Contents

ABSTRACT 2

INTRODUCTION 2

MATERIALS AND METHODS 3

Morphometric Results 5

COI Barcoding Results 5

Phenacogrammus flexus, new species 8

Phenacogrammus concolor, new species 10

DISCUSSION 11

ACKNOWLEDGMENTS 12

REFERENCES 12

AMERICAN MUSEUM NOVITATES

Number 3980, 22 pp., November 3, 2021

Copyright © American Museum of Natural History 2021

ISSN 0003-0082

Two new Phenacogrammus (Characoidei; Alestidae) from the Ndzaa River (Mfimi-Lukenie basin) of central Africa, Democratic Republic of Congo

Melanie L.J. Stiassny,1 S. Elizabeth Alter,2 Raoul J.C. Monsembula Iyaba,3 and Tobit L.D. Liyandja4

ABSTRACT

Two new Phenacogrammus are described from the Ndzaa River, a small left-bank tributary of the Mfimi-Lukenie River in the central Congo basin. They share with P. deheyni, a congener endemic to the Cuvette Centrale to the north, a prominent anterior expansion of the first pleural rib; a feature interpreted here as a synapomorphy diagnostic for this species assemblage. The two new species are readily differentiated from P. deheyni based on differences in pigmentation patterning, a lower number of scales in longitudinal series (26–28 vs. 29–33) and a longer head length (m. 24.9% SL vs. 21.7 and 23.2% SL). Phenacogrammus flexus, new species, is distinguished from all congeners in the possession of 6 (vs. 7) supraneural bones, and a characteristic zigzag pattern of black pigmentation along and below the midline extending from the posterior border of the opercle to the base of the caudal peduncle. While no unambiguous morphological autapomorphies have been located to diagnose P. concolor, new species, it is nonetheless readily distinguished from all congeners, except P. deheyni and P. flexus, in the possession of a prominent anterior expansion of the first pleural rib. It differs from both P. deheyni and P. flexus in the absence of a dominant pigmentation patterning over the flanks and caudal peduncle. Additionally, it differs from P. flexus in a shallower body depth (m. 24.9% vs. 27.0% SL) and in the possession of 7 (vs. 6) supraneurals.

The three species exhibit extensive divergence in mt-COI sequence (P. flexus vs. P. concolor 10.2%–11%; P. flexus vs deheyni 12.9%–13.5%; P. concolor vs. deheyni 11.3%–12.9%). Furthermore, analysis of shape variation utilizing geometric morphometrics indicates that each species differs significantly in body shape.

INTRODUCTION

During a survey of the fishes of the Mfimi River basin in central Congo, collections were made in the Ndzaa River, a small left-bank tributary entering the Mfimi near the settlement of Kutu at the outflow of Lake Mai-Ndombe, upstream of which the river is renamed the Lukenie (fig. 1). The Ndzaa (the Ndjua River on some maps) is a clear water stream meandering through dense riparian forest surrounded by open grasslands and savannah (fig. 2). Collections were made in July 2018, at the height of the main dry season in a short stretch of the river at about 25 km upstream of its junction with the Mfimi-Lukenie (fig. 1). In addition to samples of 27 species (table 1), seven of which had not previously been collected within the Mfimi basin (Stiassny et al., personal obs.) were specimens of two putatively undescribed alestid species belonging to the genus Phenacogrammus Eigenmann, 1907. Although the generic-level taxonomy of the Alestidae is generally poorly supported by apomorphy-based diagnoses (Stiassny and Schaefer, 2005; Schaefer, 2007; Arroyave and Stiassny, 2011) Phenacogrammus is currently diagnosed by the derived (reductive) feature of a truncated lateral line consisting of fewer pored than nonpored scales. While a truncated lateral line occurs in several other alestid taxa (Zanata and Vari, 2005) this feature, in combination with the presence of two small, usually conical, symphyseal teeth located immediately behind the multicuspidate outer row series on the dentary, serves to differentiate members of this genus from all others. Although taxon sampling within Phenacogrammus and across the family was incomplete, the molecular study of Arroyave and Stiassny (2011) provides support for the monophyly of Phenacogrammus as currently constituted (Fricke et al., 2021). Despite the shortcomings in our understanding of alestid generic composition and intrarelationships, an accurate accounting of species diversity is of central importance for phylogenetic resolution and for data-based conservation efforts (Stiassny and Mamonekene, 2007). And, as such, species discovery and description remain fundamental tasks for advancing biodiversity studies in poorly documented regions such as the central Congo basin where considerable phylogenetic diversity remains to be documented (Stiassny et al., 2011). While recognizing that future generic reassignment may be necessary, as a comparative framework for their diagnosis and description, we follow the differential generic diagnosis of Phenacogrammus provided by Poll (1967) and Zanata and Vari (2005) and assign the two new taxa from the Ndzaa River to this genus.

Prior to the present study, four species of Phenacogrammus had been reported from the central Congo basin: P. polli, P. dehenyi, P. aurantiacus, and P. interruptus (fig. 3A–D). The latter two species are reported to be widely distributed throughout the Congo basin; however, while P. interruptus appears to represent a single species across its extensive range, geographically disparate populations of P. aurantiacus exhibit morphological and coloration variation (data not shown) as well as considerable divergence in COI sequences (e.g., table 2), and likely represent multiple cryptic species. The other two species, P. deheyni and P. polli, are currently known only from a few localities in the Cuvette Centrale, located to the north of the Mfimi-Lukenie basin (Poll, 1967; Monsembula Iyaba and Stiassny, 2013.). Based on a review of morphological features among Phenacogrammus and allied genera (Zanata and Vari, 2005; Arroyave and Stiassny, 2011) we note that the two Ndzaa species uniquely share with P. dehenyi (fig. 3B) a derived expansion of the first pleural rib, absent in all congeners and comparative material examined. The expansion of the first pleural rib is a feature interpreted here as a synapomorphy diagnostic for this species assemblage. Based on examination of these and additional comparative materials, we provide formal taxonomic descriptions of the two undescribed Ndzaa Phenacogrammus.

MATERIALS AND METHODS

Morphology: Fourteen standard morphometric and eight meristic counts follow Paugy et al. (2003). Linear measurements were taken by photographing the left side of each specimen in an Ortech Professional Photobox Plus (Model 1419) on a platform with a Nikon D200 camera with a 60 mm f/2.8 AFMicro-Nikkor lens, and a 1 cm scale. Specimens were pinned to mark fins and other homologous anatomical features prior to photographing, and images uploaded into tpsDIG2 (Rolf, 2017). Measurements are presented as percentages of standard length (SL) or head length (HL) rounded to the nearest 0.1 mm. To count vertebrae and fin rays and visualize other skeletal features, specimens were X-rayed, and some were CT-scanned or cleared and stained following a modified protocol based on Taylor and van Dyke (1985). Vertebral counts include the four modified Weberian centra but exclude the terminal, hypural-bearing centrum. Longitudinal scale counts terminate at the hypural fold and do not include the few scales on the base of the caudal fin.

Geometric morphometrics: Nondistorted specimens of P. flexus new species (n = 8), P. concolor new species(n = 5), and P. deheyni (n = 11) were selected for geometric morphometric analysis. The P. deheyni sample included both adult and juvenile specimens whereas P. flexus and P. concolor were represented only by adults. Photographs were taken on the left side of each specimen as described above, and a set of 19 homologous landmarks was selected to capture overall body shape (fig. 4A). Photographs were uploaded into tpsDIG2 (Rohlf, 2017), and tpsUtil 1.70 (Rohlf, 2019) was used to generate x-y coordinate files used in downstream analyses. To remove most variation associated with size, rotate each individual to a common alignment, and generate a consensus shape by calculating the average shape of all specimens included in the analysis, a generalized Procrustes analysis (GPA) was performed in MorphoJ 1.06d (Klingenberg, 2011). A covariate matrix was then constructed to prepare data for a principal component analysis (PCA), which was also conducted in MorphoJ. To investigate shape variation across the three species, a canonical variates analysis (CVA) incorporating a permutation test for pairwise differences with 10,000 iterations was conducted in MorphoJ (data not shown).

COI barcoding: Total genomic DNA was extracted from available representatives of Phenacogrammus and from exemplars of two alestid genera (Clupeocharax, Alestopetersius) considered close relatives of Phenacogrammus (Zanata and Vari, 2005; Arroyave and Stiassny, 2011). Amplification and sequencing of partial cytochrome c oxidase subunit I (COI) was carried out as described in Stiassny and Alter (2015). We used Geneious Prime 2021.1.1 to manually edit and align sequences and to generate a distance matrix indicating the percentage identity (% of bases that are identical) in COI sequences between sampled taxa (table 2). Specimen catalog numbers, tissue codes, locality information, and GenBank accession numbers for sequences utilized in this study are provided in table 3.

Abbreviations: AMCC, Ambrose Monell Cryo Collection of the American Museum of Natural History; C&S, cleared and stained specimens; CT, micro-CT scanned specimens, SL, standard length; HL, head length; m, mean. Institutional abbreviations follow Sabaj (2016).

Morphometric Results

The first four principal components combined describe 72.7% of shape variation across the three species (fig. 4B). PC 1 accounts for 26.6% of total variation and reflects the considerable size variation between sampled specimens of P. deheyni vs. that of the two new species. Best separation among the three was obtained plotting PC2 (20.2%), which mainly describes variation in head size, against PC3 (14.9%) mainly describing variation in body depth (fig. 4B). Permutation test for Procrustes (p-value <0.005) and Mahalanobis distances (p-value <0.001) indicate that the three species are significantly different in body shape. Both the PC and CV analyses confirm that P. dehenyi can be distinguished from both P. flexus and P. concolor in possessing a longer head, whereas P. flexus can be distinguished from P. concolor by a deeper body.

COI Barcoding Results

The mitochondrial cytochrome c oxidase I gene (COI) has long been used to detect both described and undescribed fish species (e.g., Hubert et al., 2008; Ward, 2009; Decru et al., 2016), and the COI distance matrix (table 2) provides additional support for the current hypothesis of species identity for P. flexus and P. concolor as distinct from P. deheyni and other congeners. The percentage divergence in mt-COI sequence among individuals of P. flexus is 0%–0.2%, and of P. concolor is 0.6%–0.9%. In contrast sequence divergence between individuals of P. flexus and P. concolor is 10.2%–11% (bold font in table 2), between P. flexus and P. deheyni is 12.9%–13.5%, and between P. concolor and P. deheyni is 11.3%–12.9%, each of which far exceeds the traditionally employed ca. 3% sequence divergence heuristic threshold for conspecifics (Avise, 2000; Hebert et al., 2003, 2004; Arroyave et al., 2019).

Figure 1. Left, location of study area in Congo basin (black circles indicate known range of P. deheyni). Right, map of Mfimi-Lukenie region and location of Ndzaa River. Red star indicates region where collections were made. Shaded area in the Ulanzaa River catchment indicates the predicted region of highest habitat suitability based on preliminary species distribution modelling. Inset: A. Phenacogrammus flexus, B. Phenacogrammus concolor, shortly postmortem.

Figure 2. Upper panel shows a fisherman pointing to the type locality in the Ndzaa River where both new Phenacogrammus species were collected, and lower panel shows savannah/grassland surrounding the riparian cover of the river channel.

Figure 3. Phenacogrammus species previously reported from the central Congo basin (above) and posterior neurocranial and anterior axial anatomy rendered from µCT scans (below). A. P. polli, B. P. deheyni, C. P. aurantiacus, and D. P. interruptus. Arrow indicates anterior expansion of the first pleural rib.

Figure 4. A. outline showing locations of 19 homologous landmarks selected to capture overall body shape. B. Principal components analysis of body shape with deformations as implied by PC2 and PC3 scores using Procrustes superposition of the 19 landmarks. Inset: plot of % variance explained by each PC. Stars, P. flexus new species; squares, P. concolor new species; circles, P. deheyni.

Phenacogrammus flexus, new species

Figures 1, 5; table 4

Holotype: AMNH 274785, 47.2 mm SL, Democratic Republic of Congo, Mai-Ndombe Province, Ndzaa River (02° 58′ 25.0″8S 018° 7′ 55.26″E), R. Monsembula Iyaba et al., 25 July 2018.

Paratypes: AMNH 276320, 7 specimens, 2 CT-scanned, 47.0–37.8 mm SL, same data as holotype.

Diagnosis: Distinguished from all congeners except P. deheyni and P. concolor by the presence of a prominent anterior expansion of the first pleural rib. Further distinguished from all congeners in the possession of 6 (vs. 7) supraneural bones, and a characteristic zigzag pattern of black pigmentation along and below the midline extending from the posterior border of the opercle to the base of the caudal peduncle.

Description: Based on the holotype and seven paratypes. General appearance as in figure 5A; proportional measurements and meristic counts are given in table 4. A small-bodied species, maximum observed size 47.2 mm SL. Body relatively elongate, somewhat laterally compressed with greatest body depth (m. 27.0% SL) a little in front of dorsal-fin origin at level of pelvic-fin insertion. Dorsal body profile steeply convex to dorsal fin, slightly less so to caudal-fin base. Ventral profile smoothly convex between isthmus and anal-fin base, caudal peduncle longer than deep. Head length short (m. 23.4% SL), eye large, bony orbit diameter m. 43.9% HL.

Mouth terminal, lower jaw prominent and slightly prognathous, lower lip thick. Premaxilla with two teeth in outer row, each bearing 3 cusps, positioned opposite interspaces between and alternating with 4 inner row teeth bearing 4–5 cusps. Four outer row teeth on each dentary, the first 3 bearing 5 cusps, and the 4th greatly reduced in size bearing 3 cusps. Two small, conical, symphyseal teeth located immediately behind outer row series on dentary (fig. 5C). Dorsal-fin rays ii.7–8, fin positioned midway between snout and caudal-fin base, origin at, or slightly behind, pelvic-fin insertion, first two branched rays somewhat elongated but not filamentous. Anal fin iii.17–18 rays, no hypertrophy or posterior curvature of anal fin in males. Caudal fin forked, with 8 upper and 9 lower principal rays, no median filamentous extension. Scales in longitudinal series 26–27 (mode 26) to caudal flexure, 4–5 transverse scales between longitudinal series and dorsal-fin origin, 3 transverse scales between longitudinal series and pelvic-fin insertion. Pored scales in truncate lateral line 7–8 (mode 8). Circumpeduncular scales 10. Vertebral count 31–32. Twelve to 13 pleural ribs, first with prominent rounded expansion on anterior face in upper third (fig. 5B). Six tubular supraneurals located between interspaces of neural spines of vertebral centra 4–10.

Coloration: In preservation (fig. 5A), base body coloration yellowish, darker dorsally and with an overlay of silver iridescence ventrally along flanks. Snout and dorsum of head dark brownish black, cheek silver with patches of melanophores. A prominent pattern of black pigmentation encircling scales of longitudinal series along and below midline extending from posterior border of opercle to base of caudal fin, becoming broken into a zigzag band posteriorly along flanks and caudal peduncle. No trace of a humeral blotch. Dorsal and adipose fin grayish black, anal fin dusky, pectoral and pelvic fins hyaline with darkly pigmented leading edges. Median rays of caudal fin somewhat darker than above and below. Coloration and pigmentation shortly postmortem (fig. 1A) like that in preservation, but generally more intense.

Distribution: Currently known only from the type locality, a shallow pool like expansion of the Ndzaa River (fig. 2, upper panel), about 25 km upstream of the outflow of the Ndzaa into the Mfimi (fig. 1).

Etymology: Flexus, from the Latin, meaning winding, zigzag, or sharp turn. Named in reference to the prominent pattern of black pigmentation along the flanks and caudal peduncle.

Figure 5. A. Phenacogrammus flexus, new species (holotype, AMNH 274785). B. Posterior neurocranial and anterior axial anatomy rendered from µCT scans; arrow indicates location of anterior expansion of the first pleural rib. C. Premaxillary and dentary tooth insertion and cuspidation rendered from µCT scan.

Phenacogrammus concolor, new species

Figures 1, 6; table 5

Holotype: AMNH 274786, 43.9 mm SL, Democratic Republic of Congo, Mai-Ndombe Province, Ndzaa River (02° 58′ 25.0″8S 018° 7′ 55.26″E), R. Monsembula Iyaba et al., 25 July 2018.

Paratypes: AMNH 276321, 4 specimens, 2 CT-scanned, 42.9–34.5 mm SL, same data as holotype.

Differential diagnosis: While no unambiguous morphological autapomorphies have been located to diagnose P. concolor the species is nonetheless distinguished from all congeners except P. deheyni and P. flexus in the possession of a prominent anterior expansion of the first pleural rib. The species is readily differentiated from both in the absence of a dominant pigmentation patterning over the flanks and caudal peduncle. Additionally, it differs from P. flexus in a shallower body depth (m. 24.9% vs. 27.0% SL), and in the possession of 7 (vs. 6) supraneurals.

Description: Based on the holotype and four paratypes. General appearance as in figure 6A, proportional measurements and meristic counts are given in table 5. A small-bodied species, maximum observed size 43.9 mm SL. Body elongate, laterally compressed with greatest body depth (m. 24.9% SL) well in front of dorsal-fin origin, in front of the level of pelvic-fin insertion. Dorsal body profile smoothly convex to dorsal fin, slightly less so to caudal-fin base. Ventral profile smoothly convex between isthmus and anal-fin base, caudal peduncle longer than deep. Head length short (m. 21.7% SL), eye large, bony orbit diameter m. 44.8% HL.

Mouth terminal, lower jaw prominent and slightly prognathous, lower lip thick. Premaxilla with two teeth in outer row, each bearing 3 cusps, positioned opposite interspaces between and alternating with 4 inner row teeth bearing 5–6 cusps. Four robust, broad-based, outer-row teeth on each dentary, first 3 bearing 5-6 cusps, and 4th greatly reduced in size bearing 3 cusps. Two small, conical, symphyseal teeth located immediately behind outer row series on dentary (fig. 6C). Dorsal-fin rays ii.7–8, fin positioned midway between snout and caudal-fin base, with origin at, or slightly in front of, pelvic-fin insertion, first two branched rays not elongated. Anal fin iii.15–16 rays, no hypertrophy or posterior curvature of anal fin in males. Caudal fin forked, with 9 upper and 9 lower principal rays, no median filamentous extension. Scales in longitudinal series 26–28 (mode 28) to caudal flexure, 3 or 4 transverse scales between longitudinal series and dorsal-fin origin, 3 transverse scales between longitudinal series and pelvic-fin insertion. Pored scales in truncate lateral line 5–7 (mode 6). Circumpeduncular scales 10. Vertebral count 31–33. Twelve–13 pleural ribs, first with prominent rounded expansion on anterior face in upper third (fig. 5B). Seven tubular supraneurals located between interspaces of neural spines of vertebral centra 4–11.

Coloration: In preservation (fig. 6A), base body coloration grayish brown, darker dorsally and becoming pale yellow on chest and ventrum. An overlay of bluish iridescence covers most of body. Snout and dorsum of head dark brownish black, cheek silver with a few scattered melanophores. A large humeral blotch present at midbody, above pectoral fin. Other than humeral blotch no prominent pigmentation along flanks or caudal peduncle. Dorsal and adipose fin light gray, anal fin dusky distally, pectoral and pelvic fins hyaline with darkly pigmented leading edges. Median rays of caudal fin somewhat darker than above and below. Coloration and pigmentation shortly postmortem (fig. 1B) like that in preservation, but more intense.

Distribution: Currently known only from the type locality, a shallow pool like expansion of the Ndzaa River (fig. 2, upper panel), about 25 km upstream of the outflow of the Ndzaa into the Mfimi (fig. 1).

Etymology: Concolor, from the Latin, meaning of the same, or uniform, color. Named in reference to the absence of prominent pattern of black pigmentation along the flanks and caudal peduncle.

Comparative materials examined: Alestopetersius hilgendorfii: AMNH 244114, 5 specimens, 1CT, 1C&S, Democratic Republic of Congo, Salonga National Park, Yenge River at Boyenga. – MRAC 829, lectotype, Kutu. – Alestopetersius brichardi: AMNH 240416, 5 specimens, 1C&S, Republic of Congo, Cuvette Ouest, Odzala National Forest, au campement des pêcheurs. – MRAC 121105, holotype, Yangambi, Lac Yandja. – Alestopetersius nigropterus: AMNH 274754, 1 specimen, Democratic Republic of Congo, Mai-Ndombe Province, Nioki Port Market, main channel Mfimi River.

Nannopetersius ansorgii: AMNH 263093, 5 specimens, 1CT, Republic of Congo, Kouilou Province, Lake Tchibanji. – NHMUK 1910.11.28:71, lectotype, Angola, Bengo River at Cabiri and Lake Kilungu. – Nannopetersius lamberti: AMNH 253898, 5 specimens, 2C&S, Republic of Congo, Passi-passi River.

Clupeocharax schoutedeni: AMNH 274775, 1 specimen, Democratic Republic of Congo, Mai-Ndombe Province, Lomomo River near confluence with Mfimi.

Phenacogrammus aurantiacus: AMNH 274780, 3 specimens, 2CT, Democratic Republic of Congo, Mai-Ndombe Province, Ndzaa River. – AMNH 2474779, 3 specimens, Democratic Republic of Congo, Mai-Ndombe Province, Mfimi River around Nioki Port. – AMNH 260346, 5 specimens, Central African Republic, Dzanga-Sangha Protected Area. – AMNH 262366, 1 specimen, Gabon, Ogowe-Ivindo, WCTS Forestry Concession, Site 2. – AMNH 274861, 4 specimens, Republic of Congo, Kouyou River at Owando, 1 hour upstream from the bridge.

Phenacogrammus deheyni: 244045, 2 specimens, 1CT, Democratic Republic of Congo, Salonga National Park, Salonga River at Bokuma. – AMNH 244047, 1 specimen, 1CT, Democratic Republic of Congo, Salonga National Park, Yenge River at Boyenga. – AMNH 252208, 2 specimens, 1CT, Democratic Republic of Congo, Salonga National Park, Bionga Bionga. – MRAC 57789, holotype, Mumbia, Territory of Lisala. – AMNH 252277, 3 specimens, Democratic Republic of Congo, Mpongo stream ca. 2 km from Luilaka River. – AMNH 252194, 1 specimen, Democratic Republic of Congo, Ta’Simon River at Boaugi.

Phenacogrammus interruptus: AMNH 274784, 2 specimens, 2CT, Democratic Republic of Congo, Mai-Ndombe Province, Lomomo River near Kilako. – AMNH 256217, 3 specimens, 1C&S, Democratic Republic of Congo, Kwilu River at Kwilu beach. – AMNH 268936, 1 specimen, Democratic Republic of Congo, Kinshasa Province, Pool Malebo at Kinkole. – MRAC 817, lectotype, Stanley Pool. – CU 89868, 1 specimen, Republic of Congo, Lekoli River, Odzala National Park. – AMNH 247289, 1 specimen, Democratic Republic of Congo, Kongo Central Province, rockes below CO8-028 near Bulu. – AMNH 233442, 1 specimen, Aquarium purchase, no locality data.

Phenacogrammus major: AMNH 236514, 1 specimen, 1CT, Cameroon, Pont So’o at confluence of the So’o and Fala Rivers.

Phenacogrammus polli: AMNH 240816, 3 specimens, 2CT, Democratic Republic of Congo, Salonga National Park, Luilaka River at Nkombe-Dunda. – AMNH 273785, 4 specimens, Democratic Republic of Congo, Salonga National Park, Luilaka River at Ifumu. – AMNH 240793, 5 specimens, 2C&S, Democratic Republic of Congo, Salonga National Park, Luilaka River at Boangi Village. – MRAC 125500, holotype, River Lomela at Lomela.

Phenacogrammus taeniatus: ZSM 36901, Cameroon, Southeast Province, Sangha River at Libongo.

Phenacogrammus urotaenia: AMNH 253971, 5 specimens, 1CT, Republic of Congo, Kouilou Province, Lebayi River. – AMNH 262979, 5 specimens, 1C&S, Gabon, Ogowe-Ivindo, small affluent of the Gniabale River. – AMNH 262365, 1 specimen, Gabon, Ogowe-Ivindo, WCTS Forestry Concession, Site 2.

Figure 6. A. Phenacogrammus concolor, new species (holotype, AMNH 274786). B. Posterior neurocranial and anterior axial anatomy rendered from µCT scans; arrow indicates location of anterior expansion of the first pleural rib. C. Premaxillary and dentary tooth insertion and cuspidation rendered from µCT scan.

DISCUSSION

In a recent survey of the Mfimi River, collections were made principally at sites along the main channel, but some peripheral habitats were also sampled and among these were a small collection made in the Ndzaa River. Although a full accounting of fish diversity and distribution throughout the region remains to be made, preliminary data indicate that small left bank tributaries of the Mfimi-Lukenie appear to harbor a high proportion of the region’s fish diversity (Stiassny et al., personal obs.). Although just two collections were made in the Ndzaa, and despite its relatively small size, 27 species are recorded, of which nine were not collected elsewhere within the basin (indicated in bold font, table 1), and among these were the two new species of Phenacogrammus described here. These first ichthyological collections in the Ndzaa hint at the potential for additional undescribed diversity in these affluent small stream habitats. Anthropogenic impacts in Lake Mai-Ndombe and along the Mfimi-Lukenie main channel include poorly regulated fisheries, extensive clearance of seasonally inundated forests for rice cultivation and logging, and all are increasingly impacting the aquatic system (Thieme et al., 2005; Stiassny et al., 2011). Currently such anthropogenic impacts appear to be minimal in many of the smaller southern tributaries, presenting an opportunity to fully document the ichthyofaunal diversity of the region while providing baseline data for focused conservation efforts.

The genus Phenacogrammus is one of the more poorly studied of alestid clades, and studies that have investigated phylogenetic relationships within the Alestidae, or more inclusively within the Characoidei, have been based exclusively on molecular data and incorporated few, if any, Phenacogrammus species (e.g., Calcagnotto et al, 2005; Arroyave and Stiassny, 2011; Melo et al., 2021), leaving generic intrarelationships unresolved. Our preliminary investigation of morphological variation within the genus recognizes a peculiar expansion of first pleural rib present uniquely in P. flexus, P. concolor, and P. deheyni, a feature interpreted here as a synapomorphy diagnostic of this small central Congolese subgroup. In terms of overall phenetic similarity, P. concolor is closer to P. deheyni than to P. flexus, but additional morphological and molecular data are needed to definitively resolve these relationships, and a comprehensive taxonomic revision is necessary to properly delineate all species within the genus.

As noted previously P. flexus and P. concolor are currently known only from their type locality in the Ndzaa River, however a preliminary SDM (species distribution modelling) analysis conducted in Wallace, a modular, R-based platform for modeling of species niches and distributions (Kass et al. 2018), predicts the presence of highly suitable habitat for both species in the nearby headwaters of the Ulanzaa River basin (fig. 1). Future surveys planned in both the Ndzaa and Ulanzaa catchments will help test this prediction and provide additional data for modelling these and other species distributions throughout this poorly documented, yet potentially species-rich region.

ACKNOWLEDGMENTS

We thank the fishing communities of Kutu and surrounding villages for their help with this project. For logistical support and the issuance of collection and exportation permits we are grateful to the Ministère de l’Agriculture et du Développement Rural, Sécretariat Général de l’Agriculture, Pêche et Elevage, Direction des Pêches, and the Université de Kinshasa, Cabinet du Recteur, Democratic Republic of Congo. Zahra Alkaifi and Laraub Tariq (York College, CUNY) are gratefully acknowledged for their help with DNA extractions and preliminary PCR during an undergraduate internship supervised by S. Elizabeth Alter. Our thanks to the collections staff at the AMNH (Radford Arrindell, Tom Vigliotta, and Chloe Lewis) for accessioning and cataloging materials from the Mfimi basin collections. We are grateful to Jairo Arroyave (Universidad Nacional Autónoma de México), who provided some COI data, and to Ulrich Schliewen and Dirk Neumann (Zoologische Staatssammlung, Munich), and Casey Dillman (Cornell University Museum of Vertebrates, Ithaca) for the gift of tissues for use in the present study. Kimberly Bernotas (AMNH) is acknowledged for her help with morphometric and meristic analyses. Financial support for the study was provided by the US National Science Foundation (DEB 1655227) and the AMNH Axelrod Research curatorship. Finally, our thanks to John Sullivan and an anonymous reviewer for helpful input.

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1 American Museum of Natural History, Department of Ichthyology, New York.

2 California State University, Monterey Bay, Biology & Chemistry Department, Seaside, CA; American Museum of Natural History, Department of Ichthyology, New York.

3 University of Kinshasa, Department of Biology, Democratic Republic of Congo.

4 Richard Gilder Graduate School, American Museum of Natural History, New York; American Museum of Natural History, Department of Ichthyology, New York; University of Kinshasa, Department of Biology, Democratic Republic of Congo.