Potamonautes alluaudi Bouvier 1921

Potamonautes alluaudi (Bouvier, 1921)

(Figs. 30–42, 45, Table 1)

Potamon (Geothelphusa) alluaudi Bouvier, 1921: 46 –49, figs. 1–3. Potamon alluaudi Balss, 1929: 350; Chace, 1942: 240.

Type material examined: KENYA: holotype, Potamon (Geothelphusa) alluaudi Bouvier, 1921, female subadult (CW 26.5, CL 18.9, CH 9.7, FW 7.8 mm), Ruaha River. Nairobi Forest (0°24'0''S, 36°59'0''E), 1,000 m asl, 25 km from Nairobi, Central Province, 22.ii.1911 (Ch. Alluaud and R. Jeannel coll.) (MNHN B- 17); specimen illustrated is the adult male (CW 54 mm) from the Aberdare Mountains (T.R. Williams coll.) (NMU TRW 1963.02a.1-2).

Additional material examined: KENYA: Amboni River, southern Aberdare Mountains, subadult female (CW 21 mm) (damaged); Kiganjo, northern Mt Kenya, Chania River at road bridge below Nyeri (0°06'S, 37°52'E) river 6.1 m wide, fast, river bed with small boulders and stones, 7 juveniles (CW 12.7 to CW 21 mm), juvenile female (CW 22.2 mm), male (CW 32.2 mm), subadult female (CW 34.9 mm), male (damaged), 7.iii.1962 (T. R. Williams coll.) (NMU TRW EA62.75); Mt Kenya, Kiganjo, Nairobi River on road to Trout Research Station, river bed sandy with large stones, and occasional small waterfalls flowing over outcrops and large rocks, 7.iii.1962 (T. R. Williams coll.) (NMU TRW EA62.76); Meru, northern Mt Kenya, Kiganjo, first stream north of last station, 2–2.5 m wide, open, stream bed sandy with some embedded large stones and a cattle watering pool, 9.iii.1962 (T. R. Williams coll.) (NMU TRW EA62.85); Nyambeni Hills, north of Mt Kenya, Thangatha River (0°07'S, 38°13'E), 1,585 m asl, 8.4 km from Mikinduri (00°07'N, 37°50'E) river 3.7–4.6 m wide, river bed stony with some boulders, shaded, at forest edge, about 90 juveniles, 10.iii.1962 (T. R. Williams coll.) (NMU TRW EA62.89); Nyambeni Hills, north of Mt Kenya, Thangatha River, 1,585 m asl, from mid stream, adults, 10.iii.1962 (T. R. Williams coll.) (NMU TRW EA62.91); Nyambeni Hills north of Mt Kenya, unnamed stream, 1.2 km nearer Mikindwi than Thangatha River, 0.7 m wide, shallow, river bed with occasional boulders, otherwise sandy with a few stones, crab collected from beneath a boulder, adult male, 10.iii.1962 (T. R. Williams coll.) (NMU TRW EA62.93); Nyembeni Hills, north of Mt Kenya, Ngobit River at Ngobit on road between Naro Moru and Rumuruti (1°17'N, 36°47'E), a tributary of the Ewaso Ngiri River, river 3.7–4.6 m wide, up to 0.7 m deep, river bed with gravel embedded in silty clay, occasional large stones, crab collected from rubble at sides of river below bridge, juvenile male (CW 27.9 mm), 16.iii.1962 (T. R. Williams coll.) (NMU TRW EA62.106); Mt Kenya, juvenile female (CW 19.9 mm), 1972 (Joy coll.) (NMU JOY 07.2001.a.1) Mt Kenya, juvenile male (CW 32.4 mm), juvenile female (damaged) i.1972 (Joy coll.) (NMU TRW 01.1972.2); Chania River, Nyeri, Aberdare Mountains (M. J. Clarkson coll.) adult male (CW 46.8 mm) adult female (CW 51.5 mm) (NMU TRW 1963.02b.1–2); Meru, Mt Kenya, 10.iii.1962 (NHM 10.III. 1962.1); Murang’a (formerly Fort Hall), male (USNM 82309); Mt Kenya, 2 females (CWs 54.5, 50.0 mm), male (CW 48.9 mm), 29.ix.1909 (Smithsonian African Expedition coll.) (USNM 82317); Mt Kenya, Murang'a (formerly Ft. Hall), 2,833 m asl, juvenile (damaged), 4 males (CW 23.1 to CW 48.9 mm), 2 juvenile males (CW 17.2 to CW 21.8 mm), 6 females (CW 23.7 to CW 36 mm), 2 juveniles (CW14.6 to CW 15.6 mm), ovigerous female (CW 51.8 mm), x.1909 (Smithsonian African Expedition coll.) (USNM 82318); Kasarongai River, west of Mt Kenya, 15 males (CW 27.1 to CW 48.7 mm), 9 females (CW 24.9 to CW 35.1 mm), 3 juveniles (CW 7.4 to CW 21.9 mm) x.1909 (Smithsonian African Expedition coll.) (USNM 82319); Kasarongai River, west of Mt Kenya, adult male, 3 subadult males, 3 juvenile males, 3 adult females (CW 46.6 to CW 53 mm), 2 adult females with hatchlings, ovigerous female, 6 subadult females, 9 juvenile females, 7 juveniles, x.18 –19.1909 (Smithsonian African Expedition coll.) (USNM 82320); Mt Kenya to Murang'a (formerly Fort Hall), 2,833 m asl, 15 males (CW 18.8 to CW 41 mm), 7 females (CW 21.5 to CW 51.8 mm), 3 juvenile females (CW 18 to CW 25.3 mm), 11 juvenile males (CW 17.5 to CW 23.7 mm), 25 juveniles (CW 12.6 to CW 24.4 mm), juvenile (damaged), female subadult (damaged), x.1909 (Smithsonian African Expedition coll.) (USNM 82321); Kasarongai River, west of Mt Kenya, 27 males, 32 females (17 juveniles), 18–19.x.1909 (E. A. Mearns) (USNM 82322); between Mt Kenya and Murang'a (formerly Fort Hall), 2,833 m asl, 27 juveniles (CW 10.3 to CW 25.9 mm), 14 juvenile females (CW 19.4 to CW 31.8 mm), 5 juvenile males (CW 20.9 to CW 32.7 mm), subadult female (CW 38.5 mm), female subadult (CW 39.5 mm), adult female (CW 47 mm), female with hatchlings (CW 44.3 mm), 2 juveniles (damaged), x.1909 (Smithsonian African Expedition coll.) (USNM 82323).

Diagnosis. Carapace medium height (CH /FW 1.3), smooth; anterolateral margin granular; postfrontal crest distinct, completely crossing carapace, granular at junction with anterolateral margins; exorbital, epibranchial teeth low; carapace sidewalls smooth; third maxilliped ischium with deep vertical sulcus; thoracic sternal sulcus s3/s4 not complete, v-shaped, tapering inward at sides, absent across middle; episternal sulcus s4/e4 absent, s5/e5, s6/e6, s7/e7 complete; dactylus of major cheliped broad, highly arched, closed fingers enclosing oval interspace; first carpal tooth on carpus of cheliped large, blunt; second carpal tooth reduced to small granule, followed by several other small granules; ventral margins of pereiopod 1 merus granulated; distal meral tooth pointed; terminal article of G1 curving outward at 60° angle to longitudinal axis of gonopod; terminal article broad proximally (lateral, medial folds high in basal half), terminal article outwardly curved, narrowing distally, tapering to point; distal margin of subterminal segment highest on medial side (forming rounded shoulder) lowest on lateral side; dorsal membrane broad, widest at lateral edge, narrowest at medial edge.

Size. A large-sized species, pubertal molt between CW 44 and CW 54.5 mm.

Type locality. Ruaha River, Nairobi Forest, Kenya.

Distribution. Kenya, in the region of Mt Kenya and the Aberdare Mountains, Central Province. The records from the NMU collection combined with those the USNM collected by the Smithsonian African Expedition in 1909 that have been re-identified here define the species as endemic to the highlands of the Central Province of Kenya, in the region of Mt Kenya and the Aberdare Mountains. Bouvier (1921) also included in this species an adult male syntype from the Amboni River (1,800 m asl) in the Aberdare Mountains collected on January 13, 1912. However, Bouvier’s (1921) further inclusion in this species of two male specimens from the Ngare Rongai (now Ngare Longai) River in the grasslands east of Mt. Kilimanjaro is considered doubtful, and these are not listed under this species in the present work.

Natural history. This large-sized species lives in the rivers and streams of Mt Kenya and the Aberdare Mountains.

Remarks. The specimens included in the present study were assigned to P. alluaudi because they all conform strongly to the characters of the female subadult holotype of Potamon (Potamonautes) alluaudi Bouvier, 1921 (CW 26.5, CL 18.9, CH 9.7, FW 7.8 mm) from Nairobi Forest (MNHN B-17) and to Bouvier’s (1921) detailed account of its carapace, chelipeds, mouthparts, and pereiopods. Bott (1955) treated P. alluaudi as Potamonautes (Arcopotamonautes) suprasulcatus alluaudi, a subspecies of T. suprasulcatus Hilgendorf, 1898. However, comparison of G1 and carapace characters of the adult male specimen of P. alluaudi from the Nyambeni Hills north of Mt Kenya with the adult male lectotype of T. suprasulcatus Hilgendorf, 1898, from Tanzania (CW 54.6, CL 37.2, CH 17.4, FW 14.5 mm) (ZMB 9037) indicates that these two taxa are not conspecific. Potamonautes alluaudi (and P. suprasulcatus) are both treated here as valid species, based on characters of the carapace, carpus of the cheliped, and the first gonopod (Reed & Cumberlidge 2006).

All three species of freshwater crabs found in the region of Mt Kenya and the Aberdare Mountains (P. jeanneli, P. odhneri, and P. alluaudi) are similar in that they all have exorbital and epibranchial teeth that are reduced to small granules, a carapace sidewall that is completely smooth, and an s3/s4 groove on the thoracic sternum that is reduced to two side notches. The species can be distinguished from each other as follows (Table 1).

Potamonautes jeanneli and P. odhneri are similar in that both are small to medium-sized species (adult at CW 22 and 32 mm respectively), and both have exorbital and epibranchial teeth that are reduced to small granules, anterolateral margins of the carapace that are completely smooth lacking teeth of any kind, carapace sidewalls that are completely smooth, and an s3/s4 groove that is reduced to two side notches. However, P. jeanneli has a completely smooth carapace lacking a postfrontal crest, while P. odhneri has a sharp postfrontal crest; the episternal sulci s4/e4, s5/e5, s6/e6, s7/e7 of P. jeanneli are missing and not visible (smooth), whereas these sulci are all deep and distinct in P. odhneri; the ischium of the third maxilliped in P. jeanneli is smooth and lacks a vertical groove, whereas it is deep in P. odhneri; the first tooth on the carpus of the cheliped of P. jeanneli is blunt and low, whereas it is small but pointed in P. odhneri. Finally, the terminal article of G1 of P. jeanneli is straight and slim (its lateral and medial folds are equal and low) and it is not turned outward, whereas in P. odhneri the terminal article of G1 is broadened basally and curves outward at a 45° angle to the longitudinal axis of gonopod (Figs. 43, 44).

P. alluaudi P. odhneri P. jeanneli GO1: terminal article Broad based; curving outward Broad based; curving outward Slim; straight

60o 45o

GO1: subterminal segment Pronounced shoulder No shoulder No shoulder Potamonautes alluaudi can be distinguished from the other two species found on Mt Kenya by the size of adult specimens (Williams 1991; Cumberlidge 1997, 1998). With an adult size range from CW 44 to 55 mm, P. alluaudi is a much larger species than either P. jeanneli or P. odhneri that are both adult at CW 22 and 32 mm respectively, whereas a specimen of P. alluaudi in this size range would only be a subadult (Table 1). Potamonautes alluaudi can be distinguished from P. jeanneli as follows: a sharp postfrontal crest whereas P. jeanneli has a completely smooth carapace lacking a postfrontal crest; the third maxilliped ischium has a deep vertical groove, whereas in P. jeanneli the ischium of the third maxilliped is smooth and lacks a vertical groove; the first carpal tooth of the cheliped carpus is large and pointed whereas in P. jeanneli the tooth is blunt and low; the distal meral tooth on the cheliped merus is large and pointed whereas in P. jeanneli this tooth is blunt and low; and the terminal article of G1 of P. alluaudi curves sharply outward (at a 60° angle to the longitudinal axis of the gonopod) and is broadened basally, whereas the terminal article of G1 of P. j e a n - neli is straight and slim (Figs. 43, 45).

Potamonautes alluaudi is similar to P. odhneri in that they both have a sharp postfrontal crest, a deep vertical groove on the ischium of the third maxilliped, a first carpal tooth on the carpus of the cheliped that is large and pointed, a pointed distal meral tooth, and a terminal article of G1 that curves outward rather than continuing straight. Potamonautes alluaudi can be distinguished from P. odhneri as follows. The terminal article of G1 of P. alluaudi curves sharply outward at a 60° angle to the longitudinal axis of the gonopod, and the distal margin of the subterminal segment is highest on the medial side forming a pronounced, rounded shoul- der; whereas the terminal article G1of P. odhneri curves outward at a 45° angle to longitudinal axis of the gonopod, and the distal margin of the subterminal segment does not form a pronounced shoulder (Figs. 44, 45).

Conservation status. The conservation status of P. alluaudi is categorized as least concern (LC) because it has an extent of occurrence and an area of occupancy are both in excess of the thresholds for vulnerable (VU) and because there are no known threats (IUCN 2004; Cumberlidge et al. 2009). Its population is estimated to be stable based on indirect measurements such as its representation in museum collections, although its most recent collection dates back to 1962.