected with the holotype; ANSP 131337, 1, 92 mm SL, río Metica , ca. 1.5 km E of Rajote (Plancha 267), 3°56' N, 73°03' W, 19 Mar 1973, W. Saul and W. Smith-Vaniz; ANSP 148142, 2, 77-116 mm SL, río Metica at El Aviso, 3°59' N, 72°59' W, 30 Mar 1975, W. Saul and L. Fuiman; ANSP 148180, 1 C&S, 98 mm SL, río Metica , ca. 3 km SE Hacienda Mozambique, 3°57' N, 73°02' W, 30 Mar 1975, J. Böhlke et al.; ANSP 148181, 1, 95 mm SL, río Metica , ca. 3 km SE Hacienda Mozambique, 3°57' N, 73°02' W, 30 Mar 1975, J. Böhlke et al. Venezuela, Amazonas State: ANSP 160744, 1, 77 mm SL, río Orinoco : shores of Isla de Raton , 5°05' N, 67°48' W, 14 Nov 1985, B. Chernoff et al.; ANSP 162484, 3, 29-77 mm SL, río Casiquiare (main channel) ca. 1.5 hr. from its confluence with río Orinoco , 16 Mar 1987, H. Lopez and O. Castillo; ANSP 162486, 22, 25-48 mm SL (3, 43- 48 mm SL), río Casiquiare from mouth of río Pamoni to 4.0 km below mouth, 2°48’00" N, 65°57’00" W, 17 Mar 1987, B. Chernoff et al.; ANSP 185144, 6, 37-74 mm SL (3, 38- 74 mm SL), río Orinoco (Atlantic Dr.), island W of Puerto Venado, 4.5 km S of Samariapo, 56.5 km SW of Puerto Ayacucho, 5°12’25" N, 67°48’32" W, 28 Feb 2005, M. Sabaj et al.; AUM 43048, 13, 47-50 mm SL (2, 47- 50 mm SL), same data as ANSP 185144; AUM 43845, 2, 79-94 mm SL, río Orinoco , beach, 16.1 km E of La Esmeralda, 25 Mar 2005. Apure State : ANSP 160186, 2, 84-87 mm SL, río Meta , ca 40 min upstream from confluence of río Orinoco , 6°18' N, 67°37' W, 27 Nov 1985, B. Chernoff et al.; ANSP 165236, 5, 61-78 mm SL, río Apure : between río Portuguesa mouth and S. Fernando de Apure airport, 7°54’00" N, 67°32’00" W, 4 Nov 1989, S. Schaefer et al.; ANSP 187450, 4 alcohol, 1 C&S, 59-73 mm SL, río Apure between San Fernando and río Portuguesa, collected with 3 m bottom trawl, 19 Jul 1984, Lundberg et al. Bolivar State: ANSP 160407, 2, 66-71 mm SL, río Orinoco - río Caura confluence beaches, canals, lagoons & islands, vicinity Puerto Las Majadas, 7°38’36" N, 64°50’W, 23 Nov 1985, B. Chernoff et al.; ANSP 160857, 1, 57 mm SL, río Cuchivero at Cuchivero ferry crossing, 7°29' N, 65°53' W, 17 Nov 1985, B. Chernoff et al.; ANSP 187451, 1, 71 mm SL, río Orinoco at Ciudad Bolivar, collected with 3 m bottom trawl in 10 m channel, 8°09' N, 63°32' W, 8 Nov 1979, E. Marsh et al., field number ECM 1-79 .

Diagnosis. A species of the subgenus Eretmomegalonema distinguished from others by the following combination of features. Supraoccipital posterior process ( Fig. 3d ) broadly triangular in outline, its basal width equal to or greater than its length, and its sides tapering straight to a nearly pointed or bifid tip nearly reaching supraneural; adipose fin ( Fig. 7a ) smaller, its margin anteriorly rising at a shallow angle from back and usually in a weakly concave curve to its apex, and adipose fin terminating at or anterior to vertical through tips of adpressed anal-fin rays; total vertebrae modally 44, range 43-45 ( Table 1); eye large, horizontal diameter 209-337 mils of HL; anal-fin base relatively short, 93-117 mils of SL ( Fig. 5 ); width between posterior nostrils relatively broad, 196-246 mils of HL ( Fig. 5 ); pectoral spine with numerous erect dentations along posterior margin; premaxilla extremely narrow with a single row of teeth or teeth absent; and 28-29 gill rakers on first arch.

Description. Meristic data for 4 to 35 specimens are in Table 1, and morphometric data for 28 to 40 specimens (a few specimens lacking data for damaged parts) are in Table 2. Megalonema orixanthum ( Fig. 7 ) is a medium sized pimelodid with a maximum length known to us of 115.8 mm SL. Dorsal profile of head and nape convex from snout tip to vertical at posterior nostril, then nearly straight to origin of dorsal fin, scarcely convex along dorsal-fin base, then less convex to posterior insertion of adipose-fin, and gently concave across caudal peduncle. Ventral profile slanting convexly ventrally from snout tip to end of gill region, slightly convex to straight to pelvic-fin origin, stepped dorsad posterior to pelvic girdle, then convex onto anal-fin base, slanting dorsally to caudal peduncle and concave along caudal peduncle.

Cross-sectional shape roughly trapezoidal from snout to supraoccipital, then deeply and broadly triangular to dorsalfin origin, and increasingly compressed to caudal fin. Maximum body depth at dorsal-fin origin contained 4.4-6.6 times in SL. Maximum body width across cleithra in front of pectoral spine insertion, contained 6.5-8.7 times in SL.

Head of moderate length, contained 4.1-4.7 times in SL and relatively deep, its depth at base of supraoccipital posterior process slightly less than body width. Head covered by thin skin, revealing smooth, unornamented skull roofing bones. Snout moderate, contained 2.1-2.6 in head length, and with broadly rounded margin in dorsal and ventral views. Snout and upper jaw projecting; with mouth closed less than half of premaxillary dentition exposed. Anterior nostril behind snout margin in a shallow, circular depression, its aperture dorsally directed and encircled with a low fleshy rim; width between anterior nostrils contained 0.9-1.5 times in distance between anterior and posterior nostrils. Posterior nostril aperture ovoid to subtriangular, preceded by thin, semicircular membrane as large as its aperture; width between posterior nostrils contained 0.7-1.0 times in distance between anterior and posterior nostrils. Distance between anterior and posterior nostrils equal to distance between posterior nostril and eye.

Eye large and somewhat bulging, about 1.5-2 times longer than deep, placed more laterally than dorsally, centered on middle of bony head length; interorbital space convex. Interorbital narrow, containing horizontal eye diameter 0.6- 1.2 times. Anterior cranial fontanelle narrow, parallel-sided, originating behind level of posterior nostrils and terminating between eyes at epiphyseal bar. Posterior cranial fontanelle closed anteriorly, persisting as oval aperture in center of supraoccipital. Supraoccipital posterior process broad-based (its basal width equal to or greater than its length), dorsally slightly convex, not angular or keeled, parabolic in outline, failing by less than half its own length to contact supraneural (anterior nuchal plate obsolete).

Mouth subterminal, opening anteriorly and widely; gape broad, its width across inner surface of ricti twice greater than interorbital width. Lips thin-skinned and smooth. Rictal fold well defined but not fleshy or swollen, subtended above and below by deep folds respectively reaching base of maxillary barbel and about one third distance to mandibular symphysis. Premaxillary toothless or with a single row of a few fine, conical teeth. Dentary teeth slender, arranged in about 3 rows at symphysis and one row posterolaterally.

Origin of maxillary barbel in depression close to base of anterior nostril and above rictus, continuing to below eye; maxillary barbel reaching onto but not beyond caudal fin. Outer mental barbel reaching posterior end of or just beyond adipose fin. Tip of inner mental barbel reaching pelvic-fin insertion to half of the length of depressed pelvic fin. Branchiostegal membranes anteriorly united to isthmus and overlapping before diverging. Branchiostegal rays 8. Gill rakers long, slender bladelike, 28-29 on first gill arch (in four specimens): 6-8 rakers on upper limb, 1 at cartilaginous angle, 20- 21 on lower limb.

Dorsal-fin lepidotrichia I,7; spinelet absent or vestigial; dorsal-spine base with much reduced articulating processes. Dorsal-fin spine and first branched fin ray distinctly longer than remaining, progressively shorter rays. Dorsal spine slender, its distal half or more segmented and flexible, its shaft smooth and non-serrate. Dorsal-fin origin at vertical through middle of adpressed pectoral fin; its posterior insertion above pelvic-fin origin. Only the longest anterior fin rays and dorsalfin spine of adpressed dorsal fin reach adipose fin. Adiposefin origin located at vertical posterior to tips of last four dorsalfin rays. Adipose fin large, anteriorly rising with a straight or barely convex margin at a low angle to its base, its apex at vertical near anal-fin origin; terminating at or near tips of adpressed anal-fin rays; 1.4-1.7 times longer than head, relatively high, its height 4.1-6.4 times in its base.

Caudal fin deeply forked; pointed upper lobe with filamentous unbranched principal ray, the lower lobe narrowly rounded and a little shorter, and none of its rays prolonged; inner margins of both lobes straight to concave. Principal caudal-fin rays 1+7+8+1. Anal-fin origin below middle of adipose-fin base, posterior insertion before end of adipose fin, distal margin of anal fin slightly concave.Anal-fin rays 12-15, modally 13, in 27 specimens.

Pectoral fin I,11-13, modally I,12, in 9 specimens. Pectoral spine slender, its distal half or more segmented and flexible, its shaft smooth anteriorly and with numerous erect dentations along posterior margin. Pectoral-spine base with much reduced articulating (fin-locking) processes. Pectoral spine and outer few branched pectoral rays prolonged but none filamentous. Pectoral-fin margin concave. Pectoral axillary gland pore absent. Posterior cleithral process obsolescent. Pelvic fin as described for subgenus. Also, pelvic-fin insertion below posterior insertion of dorsal fin. Tip of pelvic fin close to or reaching anal fin.

Urogenital papilla small located behind anus near base of inner pelvic-fin rays; no indication of sexual dimorphism.

Lateral line straight with side branches alternating dorsally and ventrally, complete, and terminating near the ends of middle caudal-fin rays.

Total vertebrae in 35 specimens ( Table 1), modally 44, range 43-45 including Weberian complex; in 36 specimens 16-18 precaudal vertebrae, modally 17, and 26-29 caudal vertebrae, modally 27.

Coloration in alcohol. Head and body yellowish to tan in background color; skin translucent often revealing broad subcutaneous silvery band along sides and lower flanks. Top and upper sides of head and body lightly to moderately covered with small brownish chromatophores. Top of head with dense concentrations of pigment at and posterior to maxillary barbel insertion, on the midline above mesethmoid and between light olfactory organs, along anterior cranial fontanelle except between eyes and over oval posterior cranial fontanelle in supraoccipital. Deep lying, quadrangular dark spot across midline between level of eyes and posterior cranial fontanelle. Supraoccipital posterior process pallid, otherwise nape darker. Dark area of variable extent near or on supraneural, middle and posterior nuchal plates, and at insertion of dorsal fin. A variable series of dark spots at bases of dorsal-fin rays. Tympanic area often darker than surrounding sides behind pectoral girdle. Sclerotic coat of eyes dark or silvery. Upper caudal spot generally present. Rayed fins with hyaline rays and transparent membranes; some specimens with chromatophores along dorsal-, caudal-, anal-, pectoral and pelvic-fin rays and on membranes of dorsal and caudal fins. Adipose fin hyaline, with small, widely dispersed chromatophores.

Distribution. Orinoco River basin, Colombia and Venezuela.

Etymology. The name orixanthum refers to the distribution of the species in the Orinoco basin and its relationship to the species M. xanthum .

Figures

Fig. 7. Megalonema orixanthum, holotype, Colombia, Meta State, río Metica, ca. 3 km SE of Hacienda Mozambique. ANSP 187449, 100 mm SL, in a, lateral, b, dorsal and c, ventral view; distal ends of barbels clipped from image c.

Fig. 3. Details of supraoccipital processes of a, Megalonema cf. platycephalum ANSP 178450, b, M. xanthum CAS 63674, c, M. amaxanthum, holotype, CBF 11896 (ex FMNH 106769), and d, M. orixanthum ANSP 148180, paratype.Paired lines on each image show the gap between the tip of supraoccipital process and supraneural anteromedial point in or just below skin.

Fig. 5. Scatter plots illustrating: a, anal-fin base length relative to standard length, and b, width between posterior nostrils relative to head length, in Megalonema xanthum (squares, n=23), M. amaxanthum (triangles, anal-fin base n=71, width between posterior nostrils n=70), and M. orixanthum (circles, n=41). In t-tests of the residuals from regression, M. amaxanthum proved to have a significantly longer anal-fin base length than M. orixanthum and M. xanthum (p-values <0.0001 for pairwise comparisons), and all three species are significantly different from each other in width between posterior nostrils with increasing relative widths from M. xanthum, M. amaxanthum to M. orixanthum (p-values for pairwise comparisons <0.0001).