Megalonema amaxanthum, 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 : 447-450

Megalonema amaxanthum , new species Fig. 6

Holotype. CBF 11896 (ex FMNH 106769 ), 98.0 mm SL, Bolivia, Pando State, río Tahuamanu from Boca Nareuda to below Cachuelita , 11°18' S, 68°44' W, 13 Sep 1996, H. Ortega et al.

Paratypes. All records within the Amazon drainage basin. Bolivia, Pando State : ANSP 187452 , (ex FMNH 106769 ), 2, 39- 42 mm SL , FMNH 106769 , 48 , 47-98 mm SL (8, 47- 97 mm SL) , USNM 393558 , (ex FMNH 106769 ), 2, 81- 95 mm SL, all collected with the holotype ; AUM 23777, 3 , 67-85 mm SL, río Acre at Cobija, 7- 9 Feb , 1982, C. K. Swing et al. Brazil, Acre State : MCP 36198, 1 , 85 mm SL, rio Acre , city of Xapuri , at sand extraction encampment, 10°39’51" S, 68°30’45" W, 22 Jul 2004 , R. Reis et al. Amazonas State : ANSP 179213 , 1 , 81 mm SL, rio Solimões (Amazonas Dr.) below rio Purus , upriver of Manacapuru , collected with 3 m bottom trawl in channel, 3°27’02" S, 60°45’07" W, 1 Aug 1996, A. Zanata et al . ANSP 187453 , 2 , 42-77 mm SL, rio Solimões below rio Içá , collected with 3 m bottom trawl in 1 m channel, 3°00’26.4" S, 67°52’27.1" W, 23 Nov 1993, J. Lundberg et al., field number JGL 93-122 ; ANSP 187454 , 1 , 65 mm SL, rio Solimões below rio Jutaí , collected with 3 m bottom trawl in 2.5-5.5 m channel, 2°31’32.7" S, 66°36’34.8" W, 12 Nov 1993, J. Lundberg et al., field number JGL 93-45 ; ANSP 187455 , 6 , 32-60 mm SL (2, 40- 59 mm SL), rio Jutaí above rio Solimões , collected with 3 m bottom trawl in 5-10 m channel, 2°52’55.7" S, 66°57’37.2" W, 16 Nov 1993, J. Lundberg et al., field number JGL 93-69 ; ANSP 187456 , 10 alcohol and 2 C&S, 35-64 mm SL (4, 49- 64 mm SL), rio Jutaí above rio Solimões , collected with 3 m bottom trawl in 3.7-12.7 m channel, 2°57’03.8" S, 67°00’26.8" W, 16 Nov1993, J. Friel et al., field number JPF 93- 76 ; ANSP 187457 , 1 , 50 mm SL, rio Jutaí above rio Solimões , collected with 3 m bottom trawl in channel, 2°49’33.3" S, 66°55’06.8" W, 13 Nov 1993, J. Sullivan et al., field number JPS 93-48 ; CU 84562, 3, 47.1-64.4 mm SL, rio Jutaí above rio Solimões , collected with 3 m bottom trawl in 8.2-11.1 m channel, 2°53’42.7" S, 67°00’34.5" W, 16 Nov 1993, J. Friel et al., field number JPF 93- 82 ; INPA 29490 , 1 , 48 mm SL, rio Solimões below rio Içá , collected with 3 m bottom trawl in channel, 2°57’58.3" S, 67°49’44.4" W, 23 Nov 1993, J. Sullivan et al., field JPS 93-77 ; INPA 29491 , 3 , 48-62 mm SL, rio Negro , collected with 3 m bottom trawl in 3.6-5.6 m channel, 2°21’44" S, 49°28’54" W, 10 Dec 1993, M. Garcia et al., field number MG 93-37 ; MUSM 32629 , 1 , 63 mm SL, rio Japurá above rio Solimões , collected with 3 m bottom trawl in 2.9-6.7 m channel, 3°02’58.3" S, 64°46’39.2" W, 29 Oct 1993, O. Oyakawa et al., field number OTO 93-1 ; MZUSP 99744 , 1 , 65 mm SL, rio Solimões , collected with 3 m bottom trawl in channel, Nov 1993, J. Friel et al., field number JPF 93-143 ; MZUSP 99745 , 1 , 51 mm SL, rio Negro below Curidique , collected with 3 m bottom trawl in 5- 8.5 m channel, 1°58’40.6" S, 61°14’15.6" W, 5 Dec 1993, J. Friel et al., field number JPF 93-154 ; MZUSP 99746 , 1 , 70 mm SL, rio Negro , collected with 3 m bottom trawl in 7.7-9.8 m channel, 1°41’37.0" S, 61°26’50.1" W, 6 Dec 1993, J. Friel et al., field number JPF 93-158 ; MZUSP 99747 , 4 , 42-67 mm SL (3, 51- 67 mm SL), same data as CU 84562. Pará State : ANSP 187458 , 1 , 48 mm SL, rio Tocantins above rio Pará, collected with 3 m bottom trawl in 6-15 m channel, 2°21’44" S, 49°28’54" W, 19 Nov 1994, A. Zanata et al., field number AMZ 94-140; ANSP 187459 , 4 , 40-45 mm SL (2, 40- 45 mm SL), rio Tocantins above rio Pará, collected with 3 m bottom trawl in 5.5-6.9 m channel, 2°23’40.2" S, 49°28’06.3" W, 19 Nov 1994, J. Lundberg et al., field number JGL 94-116; ANSP 189096 , 4 , 29.5-38.8 mm SL, rio Tocantins above rio Pará, collected with 3 m bottom trawl in 3-5 m channel, 2°24’17.5" S, 49°27’55.5" W, 19 Nov 1994, J. Lundberg et al., field number JGL 94-114; IAvH-P 11020, 14, 30-54 mm SL (2, 50- 54 mm SL), same data as ANSP 189096 ; MBUCV V-35377, 4, 54-60 mm SL, rio Tocantins above rio Pará, collected with 3 m bottom trawl in 15.5-25.5 m channel, 2°26’52" S, 49°26’38" W, 19 Nov 1994, A. Zanata et al., field number AMZ 94-145. Rondônia State : MCP 36200, 1 , 69 mm SL, rio Mamoré, frente ao bairro Cristo Rei no município de Guajará-Mirim , collected with 3 m bottom trawl in channel, 10°47’03" S, 65°20’58" W, 25 Jul 2004 , R. Reis et al. Roraima State : AMNH 246010 , 5 , 60-68 mm SL, rio Branco above rio Negro , collected with 3 m bottom trawl in 3.7-7.9 m channel, 1°15’29.0" S, 61°50’37.1" W, 8 Dec 1993, J. Baskin et al., field number JNB 93-17 ; FMNH 117835 , 1 , 51 mm SL, rio Branco above rio Negro , collected with 3 m bottom trawl in 4.3-6.7 m channel, 1°18’31.4" S, 61°51’52.3" W, 8 Dec 1993, J. Lundberg et al., field number JGL 93-170 ; USNM 393559 , 1 , 67 mm SL, rio Solimões below rio Içá , collected with 3 m bottom trawl in 5.5-9.2 m channel, 1°17’55.4" S, 61°51’21.0" W, 23 Nov 1993, J. Lundberg et al., field number JGL 93-174. Guyana, Rupununi : ANSP 180494 , 2 , 72-83 mm SL, Takutu River ( R. Branco-Negro Dr.), 3.77 km SSW of Lethem. 1 Nov 2003, M. Sabaj et al. Peru, Loreto Department : ANSP 182732 , 10 , 44-69 mm SL (7, 54- 69 mm SL), río Nanay (Amazonas Dr.), large left bank beach upstream from mouth, N of Iquitos, 15 Aug 2005, M. Sabaj et al .; INHS 36643 , 1 , 49 mm SL, río Napo (río Amazonas Dr.) mouth of río Mazan , near town of Mazan , 20 Jul 1995, L. Page et al .; INHS 44116 , 1 , 74 mm SL, río Napo and Quebrada ( río Amazonas Dr. ), Mazan , 33.3 km NE Iquitos at bearing 34°, 3°29’32.5" S, 73°05’11.9" W, 2 Aug 1997, M. Sabaj et al .; MUSM 32833 , 3 , same data as ANSP 182732. Madre de Dios State : ANSP 180628 , 2 , 79.8-81 mm SL , MUSM 32834 , 1 , 77 mm SL, río Tahuamanu (Orton - Madre de Dios Dr. ), vicinity of San Lorenzo , 1 Aug 2004, M. Sabaj et al .; ANSP 180633 , 1, 106 mm SL , AUM 47794, 1 , 93.4 mm SL , INPA 30248 , 1 , 90 mm SL , MUSM 32835 , 1 , 97.5 mm SL, all río Acre (Purus Dr.), at town of Inapari on border with Brazil , 2 Aug 2004, M. Sabaj et al .

Non-types. All from the Amazon drainage basin. Brazil, Amazonas State: ANSP 187460 , 1 , 28 mm SL, rio Jutai , collected with 3 m bottom trawl in 9.2-15.2 m channel, 2°52’08.5" S, 66°55’45.2" W, 16 Nov 1993, Lundberg et al., field number JGL 93-70 ; ANSP 187461 , 5 , 31-42 mm SL, rio Jutai , collected with 3 m bottom trawl in 1.7-7.8 m channel, 2°49’36.4" S, 66°54’57.8" W, 13 Nov 1993, Sullivan et al., field number JPS 93-49 ; ANSP 187462 , 1 , 33 mm SL, rio Jutai , collected with 3 m bottom trawl, 2°52’48.0" S, 66°57’03.2" W, 13 Nov 1993, Sullivan et al., field number JPS 93- 45 ; ANSP 187463 , 1 , 31 mm SL, rio Jutai collected with 3 m bottom trawl in 2-4.3 m channel, 2°49’26.1" S, 66°54’40.2" W, 13 Nov 1993, Oyakawa et al., field number OTO 93-33 ; ANSP 187464 , 2 , 32-36 mm SL, rio Jutai , collected with 3 m bottom trawl in 4.9-6.7 m channel, 4.9-6.7, 2°52’13.9" S, 66°58’18.1" W, 16 Nov 1993, Oyakawa et al., field number OTO 93-66. Pará State : ANSP 187465 , 2 , 28-33 mm SL, rio Tocantins , collected with 3 m bottom trawl in 4.6-5.7 m channel, 2°24’05.4" S, 49°27’58.9"W, 19 Nov 1994, Lundberg et al., field number JGL 94-115. Roraima State : ANSP 187466 , 4 , 31-45 mm SL, rio Branco , collected with 3 m bottom trawl in 3.9 m channel, 1°20’33.6" S, 61°52’21.3" W, 8 Dec 1993, Lundberg et al., field number JGL 93-176 .

Diagnosis. A species of the subgenus Eretmomegalonema distinguished from others by the following combination of features. Supraoccipital posterior process ( Fig. 3c ) bell-shaped in outline, broad, its basal width equal to or greater than its length, and its sides convexly tapering to a blunt tip separated by a distinct gap from supraneural; adipose-fin ( Figs. 6a,b ) 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 42-43, range 42-45 ( Table 1); eye large, horizontal diameter 214-321 mils of HL; anal-fin base relatively long in larger individuals, 93-142 mils of SL ( Fig. 5 ); width between posterior nostrils relatively narrow, 162-228 mils of HL ( Fig. 5 ); pectoral spine lacking dentations; premaxilla extremely narrow with a single row of teeth; and 25-29 gill rakers on first arch.

Description. Meristic data for 4 to 61 specimens are in Table 1, and morphometric data for 59 to 70 specimens are in Table 2. Megalonema amaxanthum ( Fig. 6 ) is a medium-sized pimelodid with a maximum length known to us of 106 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, abruptly stepped dorsad posterior to pelvic girdle, then straight or convex onto analfin base, slanting dorsally to caudal peduncle and slightly 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 4.3-6.2 times in SL. Maximum body width across cleithra in front of pectoral spine insertion, contained 6.1-8.5 times in SL.

Head of moderate length, contained 3.9-4.9 times in SL and relatively deep, its depth at base of supraoccipital posterior process equal to body width. Head covered by thin skin, revealing smooth, unornamented skull roofing bones. Snout moderate, contained 2.1-2.7 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 1.0-1.4 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 0.8-1.2 times in distance between anterior and posterior nostrils. Distance between anterior and posterior nostrils equal to or a little greater than 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 0.7- 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 greater than in its length), dorsally slightly convex, not angular or keeled, bell-shaped in outline, with a blunt or rounded tip, failing by about half or more of 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 dentition reduced to a single narrow row of fine, conical teeth. Dentary teeth similarly 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 at least posterior half 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, 25-29 on first gill arch (in four specimens): 6-7 rakers on upper limb, 1 at cartilaginous angle, 18- 22 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; 1.4-1.8 times longer than head, relatively high, its height 3.9-6.1 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 12-16, modally 13, in 40 specimens.

Pectoral fin I, 11-13, modally I, 12, in 15 specimens. Pectoral spine slender, its distal half or more segmented and flexible, its shaft entirely smooth without dentations. Pectoralspine 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 61 specimens ( Table 1), modally 42-43, range 42-45 including Weberian complex; in 61 specimens 16- 18 precaudal vertebrae, modally 16, and in 60 specimens 25- 28 caudal vertebrae, modally 27.

Coloration in alcohol. Head and body yellowish to tan in background color. 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 between light olfactory organs, and along anterior cranial fontanelle except between eyes. Oval posterior cranial fontanelle dark or covered with hyaline skin. 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. Amazon River basin, Brazil, Guyana (Takutu River), Peru, Bolivia, and probably Colombia and Ecuador.

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

Figures

Fig. 6. Megalonema amaxanthum, a, lateral view of holotype, CBF 11896, 98 mm SL, b, lateral view of juvenile paratype,ANSP 187452, 39 mm SL, c, dorsal and d, ventral view of holotype; distal ends of barbels clipped from images c and d.

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).