Megalonema 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 : 440-441

Megalonema Eigenmann, 1912

Megalonema Eigenmann, 1912a: 150 . Gender neuter.

Type species. Megalonema platycephalum Eigenmann, 1912a , by original designation.

Diagnosis. Eigenmann’s (1912a) diagnosis of Megalonema is a mixture of derived and plesiomorphic character states, each with a checkered distribution among pimelodids. Accordingly, we offer an alternative diagnosis emphasizing unique or unambiguous synapomorphies. Thus, Megalonema is characterized by the following. A unique structure of the gas bladder and its encapsulating processes of the Weberian complex ( Fig. 1 ) including: reduced, dumbbell-shaped gas bladder with distended lateral lobes connected by a tubular commissure ventrally crossing Weberian compound centrum; paired, ventrally incomplete bony capsules surrounding lateral lobes of gas bladder formed by deeply down-curved 4 th transverse processes of Weberian complex; a prominent ventral flange of compound centrum immediately anterior to tubular commissure of gas bladder and sutural joint with 5 th centrum. Paired bony canals of passage for posterior cardinal veins and median aortic canal all very short, formed by ventral flange crossing compound centrum. Bony aortic canal, in some specimens, continued anterior to ventral flange by a second thin midventral bridge ( Fig. 1 ). Transformator process of tripus reduced to a thin tapering splint on anteromedial wall of gas bladder lobe. Vertebral centra five and six articulate by a in Megalonema , small median sac with anterior projections in the Calophysus group, and a pair of small separate sacs in Hypophthalmus .

We also note that Megalonema may have a possibly diagnostic dark caudal spot embedded within the base of upper lobe of the caudal fin, internal to uppermost 4-5 branched fin rays. Some specimens show a matching spot on the lower lobe. We have observed the upper caudal spot in specimens of all species except M. argentina (no specimens examined) and M. xanthum for which the only available specimens are the faded types collected nearly a century ago. It is possible that M. xanthum has the caudal spot in life but it is bleached out as it is in faded specimens of other species. Upper lobe caudal spots in a similar position are also found in Pimelodidae in Hemisorubim , Platysilurus and Platystomatichthys , but in these the pigmentation is chromatophores in the skin, external to the fin rays.

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.