Potamonautes johnstoni ( Miers, 1885 )

Reed, Sadie K. & Cumberlidge, Neil, 2006, Brachyura: Potamoidea: Potamonautidae, Platythelphusidae, Deckeniidae (), Zootaxa 1262 (1), pp. 1-139 : 1-139

4. Potamonautes johnstoni ( Miers, 1885) (Figs. 31–40, 151–152, 176, plate IV)

Thelphusa depressa var. johnstoni Miers, 1885: 237–239 .

Telphusa hilgendorfi Pfeffer, 1889: 32 .

Potamon (Potamonautes) ambiguus Rathbun, 1904 : pl. 14, fig. 7; 1905: 171.

Potamon (Potamonautes) mrogoroensis — Rathbun, 1905: 173 (not Hilgendorf, 1898).

Potamon (Potamonautes) Johnstoni — Rathbun, 1905: 170.

Potamonautes johnstoni — Balss, 1929: 343–344; 1936: 180–182, fig. 17; Cumberlidge, 1997: 580– 581; 1998: 200.

Potamonautes johnstoni f. typical — Pesta, 1937: 157.

Potamon johnstoni — Chace, 1942: 214.

Potamon (Potamonautes) hilgendorfi — Rathbun, 1933: 256; 1935: 6; Chace, 1942: 186.

Potamonautes (Lirrangopotamonautes) johnstoni johnstoni — Bott, 1955: 265–267: pl. XV, fig. 2a– d, fig. 36a, b.

Type material examined: TANZANIA: Thelphusa depressa var. johnstoni Miers, 1885 : Kilimanjaro, male type (cw 57, cl 40, ch 24.2, fw 16.2 mm) ( BMNH 1885.2). Potamon (Potamonautes) ambiguus ( Rathbun, 1905) : female type, Kilimanjaro, Saumi River, 1,000 to 1,600 m asl ( USNM 30008). Holotype of Telphusa hilgendorfi Pfeffer, 1889 (subadult female, cw 20.8, cl 15.1, ch 7.2, fw 6.1 mm; ZMB 11377).

Additional material examined: TANZANIA: West Usambara Mountains, Garaya River near Dindila Factory , adult male (cw 71.6 mm), adult female (cw 84.1 mm), 5.ix.1964 (J. N. Raybould) ( NMU TRW1966.05 ) ; south slopes of Kilimanjaro, Kondeni River near Marangu , adult male (cw 52.3 mm), 6.v.1966 (J. N. Raybould) ( NMU TRW1966.13 ) ; 16 km east of Arusha, Usa River (tributary of Kikuletwa River, itself a tributary of the Pangani River ) male (cw 53 mm), female (cw 48.7 mm) 26.v.1966 (J. N. Raybould) ( NMU TRW1966.14 ) ; Kibalwa, near Taveta, canal from Lumi River , subadult female (cw 46.5 mm), 1.ii.1967 (J. E. Hudson) ( NMU TRW1968.10 ) ; Kibalwa, near Taveta, Canal from Lumi River , male (cw 53.5 mm), 1.ii.1967 (J. E. Hudson) ( NMU TRW1968.12 ) ; East Usambara Mountains, Amani , male (cw 76.4 mm) 6.v.1966 (J. N. Raybould) ( NMU TRW1969.02 ) ; East Usambara Mountains, Amani , subadult female (cw 64.6 mm), 2 males (cws 61.1, 62 mm), 1970 ( T. R. Williams) ( NMU TRW1970.01 ) ; male (soft­shelled), 6.vii.1970 ( T. R. Williams) ( NMU 076.1970.1); East Usambara Mountains , Amani, male (cw 56.8 mm), 1970 ( T. R. Williams) ( NMU TRW1970.03 ) ; East Usambara Mountains, Amani , subadult female (cw 48 mm), 27.iii.1971 ( NMU TRW1970.05 ) ; canal near Taveta, male (cw 65.5 mm), adult female (cw 69.4 mm), 1971 ( NMU TRW1971.11 ) ; Kilimanjaro area, Mue River, station 2, 3 males (cw 45.9 to cw 50.1 mm) ( NMU TRW EA62.67); East Usambara Mountains , Amani , Dodwe River , female with hatchlings (cw 77.9 mm), 1962 ( T. R. Williams ) ( NMU TRW­EA62.25); stream on Mount Kilimanjaro , 5 juvenile males (cw 21.9 to cw 38.9 mm), iv.1967 (J. E. Hudson) ( NMU TRW1968.11 ) ; 3 juvenile males (cw 18.5 to cw 36.3 mm), 2 juvenile females (cw 25.5, 34.3 mm) ( NMU 03.29.1972.1–5); canal west of Taveta, 4 males (cw 48.9, 57 mm, 2 damaged) 2 females (cw 56.5 mm; 1 damaged), 2 juvenile females (cw 32.9, 33.2 mm) (J. E. Hudson) ( NMU 07.2001 .c) . KENYA: stream above Kibo , 3 males (cw 42 to cw 63.7 mm) (J. E. Hudson) ( NMU TRW1971.13 ) ; stream above Kibo , 22 juveniles (J. E. Hudson) ( NMU TRW1971.14 ) ; “Afrique Orientale” (either Boura [Taita] or Kilimanjaro ), male (cw 47.4 mm), 1904 (C. H. Alluaud) ( USNM 32297 ) ; Mount Mbolo , Taita, male, iv.1934 (A. Loveridge) ( USNM 70913 ) .

Diagnosis. Postfrontal crest sharp­edged distinct, complete; exorbital tooth small, low; epibranchial tooth small granule, anterolateral margin immediately behind epibranchial tooth smooth; ischium of third maxilliped with vertical sulcus; thoracic sternal sulcus s3/ s4 complete, deep; sternite s4 lacking raised marginal ridges at points where chelipeds articulate; episternal sulci s4/e4–s7/e7 all clearly marked; dactylus of major cheliped of adult males broad, curving but not arched; first carpal tooth on carpus of cheliped sharp spine; second carpal tooth sub­equal spine; medial inferior margin of merus of pereiopod 1 lined with teeth, lateral inferior margin granulated; distal meral tooth large, pointed; proximal half of terminal article of gonopod 1 straight, second half bending outward at 45° angle; lateral side of terminal article of gonopod 1 widened in middle by distinctly raised lateral fold forming long crest; medial fold small, low; base of terminal article concave at dorsal membrane; distal margin of subterminal segment at dorsal membrane u­shaped, medial, lateral sides equally high; dorsal membrane subcircular, narrowest at medial, lateral margins.

Size. Large, with an adult size range between cws 45–50 mm. The largest specimen examined was an adult female, cw 84.1 mm.

Type locality. Kilimanjaro, Tanzania .

Distribution. Kenya and Tanzania.

Remarks. All specimens of P. johnstoni were identified based on comparisons with detailed illustrations of the carapace, mouthparts, sternum, abdomen and gonopods of the male type specimen of T. depressa var. johnstoni Miers, 1885 (cw 57 mm) (BMNH 1885.2) from Kilimanjaro. Unfortunately, Calman (1913) used the same name to describe a specimen from the Ruwenzori Mountains in Uganda. Cumberlidge (1997, 1998) examined the type material of both of these taxa and concluded that the specimens from Kilimanjaro and the Ruwenzoris belong to two distinct species. The specimens from Kilimanjaro retain the name Potamonautes johnstoni Miers, 1885 by priority, while the type of Calman’s (1913) species from the Ruwenzoris corresponds to Potamonautes aloysiisabaudiae ( Nobili, 1906) .

Bott (1955) synonymized P. johnstoni with seven different taxa, but Cumberlidge (1997, 1998) found all of these synonymies to be questionable. The carapace, sternum, mandibles and cheliped of the type specimens of Telphusa reichardi Hilgendorf, 1898 (a subadult female, cw 33.8, cl 23.7, ch 11.4, fw 10 mm; ZMB 7463), Telphusa suprasulcata var. pseudoperlata Hilgendorf, 1898 (a subadult male, cw 32.1, cl 24.3, ch 9.9, fw 9.2 mm; ZMB 9348) and Telphusa mrogoroense Hilgendorf, 1898 (a subadult female, cw 20.8, cl 15.1, ch 7.2, fw 6.1 mm; ZMB 11377) were examined in the present study by NC. In each of these taxa, the characters correspond well with those of subadult specimens of P. suprasulcatus ( Hilgendorf, 1898) and all are treated here as junior subjective synonyms of P. suprasulcatus . However, the type of Telphusa hilgendorfi Hilgendorf, 1898 (a subadult female, cw 20.8, cl 15.1, ch 7.2, fw 6.1 mm; ZMB 11377) was found to correspond well with P. johnstoni and is considered here to be a junior synonym of P. johnstoni .

Bott (1955) also synonymized P. johnstoni with Potamon (Potamonautes) unisulcatus Rathbun, 1933 and with Potamon (Potamonautes) montivagus Chace, 1953 . Comparison of P. johnstoni to the types of P. (P.) unisulcatus (an adult male, cw 33, cl 21, ch 9, fw 10 mm; MCZ 7678a) and of P. (P.) montivagus (an adult male, cw 65.5, cl 44.2, ch 21.8, fw 16.6 mm; MCZ 12611) found the characters of the carapace, sternum, mandibles and cheliped to be unique in each taxon. Therefore, Bott’s (1955) opinion that P. (P.) unisulcatus and P. (P.) montivagus are junior subjective synonyms of P. johnstoni is not accepted in the present work. Finally, Bott (1955) regarded Potamon (Potamonautes) ambiguus to be a junior synonym of P. suprasulcatus , but examination of the characters of the carapace, chelipeds and first gonopods of the type of P. (P.) ambiguus (USNM 30008) support the conclusion that this taxon should properly be treated as a junior synonym of P. johnstoni ( Miers, 1885) .

Natural history and conservation status. The conservation status of P. johnstoni is categorized as vulnerable (VU) ( Table 4) because it has a narrow range of occurrence and a restricted area of occupancy that are both below the thresholds for vulnerable (VU) ( IUCN 2004). It is difficult to estimate the population status and trends of this species, but its population is estimated to be declining based on indirect measures such as lack of recent collections of specimens, a relatively poor representation in museum collections and increasing habitat disturbance associated with growing human populations in the region.