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439
Neotropical Ichthyology, 6(3):439-454, 2008
Copyright © 2008 Sociedade Brasileira de Ictiologia
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)
John G. Lundberg and Wasila M. Dahdul
A revised diagnosis of the pimelodid catfish genus Megalonema is given based on synapomorphic features of the Weberian
complex and gas bladder. Megalonema xanthum from the Magdalena River is redescribed. Two new cis-Andean species of
Megalonema are described, M. amaxanthum n. sp. from the Amazon River basin, and M. orixanthum n. sp. from the Orinoco
River basin. These three species are differentially diagnosed by shape and size of the supraoccipital posterior process,
adipose-fin shape, vertebral counts, eye size, premaxillary bone shape and dentition, length of the anal-fin base, width between
the posterior nostrils and presence/absence of dentations on the pectoral spine. Eretmomegalonema new subgenus is
established for M. xanthum, M. amaxanthum and M. orixanthum and supported by the uniquely synapomorphic paddle-like
structure of its pelvic fin and hypertrophied basipterygium. Unambiguous synapomorphies indicate a sister-group relationship
between M. amaxanthum and M. orixanthum, with M. xanthum basal to this pair. This topology is congruent with the
Neogene origins of separate Magdalena, Amazon and Orinoco basins suggesting vicariant control of diversification of
Eretmomegalonema.
Uma diagnose do gênero Megalonema é fornecida baseada em caracteres sinapomórficos do aparelho de Weber e da bexiga
natatória. Megalonema xanthum do rio Magdalena, é redescrita. Duas novas espécies cis-Andinas de Megalonema são
descritas: M.amaxanthum sp. n. da bacia Amazônica, e M.orixanthum da bacia do rio Orinoco. Estas três espécies são
diagnosticadas pela forma e tamanho do processo supraoccipital posterior, forma da nadadeira adiposa, contagem do número
de vértebras, tamanho do olho, forma do premaxilar e dentição, comprimento da base da nadadeira anal, distância entre as
narinas posteriores, e presença/ausência de dentições no espinho da nadadeira peitoral. Eretmomegalonema, novo subgênero,
é estabelecido para M.xanthum,M.amaxanthum eM.orixanthum e suportado pelas únicas estruturas sinapomórficas da
nadadeira pélvica em forma de remo, e do basipterígio hipertrofiado. Claras sinapomorfias indicam uma relação de grupo irmão
entre M.amaxanthum e M.orixanthum, com M.xanthum basal a esse grupo. Esta topologia é congruente com a origem
Neogênica das distintas bacias do Magdalena, Amazonas e Orinoco, sugerindo um evento vicariante de diversificação de
Eretmomegalonema.
Key words: Eretmomegalonema,Megalonema amaxanthum,Megalonema orixanthum.
Department of Ichthyology, The Academy of Natural Sciences, 1900 Benjamin Franklin Parkway, Philadelphia, PA 19103-1195, USA.
lundberg@ansp.org (JGL), dahdul@ansp.org (WMD)
Introduction
The pimelodid catfish genus Megalonema has an ample
distribution in South America throughout the Paraná, Amazon,
Essequibo, Orinoco, Maracaibo and Magdalena basins. The
species of Megalonema inhabit medium to large rivers mostly
in flowing marginal reaches and over fine substrates of mud
or sand. These are streamlined catfishes of modest size with
adult lengths ranging among species from ca. 10-30 cm SL.
Their clear to hyaline fins are gracefully shaped and often
filamentous, with flexible fin spines in the dorsal and pectoral
fins, and an expansive adipose fin. In color Megalonema are
mostly plain yellowish to tan and generally without prominent
marks except for the common presence of an embedded dark
spot in the base of the upper caudal-fin lobe, and sometimes
also the lower lobe.
Description of a new subgenus and two new species of Megalonema
440
Carl Eigenmann (1912a) established the genus
Megalonema for his newly described species M.
platycephalum from the Essequibo River, Guyana. Later in
the same year he described a second species, M. xanthum,
from the río Magdalena, Colombia (Eigenmann, 1912b).
Subsequent species descriptions and taxonomic transfers
have raised the number of presumed valid species in
Megalonema to six (Lundberg & Littmann, 2003). The
existence, however, of at least one undescribed cis-Andean
species has been known for over 25 years and our recent
study of Amazon and Orinoco populations indicates
additional undescribed species. The purpose of this paper is
to describe two new species that are related to M. xanthum,
and to recognize these three as a monophyletic subgroup of
Megalonema characterized by oddly hypertrophied pelvic
fins. In a subsequent paper we are revising the remaining
species of the genus.
Material and Methods
Measurements and counts follow Lundberg et al. (1991),
Lundberg & Parisi (2002) and Lundberg & Akama (2005) with
the addition of the distance from snout tip to insertion of
outer pelvic-fin ray, and least distance from orbital margin to
posterior nostril. Measurements are straight-line distances
taken with a digital caliper to the nearest 0.1 mm from a sample
of 136 specimens ranging in standard length (SL) from 38.2 to
120.0 mm. Measurement ratios are expressed in thousandths
(mils) in text and Table. We used least squares regression to
analyze variation in log10 transformed variables, and t-tests
of the residuals from regression lines to assess taxonomic
differences in proportional measurements. Vertebral counts
include the five elements united in the Weberian complex; the
first caudal vertebra is that immediately posterior of the
visceral cavity and recognized by its fully developed hemal
spine; the compound centrum of the caudal skeleton is counted
as one. Radiographed (R), cleared and stained (C&S), and
articulated dry skeletal (Sk) specimens were used for counts
of vertebrae and median-fin rays. Institutional acronyms are
as listed on the Catalog of Fishes website at http://
www.calacademy.org/research/ichthyology/catalog/
abtabr.html.
Results
The genus Megalonema possesses nearly all of the
diagnostic synapomorphies for the following nested groups
(Lundberg et al., 1991: 842, fig. 1; see also Hardman &
Lundberg, 2006): Pimelodidae sensu Lundberg & Littmann
(2003), non-phractocephaline pimelodids (i.e., all Pimelodidae
except Steindachneridion,Phractocephalus,Leiarius,
Perrunichthys), and the Calophysus-Pimelodus clade.
Megalonema is exceptional among Pimelodidae in lacking a
deep sutural joint between the fifth and sixth centra. In the
framework of its inferred phylogenetic position in Pimelod-
idae, the presence in Megalonema of a normal intervertebral
joint between these centra has been interpreted as a reversal
or secondary loss of the synapomorphic sutural joint (Stewart,
1986; Lundberg et al., 1991).
Beyond the aforementioned, the relationships of
Megalonema are unsettled. Driver (1919) interpreted
similarities of the reduced gas bladders, encapsulating
Weberian complexes and external morphology among
Megalonema,Luciopimelodus and Pinirampus (as Perugia
and including M. xanthum) as indicative of relationship,
placing these taxa together as the Luciopimelodinae.
Alternatively, Stewart (1986) described a number of
unambiguous synapomorphies supporting his Calophysus
group: Calophysus,Pinirampus,Luciopimelodus,
Pimelodina and Aguarunichthys. Stewart found that
Megalonema exhibits unique features of the gas bladder and
Weberian vertebral structure that set it apart from the
Calophysus group, although he pointed out other possible
synapomorphies in the reduced structure of the pectoral spine,
elevated pectoral-fin ray count and loss of the posterior
cleithral process. Lundberg et al. (1991) tentatively leaned
toward placing Megalonema basal to the Calophysus group.
We are pursuing the question of the systematic position of
Megalonema with both morphological and molecular data.
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 4th
transverse processes of Weberian complex; a prominent
ventral flange of compound centrum immediately anterior to
tubular commissure of gas bladder and sutural joint with 5th
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
J. G. Lundberg & W. M. Dahdul 441
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
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 Pimelod-
idae in Hemisorubim,Platysilurus and Platystomatichthys,
but in these the pigmentation is chromatophores in the skin,
external to the fin rays.
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.
Diagnosis.Eretmomegalonema is uniquely characterized by
the paddle-like structure of its pelvic fin and hypertrophied
basipterygium (Figs. 2a-b). Both sexes and juveniles exhibit
these features. The outer three pelvic-fin rays (1st-3rd) are
elongated and a little thickened to form a prominent paddle-
like lobe. These rays exceed twice the length of the inner-
most (6th) pelvic ray, and about 1.4 times the length of the 4th.
The 1st pelvic ray is proximally unbranched and unsegmented
for about half its length and its segments are foreshortened
(square). The 2nd 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 2nd 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 3rd 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
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.
Description of a new subgenus and two new species of Megalonema
442
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 pelvic-
fin 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. 2a-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 anal-
fin 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 distribu-
tion……………………………………………………………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 Ba-
sin ……………………………… Megalonema orixanthum
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.
J. G. Lundberg & W. M. Dahdul 443
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 [ho-
lotype 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 ver-
tical through tips of adpressed anal-fin rays; total vertebrae
modally 47, range 45-48 (Table 1); eye relatively small, hori-
zontal 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); pecto-
ral 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 dorsal-
fin 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
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.
soft pectoral rays
11 12 13 14
M.xanthum 1 6 5
M.amaxanthum 5 9 2
M.orixanthum 1 6 3
gill rakers on first branchial arch
18 19 20 / 25 26 27 28 29
M.xanthum 1 3 1
M.amaxanthum 1 1 2
M.orixanthum 1 3
total vertebrae
42 43 44 45 46 47 48
M.xanthum 1 1 18 3
M.amaxanthum 21 21 14 5
M.orixanthum 12 21 2
Table 1. Frequency distributions for counts of soft pectoral-
fin rays, gill rakers on first branchial arch, and total vertebrae.
Description of a new subgenus and two new species of Megalonema
444
by thin skin, skull roofing bones relatively smooth, not orna-
mented with tubercles or ridges. Snout moderately long, con-
tained 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 con-
tained 1.2-1.7 times in distance between anterior and poste-
rior nostrils. Long axis of posterior nostril oblique to longitu-
dinal body axis; posterior nostril preceded by thin, semicircu-
lar membrane as large as nostril aperture; width between pos-
terior nostrils contained 1.2-1.6 times in distance between
anterior and posterior nostrils. Distance between anterior and
Fig. 4. Megalonema xanthum, CAS 63674, paratype, 118 mm SL, in a, lateral, b, dorsal and c, ventral view.
J. G. Lundberg & W. M. Dahdul 445
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 cra-
nial fontanelle narrow, parallel-sided, originating behind level
of posterior nostrils and terminating between eyes at epiphy-
seal bar. Posterior cranial fontanelle closed anteriorly, per-
sisting as oval aperture in center of supraoccipital. Supraoc-
cipital posterior process narrow-based (its basal width con-
tained 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 pelvic-
fin 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
Megalonema xanthum Megalonema amaxanthum Megalonema orixanthum
Measurement Mean N Range Hol Mean N Range Hol Mean N Range
Standard length in mm 92.1 23 64.3-120 98 66 70 40-106 100.3 75.4 40 38.2-115.8
Predorsal length (SL) 336 23 309-354 320 343 69 326-362 337 342 40 326-361
Preanal length (SL) 710 23 688-730 694 711 70 685-741 722 709 40 684-736
Prepectoral length (SL) 226 23 192-424 194 221 70 191-273 210 218 40 196-257
Prepelvic length (SL) 435 23 415-446 436 454 70 429-499 440 443 40 424-471
Head length (SL) 229 23 208-246 214 228 70 205-257 218 226 40 211-246
Caudal peduncle length (SL) 204 23 186-218 219 195 70 178-218 191 203 40 187-219
Caudal peduncle depth (CPL) 410 23 336-474 374 423 70 336-487 385 394 40 326-452
Dorsal spine length (SL) 266 20 243-287 297 290 59 244-371 299 288 28 254-320
Dorsal fin base (SL) 140 23 129-151 153 136 70 108-151 142 136 40 125-147
Pectoral spine length (SL) 216 21 196-230 199 217 64 191-257 217 215 37 192-235
Pelvic fin length (SL) 248 23 233-267 249 268 70 223-327 255 266 40 230-311
Anal fin height (AFB) 1490 20 1311-1637 1252 1341 66 1088-1673 1590 1475 38 1258-1655
Anal fin base (SL) 104 23 94-117 106 112 70 93-142 100 104 40 93-117
Adipose length (SL) 393 23 361-419 366 358 69 311-415 370 361 40 327-387
Adipose height (ADL) 185 23 165-211 185 207 68 165-257 186 199 40 156-246
Dorsal to adipose distance (SL) 55 21 42-78 64 63 69 25-94 44 56 40 38-76
Body depth (SL) 191 22 158-221 191 189 69 161-233 169 185 40 152-226
Body width (SL) 161 22 146-171 141 139 70 118-165 133 136 39 115-155
Bony interorbital (HL) 233 23 204-254 268 232 70 183-285 228 231 40 196-279
Eye diameter horizontal (HL) 168 23 154-195 244 263 70 214-321 260 258 40 209-337
Eye diameter vertical (HL) 110 23 93-136 148 158 70 112-214 169 150 40 105-194
Snout length (HL) 476 23 440-506 450 432 70 373-487 411 429 40 383-483
Internarial length (HL) 228 23 198-260 191 188 69 149-214 183 184 40 145-210
Internarial anterior width (HL) 151 23 125-162 148 156 70 131-177 137 149 40 134-174
Internarial posterior width (HL) 160 23 138-179 177 199 69 162-228 224 221 40 196-246
Eye to posterior nostril (HL) 194 23 172-226 220 183 69 140-227 164 183 40 128-220
Gape width (HL) 455 23 393-497 502 457 70 346-538 425 450 40 372-488
Table 2. Measurement data for Megalonema xanthum,M.amaxanthum, and M.orixanthum; expressed in thousandths of the
standard dimensions given in parentheses. SL=standard length, CPL=caudal-peduncle length, AFB=anal-fin base,
ADL=adipose-fin length, HL=head length.
Description of a new subgenus and two new species of Megalonema
446
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, mod-
ally 14, in 19 specimens.
Pectoral fin I, 12-14, modally I, 13, in 11 specimens. Pecto-
ral spine slender, its distal half or more segmented and flex-
ible, its shaft anteriorly smooth, posteriorly with small erect
dentations, one per segment where segments are discernable.
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 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
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).
J. G. Lundberg & W. M. Dahdul 447
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.
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.
Description of a new subgenus and two new species of Megalonema
448
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 usu-
ally 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.
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.
J. G. Lundberg & W. M. Dahdul 449
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 anal-
fin 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 dorsal-
fin 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 pos-
terior nostrils. Distance between anterior and posterior nos-
trils equal to or a little greater than distance between poste-
rior 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 maxil-
lary barbel and about one third distance to mandibular sym-
physis. 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 speci-
mens): 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 dorsal-
fin spine of adpressed dorsal fin reach adipose fin. Adipose-
fin origin located at vertical posterior to tips of last four dorsal-
fin 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, rela-
tively 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 adi-
pose-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. Pecto-
ral spine slender, its distal half or more segmented and flex-
ible, its shaft entirely smooth without dentations. Pectoral-
spine base with much reduced articulating (fin-locking) pro-
cesses. Pectoral spine and outer few branched pectoral rays
prolonged but none filamentous. Pectoral-fin margin concave.
Pectoral axillary gland pore absent. Posterior cleithral pro-
cess 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 dor-
sally and ventrally, complete, and terminating near the ends
of middle caudal-fin rays.
Total vertebrae in 61 specimens (Table 1), modally 42-43,
Description of a new subgenus and two new species of Megalonema
450
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.
Megalonema orixanthum, new species
Fig. 7
Holotype. ANSP 187449 (ex ANSP 148143), 100.3 mm SL, Co-
lombia, 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. Colom-
bia, 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,
J. G. Lundberg & W. M. Dahdul 451
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 dorsal-
fin origin, and increasingly compressed to caudal fin. Maxi-
mum body depth at dorsal-fin origin contained 4.4-6.6 times
in SL. Maximum body width across cleithra in front of pecto-
ral spine insertion, contained 6.5-8.7 times in SL.
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.
Description of a new subgenus and two new species of Megalonema
452
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 con-
tained 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 speci-
mens): 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 dorsal-
fin spine of adpressed dorsal fin reach adipose fin. Adipose-
fin origin located at vertical posterior to tips of last four dorsal-
fin 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, rela-
tively 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 adi-
pose-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
J. G. Lundberg & W. M. Dahdul 453
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.
Discussion
Two unambiguous synapomorphies described in the spe-
cies diagnoses indicate a sister-group relationship between
M. amaxanthum and M. orixanthum within Eretmomegalo-
nema: broad and short supraoccipital posterior process (Figs.
3c,d) and greatly reduced premaxilla and upper jaw dentition.
As inferred from its similarities to species of Megalonema
not belonging to Eretmomegalonema, M. xanthum has the
plesiomorphic conditions of these elements, i.e. long, narrow
supraoccipital posterior process (Fig. 3b, compare Fig. 3a)
and broad, well-toothed premaxilla. Thus, trans-Andean M.
xanthum is considered to be phylogenetically basal within
Eretmomegalonema to the cis-Andean species pair in the
Amazon and Orinoco. The sequence of two cladistic events
in the tree topology of Eretmomegalonema matches the se-
quence of origin of drainage divides separating the
Magdalena, Amazon and Orinoco basins (Lundberg et al.,
1998; Lundberg, 2005). The Magdalena watershed was formed
and isolated from the Amazon and Orinoco by about 10 Ma in
the Middle to Late Miocene when the Eastern Andes was
uplifted. The low divides that now separate the Amazon and
Orinoco originated later in the Miocene at about 8 Ma. The
picture that emerges is one of vicariant control of cladogen-
esis and divergence of the three species in Eretmomegalo-
nema, and the persistence of their allopatric distributions.
Comparative material. Megalonema platycephalum Eigenmann,
1912, Guyana, Demerara-Berbice: ANSP 179249, 1 Sk, 145 mm
SL, Essequibo River (east bank) at Kurukupari, 04°39’41"N,
058°40’31"W. Rupununi: ANSP 179697, 2, 92-133 mm SL,
Rupununi River (Essequibo Dr.) at Kwatamang, 4 km SE of Annai,
03°55’03"N, 059°06’01"W. Megalonema cf. platycephalum.Peru,
Loreto Department: ANSP 178450, 2 alcohol, 1 C&S, 105-110 mm
SL, río Nanay (tributary río Amazonas) at Pampa Chica, village
4.54 km W of Iquitos (large beach along N bank), 3°45’09"S,
73°17’00"W; ANSP 178515, 1 Sk, río Napo, near town of Mazan,
3°29’10"S, 73°06’24"W. Madre de Dios State: ANSP 187323, 1
Sk, 240 mm SL, río Tahuamanu (Orton–Madre de Dios Dr.), vicin-
ity of San Lorenzo. Colombia, Meta State: ANSP 131341, 45-81
mm SL, río Metica, upstream from entrance to Lake Mozambique,
halfway to entrance to Laguna ‘Arrotas.’ Venezuela, Apure State:
ANSP 189040 (ex DU F998), 6 alcohol, 1 C&S, 53-76 mm SL, río
Apure between San Fernando and río Portuguesa, collected with 3
m bottom trawl, 19 Jul 1984, Lundberg et al. Megalonema platanum
(Günther, 1880), Brazil, Rio Grande do Sul: MZUSP 78465, 1 Sk,
210 mm SL, rio Uruguai, em frente ao porto de São Borja. Paraguay,
Asunción: UMMZ 207635, 2, 180-205 mm SL, Pettirossi fish
market (=Mercado Cuatro), Asunción (from nearby río Paraguay).
Acknowledgements
For loans of specimens we thank David Catania (CAS,
San Francisco), Osvaldo Oyakawa (MZUSP, São Paulo),
Francisco Provenzano (MBUCV, Caracas), Mary Anne Rogers
(FMNH, Chicago), Roberto Reis (MCP, Porto Alegre), Mike
Retzer (INHS, Champaign), David Werneke and Jonathan
Armbruster (AUM, Auburn), and Jeff Williams and Richard
Vari (National Museum of Natural History). We are most
grateful to Kyle Luckenbill (ANSP) for his expert preparation
of the illustrations, and to Mark Sabaj Pérez (ANSP) for his
assistance with spotting and management of specimens, and
many helpful comments on this study. Two anonymous
reviewers provided valuable corrections and comments on
the manuscript, and we are indebted to the one who voluntarily
translated our abstract into Portuguese. Support for travel
and publication was provided by the All Catfish Species
Inventory (ACSI, US National Science Foundation DEB-
0315963) and a research award to JGL (US National Science
Foundation, DEB-0089612).
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Accepted July, 2008
Published September 30, 2008
