Eretmomegalonema, 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 : 441-442

Eretmomegalonema , new subgenus

Type Species. Megalonema xanthum Eigenmann, 1912 , by original designation. We establish this taxon for a subgroup of the genus Megalonema containing M. xanthum , M. amaxanthum new species and M. orixanthum new species. typical intervertebral joint without interdigitating sutures, processes or superficial ossification ( Fig. 1 ).

Some other pimelodids possess modifications of the fourth transverse processes for encapsulation of gas bladder: Hypophthalmus (Howes, 1983) and the genera of the Calophysus group (Stewart, 1986; Driver, 1919). Many osteological details of the Weberian complex, however, differ among these groups and Megalonema . Further, the gas bladder morphologies differ strikingly among them: bilobed Diagnosis. Eretmomegalonema is uniquely characterized by the paddle-like structure of its pelvic fin and hypertrophied basipterygium ( Figs. 2 a-b). Both sexes and juveniles exhibit these features. The outer three pelvic-fin rays (1 st- 3 rd) are elongated and a little thickened to form a prominent paddlelike lobe. These rays exceed twice the length of the innermost (6 th) pelvic ray, and about 1.4 times the length of the 4 th. The 1 st pelvic ray is proximally unbranched and unsegmented for about half its length and its segments are foreshortened (square). The 2 nd pelvic ray is similarly unbranched and unsegmented for about half its length, then distally it is asymmetrically branched and segmented as follows. Distal to its first division, the 2 nd ray’s outer primary ramus does not further divide and its segments are square; the inner primary ramus divides into secondary rami of which the outer ramus remains undivided and with short segments, whereas the inner secondary ramus divides into thin, tertiary rami with more elongated (rectangular) segments. The 3 rd ray is unbranched and unsegmented for about 40% of its length; its primary rami further divide twice and have slightly elongated segments. In contrast to the outer rays, the inner three rays are slender, branched before their midlength, and their segments are elongate rectangles.

The basipterygium is elongated with an overall bony length nearly twice the combined width of both basipterygia. Also, the midline symphysis is complete along the entire length of the anteromedial process, basal plate and posterior process. The posterior process is long and has a triangular shaped tip. On the ventral side, the anteromedial process has a proximal lateral-margin crest. Dorsally, the anterolateral process has on its lateral-margin a prominent high crest that curves medially near its tip. The crests are associated with the arrector, adductor and abductor muscles of the elongated outer pelvic-fin rays.

In marked contrast to Eretmomegalonema , other species of Megalonema ( Figs. 2 c-d) and other pimelodids lack pelvicfin lobes, although in some the pelvic fins may be generally elongated. Also, no other Megalonema ( Figs. 2 c-d) or other pimelodids have the high dorsal crest on the anterolateral process (compare Fig. 2b ).Also, other species of Megalonema have a wide gap across the midline between the anteromedial processes ( Figs. 2 c-d), but a completely bony basipterygial symphysis including the anteromedial processes (compare Figs. 2 a-b) is found in some other pimelodids (Parisi et al., 2006; Rocha et al., 2007): Propimelodus , Exallodontus , Pimelodus altissimus , Duopalatinus peruanus , Cheirocerus and the Calophysus group.

Etymology. Subgenus Eretmomegalonema , from Greek eretmon, oar, in reference to the paddle-like pelvic fins, and megalonema, name of the catfish genus to which the new taxon belongs. Gender Neuter.

Key to the species of Subgenus Eretmomegalonema

1. Supraoccipital posterior process narrow, its basal width less than half its length, its sides gently tapering ( Fig. 3b ); adipose fin rising steeply and terminating posterior to vertical through tips of adpressed analfin rays; eye small, horizontal diameter contained 5-6 times in head length; pectoral spine with numerous, erect dentations along posterior margin; premaxillary tooth band broad with about 8-10 rows of teeth; 18- 20 gill rakers on first arch. Magdalena River Basin ……………………………………… Megalonema xanthum

1’. Supraoccipital posterior process broad based, its basal width equal to or greater than its length, and its sides abruptly tapering ( Figs. 3c,d ); adipose fin rising at a shallow angle from back and terminating at or anterior to vertical through tips of adpressed anal-fin rays; eye large, horizontal diameter contained 3-5 times in head length; premaxilla extremely narrow with one row of teeth or none; 25 or more gill rakers on first arch. Cis-Andean distribution……………………………………………………………2

2. Supraoccipital posterior process bell-shaped in outline, its tip blunt and separated by a distinct gap from supraneural ( Fig. 3c ); pectoral spine lacking dentations, premaxilla with a single row of teeth. Amazon River Basin ………………………………… Megalonema amaxanthum

2’. Supraoccipital posterior process triangular in outline, its tip pointed or bifid and nearly reaching supraneural ( Fig. 3d ); pectoral spine with numerous, erect dentations along posterior margin; premaxilla with a single row of teeth or teeth absent. Orinoco River Basin ……………………………… Megalonema orixanthum

Figures

Fig. 1. a, Weberian complex, post-Weberian vertebrae, basioccipital region and upper bones of shoulder girdle of Megalonema cf. platycephalum, ANSP 178515. b, Weberian complex and gas bladder of M. orixanthum, ANSP 187450. aoc = bony aortic canal; bocc = basioccipital; ccfl = ventral flange of compound centrum; tcom = position of gas bladder tubular commissure posterior to ventral flange; clr = cleithral ring; cpcv = position of bony canal for passage of posterior cardinal vein; gbld = lateral distention of gas bladder; gbtc = transverse commissure of gas bladder; ij5-6 = typical intervertebral joint between c5 and c6; tp4 = ventrally incomplete bony capsules surrounding lateral lobes of gas bladder formed by deeply down-curved 4th transverse processes of Weberian complex; c1, c5, c6, c7 = vertebrae designated by their ordinal numbers.

Fig. 2. Pelvic fins and basipterygia in Megalonema orixanthum, ANSP 187450, a, ventral view, b, dorsal view, and M. platycephalum, ANSP 189040, c, ventral view, d, dorsal view. alp = anterolateral process; amp = anteromedial process; bp = basal plate; dc = dorsal crest; pp = posterior process; r1-3 = pelvic-fin rays; vc = ventral crest.

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.