Megalonema xanthum Eigenmann, 1912

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 : 443-446

Megalonema xanthum Eigenmann, 1912 Fig. 4

Megalonema xanthum Eigenmann, 1912b: 16 [original description; type locality: Colombia, Cudinamarca Department, Girardot, Magdalena River]. - Stewart, 1986; 670 [anatomy]. - Maldonado-Ocampo et al., 2005: 170, fig. 160, map 162 [description, distribution, specimen list].

Perugia xanthus . -Driver, 1919:453, pl. 2, figs. A, C, D, pl. 3, fig. 5 [anatomy of Weberian complex and gas bladder, in key, relationships]. -Eigenmann, 1922: 35, pl. 3, fig. 3 [holotype photograph].

Diagnosis. A species of the subgenus Eretmomegalonema distinguished from others by the following combination of features. Supraoccipital posterior process ( Figs. 4b , 3b ) narrow, its basal width less than half its length, its sides gently tapering to a sharp tip separated by a distinct gap from supraneural; adipose-fin ( Fig. 4a ) large, its margin anteriorly rising steeply and straight or in a broad convex curve to a gently curved apex; adipose fin terminating posterior to vertical through tips of adpressed anal-fin rays; total vertebrae modally 47, range 45-48 ( Table 1); eye relatively small, horizontal diameter 154-195 mils of HL; anal-fin base relatively short, 94-117 mils of SL ( Fig. 5a ); width between posterior nostrils relatively narrow, 138-179 mils of HL ( Fig. 5b ); pectoral spine with numerous, erect dentations along posterior margin; premaxillary tooth band broad with about 8-10 rows of teeth; and 18-20 gill rakers on first arch.

Description. Meristic data for 5 to 23 specimens are in Table 1, and morphometric data for 20 to 23 paratypes are in Table 2. Megalonema xanthum ( Fig. 4 ) is a medium sized pimelodid with a maximum length known to us of 159.3 mm SL in the holotype (note that the length of 202 mm reported by Eigenmann (1912b) is consistent with Total Length). Dorsal profile of head and nape gently convex from snout tip to vertical at insertion of maxillary barbel, then nearly straight to origin of dorsal fin, then body profile scarcely convex along dorsal-fin base, straight to posterior insertion of adipose fin, and a little concave across caudal peduncle. Ventral profile slanting convexly ventrally from snout tip to mental barbels, nearly straight to pectoral-fin origin, straight or convex across abdomen to anal-fin origin, then 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 4.5-6.3 times in SL. Maximum body width across cleithra in front of pectoral spine insertion, contained 5.8-6.8 times in SL.

Head of moderate length, contained 4.1-4.8 times in SL and relatively deep, its depth at base of supraoccipital posterior process slightly less than body width. Head covered soft pectoral rays 11121314 M. xanthum 1 6 5 M. amaxanthum 5 9 2 M. orixanthum 1 6 3

gill rakers on first branchial arch

181920 / 2526272829 M. xanthum 1 3 1 M. amaxanthum 1 1 2 M. orixanthum 1

total vertebrae 42434445464748 M. xanthum 1 1 18 3 M. amaxanthum 21 21 14 5 M. orixanthum 12 21 2

by thin skin, skull roofing bones relatively smooth, not ornamented with tubercles or ridges. Snout moderately long, contained 2-2.3 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 1.2-1.7 times in distance between anterior and posterior nostrils. Long axis of posterior nostril oblique to longitudinal body axis; posterior nostril preceded by thin, semicircular membrane as large as nostril aperture; width between posterior nostrils contained 1.2-1.6 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 relatively small, about 1.5-2 times longer than deep, placed dorsolaterally and centered on midpoint of bony head length; interorbital space scarcely convex. Interorbital wide, containing horizontal eye diameter 1.2-1.6 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 narrow-based (its basal width contained about 3 times in its length), dorsally flat, not angular or keeled, but prominent with sides scarcely tapering to pointed tip, failing by about half 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 tooth band of uniform length along transverse width, with bluntly rounded posterolateral corner. Premaxillary teeth conical, fine, in about 8-10 irregular rows, more numerous in larger specimens. Dentary teeth similarly slender, arranged in about 6-8 rows at symphysis and fewer posterolaterally. No prevomerine, metapterygoid or accessory tooth plates present on the palate.

Origin of maxillary barbel in depression close to base of anterior nostril and above rictus, continuing to below eye; maxillary barbel reaching just beyond posterior end of adipose fin to base of caudal fin. Outer mental barbel reaching near or onto anal fin, not beyond adipose fin. Tip of inner mental barbel reaching near pelvic fin insertion. Branchiostegal membranes anteriorly united to isthmus and overlapping before diverging. Gill rakers long, slender and bladelike, 18-20 on first gill arch (in 5 specimens): 4-5 rakers on upper limb, 14- 15 on lower limb.

Dorsal-fin lepidotrichia I,7; spinelet absent or vestigial; dorsal-spine base with much reduced articulating processes. Dorsal 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 pelvicfin origin. Adpressed dorsal fin reaches onto adipose fin. Adipose-fin origin located at vertical before or at tip of last dorsal-fin ray.Adipose fin expansive, anteriorly rising straight or convexly to its gently curved apex at vertical about midway between pelvic-fin base and anal-fin origin; terminating posterior to tips of adpressed anal-fin rays; 1.5-2 times longer than head, relatively high, its height 4.7-6 times in its base.

Caudal fin deeply forked with sharply pointed lobes, the lower lobe shorter, the uppermost two principal rays a little prolonged; inner margins of both lobes straight to concave. Principal caudal-fin rays 1+7-8+1.

Anal-fin origin at vertical through middle of adipose-fin base, posterior insertion before end of adipose fin, distal margin of anal fin slightly concave.Anal-fin rays 13-16, modally 14, in 19 specimens.

Pectoral fin I, 12-14, modally I, 13, in 11 specimens. Pectoral spine slender, its distal half or more segmented and flexible, its shaft anteriorly smooth, posteriorly with small erect dentations, one per segment where segments are discernable. Pectoral-spine base with much reduced articulating (finlocking) 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 not 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 reaching at least onto caudal-fin base.

Total vertebrae in 23 specimens ( Table 1), modally 47, range 45-48 including Weberian complex; in 20 specimens 18-19 precaudal vertebrae, modally 18, and 27-30 caudal vertebrae, modally 29.

Coloration in alcohol. All available specimens are types collected in 1912 that have faded or darkened in preservative. In addition to the dark retina of the eye, small, dark chromatophores are scattered over the dorsum of head and body. Many specimens show a subcutaneous silvery layer along the lower flanks. No specimens show the upper caudal lobe spot that is present in all other species of the genus, but this may be due to its loss in preservative.

In the original description, Eigenmann (1912b: 17), who collected several specimens, described the color of the species as “Plumbeous, yellow in life.” An artist’s illustration of M. xanthum in Maldonado-Ocampo et al. (2005: 291, fig. 160) shows the ground color as gray and otherwise without pigment pattern.

Distribution. Endemic to the Magdalena River basin, Colombia. Maldonado-Ocampo et al. (2005: 341, map 162) plot two localities for the species in or near the río Magdalena mainstem above the 90 m elevation contour. All of the types are from localities above 90 m.

Material examined. 48 specimens, all from Colombia, Cundinamarca Department. FMNH 56032 , holotype, 159.3 mm SL, Girardot (see digital images and radiographs online at http:// acsi.acnatsci.org/base/index.html); BMNH 1920.12.20.112-113, paratypes 2, 74- 77 mm SL; CAS 63674 [ex IU 12681-82 ], paratypes 11, 70- 118 mm SL (4, 70- 118 mm SL), Girardot; FMNH 10285 , paratype 1, 107 mm SL, Apulo; FMNH 10289 , paratype 1, 69 mm SL, Apulo; FMNH 77890 , 2 , 101 - 126.5 mm SL, Apulo; FMNH 77906 , 1, 108 mm SL, Apulo; USNM 76930 , paratypes 24, 64- 120 mm SL (8, 64- 111 mm SL), Apulo; USNM 167852 , paratypes 5, 88- 114 mm SL, Apulo .

Figures

Fig. 4. Megalonema xanthum, CAS 63674, paratype, 118 mm SL, in a, lateral, b, dorsal and c, ventral view.

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).