A new species of the freshwater crab genus Potamonemus Cumberlidge & Clark, 1992 (Crustacea, Potamonautidae) endemic to the forested highlands of southwestern Cameroon, Central Africa

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. Potamonemusmansp. 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.

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.

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;
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 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;. 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 bran-chiostegal 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   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).
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. 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 Mvogo Ndongo et al. 2017a, d, c, 2018, 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 the forested highlands and lowlands of southwestern Cameroon (extent of occurrence (EOO) 43,291 km 2 ). 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 km 2 ) 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.
Key to the species of Potamonemus Cumberlidge & Clark, 1992 1 Dactylus of major cheliped highly arched (Fig. 5a)  . 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.