Accepted by J. Friel: 11 Jul. 2013; published: 31 Jul. 2013
Licensed under a Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0
ISSN 1175-5326 (print edition)
ISSN 1175-5334 (online edition)
Copyright © 2013 Magnolia Press
Zootaxa 3693 (4): 479–490
A new ectoparasitic distichodontid of the genus Eugnathichthys
(Characiformes: Citharinoidei) from the Congo basin of central Africa,
with a molecular phylogeny for the genus
MELANIE L.J. STIASSNY1 JOHN S.S. DENTON1,2 & RAOUL J.C. MONSEMBULA IYABA3
1American Museum of Natural History, Department of Ichthyology, Central Park West at 79th Street, New York, New York 10024.
2Richard Gilder Graduate School, American Museum of Natural History, Central Park West at 79th Street, New York, New York 10024.
3Faculté des Sciences, Département de Biologie, Université de Kinshasa B.P 190 Kin XI, Democratic Republic of Congo.
A new species of ectoparasitic distichodontid, Eugnathichthys virgatus, is described from localities in the central and
western Congo basin. The new species is a fin-eater even at small sizes and, in common with congeners, is capable of
biting off sections of heavily ossified fin-rays of large prey species. Prior to the present study, two species were included
in this distinctive distichodontid genus: the type species, Eugnathichthys eetveldii, and a second species, E. macroterole-
pis, both of which are widely distributed throughout much of the Congo basin. Morphologically, E. virgatus is readily dis-
tinguished from its two congeners based on a combination of meristic and morphometric attributes. The new species
possesses a unique pigmentation pattern, a reduced number of pectoral-fin rays, and a markedly reduced dentition on the
fifth ceratobranchial elements of the pharynx, all of which are derived features considered diagnostic for the new species.
With molecular data the species is further diagnosed by four apomorphic, non-synonomous nucleotide transitions in two
sampled genes (NADH dehydrogenase subunit 2 and glycosyltransferase). Phylogenetic analysis of those mtDNA and
ncDNA markers supports a sister-group relationship between E. virgatus and E. eetveldii rather than with E. macroterole-
pis, the species with which it bears closest phenetic similarity.
Key words: Eugnathichthys virgatus, new species, ectoparasite, generic intrarelationships
Une nouvelle espèce de distichodontid ectoparasite, Eugnathichthys virgatus, est décrite à partir des localités du centre et
de l’ouest du bassin du Congo. La nouvelle espèce se nourrit de nageoire, et cela même à petites tailles. Elle a en commun
avec les autres espèces congénères la capacité de manger, par morsure, de morceaux de nageoire fortement ossifiés d’es-
pèces proies de grande taille. Avant la présente étude, deux espèces avaient été incluses dans ce genre distinctif de dis-
tichodontid: il s’agit des espèces type, Eugnathichthys eetveldii, et E. macroterolepis qui, tous deux, sont largement
distribuées dans une grande partie du bassin du Congo. Morphologiquement, E. virgatus se distingue facilement de ses
deux congénères sur base d’une combinaison de caractères méristiques et morphométriques. Cette espèce possède un mo-
tif de pigmentation unique, un nombre réduit de rayons des nageoires pectorales et une dentition nettement réduite sur les
éléments du cinquième cératobranchial du pharynx, qui sont tous des traits dérivés, considérés comme diagnostic pour la
nouvelle espèce. De surcroît, l’espèce est encore diagnostiquée par la présence de quatre nucléotides apomorphes de tran-
sition, non-synonymes, dans les deux gènes testés (NADH déshydrogénase sous-unité 2 et glycosyltransférase). L'analyse
phylogénétique de ces marqueurs mtADN et ncADN soutient que E. virgatus et E. eetveldii sont issus d’un même groupe
et ont un lien de parenté étroit plutôt qu'avec l’espèce E. macroterolepis qui porte la plus proche similitude phénotypique.
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Prior to the present study the distinctive distichodontid genus Eugnathichthys contained the type species, E.
eetveldii (Figs. 1A, B), and a sole congener, E. macroterolepis (Figs. 1B, C). Both species, originally described by
Boulenger in the late 1890’s from few individuals, are now known to be widely distributed throughout much of the
Congo basin and seemingly occur in sympatry over most of that range (Fig. 2). Eugnathichthys eetveldii and E.
macroterolepis are readily distinguished from one another based on non-overlapping scale and fin-ray counts
(Boulenger, 1909), as well as by distinctive pigmentation patterning and caudal fin coloration (Figs. 1B, C).
FIGURE 1. Eugnathichthys: (A) E. eetveldii (AMNH 243549, Lulua River), (B) E. eetveldii, James Chapin1 watercolor of live
specimen, Avakubi, Ituri River (specimen not retained), (C) E. macroterolepis (AMNH 254699, Nsele River), (D) E.
macroterolepis, immediately post-mortem (AMNH 253624, Kwilu River). Reproduction of previously unpublished Chapin
watercolor courtesy of the American Museum of Natural History, Library Archive.
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FIGURE 2. Distributional Ranges (modified after Brooks et al., 2011), with Congo basin delimited in dark grey: (A)
Eugnathichthys eetveldii, white circles indicate syntypical localities (B) E. macroterolepis, white circle indicates type locality,
and stars indicate areas of occurrence of E. virgatus (1= Lengoué River, 2= Lomako River, 3= Luilaka and Yenge Rivers in
Salonga National Park). “??” denotes possible occurrence in Mossapoula River (Ubangi basin), Central African Republic (see
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Both Eugnathichthys species are reportedly ectoparasitic, feeding exclusively on fish fins as adults, although
smaller individuals of E. eetveldii have been reported to eat aquatic insect larvae (Roberts, 1990). Vari (1979)
provides a series of derived morphological features attesting to the monophyly of the genus, most notable of which
are the presence of characteristically remodeled massive jaws and associated musculature, which facilitate a
powerful bite employed during fin-feeding. Whereas nine other distichodontid species are known at least to be
facultative, fin-eaters (Roberts, 1990), the massive jaws and robust shearing dentition of Eugnathichthys apparently
allow these fishes to subsist exclusively on more robust, highly ossified fins than other fin-eating distichodontids,
many of which are found to occur in sympatry with them.
In the course of a multi-locus molecular study of distichodontid intergeneric relationships four individuals
identified as E. macroterolepis were included as terminals in that analysis (Arroyave et al., in press). An
unanticipated degree of molecular divergence was observed between two individuals from the southern part of the
species’ range (Nsele and Kasai Rivers) and two individual from more northerly locations (Lengoué and Lomako
Rivers). Morphological reexamination of these and additional specimens in comparison with type materials,
indicates that the initial identification of the Lengoué and Lomako River specimens as Eugnathichthys
macroterolepis was in error, and that these fishes are in fact members of an undescribed species. Additional
specimens of the undescribed species from the Luilaka River in the Salonga National Park (SNP) in central Congo
and of E. macroterolepis and E. eetveldii were sequenced in order to confirm the conspecificity of the SNP
specimens with the Lengoué and Lomako individuals, and also to estimate intrageneric relationships. A formal
taxonomic description of the new species is provided below.
Material and methods
Species description. Fifteen morphometric measurements and 12 meristic counts were taken following Zengeya et
al. (2011). In order to accurately count vertebral and fin-ray elements and to visualize other skeletal features,
specimens were radiographed and some were cleared and stained following a modified protocol based on Taylor and
Van Dyke (1985). C/S indicates cleared and stained specimens; SL denotes standard length, and HL head length.
Total vertebral counts include the four modified Weberian centra but exclude the terminal, hypural-bearing
centrum. Gill raker counts include only the rakers on the ceratobranchial of the first arch and exclude
hypobranchial rakers and the raker in the angle of the arch. Lateral line counts exclude the small pored scales on
the caudal fin distal to the point of caudal flexion. Institutional abbreviations follow Leviton et al. (1985).
Molecular analysis. Partial sequence fragments of glycosyltransferase (glyt, ncDNA) and NADH
dehydrogenase subunit 2 (ND2, mtDNA) were amplified from total genomic DNA via polymerase chain reaction
using illustra PuReTaq Ready-To-Go PCR Beads (GE Healthcare) Sequence fragments for glyt (Li et al., 2007)
were amplified using a 1:20 dilution nested PCR, with the primers Glyt_F559 (5'-GGACTGTCMAAGATGACCA
CMT-3') and Glyt_R1562 (5'-CCCAAGAGGTTCTTGTTRAAGAT-3') for round 1, and Glyt_F577 (5'-ACATGG
TACCAGTATGGCTTTGT-3') and Glyt_R1464 (5'-GTAAGGCATATASGTGTTCTCTCC-3') for round 2. Both
rounds of nested PCR utilized the protocol 95°C initial denaturation for 1:00, followed by two elongation cycles of
20x (98°C, 0:10; 57°C, 0:30; 72°C, 0:45) and 15x (98°C, 0:10; 55°C, 0:30; 72°C, 0:45), and a final elongation of
72°C for 5:00. Sequence fragments for ND2 were amplified from a single round of PCR, using the primers
nd2_Dist_f (5’- AGCTTTTGGGCCCATACCCCA-3’) and nd2_Dist_r (5’- AGGRACTAGGAGATTTTCACTCC
TGCT-3’) (Arroyave et al., in press) and the protocol 95°C initial denaturation for 1:00, followed by an elongation
cycle of 35x (95°C, 1:00; 58°C, 1:00; 72°C, 2:00) and a final extension of 72°C for 10:00. Contigs of sequence
chromatograms for corresponding forward and reverse reactions were assembled in Geneious v.5.1.7 (Biomatters
Ltd., Aukland, NZ), checked for miscalled bases and trimmed to begin in the first codon position. Sequences were
aligned by corresponding translation product using the Translation Align function in Geneious. All sequences are
deposited in Genbank (Accession numbers: KF366267-KF366293).
The resulting 2-gene alignment, of length 1812 (variable sites: 234, parsimony-informative sites: 187), was
partitioned by gene and codon position. Partition-optimal models of nucleotide substitution were selected using the
AICc in TreeFinder (Jobb et al., 2004). A maximum likelihood analysis was conducted in TreeFinder using search
depth 2. After an initial topology search, 20 starting trees at nearest neighbor interchange (NNI) distances of 50
from the tree found were generated, and the analysis was re-run. Support values for the final tree were calculated
using 1000 replicates of LR-ELW (Strimmer & Rambaut, 2002). A parsimony analysis was also conducted in TnT
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NEW SPECIES OF EUGNATHICHTHYS
v1.1 (Goloboff et al., 2008), using the command Xmult=hits 50000 rss css fuse level 5 chklevel 10. Three equally
parsimonious trees with score 302 were found. A standard parsimony bootstrap of 100 replicates was conducted on
the parsimony tree.
Eugnathichthys virgatus, new species
Figures 3–5; Tables 1 & 2
Holotype: AMNH 241648, ♀, 93.00 mm SL, tissue voucher, Democratic Republic of Congo, Equateur Province,
Luilaka River at confluence of small stream flowing out of Salonga National Park, 2.6713° S, 21.71452° E, Coll.
R. Monsembula and R. Schelly, 11 July 2006.
Paratypes: AMNH 245002, 69.1 mm SL, Republic of Congo, Sangha Province, Lengoué River near Ouesso,
Coll. V. Mamonekene, 2 September, 2007.—AMNH 246319, 51.9 mm SL, tissue voucher, Republic of Congo,
Sangha Province, Lengoué River near Ouesso, Coll. V. Mamonekene, 8 September 2007.—AMNH 249790, 106.0
mm SL, tissue voucher, Democratic Republic of Congo, Equateur Province, Lomako River at Isake, 0.87944° N,
20.794722° E, Coll. Worldfish Center personnel, February 2009.—AMNH 241650, C/S, 94.8 mm SL, Democratic
Republic of Congo, Equateur Province, Yenge River, Salonga National Park, 1.04605° S, 20.73328° E, Coll. R.
Monsembula and R. Schelly, 28 July 2006.—MRAC B3-15-P1, 92.4 mm SL, same data as AMNH 241650.—
AMNH 241649, 2 specimens, 52.2-91.4 mm SL, Democratic Republic of Congo, Equateur Province, Luilaka River
at Nkema Asondji, 2.03694° S, 20.99683° E, Coll. R. Monsembula and R. Schelly, 16 July 2006.—AMNH
252256, 3 specimens, 41.0-48.0 mm SL, 3 tissue vouchers, Democratic Republic of Congo, Equateur Province,
Luilaka River at Bosombangwa, Salonga National Park, 2.22435° S, 21.1851° E, Coll. R. Monsembula, 26 May
2010.—AMNH 241647, C/S, 32.1 mm SL, Democratic Republic of Congo, Equateur Province, Luilaka River at
Nkomba Dumbe, 2.67111° S, 21.7211° E, Coll. R. Monsembula and R. Schelly, 09 July 2006.
Diagnosis. Eugnathichthys virgatus, new species, is unique among congeners in the possession of body
pigmentation dominated by a broad lateral band intersected by numerous vertical bars resulting in a midlateral,
checkerboard-like pigmentation patterning both in life and in preservation. Internally it is characterized by a
marked reduction in dentition on the fifth ceratobranchial elements of the pharynx. Externally it differs from both
congeners in the possession of a reduced total number of pectoral-fin rays (13–15 vs. 16–18). Eugnathichthys
virgatus is further readily distinguished from E. eetveldii by the possession of 66–72 (vs. 96–103) pored lateral line
scales from opercle to caudal flexion, 10–12 (vs. 14 or 15) scale rows between the lateral line and the dorsal-fin
origin, and 8 or 9 (vs. 10) scale rows between the lateral line and pelvic-fin insertion. It differs from E.
macroterolepis, the species that bears closest phenetic similarity, in the possession of 8 or 9 (vs. 6 or 7) scale rows
between the lateral line and the pelvic-fin insertion. Diagnostic molecular characters include 4 non-synonymous,
nucleotide transitions in ND2: TC (site 222); CA (site 530); AG (site 601); AG (site 672).
Description. A Eugnathichthys attaining maximum-recorded size of 106.0 mm SL (mature female, AMNH
249790), with general body shape and appearance as in Figures 3 and 4. Tables 1 and 2 summarize morphometric
and meristic attributes with comparative ranges for congeners. Relatively deep-bodied, body depth 18.0–24.4 % SL
(mean 20.8), greatest depth at vertical midway between pectoral and pelvic-fin insertions. Head length 29.0–31.3
% SL (mean 30.0), eyes large, bony orbit diameter 22.6–26.2 % HL (mean 24.5). Dorsal head profile straight from
snout to top of head, strongly convex over nape to dorsal-fin origin. Dorsal body profile gently convex along
dorsal-fin base to caudal-fin base, ventral body profile gently convex between isthmus and anal-fin base, caudal
peduncle almost twice as long as deep.
Snout elongate; mouth terminal and jaws relatively massive. Contralateral premaxillae and dentaries
immovably united by strongly interdigitating sutures. Upper and lower jaws both with two tooth rows. Inner row
teeth small, elongate bicuspids moveably implanted in a connective tissue sheath within replacement tooth trench.
Inner row tooth shafts horizontally oriented spanning tooth trench; recurved cusps vertically oriented. Outer row
teeth stout, erect and closely apposed bicuspids with flattened, expanded crowns each bearing a small, blunt
anterior cusp and a prominent, enlarged posterior cusp. Premaxillae and dentaries each with 15–17 outer row teeth
firmly ankylosed to anterior margin of replacement trench. Premaxillary teeth overlie those on dentaries when
mouth closed, resulting in a shearing bite.
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TABLE 1 . Morphometric data for type series of Eugnathichthys virgatus, new species with comparative ranges for
TABLE 2 . Meristic data for type series of Eugnathichthys virgatus, new species with comparative ranges for congeners.
Dorsal fin iii–iv, 12 or 13 (mode 12), anal fin iii, 8 or 9 (mode 9) with first unbranched rays diminutive, often
clearly apparent only in x-rays or cleared and stained specimens. Dorsal-fin origin located well in advance of
vertical through pelvic-fin insertion; first dorsal pterygiophore inserted between neural spines of vertebral centra 11
or 12. Caudal fin deeply forked, upper lobe slightly longer than lower; principal caudal-fin rays 10+9. Pectoral fin
short and narrow, mean length 14.6 % HL; reduced number of 13–15 rays (count includes leading unbranched ray,
Fig. 5A vs. 16–18 in congeners, Fig. 5B).
Body covered with small, regularly imbricate, distally dentate (pseudo-ctenoid) scales. Lateral line complete,
in straight midlateral line from opercle to anterior margin of caudal fin, 66–72 pored scales to caudal flexion (+2–6
on caudal-fin base), 10–12 scale rows between lateral line and dorsal-fin insertion, 8 or 9 between lateral line and
pelvic-fin insertion, 24 or 25 circumpeduncular scales.
Eugnathichthys virgatus, new species E. macroterolepis (n =12) E. eetveldii (n =10)
Holotype N Mean Range SD Mean Range Mean Range
Standard Length (mm) 93.0 12 32.1–106 99.0 63.4–157.0 84.3 42.3–265.0
Body Depth 22.8 10 20.8 18.0–24.4 2.2 23.2 20.8–27.6 18.3 16.2–20.1
Head Length 29.0 10 30.0 29.0–31.3 0.9 29.4 27.6–30.8 27.6 25.0–30.3
Predorsal Length 48.4 10 48.9 47.6–50.5 1.0 49.4 48.3–51.6 47.7 45.6–50.0
Preanal length 76.7 10 75.0 73.0–77.1 1.5 74.3 71.5–76.3 73.9 71.4–75.5
Prepelvic length 51.8 10 52.4 50.8–55.0 1.2 52.9 51.0–55.4 52.6 51.2–55.0
Dorsal-adipose 19.1 10 19.8 18.5–20.5 1.1 20.9 19.7–22.7 17.7 17.0–18.5
Caudal peduncle length 14.3 10 14.8 13.1–16.5 1.0 15.1 13.0–16.9 16.6 14.4–19.6
Caudal peduncle depth 7.8 10 7.9 7.6–8.5 0.3 8.6 8.0–9.4 7.9 7.6–8.2
Pectoral fin length 13.7 10 14.6 13.7–16.8 0.9 15.8 14.5–16.8 14.2 13.5–15.2
Eye diameter 22.6 10 24.5 22.6–26.2 1.5 22.3 20.0–25.4 26.3 24.1–28.9
Snout Length 34.1 10 32.9 30.5–34.2 1.3 32.9 30.1–35.0 31.3 28.9–34.0
Upper Jaw length 42.2 10 41.5 38.0–45.0 1.9 43.7 40.8–45.7 44.1 40.6–46.4
Interorbital width 27.0 10 24.0 21.3–25.3 2.1 24.9 21.1–28.1 25.7 23.4–29.1
Postorbital length 44.8 10 44.4 42.9–45.2 0.7 46.1 42.9–48.4 43.9 41.4–45.6
(*=modal count) E. virgatus (n=12) E. macroterolepis (n=20) E. eetveldii (n=10)
Branched dorsal-fin rays 12–13 (*12) 12–13 (*12) 14–15 (*14)
Branched anal-fin rays 8–9 (*9) 9–10 (*10) 9–11 (*10)
Pectoral-fin rays (total) 13–15 (*15) 16–18 (*17) 17–18 (*17)
Gill rakers (first ceratobranchial) 9–10 (*9) 9–12 (*10) 14–17 (*16)
Lateral line scales (to flexion) 66–72 (*68) 64–68 (*66) 96–103 (*99)
Lateral line-dorsal fin scale rows 10–12 (*12) 9–12 (*10) 14–15 (*15)
Lateral line-pelvic fin scale rows 8–9 (*9) 6–7 (*7) 10–11 (*10)
Circumpeduncular scale rows 24–25 (*24) 21–24 (*24) 30–32 (31)
Total number of vertebrae 42–43 (*43) 43–45 (*44) 49
Abdominal vertebrae 26–7 (*27) 26–28 (*27) 31
Caudal vertebrae 15–16 (*16) 16–18 (*17) 18
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Total of 9 or 10 gill rakers on ceratobranchial of first arch, rakers reduced to flattened toothplates along
proximal portion of arch, becoming somewhat enlarged towards angle of arch (Fig. 6A). Toothplates on fifth
ceratobranchial elements of pharynx greatly reduced, restricted to distal margin of bone and dentition limited to 2
or 3 tooth rows (Fig. 6A). Total vertebrae, 42 or 43 consisting of 27 abdominal plus 15 or 16 caudal vertebrae
(holotype: 27+16). Eight supraneurals interdigitating with neural spines of last Weberian vertebra and first 7 rib-
bearing vertebrae anterior to first dorsal-fin pterygiophore.
FIGURE 3. Eugnathichthys virgatus, new species (holotype, AMNH 241648): (A) preserved, (B) immediately post-mortem.
FIGURE 4. Eugnathichthys virgatus, new species (paratypes): (A) AMNH 246319, Lengoué River, (B) AMNH 249790,
Pigmentation and coloration. In preservation (Fig. 3A), base body coloration creamy brown, darker above
midlateral line than below. Snout, upper jaw, and top of head dark brown; well marked postorbital streak passing
diagonally across infraorbital 4 onto posterior margin of opercle. Lower jaw, cheek and branchiostegal membrane
pale cream. One or two rows of irregular, often indistinct, oblong blotches on nape; 12–14 vertically oriented
oblong bars along midlateral line, bars extended onto dorsum from mid-body to caudal peduncle. Vertical bars
intersected medially by broad midlateral band resulting in characteristic checkerboard-like pigmentation
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patterning. Dorsal fin pale, creamy white with three thin black stripes, adipose fin with black basal blotch and distal
spotting; caudal fin with alternating black and white stripes. Remaining fins creamy white. In life (Fig. 3B), base
body coloration with slight pinkish hue; iridescent silver reflections on scales. Head and body pigmentation
patterning as for preserved specimens. Markings on dorsal and caudal fin as in preserved specimens but interspaces
between black bands bright orange-red. Adipose fin pale orange with black spotting distally, remaining fins
hyaline, with slight dusky overlay.
FIGURE 5. Pectoral fin and girdle (lateral view): (A) Eugnathichthys virgatus, AMNH 253476, 94.8 mm SL, (B) E.
macroterolepis, AMNH 243084, 88.2 mm SL.
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FIGURE 6. Branchial apparatus (dorsal view) fifth ceratobranchial elements shaded grey, gill rakers on first ceratobranchial,
black: (A) Eugnathichthys virgatus, AMNH 253476, (B) E. eetveldii, AMNH 252993, isolated first ceratobranchial and gill
rakers (black), (C) E. macroterolepis, AMNH 243084, isolated fifth ceratobranchials, (D) E. eetveldii, AMNH 252993, isolated
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FIGURE 7. Eugnathichthys species relationships. Indices at nodes indicate LR-ELW resampling support >50. Species
exemplars inset: Mesoborus crocodilus (AMNH 251761), Eugnathichthys macroterolepis (holotype, BMNH 1899.6.28:16), E.
eetveldii (syntype, MRAC 148), and E. virgatus (holotype, AMNH 241648).
Geographical variation. Preceding description based on holotype and paratypes from SNP. Specimens from
Lengoué River (Fig. 4A) and Lomako River (Fig. 4B) are darker than Salonga conspecifics, interspaces on dorsal
fin greatly reduced, and in Lengoué specimens ventrum, pectoral, pelvic and anal fins are sooty black.
Additionally, caudal fin pigmentation of Lomako specimen is strongly reticulate not striped. Despite these
differences, all populations exhibit body pigmentation dominated by a broad lateral band intersected by vertical
bars, here considered diagnostic of the species.
Distribution. Currently known only from three regions in western and central Congo basin; Lengoué River
near Ouesso, Republic of Congo (Fig. 1, area 1), Lomako River at Isake, Democratic Republic of Congo (Fig. 1,
area 2), and localities along the Luilaka and Yenge Rivers in the SNP, Democratic Republic of Congo (Fig. 1, area
3). While most of these areas are poorly known and their ichthyofaunas have yet to be fully documented (Stiassny
et al., 2011; Monsembula Iyaba & Stiassny, 2013), from collections housed at the AMNH it is evident that E.
macroterolepis also occurs in the Lengoué. Yet despite relatively intensive recent collecting efforts in the SNP E.
virgatus is the only Eugnathichthys currently recorded from that region. Given the disjunct locations from which
the species has been recorded it is probable that E. virgatus is more widespread than currently known. We note, for
example that Roberts (1990: Fig. 3) provides a photograph of a 70 mm individual identified as E. macroterolepis
from the Mossapoula River (Ubangi drainage) (Fig. 1, region depicted with “??”). We have been unable to locate
this specimen in a museum collection, but from the illustration it clearly exhibits the characteristic pigmentation
patterning of E. virgatus—a broad midlateral band intersected by numerous vertical bars—suggesting the range of
E. virgatus extends into the Ubangi basin.
Feeding. While field observations were not made, gut morphology and contents suggest that the species feeds
exclusively on fish fins. The stomach is a large, simple sac of length 20–25 % SL, from which a short gut issues
basally and coils twice before exiting the body cavity. The unraveled combined length of stomach and intestine is
about 1.25 times SL. All specimens regardless of size or geographical location, contained pieces of fish fin
packaged together and often filling the stomach; no other food items were found. Roberts (1990) observed that
young E. eetveldii feed on aquatic insect larvae, and that the smallest specimen with fins observed in the stomach
was an individual of 96 mm SL. In this study, a juvenile E. eetveldii (AMNH 247243, 41 mm SL) was found to
have a mixture of fish fins and aquatic insect larvae in its stomach, indicating that E. eetveldii may undergo an
ontogenetic shift in trophic strategy. This does not appear to be the case for E. virgatus as all juveniles, even the
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smallest examined (32.1 mm SL), apparently feed exclusively on fins. Both caudal and unpaired fin fragments
were recovered but unfortunately it was not possible to discern the prey species. We note that larger individuals of
E. virgatus are missing large portions of their dorsal fins (see Figs 3 and 4), but whether this is a result of
intraspecific ectoparasitism, or from attack by sympatric fin-biting distichodontids is unclear.
Ecology and habitat. All specimens of E. virgatus were collected exclusively in heavily shaded, highly humic
Etymology. Virgatus, from the Latin, in reference to the conspicuous midlateral band or streak forming the
characteristic pigmentation patterning of the species in life and in preservation.
Molecular phylogenetic analysis
In order to confirm that SNP specimens were conspecific with the Lengoué and Lomako individuals, and to
investigate species relationships within the genus, a small-scale phylogenetic analysis was undertaken. Based on
the phylogenetic hypothesis of Vari (1979) and our own preliminary analyses, the genus Mesoborus was selected as
an appropriate outgroup for rooting the Eugnathichthys tree. Results of a maximum likelihood analysis in
TreeFinder (Jobb et al., 2004), based on a 2-gene alignment partitioned by gene and codon position, are presented
in Figure 7, and an identical topology was retrieved in parsimony reconstruction in TnT (Goloboff et al., 2008).
The SNP samples cluster together with the Lengoué and Lomako individuals forming two somewhat
geographically differentiated subclades within a strongly supported monophyletic E. virgatus. Eugnathichthys
virgatus is further resolved as sister to E. eetveldii, although support is somewhat lower than that supporting
monophyly of E. macroterolepis and E. virgatus.
Eugnathichthys eetveldi: MRAC 148, Syntype, Upoto.—MRAC 121, Syntype, Leopoldville.—MRAC 26.
Syntype, Boma.—ZMB 19043, Holotype of Eugnathichthys intermedius, Aruwimi River at Basoko.—AMNH
5804, 1 specimen, Stanleyville, junction of Lualaba with Congo River.—AMNH 5899, 1 specimen, Stanleyville,
junction of Lualaba with Congo River.—AMNH 227473, 1 specimen, Bobongo creek into Sangha River.—AMNH
227513, 1 specimen, Middle of Sangha River at Bayonga.—AMNH 243649, 1 specimen, Katende, Lulua River.—
AMNH 252993, 1 specimen, C/S, Lulua River at Nsanga Nyembo.—AMNH 253161, 1 specimen, Lulua River at
Nsanga Nyembo.—AMNH 247243, 1 specimen, Lulua River upstream of confluence with Kasai River.
Eugnathichthys macroterolepis: BMNH 1899.6.28:16, Holotype, Chiloango, Angola.—AMNH 5810, 1
specimen, Stanleyville, junction of Lualaba with Congo River.—AMNH 6148, 1 specimen, Avakubi, Ituri River.—
AMNH 6069, 1 specimen, Rungu, Bomokandi River.—AMNH 6347, 1 specimen, Poko, Bomokandi River.—
AMNH 5999, 1 specimen, Faradji, Dungu River.—AMNH 6332, 1 specimen, Malela, mouth of Congo River.—
AMNH 12421, 1 specimen, Lulua River, Luluabourg.—AMNH 238341, 2 specimens, Upstream of Boma, Congo
River.—AMNH 245003, 2 specimens, Lengoué River, Upstream of Ouesso.—AMNH 245004, 1 specimen,
Sangha River at Point 9.—AMNH 245588, 1 specimen, Nsele River at market.—AMNH 252994, 1 specimen,
Lulua River at Nsanga Nyembo.—AMNH 252805, 1 specimen, Lulua River at Mukundulu.—AMNH 253084, 1
specimen, C/S, Kasai River at Tshikapa.—AMNH 253624, 3 specimens, 1C/S, Kwilu River at Kwilu beach.—
AMNH 252378, 1 specimen, Nsele River at Koke ya Mbila.—AMNH 252584, 1 specimen, Kwilu River at Kikwit.
K. Hartel and A. Williston (MCZ), and P. Bartsch (ZMB) kindly made available photographs and radiographs of
specimens in their collections. J. Arroyave (AMNH) provided preliminary data from an ongoing molecular
phylogenetic study of distichodontid fishes for which we are extremely grateful. Our thanks also to Barbara Brown
and Radford Arrindell (AMNH) for their help with materials examined during the course of this study. The Herbert
and Evelyn Axelrod Curatorship (MLJS) provided financial support, and facilitated study visits of RJCM to the
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