Two new species of freshwater crabs from the highlands of northern Uganda, East Africa and a redescription of Potamonautes amalerensis (Rathbun, 1935) stat. rev. from Mount Kadam (Brachyura: Potamoidea: Potamonautidae)

Two new species of potamonautid freshwater crabs are described from the Imatong Mountains and Mount Moroto in northern Uganda, East Africa, and a third highland species, Potamonautes amalerensis (Rathbun, 1935) stat. rev. from Mount Kadam is re-diagnosed based on examination of the holotype. All three species are endemic to a different mountain range and their collection localities indicate a distinct preference for higher altitudes. Diagnoses, illustrations and distribution maps are provided for these taxa, and they are compared to similar species from the region. The conservation status of all three species is discussed.


Introduction
Freshwater crabs from the highland regions of northern Uganda collected by Trefor R. Williams (formerly University of Liverpool, United Kingdom) and his colleagues in the 1960s were made available to the authors for examination.This material proved to include two new species that are described here based on a unique combination of adult male characters including the fi rst gonopod, carapace, thoracic sternum and chelipeds.The two new species were collected from streams and rivers draining the slopes of mountain ranges in the Acholi and Karamajo Provinces of northern Uganda during a survey aimed at a better understanding of the transmission of river blindness (Onchocerciasis) to humans (Barnley & Prentice 1958;McMahon et al. 1958;Hynes et al. 1961;Williams et al. 1964;Williams 1968Williams , 1991;;Crosskey 1990).The third species, Potamon (Geothelphusa) amalerensis Rathbun, 1935 stat. rev., is from Mount Kadam in the Karamoja Province of Uganda and was collected by Arthur Loveridge during the American Natural History Museum Expedition to Kenya andUganda in 1933-1934.This species is redescribed here as Potamonautes amalerensis (Rathbun, 1935) stat.rev.and a second locality is recorded.
Recently, Cumberlidge & Clark (2010) reported on four species of freshwater crabs from Mount Elgon, a highland region in southeast Uganda, south of Mount Kadem, one of the highland ranges included in the present study.The freshwater crab faunas of Uganda's northern and eastern highlands include highaltitude taxa that are typically small-bodied restricted-range species that are endemic to their mountain ranges, despite the relatively close proximity of some of these localities.
All of the freshwater crabs reported on here belong to Potamonautes MacLeay, 1838.Specimens of the two new species are deposited in the NHM, London, UK, while the holotype and paratypes of P. amalerensis stat.rev.are registered in the MCZ, Harvard University, Cambridge, MA, U.S.A.The re-description of P. amalerensis stat.rev. is based on examination of the type material and is necessary because it is still diffi cult for the non-specialist to distinguish between the many Potamonautes species from Uganda.This is due to an absence of modern taxonomic treatments, not least because the only available identifi cation keys (Rathbun 1921;Chace 1942;Bott 1955) are insuffi cient by modern standards and their systematics is dated.At present, the only way to make reliable identifi cations of freshwater crabs from this part of Africa is to refer to the original type series of all relevant taxa but these are held in different European and North American museums.Unfortunately, molecular data for the three species reported on here are not available, because all of this material was originally preserved in formalin.Diagnoses, illustrations, and distribution maps are provided for these taxa, and they are compared to similar species from the region.The conservation status of all three species is discussed.The addition of P. imatongensis sp.nov.and P. morotoensis sp.nov., and the recognition of a previously suppressed taxon, P. amalerensis stat.rev.raises to 13 the number of freshwater crabs known from Uganda (Cumberlidge & Clark 2010).

Material and methods
Measurements were made with digital calipers and are given in millimeters.The terminology is adapted from Cumberlidge (1999) and the higher classifi cation used here follows that of Ng et al. (2008).Line drawings were prepared using a Leica MZ 16 binocular microscope.The habitus photographs were taken with a digital camera in combination with a Leiz MZ 95 adapter.Post processing was undertaken using Adobe Photoshop CS5.

Etymology
The new species is named for the Imatong Mountains on the border between Uganda and South Sudan, which is the locality where this species was fi rst collected.

Size
Small species, adult size range CW 18.2-23 mm.

Colour
Preserved specimens uniformly light brown.

Distribution
The localities where this species is known to occur are in the Aringa River or its tributaries in northern Acholi Province of Uganda close to the border with South Sudan.The Aringa River is part of the Nile River drainage and fl ows north out of Uganda, eventually meeting the White Nile River in South Sudan (Fig. 10).The third locality from the 'Sudan (now South Sudan) / Uganda border at 7,000 feet (= 2,134 m asl)' is vague, but has been estimated here based possible places at that altitude in the Aringa River basin in the Imatong Mountains.

Ecology
This species is so far known only from high altitude localities, and has been collected from streams at over 1,500 m asl, and in the Aringa River at 2,134 m asl.

Vernacular name
The Imatong Mountains Crab.

Conservation status
An IUCN conservation assessment of Potamonautes imatongensis sp.nov.has not yet been carried out.

Remarks
Potamonautes imatongensis sp.nov. is superfi cially similar to several other Potamonautes species that share the following characters: a small body size at maturity (beginning at CW 18 mm), a complete postfrontal crest; completely reduced or absent exorbital and epibranchial teeth; smooth anterolateral margins immediately behind each epibranchial tooth; a third maxilliped ischium that lacks a visible sulcus; the fi rst tooth on the carpus of the cheliped that is strong and pointed; a distal meral tooth of the cheliped that is strong and pointed; and a slim, elongated, highly arched dactylus of the male major cheliped.East African crabs that share this suite of characters include P. mutandensis (Chace, 1942)  (southwest Uganda), P. berardi (Audouin, 1826) (Nile River basin), P. williamsi Cumberlidge & Clark, 2010 (Mount Elgon, Uganda), P. amalerensis (Rathbun, 1935) stat.rev.and P. morotoensis sp.nov.
Potamonautes imatongensis sp.nov.differs from P. mutandensis in that the second tooth on the carpus of the cheliped is weak, low and blunt (vs pointed and as large as the fi rst carpal tooth in P. mutandensis), the outer margins of sternal segment s4 are distinctly thickened (vs slim and not thickened in P. mutandensis; see Chace 1942;Cumberlidge & Meyer 2011), and the G1 terminal article of P. imatongensis sp.nov. is straight and cone-shaped and not directed only slightly outward (vs s-shaped, widened medially, and angled outward at a 45° angle to the longitudinal axis of the gonopod in P. mutandensis).
Potamonautes imatongensis sp.nov.differs from P. berardi in that the G1 terminal article is straight, cone-shaped and not directed outward (vs straight at fi rst, then bent sharply outward at 45° to the longitudinal axis in the mid-section in P. berardi) and the outer margins of thoracic sternite s4 are distinctly thickened (vs slender and not thickened in P. berardi; see Chace 1942;Cumberlidge & Meyer 2011).
Potamonautes imatongensis sp.nov.differs from P. williamsi from Mount Elgon, Uganda, in that the fi rst tooth on the carpus of the cheliped is strong and pointed (vs small and low in P. williamsi), thoracic sternal sulcus s3/s4 is deep at the edges and medially faint (vs s3/s4 is reduced to two short notches on the external margins of the sternite in P. williamsi) and the outer margins of thoracic sternite s4 are distinctly thickened (vs slender and not thickened in P. williamsi; see Cumberlidge & Clark 2010).
As for the species described here, P. imatongensis sp.nov.differs from P. morotoensis sp.nov. in that the G1 terminal article is straight, cone-shaped and tapering, with a long tube-like tip (vs straight at fi rst, bent outward at 45° to the longitudinal axis, and cone-shaped, tapering evenly to a down-turned tip in P. morotoensis sp.nov.), the fi rst tooth on the carpus of the cheliped is medium sized and pointed, and the second carpal tooth is small, broad and pointed (vs both teeth small and low in P. morotoensis sp.nov.) and the dactylus of the cheliped is highly arched, enclosing a widely oval interspace when closed (vs a cheliped dactylus that is only slightly arched and enclosing a long narrow interspace when closed in P. morotoensis sp.nov.).
Potamonautes imatongensis sp.nov.differs from P. amalerensis stat.rev. in that the G1 terminal article is straight and cone-shaped with a long tube-like tip (vs straight at fi rst, bent outward at 45° to the longitudinal axis and cone-shaped, tapering evenly to a pointed tip in P. amalerensis stat.rev.), the fi rst tooth on the carpus of the cheliped is medium sized and pointed, and the second carpal tooth is small, broad and pointed (vs a small fi rst carpal tooth and reduced granule-sized second carpal tooth in P. amalerensis stat.rev.), and the dactylus of the cheliped is highly arched, enclosing a widely oval interspace when closed (vs a cheliped dactylus that is only slightly arched and encloses a long narrow interspace when closed in P. amalerensis stat.rev.).raised anterior margin of sternoabdominal cavity; margins of s4 distinctly raised, thickened.Chelipeds of adult male unequal; movable fi nger (dactylus) and fi xed fi nger (propodus) of major cheliped of adult male slim, elongated, fi xed fi nger (propodus) with 2 large teeth proximally followed by series of small teeth; movable fi nger (dactylus) slightly arched, enclosing narrow rectangular interspace when closed.

Potamonautes morotoensis
Inferior margins of merus of cheliped lined by series of small granules, distal meral tooth small pointed; superior surface of merus granulated.Cheliped carpus fi rst and second teeth small, low, equal-sized.G1 terminal article short, straight, in line with longitudinal axis, cone-shaped, evenly tapering to pointed tip.

Etymology
The new species is named for Mount Moroto in Karamoja District, Uganda which is the only locality where this species has been collected.

Size
Small species, adult at CW 26 mm.

Colour
Preserved specimens uniformly light brown.

Ecology
This species is so far known only from a single high altitude locality, and was collected from a mountain stream at 2,438 m asl.

Conservation status
An IUCN conservation assessment of Potamonautes morotoensis sp.nov.has not yet been carried out, but given the fact that this species is known from a single specimen from a single locality it would probably be regarded as Data Defi cient.

Remarks
Potamonautes morotoensis sp.nov. is superfi cially similar to several other Potamonautes species that share the following characters: a small body size at maturity, a complete postfrontal crest; completely reduced or absent exorbital and epibranchial teeth; smooth anterolateral margins immediately behind each epibranchial tooth; a third maxilliped ischium that lacks a visible sulcus; a fi rst tooth on the carpus of the cheliped that is strong and pointed; a strong and pointed distal meral tooth; and a slim, elongated, highly arched dactylus of the cheliped.East African crabs that share these characters include Potamonautes imatongensis sp.nov., P. mutandensis (Chace, 1942) (southwest Uganda), P. berardi (Audouin, 1826) (Nile River basin), and P. williamsi Cumberlidge & Clark, 2010 (Mount Elgon, Uganda).
Potamonautes morotoensis sp.nov.differs from P. amalerensis stat.rev. in that the second tooth on the carpus of the cheliped is small and low (vs the second carpal tooth is reduced to a granule in P. amalerensis stat.rev.) and the G1 terminal article tapers evenly to a broad tip (vs G1 terminal article tapering evenly to a pointed tip in P. amalerensis stat.rev.).
For comparisons of P. morotoensis sp.nov.with P. imatongensis sp.nov., see above.(Rathbun, 1935)   sternoabdominal cavity; margins of s4 distinctly raised, thickened.Inferior margins of merus of cheliped with series of small granules, distal meral tooth pointed; fi rst cheliped carpal tooth small, second cheliped carpal tooth reduced to small granule.Fixed fi nger (propodus) and movable fi nger (dactylus) of major cheliped of adult male each with larger teeth interspersed with small teeth, dactylus slightly arched, enclosing long narrow interspace when closed.G1 terminal article short cone-shaped, directed outward at 45° angle to longitudinal axis of gonopod, evenly tapering to pointed tip.

Size
Small species, adult size range between CW 26.0-31.5 mm.

Colour
Preserved specimens uniformly light brown.

Ecology
This species was collected from high altitude mountain streams at 1,524 m asl.

Vernacular name
Mount Kadam Crab.

Conservation status
This species is known only from two localities both on Mount Kadam in Uganda, and from a total of 11 specimens collected in 1935 and 1962.Potamonautes amalerensis stat.rev.was listed as Data Defi cient (IUCN 2003;Cumberlidge et al. 2009) in view of the absence of further information on its extent of occurrence, ecological requirements, population size, population trends, and long-term threats (Cumberlidge 2008).The addition of a second locality on Mount Kadam would probably not warrant a change in assessment category.

Remarks
The type material was collected during the AMNH 1933AMNH -1934 Expedition to the then colonies of Kenya and Uganda led by Mr. Arthur Loveridge, a Fellow of the Guggenheim Memorial Foundation.Potamon (Geothelphusa) amalerensis Rathbun, 1935 stat. rev., is redescribed here as the valid species Potamonautes amalerensis stat.rev., because Bott (1955) erroneously treated this taxon as a junior synonym of Potamonautes (Rotundopotamonautes) didieri (Rathbun, 1904) from the D.R. Congo (Cumberlidge & Clark 2012).This opinion of Bott (1955) is further undermined by the fact that he re-described P. didieri based on a non-type museum specimen from 'Ethiopia' using photographs of the carapace and illustrations of the gonopods of this (unidentifi ed) specimen (ZSM, CW 37 mm).As a result, a number of subsequent authors (Cumberlidge 1997(Cumberlidge , 1998;;Ng et al. 2008;Cumberlidge et al. 2009) did not accept the opinion of Bott (1955), and recognized P. didieri and P. amalerensis stat.rev.as valid Potamonautes species.Illustrations and photographs of the holotype of P. amalerensis stat.rev.were provided by Rathbun (1935: 25, plate 2) as Potamon (Geothelphusa) amalerensis stat.rev.and by Chace (1942: 204, fi g. 8) as Potamon amalerensis stat.rev.
Potamonautes amalerensis stat.rev. is superfi cially similar to several other species of African freshwater crabs in that they share the following characters: a small body size at maturity (beginning at CW 18 mm); a complete postfrontal crest; completely reduced or absent exorbital and epibranchial teeth; smooth anterolateral margins immediately behind each epibranchial tooth; a third maxilliped ischium that lacks a visible sulcus; a cheliped merus with a strong and pointed distal meral tooth; and a slim, elongated, arched dactylus of the male major cheliped that when closed encloses either a wide oval interspace or a long narrow interspace.For comparisons of P. amalerensis stat.rev.with P. morotoensis sp.nov.and P. imatongensis sp.nov.see above.

Fig. 8 .
Fig. 8. Distribution map showing the known localities of Potamonautes imatongensis sp.nov.(black circles, not shown to scale), Potamonautes morotoensis sp.nov.(black star, not shown to scale) and Potamonautes amalerensis (Rathbun, 1935) stat.rev.(black squares, not shown to scale) in Uganda.East Africa.For exact localities see text.