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A new species of the freshwater crab genus Potamonemus Cumberlidge & Clark, 1992 (Crustacea, Potamonautidae) endemic to the forested highlands of southwestern Cameroon, Central Africa
expand article infoPierre A. Mvogo Ndongo§, Thomas von Rintelen§, Neil Cumberlidge|
‡ Université de Douala à Yabassi, Douala-Bassa, Cameroon
§ Museum für Naturkunde, Berlin, Germany
| Northern Michigan University, Marquette, United States of America
Open Access

Abstract

A new species of freshwater crab of the genus Potamonemus Cumberlidge & Clark, 1992 is described from Mount Manengouba Reserve and Bakossi National Park in the tropical rainforests of southwestern Cameroon, Central Africa. Potamonemus man sp. nov. is recognized by characters of the carapace and chelipeds. In addition, a phylogenetic analysis based on partial sequences of three mitochondrial DNA genes (COI, 12S rRNA, and 16S rRNA) that included representatives of all other freshwater crab genera found in Cameroon recovered each of the new species as a distinct lineage. A diagnosis and illustrations of the new species are provided, and it is compared to the other species of Potamonemus. Brief notes are provided on the ecology of the new species and the two other species of Potamonemus. An identification key to the species of Potamonemus is provided. The conservation status of the genus is discussed.

Keywords

Afrotropical region, conservation, identification key, mtDNA, taxonomy

Introduction

Potamonemus Cumberlidge & Clark, 1992 is one of five genera of freshwater crabs currently known from Cameroon. The other four species are Buea Cumberlidge, Mvogo Ndongo, Clark & Daniels, 2019, Louisea Cumberlidge, 1994, Potamonautes MacLeay, 1838, and Sudanonautes Bott, 1955 (Cumberlidge 1987, 1989, 1993a, b, c, 1994a, b; 1999; Cumberlidge and Boyko 2001; Cumberlidge and Clark 1992; Cumberlidge et al. 2019; Mvogo Ndongo et al. 2017a, b, c, 2018, 2019, 2020). Potamonemus was originally established as a monotypic genus for P. mambilorum Cumberlidge & Clark, 1992, and in the following year two more species were described, namely P. asylos Cumberlidge, 1993 and P. sachsi Cumberlidge, 1993. Subsequent phylogenetic analyses of the Afrotropical freshwater crab fauna (Daniels et al. 2006, 2015) supported the close relationship between P. mambilorum and P. sachsi but indicated that P. asylos formed a separate genetic lineage from these two species. Recently, P. asylos was assigned to a new genus, Buea Cumberlidge, Mvogo Ndongo, Clark & Daniels, 2019. Mvogo Ndongo et al. (2020) recently described three additional Buea species, including B. bangem Mvogo Ndongo, von Rintelen, Tomedi-Tabi & Cumberlidge, 2020, B. mundemba Mvogo Ndongo, von Rintelen & Cumberlidge in Mvogo Ndongo, von Rintelen, Tomedi-Tabi and Cumberlidge 2020, and B. nlonako Mvogo Ndongo, von Rintelen & Cumberlidge in Mvogo Ndongo, von Rintelen, Tomedi-Tabi and Cumberlidge 2020.

Cumberlidge et al. (2019) and Mvogo Ndongo et al. (2020) established that Buea is endemic to southwestern Cameroon and that Potamonemus is a more widely distributed genus, with a range including southwestern Cameroon and eastern Nigeria. Cumberlidge et al. (2019) revised the diagnostic characters of Potamonemus as a 2-segmented mandibular palp lacking a lobe or anterior flap at the junction between the two segments, a G1 with a slim, outward-curving and elongated TA (TA/SS 0.63) that lacks marginal setae and tapers to a pointed tip, and a G2 with a remarkably short TA (TA/SS 0.13). The three protected areas surveyed in the present study for freshwater decapods are located in a region of southwestern Cameroon which has been recognised as a biodiversity hotspot for several other freshwater taxa.

Extensive systematic surveys carried out from 2017 to 2020 in the lowland and upland zones of the tropical rainforests of southwestern Cameroon resulted in the collection of several new taxa, including a new species of Potamonemus. The purpose of the present study is to describe this new species using an integrative approach based on morphological characters and molecular genetic data and to provide a key to the three species now assigned to this genus. The molecular analysis based on three partial mitochondrial genes (COI, 12S rRNA, and 16S rRNA) also recovers the three species as separate genetic lineages within Potamonemus. For all the species treated in this study we also provide notes on their ecology and conservation.

Materials and methods

Sampling

Field surveys of freshwater decapods were conducted in 2017 at Mount Manengouba Reserve and Bakossi National Park, and in 2018 and 2020 at Nlonako Ecological Reserve. Survey transects were made in each study area. Crabs were collected from small rivers using nylon fishnets and dip nets, and near small permanent streams where crabs were found in puddles, under fallen leaves, under stones, and in burrows. The amount of disturbance of the habitat and the various threats to freshwater organisms, including to freshwater crabs, were evaluated during structured discussions with local people.

Morphological analyses

All measurements (in mm) were taken with digital callipers. The terminology used follows Cumberlidge (1999), and the classification follows Ng et al. (2008). Characters of the gonopods, carapace, thoracic sternum, chelipeds, third maxillipeds, and mandibles were examined in detail, and photographs were taken using a Leica microscope (model Z16A POA), LAS V4, and Helicon Focus 6.7.1 software. Post processing of the images was undertaken using Adobe Photoshop CC5 and Photo Impact. The newly collected specimens were deposited in the Museum für Naturkunde, Berlin, Germany (ZMB). Other material is deposited in the Institute of Fisheries and Aquatic Sciences, University of Douala at Yabassi (IFAS).

Abbreviations used

A pleonal (abdominal) segment or pleomere;

A5/A6 sulci between adjacent pleomeres;

a.s.l. above sea level;

CW carapace width measured at widest point;

CL carapace length measured along medial line from anterior to posterior margin;

CH carapace height measured at maximum height of cephalothorax;

E episternite;

FW front width measured along anterior frontal margin between inner angles of orbits;

G1 male first gonopod;

G2 male second gonopod;

P2–5 pereiopods 2–5 or ambulatory legs 1–4;

SS subterminal segment of G1 or G2;

S4/E4 (S4/E4, S5/E5, S6/E6, S7/E7) episternal sulci between adjacent thoracic sternites and episternites;

S thoracic sternite;

S1/S2 (or S2/S3, S4/S5, S5/S6, S6/S7) sternal sulci between adjacent thoracic sternites;

TA terminal article of G1 or G2;

TS terminal segment of mandibular palp.

Details for DNA extraction, DNA sequencing, PCR, and molecular phylogenetic analyses are given by Mvogo Ndongo et al. (2019, 2020). All sequences used in this study are given in Table 1.

Systematic account

Infraorder Brachyura Latreille, 1802

Superfamily Potamoidea Ortmann, 1896

Family Potamonautidae Bott, 1970

Subfamily Potamonautinae Bott, 1970

Potamonemus man sp. nov.

Figures 1d, 2d, 3d, 4d, 5g, h, l, 6j–l, 7d, h Common name: Man Lake freshwater crab

Holotype

Adult ♂ (CW 24.51 mm, CL 17.09 mm, CH 9.62 mm, FW 7.62 mm), Cameroon, Southwest Region, Mount Manengouba Ecological Reserve, Man Lake, Mount Manengouba (5.02414, 9.82142), 1,958 m a.s.l., 14 March 2017, coll. P.A. Mvogo Ndongo (ZMB Crust. 30320).

Paratypes

1 adult ♂ (CW 21.37 mm, CL 15.61 mm, CH 8.45 mm, FW 6.69 mm), 1 adult ♀ (CW 23.55 mm, CL 16.87 mm, CH 10.09 mm, FW 7.60 mm), Cameroon, Southwest Region, Mount Manengouba Ecological Reserve, Man Lake, Mount Manengouba (5.03604, 9.82906), 1,958 m a.s.l., 14 March 2017, coll. P.A. Mvogo Ndongo (ZMB Crust. 30324). 2 adult ♂ (CW 20.12 mm, CL 14.64 mm, CH 7.92 mm, FW 6.76 mm; CW 20.40 mm, CL 14.73 mm, CH 8.16 mm, FW 6.63 mm); 3 subadult ♂ (CW 18.46 mm, CL 13.89 mm, CH 7.40 mm, FW 6.16 mm; CW 19.38 mm, CL 13.78 mm, CH 7.78 mm, FW 6.63 mm; CW 14.05 mm, CL 10.67 mm, CH 5.84 mm, FW 5.30 mm), Cameroon, Southwest Region, Mount Manengouba Ecological Reserve, Man Lake, Mount Manengouba (5.03604, 9.82906), 1,958 m a.s.l., 14 March 2017, coll. P.A. Mvogo Ndongo (IFAS-017); 4 adult ♀ (CW 19.39 mm, CL 14.07 mm, CH 7.74 mm, FW 6.48 mm; CW 17.37 mm, CL 12.46 mm, CH 6.94 mm, FW 6.33 mm; CW 16.88 mm, CL 12.06 mm, CH 6.36 mm, FW 5.20 mm), Cameroon, Mount Manengouba Ecological Reserve, Man Lake, Mount Manengouba (5.03604, 9.82906), 1,958 m a.s.l., 14 March 2017, coll. P.A. Mvogo Ndongo (IFAS-018).

Other material

Bakossi National Park (Figs 1c, 2c, 3c, 4c, 5e, f, j, 6g–i, 7c, g). 1 adult ♂ (CW 30.41 mm, CL 20.57 mm, CH 12.50 mm, FW 9.32 mm), 1 adult ♀ (CW 27.48 mm, CL 20.06 mm, CH 11.31 mm, FW 8.19 mm), Cameroon, Southwest Region, Bakossi National Park (5.031083, 9.687528), 1,253 m a.s.l., 15 March 2017, coll. P. A. Mvogo Ndongo (ZMB Crust. 30328). 5 adult ♂ (CW 27.61 mm, CL 19.37mm, CH 11.39 mm, FW 8.65 mm; CW 26.54 mm, CL 19.09 mm, CH 10.80 mm, FW 9.13 mm; CW 25.55 mm, CL 18.68 mm, CH 10.58 mm, FW 8.41 mm; CW 25.3 mm, CL 18.05 mm, CH 10.70, FW 8.41 mm) (IFAS-014); 2 subadult ♂ (CW 22.17 mm, CL 15.76 mm, CH 9.04 mm, FW 7.35 mm; CW 22.02 mm, CL 15.67 mm, CH 9.06 mm, FW 7.49 mm; CW 22.04 mm, CL 16.00 mm, CH 9.22 mm, FW 7.65 mm; CW 21.35 mm, CL 15.19 mm, CH 8.76 mm, FW 6.81 mm), Cameroon, Southwest Region, Bakossi National Park (5.031083, 9.687528), 1,248 m a.s.l., 15 March 2017, coll. P.A. Mvogo Ndongo (IFAS-015). 2 subadult ♀ (CW 23.76 mm, CL 17.57 mm, CH 9.88 mm, FW 7.68 mm; CW 23.31 mm, CL 16.63 mm, CH 9.08 mm, FW 7.59 mm), Cameroon, Southwest Region, Bakossi National Park (05.031083, 9.687528), 1,248 m a.s.l., 15 March 2017, coll. P.A. Mvogo Ndongo (IFAS-016).

Table 1.

Species and specimens of Louisea, Buea, Potamonemus, Sudanonautes, and Potamonautes and the outgroup taxa included in the molecular analysis. All measurements in mm.

Species Locality Museum number Reference study GenBank accession number
CO1 12S rRNA 16S rRNA
Louisea nkongsamba (CW 20.0) Mt. Nlonako ZMB Crust. 31618 Mvogo Ndongo et al. 2019 MN188072 MN217386 MN217393
Louisea balssi (CW 14.8) Manengouba ZMB Crust.29628 Mvogo Ndongo et al. 2019 MN188070 MN217384 MN217391
Louisea edeaensis (CW 17.2) Lake Ossa LZUY 15-3 (T351-30) Mvogo Ndongo et al. 2017c KY964474 KY964479 KY964472
Buea mundemba. (CW 26.2) Korup N. P. ZMB Crust. 30321 Mvogo Ndongo et al. 2019 MN188069 MN217388 MN217396
Buea bangem (CW 26.5) Bakossi N.P. IFAS-010 Mvogo Ndongo et al. 2020 MT019691 MT021447
Buea asylos (CW 25.4) Buea and Kumba NHM 1994.588-591 Daniels et al. 2015 KP640489 KP640410 KP640453
Potamonemus man sp. nov Bakossi N. P. ZMB Crust. 30328 Mvogo Ndongo et al. 2019 MN188067 MN217390 MN217398
Potamonemus man sp. nov Mt. Manengouba R. ZMB Crust. 30320 Present study GenBank (submitted by the first author)
Potamonemus mambilorum Southwest Cameroon NHM 1991.183 Daniels et al. 2015 KP640409 KP640452
Potamonemus sachsi Southwest Cameroon NMU09.04.1983 Daniels et al. 2015 AY803490 AY803530
Potamonautes idjiwiensis D. R. Congo SAM A78437 Daniels et al. 2015 KP640481 KP640402 KP640446
Potamonautes obesus Tanzania Unaccessioned Daniels et al. 2015 AY803647 AY803497 AY803537
Afrithelphusa monodosa Guinea NMU 25.IV.2005.C Daniels et al. 2015 KP640469 KP640386 KP640430
Globonautes macropus Liberia NMU VII. 1988 Daniels et al. 2015 KP640391 KP640435
Sudanonautes aubryi Cameroon LZUY-06 Mvogo Ndongo et al. 2017c KY069938 KY964475 KY069950
Sudanonautes tiko Cameroon ZMB Crust.29628 Mvogo Ndongo et al. 2017c KY069941 KY964476 KY069954

Diagnosis

Carapace anterior surface smooth except for faint urogastric groove (Fig. 1d). Broad epimeral (longitudinal) suture on carapace sidewall (branchiostegite) dividing carapace sidewall into 2 regions, vertical (pleural) groove lacking (Fig. 1d). Outer lower margin of cheliped merus lined by small, blunt teeth, inner lower margin smooth, distal meral tooth distinct, pointed (Fig. 3d). Major chela dactylus straight (not arched) (Fig. 5g). Sternal sulcus S2/S3 completely traversing sternum; S3/S4 incomplete, reduced to 2 short, distinct notches on each side of sternum (Fig. 3d). G1 with long TA (TA/SS 0.66), slim, curving outward, lacking marginal setae, tapering to pointed tip; G2 TA remarkably short (TA/SS 0.13) (Fig. 6j, l). A small species, mature between CWs 20–25 mm.

Figure 1. 

Species of Potamonemus from southwestern Cameroon, whole animal, dorsal view a largest adult male (CW 29 mm) of P. mambilorum from Mount Nlonako (ZMB Crust. 32428) b adult male (CW 28 mm) of P. mambilorum from small stream on Mount Manengouba (ZMB Crust. 30326) c adult male (CW 31 mm) of P. man sp. nov. from Bakossi National Park (ZMB Crust. 30328) d adult male, holotype (CW 25 mm) of P. man sp. nov. from Man Lake, Mount Manengouba (ZMB Crust. 30320). Scale bars: 15 mm (a), 16 mm (b), 17 mm (c), 16 mm (d).

Description

Carapace ovoid, medium height (CH/FW 1.17), wide (CW/FW 3.19); carapace surface smooth; postfrontal crest distinct, completely traversing carapace, lateral ends meeting anterolateral margins (Fig. 1d); exorbital tooth low, distinct; intermediate, epibranchial teeth each reduced to small granule (Fig. 4d); anterolateral margin behind epibranchial tooth smooth (Fig. 4d). Carapace branchiostegite with prominent epimeral suture dividing wall into subhepatic/suborbital, pterygostomial regions; vertical (pleural) suture faint (Fig. 3d). Sternal sulcus S2/S3 deep, completely traversing sternum; S3/S4 incomplete, reduced to 2 short, distinct notches on each side of sternum (Fig. 3d); margins of S3, S4 raised, broad (Fig. 3d); episternal sulci S4/E4, S5/E5, S6/E6 faint or missing, S7/E7 complete (Fig. 3d). Mandibular palp 2-segmented; medium-sized anterior lobe at junction between segment (0.25 × TS length; Fig. 7h). Third maxillipeds filling entire buccal cavern, except for transversely oval, efferent respiratory openings in superior lateral corners; ischium smooth, lacking vertical groove; exopod lacking flagellum (Fig. 7d).

Figure 2. 

Species of Potamonemus from southwestern Cameroon, dorsal view a largest adult male (CW 29 mm) of P. mambilorum from Mount Nlonako (ZMB Crust. 32428) b adult male (CW 28 mm) of P. mambilorum from small stream on Mount Manengouba (ZMB Crust. 30326) c adult male (CW 31 mm) of P. man sp. nov. from Bakossi National Park (ZMB Crust. 30328) d adult male, holotype (CW 25 mm) of P. man sp. nov. from Man Lake, Mount Manengouba (ZMB Crust. 30320). Scale bars: 13 mm (a), 13 mm (b), 14 mm (c), 11 mm (d).

Male chelipeds greatly unequal, right cheliped larger than left (Figs 1d, 5g, h). Movable finger (dactylus), fixed finger (pollex of propodus) of right (major) chela both slim, elongated; fixed finger with 3 large pointed teeth (2 proximal, 1 distal); movable finger relatively stout, straight (not highly arched), with 4 small but distinct teeth (2 proximal, 2 distal; Fig. 5g). Left (minor) chela dactylus, propodus smaller than right chela, with small teeth on occluding margins (Fig. 5h). Inner inferior margin of cheliped merus lined by small teeth, outer inferior margin smooth; distal meral tooth large, pointed (Fig. 5d). Cheliped carpus inner margin with large pointed distal tooth; proximal tooth much smaller, followed by granule (Fig. 5l). Ambulatory legs (P2–5) slender, P4 longest, P5 shortest; dactyli P2–5 tapering to point, each bearing rows of downward-pointing sharp bristles, P5 dactylus shortest (Fig. 1d).

Male pleon triangular, margins not indented and lacking setae (Fig. 3d). G1 with long TA (TA/SS 0.66), slim, curving outward, lacking marginal setae, tapering to pointed tip; G2 TA remarkably short (TA/SS 0.13) (Fig. 6j, l); G1 SS, broad in basal, midsection, distal quarter tapering sharply, narrow at junction with G1 TA (Fig. 6j, l). G2 TA extremely short (G2 TA/SS 0.3; Fig. 6k).

Adult female. Right and left chelipeds subequal. Fixed, movable fingers of chela interspersed with series of smaller acute teeth along their length. Pleon wide, covering entire sternum, reaching bases of coxae of P2–5; pleon with 6 free pleomeres (A1–6) becoming gradually wider proximally, telson wide, forming near semicircle.

Figure 3. 

Species of Potamonemus from southwestern Cameroon, ventral view a largest adult male (CW 29 mm) of P. mambilorum from Mount Nlonako (ZMB Crust. 32428) b adult male, holotype (CW 28 mm) of P. mambilorum from small stream on Mount Manengouba (ZMB Crust. 30326) c adult male (CW 31 mm) of P. man sp. nov. from Bakossi National Park (ZMB Crust. 30328) d adult male, holotype (CW 25 mm) of P. man sp. nov. from Man Lake, Mount Manengouba (ZMB Crust. 30320). Scale bars: 16 mm (a), 17 mm (b), 16 mm (c), 17 mm (d).

Size. Small species, CW in mature individuals ranging from 20.0–30.4 mm.

Colour in life. Dorsal carapace and all ambulatory legs dark brown, chelipeds red.

Type locality

Stream flowing into Man Lake, Mount Manengouba, in Manengouba Ecological Reserve, Southwest Region of Cameroon.

Etymology

The species is named for Man Lake, one of a pair of small lakes in the caldera at the summit of Mount Manengouba (the other lake being Woman Lake). The species epithet is used as a noun in apposition.

Habitat

At the Man Lake locality at the summit of Mount Manengouba the species is found in a small stream flowing into the lake, and it was also collected from a small stream in the Bakossi National Park. Both of these localities are located in rainforest habitat found along the Cameroon Volcanic Line, a 1,600 km long chain of volcanoes that stretches from the islands in the Gulf of Guinea to the mountains of eastern Nigeria and western Cameroon, including Mount Cameroon.

Remarks

The new species is assigned to Potamonemus because it conforms to the genus diagnosis (Cumberlidge and Clark 1992; Cumberlidge 1993c; Cumberlidge et al. 2019). Potamonemus man sp. nov. most closely resembles P. sachsi in that the dactylus of the major cheliped of both species is straight rather than highly arched. These two species can be distinguished from each other by the smooth carapace and branchiostegal sidewalls in P. man sp. nov. (Figs 1D, 2D) (vs. patches of short setae along the anterolateral and posterolateral margins of the carapace that continue around to the sidewalls in the subhepatic and pterygostomial regions of the branchiostegite in P. sachsi (Cumberlidge et al. 2019: fig. 4c)). The highly arched dactylus of the major cheliped of P. mambilorum distinguishes it from both P. man sp. nov. and P. sachsi. Finally, P. man sp. nov. can be distinguished from P. mambilorum and P. sachsi by the body size of adult specimens: the new species and P. sachsi are adult between CW 20–30 mm and CW 23–28 mm, respectively, while P. mambilorum is the largest species (adult at CW 29–38 mm).

Figure 4. 

Species of Potamonemus from southwestern Cameroon, frontal view a largest adult male (CW 29 mm) of P. mambilorum from Mount Nlonako (ZMB Crust. 32428) b adult male, holotype (CW 28 mm) of P. mambilorum from small stream on Mount Manengouba (ZMB Crust. 30326) c adult male (CW 31 mm) of P. man sp. nov. from Bakossi National Park (ZMB Crust. 30328) d adult male, holotype (CW 25 mm) of P. man sp. nov. from Man Lake, Mount Manengouba (ZMB Crust. 30320). Scale bars: 13 mm (a), 12 mm (b), 14 mm (c), 11 mm (d).

A phylogenetic tree (Fig. 8), based on 1,848 base pairs representing the combined partial sequences of three mtDNA markers (COI, 16S RNA, and 12S RNA), recovered three species of Potamonemus as a single clade with strong BI and ML confidence values (1/100 at this node). The three species of Potamonemus (P. mambilorum, P. man sp. nov., and P. sachsi) form an independent lineage within the subfamily Potamonautinae, and all are found in the same geographical area of Cameroon. The uncorrected p-distance between Potamonemus man sp. nov. and P. mambilorum is 2.2% for 12S RNA and 0.6% for 16S RNA, and between P. man sp. nov. and P. sachsi it is 4.5% for 12S RNA and 4.6% for 16S RNA.

Figure 5. 

Frontal view of the right and left chelae of adult males of species of Potamonemus from southwestern Cameroon a, b largest adult male (CW 29 mm) of P. mambilorum from Mount Nlonako (ZMB Crust. 32428) c, d adult male (CW 28 mm) of P. mambilorum from small stream on Mount Manengouba (ZMB Crust. 30326) e, f largest adult male, holotype (CW 31 mm) of P. man sp. nov. from Bakossi National Park (ZMB Crust. 30328) g, h adult male, holotype (CW 25 mm) of P. man sp. nov. from Man Lake, Mount Manengouba (ZMB Crust. 30320). Dorsal view of the right cheliped carpus of adult males of species of Potamonemus i, largest adult male (CW 29 mm) of P. mambilorum from Mount Nlonako (ZMB Crust. 32428) j adult male (CW 28 mm) of P. mambilorum from small stream on Mount Manengouba (ZMB Crust. 30326) k adult male (CW 31 mm) of P. man sp. nov. from Bakossi National Park (ZMB Crust. 30328) l adult male, holotype (CW 25 mm) of P. man sp. nov. from Man Lake, Mount Manengouba (ZMB Crust. 30320). Scale bars: 5 mm (a, c, g), 2.5 mm (b, d, f, h), 5 mm (i, j, k, l).

Conservation

Potamonemus man sp. nov. is found in an area of great conservation interest. The aquatic habitats of this species in the Mount Manengouba Ecological Reserve and in the Bakossi National Park are both in montane tropical rainforest in the Cameroon highlands, an area with a high biodiversity and a high rate of endemism, including freshwater crabs (Cumberlidge et al. 2019; Mvogo Ndongo et al. 2017a, d, c, 2018, 2019, 2020). In Mount Manengouba Ecological Reserve the small, forested stream near Man Lake where P. man sp. nov. lives also supports a sympatric population of the endangered freshwater crab Louisea balssi (Bott, 1959). Significantly, there are no reports of any species of invertebrates (molluscs, insects, crustaceans) or vertebrates (fish, amphibians, snakes, and birds) from Man Lake itself. This inhospitality to life may be related to the unusual green colour of its waters which may be due to the accumulation of lethal compounds, which may also be a potential danger to humans (see Mvogo Ndongo et al. 2018). Both the Mount Manengouba Ecological Reserve and the Bakossi National Park are under increasing pressure from growing nearby human populations and from the associated clearance of land for agriculture. As a result, despite being found in protected areas, the habitat of P. man sp. nov. is increasingly threatened by nearby intensive agricultural practices and forest destruction for firewood collection. In addition, the farmers encroaching on these habitats use agrochemicals and pesticides on their crops, and these pollutants eventually drain into the aquatic systems, potentially poisoning the freshwater communities (Mvogo Ndongo et al. 2018).

Figure 6. 

Dorsal view of left G1 (a, d, g, j) and ventral view of left G1 (c, f, i, l) of adult males of species of Potamonemus from southwestern Cameroon. a, c largest adult male (CW 29 mm) of P. mambilorum from Mount Nlonako (ZMB Crust. 32428) d, f adult male (CW 28 mm) of P. mambilorum from small stream on Mount Manengouba (ZMB Crust. 30326) g, i adult male (CW 31 mm) of P. man sp. nov. from Bakossi National Park (ZMB Crust. 30328) j, l adult male, holotype (CW 25 mm) of P. man sp. nov. from Man Lake, Mount Manengouba (ZMB Crust. 30320). Ventral view of G2 of adult males of species of Potamonemus from southwestern Cameroon b largest adult male (CW 29 mm) of P. mambilorum from Mount Nlonako (ZMB Crust. 32428) e adult male (CW 28 mm) of P. mambilorum from small stream on Mount Manengouba (ZMB Crust. 30326) h adult male (CW 31 mm) of P. man sp. nov. from Bakossi National Park (ZMB Crust. 30328) k adult male, holotype (CW 25 mm) of P. man sp. nov. from Man Lake, Mount Manengouba (ZMB Crust. 30320). Scale bars: 2 mm (a–k).

Potamonemus mambilorum Cumberlidge & Clark, 1992

Figures 1b, 2b, 3b, 4b, 5c, d, i, 6d–f, 7b, f

Material examined

6 adult ♂ (CW 29.05 mm, CL 21.17 mm, CH 12.10 mm, FW 8.85 mm; CW 29.56 mm, CL 21.35 mm, CH 12.34 mm, FW 9.12 mm; CW 29.16 mm, CL 20.70 mm, CH 12.00 mm, FW 9.12 mm; CW 28.93 mm, CL 20.69 mm, CH 11.85 mm, FW 9.94 mm; CW 26.74 mm, CL 19.62 mm, CH 11.32 mm, FW 9.63 mm; CW 26.74 mm, CL 19.62 mm, CH 11.32 mm, FW 9.63 mm). 2 adult ♀ (CW 27.06 mm, CL 19.76 mm, CH 12.45 mm, FW 8.34 mm; CW 26.68 mm, CL 19.06 mm, CH 11.03 mm, FW 7.72 mm); Cameroon, Littoral region, Mount Nlonako Ecological Reserve (4.891820, 9.984830), 900 m a.s.l., 26 May 2018, coll. P.A. Mvogo Ndongo (ZMB Crust. 32428). 1 adult ♂ (CW 28.00 mm, CL 19.10 mm, CH 11.37 mm, FW 8.56 mm); 1 adult ♀ (CW 28.36 mm, CL 20.00 mm, CH 10.27 mm, FW 7.79 mm; Southwest Region, Mount Manengouba Ecological Reserve, small stream around the mountain (ZMB Crust. 30326) (5.034920, 9.836150), 1,958 m asl, 14 March 2017, coll. P.A. Mvogo Ndongo.

Figure 7. 

Frontal view of the left third maxilliped of adult males of species of Potamonemus from southwestern Cameroon a largest adult male (CW 29 mm) of P. mambilorum from Mount Nlonako (ZMB Crust. 32428) b adult male (CW 28 mm) of P. mambilorum from small stream on Mount Manengouba (ZMB Crust. 30326) c adult male (CW 31 mm) of P. man sp. nov. from Bakossi National Park (ZMB Crust. 30328) d adult male, holotype (CW 25 mm) of P. man sp. nov. from Man Lake, Mount Manengouba (ZMB Crust. 30320). Frontal view of the left mandible of adult males of species of Potamonemus from southwestern Cameroon e largest adult male (CW 29 mm) of P. mambilorum from Mount Nlonako (ZMB Crust. 32428) f adult male (CW 28 mm) of P. mambilorum from small stream on Mount Manengouba (ZMB Crust. 30326) g adult male (CW 31 mm) of P. man sp. nov. from Bakossi National Park (ZMB Crust. 30328) h adult male, holotype (CW 25 mm) of P. man sp. nov. from Man Lake, Mount Manengouba (ZMB Crust. 30320). Scale bars: 5 mm (a–d), 2 mm (e–h).

Remarks

The distributional range of P. mambilorum is extended in this work by the discovery of populations in Mount Nlonako Ecological Reserve in the littoral region of Cameroon. Potamonemus mambilorum was previously known from seven localities in the forested highlands and lowlands of southwestern Cameroon (extent of occurrence (EOO) 43,291 km2). The conservation status of this species was assessed as Least Concern (Cumberlidge 2008a), but this was before the threats to the freshwater ecosystems of this part of Africa were brought to light. The conservation status of P. sachsi was assessed as Vulnerable, B1ab(iii)+2ab(iii), based on its distributional range that includes the Bamenda highlands in southwest Cameroon and the neighboring Obudu plateau in southeast Nigeria, which is continuous with the Bamenda highlands (EOO 24,219 km2) and perceived threats (Cumberlidge 2008b). The areas where P. mambilorum and P. sachsi occur are now known to be at risk from a number of anthropogenic threats, including deforestation, together with intensive and encroaching agricultural practices and firewood collection, as well as release of pollutants such as agrochemicals potentially affecting the eggs, hatchling-carrying female crabs, and other aquatic organisms.

Figure 8. 

Bayesian Inference (BI) tree topology for the freshwater crab taxa from Cameroon included in this study derived from mtDNA sequences corresponding to three loci (partial 16S rRNA, COI, and 12S rRNA genes). Bayesian Inference (BI) and ML statistical values (%) on the nodes indicate bootstrap support and posterior probabilities, respectively.

Key to the species of Potamonemus Cumberlidge & Clark, 1992

1 Dactylus of major cheliped highly arched (Fig. 5a) P. mambilorum
Dactylus of major cheliped either straight or only slightly concave 2
2 Carapace sidewalls in subhepatic and pterygostomial regions smooth P. man sp. nov.
Carapace sidewalls in subhepatic and pterygostomial regions with fields of short setae (Cumberlidge 1994: figs 3a, 4c) P. sachsi

Acknowledgements

We thank the Rufford Small Grant Foundation for funding the fieldwork in the South and Southwestern Regions of Cameroon, and the Museum für Naturkunde, Berlin (ZMB) for funding the first author during a research visit to Germany. We thank Dr. Paul F. Clark (Natural History Museum, London, UK) and Prof. Dr Christoph D. Schubart (University of Regensburg, Germany) for constructive comments in the earlier manuscript. Thanks are also due to the two anonymous reviewers and the Subject Editor for their important comments to improve the manuscript.

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