Potamonautes williamsi, Cumberlidge & Clark, 2010

Cumberlidge, Neil & Clark, Paul F., 2010, Two new species of freshwater crabs from Mount Elgon, East Africa (Brachyura: Potamoidea: Potamonautidae) and a re-diagnosis of Potamonautes niloticus (H. Milne Edwards, 1837) with notes on their natural history and onchocerciasis, Journal of Natural History 44 (29 - 30), pp. 1807-1829 : 1818-1825

Potamonautes williamsi sp. nov.

( Figures 5–7 )

Material examined

Type material. Uganda. Holotype. Adult male holotype (cw 21.1), stn El. 114b, Simu tributary, Siroko sector, Mt Elgon , 1341 m asl, coll. T. R. Williams, 9 January 1961 ( NHM 2008.3331 ) ; Paratypes. eight males (cw 20–16.8), one adult female (cw 19.7), stn El. 114b Simu tributary, Siroko sector, Mt Elgon , 1341 m asl, coll. T. R. Williams, 9 January 1961 ( NHM 2008.3332 – 3339 ) ; many small specimens, stn El. 114b, Simu tributary, Siroko sector, Mt Elgon , 1341 m asl, coll. T. R. Williams, 9 January 1961 ( NHM 2008.3332 – 3339 ) ; stn El. 54, stream, tributary of the Manafwa River, Manafwa sector, Mt Elgon , 1265 m asl, coll. T. R. Williams, 1 January 1961, ( NMU TRW 1.i.1961) ; 14 males (cw 19.23–11.7), nine females (cw 20.6–12.17), four ovig. females (cw 20.7–19.6) stn El. 90, Tayaka River, Namatala sector, Mt Elgon , 1463 m asl, from soil among roots at stream edge, coll. T. R. Williams, 7 January 1967 ( NHM 2008.3056 ) ; 18 males (cw 34.71–12.36), three ovig. females (cw 20.1–18.3), four females (cw 15–13.4) stn El. 96, Namafumbole, Namatala sector, Mt Elgon , 1585 m asl, coll. T. R. Williams, 8 January 1961 ( NHM 2008.3058 – 3067 ) ; eight males (cw 20.3–13.5), one ovig. female (cw 16.6), one female (cw 17.2), seven juvs, stn El. 97, Sisiwachi, Namatala sector, Mt Elgon , 1585 m, coll. T. R. Williams, 8 January 97 ( NHM 2008.3088 – 3097 ) ; adult male stn El. 109, Simu stream, tributary of the Siroko River , Siroko sector, Mt Elgon , 1829 m asl, coll. T. R. Williams, 1960 ( NMU TRW 1960 a) ; 41 males (cw 27.4–9.1), 17 females (cw 17.4–11.2), 35 juvs, stn El. 114a, Simu tributary, Siroko sector, Mt Elgon , 1341 m asl, coll. T. R. Williams, 9 January 1961 ( NHM 2008.3072 – 3081 ) ; two males (cw 29.0, 27.3), two females (cw 31.0, 16.3), stn El. 114b, Simu tributary, Siroko sector, Mt Elgon , 1341 m asl, with P. loveni , coll. T. R. Williams, 9 January 1961 ( NHM 2008.3327 – 3330 ) ; 29 males (cw 32.1–7.8), eight adult females, all ovig. (cw 19.7–16.4), 29 females (cw 18.7–10.8) 14 juvs, stn El. 115, Simu tributary, Siroko sector, Mt Elgon , 1341 m asl, coll. T. R. Williams, 9 January 1961 ( NHM 2008.3113 – 3122 ) ; one adult male (cw 17.4, cl 12.8, ch 7.2, fw 5.9) one male, stn El. 173a, foot of Sipi Falls, Sipi River , Nyenye sector, Mt Elgon , 1737 m asl, coll. T. R. Williams, 17 January 1961 ( NHM 2008.3053 ) ; four males (cw 18.6–14.2) stn El. 173b, in stream and splash zone below Sipi Falls, Nyenye River , Mt Elgon , 1737 m asl, coll. T. R. Williams, 17 January 1961 ( NHM 2008.3133 – 3136 ) ; one male (cw 17.4), oine female (cw 16.4) stn El. 173c, foot of Sipi Falls, Sipi River , Nyenye sector, Mt Elgon , coll. T. R. Williams, 17 January 1961 ( NHM 2008.3054 – 3055 ) ; nine males (cw 17.8–9.3), 10 adult females, all ovig. (cw 20.5–17.0), 12 females (cw 16.8–9.8), 20 juvs, stn El. 173, Nyenye River, Nyenye sector, Mt Elgon , 1737 m asl, with P. loveni , in stream and splash zone below Sipi Falls , coll. T. R. Williams, 17 January 1961 ( NHM 2008.3123 – 3132 ) ; one adult, stn 173, male from stream in splash zone below Sipi Falls, Sipi River, Nyenye sector, Mt Elgon , 1738 m asl, with P. loveni , coll. T. R. Williams, 17 January 1961 ( NMU TRW 17.i.1961) ; stn El. 179, Sipi stream, Nyenye sector, Mt Elgon , 1463 m asl, coll. T. R. Williams, 1960 ( NMU TRW 1960 c) ; one adult male (cw 19.8), 1 subadult (cw 17.2), one adult female (cw 19.8), one ovig. female (cw 17.8), 2 juvs, stn El. 180, Muyembe River, Siroko sector, Mt Elgon , 1067 m asl, coll. T. R. Williams, 18 January 1961 ( NHM 2008.3082 – 3087 ) ; seven males (cw 19.6–11.9), one adult ovig. female (cw 23.3), seven females (cw 17.2–10.2), stn El. 187 Sipi stream, Nyenye sector, Mt Elgon , 1737 m asl, coll. T. R. Williams, 17 January 1961 ( NHM 2008.3098 – 3107 ) ; adult ovig. female (cw 27), stream 2, near Jackson Pool, Jinja, Mt Elgon , c. 3886 m asl, coll. M. C. Williams and A. Tjonneland, 15 January 1955, pres. Philip S. Corbet, EAFRO 5 May 1955, ( NHM 2010 . 63) ; one juv., Sasa River, Jinja, Mt Elgon , c. 3536 m asl, coll. P. S. Corbet, 29 July 1954, pres. P. S. Corbet, EAFRO, 5 May 1955, ( NHM 2010 . 64) ; two adult males (cw 23.2, 20.4), stream 1, near Jackson Pool, Mt Elgon , c. 3871 m asl, coll. Miles C. Williams and Ardfinn Tjonneland, January 1955, pres. P. S. Corbet, EAFRO, 5 May 1955 ( NHM 2010.28 – 29 ) ; Sipi Falls, Sipi River, Nyenye sector, Mt Elgon , coll. H. Löffler, Zoological Institute, Vienna, 23 May 1961 ( NMU TRW 1961.02 ) ; two males Butandiga, near Bulambuli, Mbale west of Mt Elgon (1°12′ N, 34°22′ E) ( USNM 70905 ) ; Sipi stream, Nyenye sector, Mt Elgon , 2000 m asl, coll. A. Loveridge, 18 December 1933 ( USNM 70907 ) ; Elgonyi, Mt Elgon , 2333 m asl, coll. A. Loveridge, 25 January 1934 ( USNM 70911 ) ; one male (cw 38.7), Butandiga, near Bulambuli, Mbale west of Mt Elgon , coll. A. Loveridge, 8 January 1935 ( USNM 70912 ) ; one male, Mt Elgon , Elgonyi ( USNM 82295 ) ; two adult females (cw 17.5, 17), two adult males (cw 20.5, 19.5) Elgonyi, Butandiga? Mt Elgon , coll. MCZ Expedition, 1933–34 ( MCZ 8235 ) ; one adult female (cw 18), one adult male (cw 16.5), two subadult males (cw 15.5, 14.5) Elgonyi, Mt Elgon , coll. MCZ Expedition, 1933–34 ( MCZ 8236 ) ; adult female (cw 17), one adult male (cw 18.5) Butandiga, near Bulambuli, Mbale west of Mt Elgon ( MCZ 9026 ) .

Diagnosis

Holotype. Postfrontal crest either faint or missing completely; exorbital, epibranchial teeth reduced to small granules, anterolateral margin behind epibranchial tooth not raised, completely smooth, continuous with posterolateral margin; carapace surface completely smooth; carapace medium high (ch/fw 1.2); suborbital margin raised, completely smooth; ischium of third maxilliped either smooth or with very faint vertical suture; thoracic sternal suture s3/s4 missing, except for two short side notches; thoracic episternal suture s4/e4 to s7/e7 faint or absent; dactylus of major cheliped thickened, medium arched, enclosing oval interspace; first carpal tooth on carpus of cheliped weak, low, blunt; second carpal tooth smaller, also weak, low, blunt, followed by several small granules; ventral margins of merus of pereiopod 1 both weakly granulated; distal meral tooth either low or missing; terminal article of GO1 straight-sided, slim, cone-shaped, angled at about 60° to longitudinal axis of gonopod, tapering evenly to pointed tip; terminal article of GO1 not widened in middle, medial, lateral lobes both low, even; dorsal membrane broad; margins of subterminal segment of GO1 setose; small species, adult size range beginning at cw 20 mm.

Description

Carapace ovoid, wide (cw/fw 3.15), medium high (ch/fw 1.2); surface completely smooth semi-circular, urogastric, transverse branchial grooves faint. Front straight, relatively narrow, about one-third carapace width (fw/cw 0.32), anterior margin sharply deflexed. Postfrontal crest either faint or missing completely. Anterolateral margin between exorbital, epibranchial teeth smooth, lacking intermediate tooth; exorbital, epibranchial teeth reduced to small granules, anterolateral margin completely smooth, continuous with posterolateral margin. Suborbital margin raised, completely smooth. Suborbital, subhepatic, pterygostomial regions of carapace sidewall all completely smooth; sidewall divided into three parts by longitudinal (epimeral) suture (dividing suborbital, subhepatic regions from pterygostomial region), and by vertical (pleural) groove (dividing suborbital from subhepatic regions), dorsal end of vertical groove meeting epibranchial tooth. First thoracic sternal suture s1/s2 obscure; second suture s2/s3 deep, running horizontally across sternum; third suture s3/s4 reduced to two short notches on sides; episternal sulci s4/e4 to s7/e7 faint/ absent. Third maxillipeds filling entire oral field, except for transversely oval respiratory openings at superior lateral corners; long flagellum on exopod of third maxilliped, ischium of third maxilliped either smooth or with very faint vertical suture. Epistomial tooth prominent, smooth, triangular. Mandibular palp two-segmented; terminal segment single, undivided, with setae (but no hard flap) at junction between segments. Dactylus of adult male major cheliped thickened, medium arched, enclosing oval interspace. First carpal tooth of cheliped weak, low, blunt; second carpal tooth smaller, also weak, low, blunt, followed by several granules; ventral margins of merus of pereiopod 1 both weakly granulated; distal meral tooth either low or missing; superior surface of merus smooth. Pereiopods p2–p5 slender, p3 longest, p5 shortest, dactyli of p2–p5 tapering to point, each bearing four rows of downward-pointing short, sharp spines. Adult male abdomen triangular, somites a1–a6 of male abdomen four-sided, telson (a7) a broad triangle with rounded apex; somites a5–a6 broadest. Terminal article of GO1 straight-sided, slim, cone-shaped, angled at about 60° to longitudinal axis of gonopod, tapering evenly to pointed tip; terminal article of GO1 not widened in middle, medial, lateral lobes both low, even; dorsal membrane broad; margins of subterminal segment of GO1 setose. This is a relatively small species, adult size range beginning at cw 17 mm.

Distribution

This species is known only from the western slopes of Mt Elgon in Uganda (01°8′ N, 34°33′ E) at medium altitudes between 1128 and 2333 m asl .

Comparisons

Adult egg-bearing females of P. williamsi measure between cw 17 and 21 mm, which makes this a considerably smaller species than P. niloticus and P. loveni (where specimens with cws of 20 mm would be juveniles). Potamonautes williamsi is closest to P. loveni , a medium-sized species of freshwater crab from Mt Elgon that has an adult size range from cw 35 to 49 mm ( Cumberlidge and Clark 2009). Potamonautes williamsi and P. loveni are similar in that both species have a completely smooth carapace lacking a postfrontal crest, their exorbital and epibranchial teeth are reduced to small granules, the anterolateral margins of the carapace are completely smooth, lacking teeth of any kind, the thoracic sternal groove s3/s4 is reduced to two side notches, and a similar first carpal tooth on the carpus of the cheliped which is small, low and rounded ( Cumberlidge and Clark 2009). However, there are a number of characters that distinguish them, for example, the terminal article of GO1 of adult male of P. williamsi is straight-sided, slim, and cone-shaped, and tapers evenly to a pointed tip (without widening in the middle), whereas that of P. loveni is slightly sinuous and is widened in the middle by an enlarged medial lobe; the subterminal segment of GO1 of P. williamsi has setose margins, whereas these margins are smooth in P. loveni ; the dactylus of the cheliped of P. williamsi is thickened, has large teeth, and forms a medium arch that encloses an oval interspace, whereas the dactylus of the cheliped of P. loveni is slim, lacks teeth, and forms a high arch that encloses a round interspace.

The adult male holotype of P. williamsi was compared here with an adult male specimen of P. niloticus from Kenya. The two taxa can be distinguished as follows. The anterolateral margin of P. niloticus behind the epibranchial tooth is clearly toothed (whereas this margin is completely smooth in P. williamsi ); the first and second carpal teeth on the carpus of the cheliped of P. niloticus are each produced into sharp spines (whereas these teeth are small and low in P. williamsi ); the ventral margins of the merus of pereiopod 1 of P. niloticus are both heavily granulated (whereas these margins are smooth in P. williamsi ); the distal meral tooth of P. niloticus is a small sharp spine (whereas this tooth is reduced to a small granule in P. williamsi ); and P. niloticus (whose adult size range is from cw 46 to 65 mm) is a much larger species than P. williamsi .

Etymology

The species is named for Trefor R. Williams (retired, formerly of the University of Liverpool, UK), in recognition of his significant contributions to freshwater crab biology and to advances in the study of onchocerciasis in East Africa .

Habitat and ecology

The following notes are based on those provided by Hynes et al. (1961) and Williams et al. (1961). Potamonautes williamsi is found in the forest zone of Mt Elgon at midaltitudes from 1100 to 1800 m. Female crabs with either eggs or hatchlings attached to their abdomen are common in January, indicating that the breeding season of P. williamsi is between November and February (Cumberlidge 1999). Potamonautes williamsi spends little time in the water, and in the forest was frequently seen on the banks of rivers, streams and (most commonly) small trickles. This species typically burrows into the loose soil of the banks of small forest-streams and is found among the roots of tree ferns in places where the stream banks are sufficiently damp and soft, and it often digs burrows some distance from streams. Several very dense populations of P. williamsi were found in areas of loose earth and stones in the damp soil of the splash zone of waterfalls. The Sipi Falls is the type locality and consists of a series of three waterfalls in the foothills of Mt Elgon located close to, but not inside, the boundary of the Ugandan Mt Elgon National Park, about 55 km north of Mbale, and not far from the town of Kapchorwe. It would appear that P. williamsi requires the extremely humid conditions that are typical of the wet forested areas at medium altitudes on Mt Elgon, because it was not found on the drier northern slopes, or at lower altitudes (i.e. below 1100 m) in the cleared farmlands below the forest.

Ecological notes Mount Elgon is an ancient eroded volcano (4321 m asl with an area of 169 km 2) situ-

ated on the Kenya – Uganda border, about 140 km north-east of Lake Victoria. The Ugandan sector of Mt Elgon is an important watershed for the Kiriki, Nyenye, Siroko, Namatala, Manafua and Malawa Rivers (which flow south into Lake Victoria). The Kenyan sector is drained by the Nzoia River (which flows south into Lake Victoria) and the Suam River (which flows north into Lake Turkana) ( Williams 1991). At lower altitudes, Mt Elgon’s slopes support montane forest (some of which has been cleared for agriculture) that gives way at higher altitudes to open moorland with giant lobelia and groundsel plants.

The four species of freshwater crabs found on Mt Elgon ( P. niloticus , P. loveni , P. elgonensis and P. williamsi ) have different altitudinal preferences that overlap. Potamonautes niloticus is found in the warm waters at lower altitudes up to 1400 m, P. williamsi is found at mid-altitudes between 1100 and 1800 m, P. elgonensis is found at mid-altitudes between 1402 and 2134 m, and P. loveni is found in cooler waters between 1000 and 3000 m. Potamonautes loveni also occurs in the upper reaches of rivers on the highlands of western Kenya and eastern Uganda ( Cumberlidge and Clark 2009). At medium altitudes, all four species are found together on the slopes of Mt Elgon. Within the range of altitudes between 1280–1950 m asl, where the water is cooler than the lowland streams but warmer than at higher altitudes, P. loveni coexists with P. niloticus . Potamonautes loveni reaches its lowest altitudes on Mt Elgon in the Namatala River drainage sector where the forest remains largely intact, below which it is replaced by P. niloticus , which prefers warmer waters ( Williams 1991).

Potamonautes niloticus is completely dependent on rivers and (unlike P. williamsi and P. loveni ) it rarely, if ever leaves the water. The ranges of P. niloticus , P. williamsi and P. loveni overlap in several rivers at medium altitudes between 1158 and 1402 m, presumably because these heights represent the upper limits of the warmer waters favoured by P. niloticus . Potamonautes niloticus is a widespread species associated with the Nile River basin ( Bott 1955; Cumberlidge 1997, 1998, 2009a; Cumberlidge et al. 2008) and occurs in a range of aquatic habitats including the major lowland rivers and lakes, small clear fast-flowing mountain streams with rocky beds, and sluggish lowland streams with muddy bottoms.

Crabs and human river blindness

Potamonautes niloticus , P. elgonensis and P. loveni are associated with onchocerciasis on Mt Elgon, while P. loveni is linked with this disease elsewhere in the highland areas of western Kenya ( Williams 1968, 1991; Cumberlidge and Clark 2009). In eastern Kenya, in the region of Mount Kenya, P. alluaudi (Bouvier, 1921) has also been linked with river blindness ( Williams 1968; Cumberlidge 2009a). The larval stages of Simulium neavei , the blackfly vector of the parasite Onchocerca volvulus that causes human river blindness, attach to the carapace of freshwater crabs living in the cold well-oxygenated waters of fast-running highland streams ( Williams 1991). Wherever P. niloticus occurs in fast-flowing streams with turbulent water it is often associated with S. neavei ( Williams et al. 1961; Hynes et al. 1961). Simulium neavei was never found associated with P. williamsi and it is unlikely that this species is involved with the focus of onchocerciasis on Mt Elgon ( Williams et al. 1961).

Conservation

None of the collection localities for any of the species found on Mt Elgon fall within any of the three protected areas on the mountain – the Ugandan and Kenyan Mt Elgon National Parks, and the Mt Elgon Forest Reserve in Kenya. It is encouraging to note that much of Mt Elgon’s montane ecosystem now appears to be recovering from previous episodes of human disturbance. This contrasts with other parts of East Africa where the montane forests are experiencing rapid habitat destruction and degradation, and encroachment from growing human populations that now endangers much of the rich and scarcely known biota of this unique region.

Remarks

The present work adds two new species to Potamonautes , an important and widespread genus of African freshwater crabs that is found throughout Africa from Senegal to the Horn of Africa, and from Egypt to South Africa. The recent upsurge in interest in the Afrotropical freshwater crabs has seen the description of a number of new species of Potamonautes and this widespread genus now has close to 70 species ( Coulter 1991; Stewart 1997a, 1997b; Stewart and Cook 1998; Daniels et al. 1998; Cumberlidge 1999; Cumberlidge et al. 1999; Daniels et al. 2000, 2002; Corace et al. 2001; Gouws et al. 2001; Cumberlidge et al. 2002; Cumberlidge and Vannini 2004; Reed and Cumberlidge 2004, 2006; Cumberlidge and Tavares 2006; Cumberlidge and Dobson 2008; Cumberlidge 2008, 2009b; Cumberlidge and Clark 2009; Ng et al. 2008). It is likely that this genus will have even more species attributed to it as taxonomic discrimination improves and as biodiversity studies survey the continent in more detail.

Figures

Figure 5. Potamonautes williamsi sp. nov. Adult male, paratype, cw 20 mm, from Mount Elgon, foot of Sipi Falls, Sipi River, Nyenye sector (NHM 2008.3332–3339). (A) Carapace, dorsal view; (B) cephalothorax, frontal view; cheliped, (C) right frontal view; (D) left frontal view; (E) carpus and merus of right cheliped, dorsal view; (F) carpus and merus of right cheliped, inferior view. Scale bar represents 5.7 mm (A,B) or 3.7 mm (C–F).

Figure 6. Potamonautes williamsi sp. nov. (A,B) Adult male, paratype, cw 20 mm, from Mount Elgon, foot of Sipi Falls, Sipi River, Nyenye sector (NHM 2008.3332–3339): (A) left third maxilliped; (B) sternum; (C,D) adult male Mount Elgon, stn El.109, Simu stream, tributary of the Siroko River, (NMU TRW 1960a): (C) GO1 ventral view; (D) GO1, dorsal view. Scale bar represents 4.5 mm (A,B) or 3 mm (C,D).

Figure 7. Potamonautes williamsi sp. nov. Adult male, paratype, cw 20 mm (NHM 2008.3332– 3339). Cheliped: (A) right (major) frontal view; (B) left (minor) frontal view; whole animal: (C) dorsal view; (D) ventral view; (E) carapace, frontal view. Photographs Phil Crabb, NHM Photographic Unit.