Eugnathichthys virgatus Denton & Iyaba, 2013, new species

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, 0 9 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: T+C (site 222); C+A (site 530); A+G (site 601); A+G (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.

Eugnathichthys virgatus, new species E. macroterolepis (n =12) E. eetveldii (n =10) 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. 5 A vs. 16–18 in congeners, Fig. 5 B).

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.

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. 6 A). 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. 6 A). 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 ribbearing vertebrae anterior to first dorsal-fin pterygiophore.

Pigmentation and coloration. In preservation (Fig. 3 A), 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 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. 3 B), 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.

Geographical variation. Preceding description based on holotype and paratypes from SNP. Specimens from Lengoué River (Fig. 4 A) and Lomako River (Fig. 4 B) 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 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 forest rivers.

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.