Potamonemus Cumberlidge and Clark, 1992

Cumberlidge, Neil, Mvogo Ndongo, Pierre A., Clark, Paul F. & Daniels, Savel R., 2019, A new genus for the freshwater crab Potamonemus asylos Cumberlidgeı 1993 ı (Brachyura: Potamoidea: Potamonautidae) from Cameroonı Central Africaı with a key to the genera of the Potamonautinae, Journal of Natural History 53 (11), pp. 659-676 : 667-674

Potamonemus Cumberlidge and Clark, 1992

( Figures 2 (a,b), 3(a,b), 4(b,c), 5(c – f), (h,i), 6(c – f), 7(b,c,e,f) and 8(b,c,e,f,h,i))

Potamonemus Cumberlidge and Clark, 1992: 49 , figs 1 – 3, tables 1 – 2, pl. 1; Cumberlidge, 1999: 208 – 209, fig. 64D

Rediagnosis

Mandibular palp 2-segmented, lacking flap at junction between segments ( Figure 8 (h,i)); G1 TA slim, curving outward, long (TA/SS 0.63), lacking setae, tapering to pointed tip ( Figure 7 (b,c,e,f)); G2 TA distinctly shortened (TA/SS 0.13, Figure 8 (b,c)).

Description

See Cumberlidge and Clark (1992) and Cumberlidge (1999). The carapace proportions of all known specimens of P. mambilorum have been recalculated in the present study (CW/FW 3.08, CW/FW 2.22, and CW/FW 0.96), and the relative lengths of the TAs of G1 and G2 of P. mambilorum (TA/SS of 0.63 and 0.13 respectively) are presented here for the first time.

Type species

Potamonemus mambilorum Cumberlidge and Clark, 1992 , by original designation.

Type locality

Somié, Cameroon.

Species included

Potamonemus mambilorum and P. sachsi Cumberlidge, 1993 .

Distribution

Potamonemus is endemic to the rainforest zone of south-western Cameroon and southeastern Nigeria ( Cumberlidge 1993, 1999).

Remarks

The carapace proportions of all known specimens of P. sachsi have been recalculated in the present study (see above), as have the relative lengths of the terminal articles of G1 and G2 of P. sachsi , and these characters can be used to distinguish the two species of Potamonemus from B. asylos comb. nov.

Biogeography

The freshwater crab fauna of Cameroon is dominated by species of Sudanonautes , and is most diverse in south-west Cameroon where there are two endemic genera ( Louisea and Buea gen. nov.), two species of Potamonemus , and a single species of Potamonautes (N. Cumberlidge unpub. data). Biogeographically, the affinities of the freshwater crabs of Cameroon lie with those of its neighbouring countries ( Nigeria, Central African Republic, Congo, Gabon, and Equatorial Guinea /Bioko) which occupy a forested part of Africa to the north of the Congo River that is bounded by the north-west coastline of Central Africa. The freshwater crab fauna of this part of Africa is dominated by species of Sudanonautes and is remarkable for the almost complete absence of species Potamonautes (Cumberlidge 1999) , which is the most species-rich and most widely distributed genus in sub-Saharan Africa (N. Cumberlidge, unpub. data). The freshwater crab fauna of Cameroon is unusually diverse (15 species in five genera), especially given the relatively small size of this Central African country (475,440 km 2). This species richness may be attributed to the abundance of stable lowland and highland equatorial forests and the numerous year-round water courses that are sustained over historical time by the high rainfall totals recorded for this part of Africa (Cumberlidge 1999; Mvogo Ndongo et al. 2017a; 2017b, 2017c, 2018) ( Table 1 ). The present study underlines the importance of Cameroon as a biodiversity hotspot because it has the highest number of freshwater crab genera in the whole of the African continent (Cumberlidge 1999; Daniels et al. 2015; Mvogo Ndongo et al. 2017a, 2017b, 2017c, 2018), and only Madagascar (with seven genera) is more diverse in the Afrotropical region as a whole ( Cumberlidge and Sternberg 2002; Cumberlidge et al. 2017).

Amended key to the genera of the African Potamonautinae

1 The recognition of the new genus and the re-diagnosis of the reorganised Potamonemus make it necessary to provide an amended key to the genera of the African Potamonautinae .

1a Carapace frontal margin lined by small teeth or distinct granules; external angles of front marked by either sharp forward-pointing spine or small granule; triangular downward-pointing tooth beneath each external angle … ............ ........... Platythelphusa

1b Carapace frontal margin smooth, external angles of front smooth (lacking spine, granule, or downward-pointing tooth) ................................................................................ 2

2a Carapace outline subhexagonal; outer margin of cheliped carpus with 2 or 3 sharp teeth; carpi and propodi of p2 – p5 with marginal spines................................ Erimetopus

2b Carapace outline transversely oval; outer margin of carpus of cheliped smooth, lacking teeth; carpi and propodi of p2 – p5 smooth, lacking marginal spines .... 3 3a G1 TA curving inwards towards medial line................................................ Liberonautes 3b G1 TA either straight or curving outwards away from medial line......................... 4

4a Third maxilliped exopod with long flagellum ................................................................... 5 4b Third maxilliped exopod lacking flagellum ........................................................................ 6

5a Intermediate tooth on anterolateral margin between exorbital and epibranchial teeth .......................................................................................................................... Sudanonautes

5b No intermediate tooth on anterolateral margin between exorbital and epibran- chial teeth................................................................................................................ Potamonautes

6a G1 TA long (TA/SS 0.63 – 0.86), curving outwards, ending in pointed tip; G2 TA much shorter (TA/SS 0.13 – 0.23) ............................................................................................. 7

6b G1 TA weakly S-shaped, tube-like, ending in rounded tip; G2 TA very long, flagellum- like (TA/SS 0.64).......................................................................................................................... Louisea

7a G1 TA long (TA/SS 0.86), broadened in mid-section, with long marginal setae......... ........................................................................................................................................... Buea gen. nov.

7b G1 TA short (TA/SS 0.63), slim along entire length, lacking long marginal setae..... ............................................................................................................................................. Potamonemus

Figures

Figure 2. Potamonemus mambilorum Cumberlidge and Clark, 1992, holotype, adult ♂, CW 34.5 mm, Somié, Cameroon (NHM 1991:183). (a) Whole animal, dorsal view; (b) whole animal, ventral view. Photographs by Phillip Crabb, NHM, London. Scale bar: a = 21 mm; b = 28 mm.

Figure 7. Right G1, ventral view (a–c) and dorsal view (d–f). (a,d) Buea asylos (Cumberlidge, 1993) comb. nov., holotype, adult ♂ CW 27.6 mm, between Buea and Kumba, Cameroon, total G1 length 5.9 mm (TA 2.7 mm, SS 3.2 mm) (NHM 1994.587); (b,e) Potamonemus mambilorum Cumberlidge and Clark, 1992, holotype, adult ♂, CW 34.5 mm, Somié, Cameroon, total G1 length 9.7 mm (TA 3.6 mm, SS 6.1 mm) (NHM 1991:183); (c,f) P. sachsi Cumberlidge, 1993, adult ♂, CW 31.6 mm, Obudu Plateau, Nigeria), total G1 length 9.0 mm (TA 3.4 mm, SS 5.6 mm) (NMU 8.IV.1983A). Scale bars: 2.0 mm.

Figure 8. Right G2 ventral view (a–c); left third maxilliped, frontal view (d–f); left mandible, frontal view (g–i). (a) Buea asylos (Cumberlidge, 1993) comb. nov., holotype, adult ♂ CW 27.6 mm, between Buea and Kumba, Cameroon, total G2 length 4.3 mm (TA 0.9 mm, SS 3.4 mm, NHM 1994.587); (b) Potamonemus mambilorum Cumberlidge and Clark, 1992, holotype, adult ♂, CW 34.5 mm, Somié, Cameroon, total length G2 5.3 mm (TA 0.9 mm, SS 4.4 mm, NHM 1991:183); (c) P. sachsi Cumberlidge, 1993, adult ♂, CW 31.6 mm, Obudu Plateau, Nigeria, total length G2 6.8 mm (TA 0.8 mm, SS 6.0 mm, NMU 8.IV.1983A), (d) Buea asylos (Cumberlidge, 1993) comb. nov., holotype, adult ♂ CW 27.6 mm, between Buea and Kumba, Cameroon (NHM 1994.587); (e) P. mambilorum, holotype, adult ♂, CW 34.5 mm, Somié, Cameroon (NHM 1991:183); (f) P. sachsi, adult ♂, CW 31.6 mm, Obudu Plateau, Nigeria (NMU 8.IV.1983A); (g) Buea asylos (Cumberlidge, 1993) comb. nov., holotype, adult ♂ CW 27.6 mm, between Buea and Kumba, Cameroon (NHM 1994.587); (h) Potamonemus mambilorum Cumberlidge and Clark, 1992, holotype, adult ♂, CW 34.5 mm, Somié, Cameroon (NHM 1991:183); (i) P. sachsi Cumberlidge, 1993, adult ♂, CW 31.6 mm, Obudu Plateau, Nigeria (NMU 8.IV.1983A). Figures (a–i) not shown actual size; the size of these structures has been adjusted for comparative purposes. Scale bar: a = 1.57 mm; b = 1.76 mm; c = 1.94 mm; d = 5.5 mm; e = 5.9 mm; f = 4.4 mm; g = 1.6 mm; h = 2.4 mm; i = 2.3 mm.