Potamonautes platycentron ( Hilgendorf, 1897 )

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

9. Potamonautes platycentron ( Hilgendorf, 1897) (Figs. 82–92, 161–162, 181, plate IX)

Telphusa platycentron Hilgendorf, 1897: 81–85, 1898: 11–12 , pl. fig. 4, 4a–c.

Potamon platycentron — de Man, 1898: 437; Chace, 1942: 220.

Potamon (Potamonautes) platycentron — Rathbun, 1905: 173.

Potamonautes platycentron — Balss 1929: 349.

Potamonautes (Lirrangopotamonautes) johnstoni platycentron — Bott, 1955: 267–268, pl. XVI, fig. 1a–d, fig. 37.

Type material examined: TANZANIA: Lake Chala, Kilimanjaro, Tanzania , subadult male lectotype ( ZMB 9656 ), cw 46, cl 34, ch 22, fw 13 mm .

Additional material examined: KENYA: Lake Chala , near Taveta, from the margins of the lake, lake­bed stony, 762 m asl, 4 subadult females (cw 49.4 to cw 55.5 mm), 2 subadult males (cws 50. 8, 51.7 mm), vii.1967 (J. E. Hudson) ( NMU TRW1991.12 ) .

Diagnosis. Postfrontal crest sharp­edged meeting anterolateral margins at epibranchial teeth; exorbital tooth sharp, triangular; epibranchial tooth small, granular; anterolateral margins granulated curving inward over carapace; all three regions of carapace sidewall smooth; sternal sulcus s3/s4 complete; lateral margins of s4 thin, not raised; episternal sulci s4/e4, s5/e5, s6/e6, s7/e7 all deep; ischium of third maxilliped with vertical sulcus; dorsal surface of carpus of major cheliped with carinae; first carpal tooth large, thick; second carpal tooth short, with one small tooth/granule behind it; medial inferior margin lined with teeth; distal meral tooth large, sharp; lateral inferior margin granulated; dactylus of closed major cheliped broad meshing with fixed finger of propodus; lower margin of propodus of cheliped straight; propodus granulated on posterior portion; first half of terminal article of gonopod 1 straight, second half bent at close to 90° angle to longitudinal axis of gonopod, tapering to pointed, upcurved tip; terminal article of gonopod 1 widened in middle by lateral fold forming high longitudinal crest; medial fold small, low; basal margin of terminal article straight at dorsal membrane, but slanted toward medial side; distal margin of subterminal segment v­shaped at base of dorsal membrane; dorsal membrane broad at both medial, lateral margins.

Size. A large­bodied species. All females observed were subadult, with the abdomen of the largest specimen (cw 55.5 mm) very close to maturity.

Variation. Potamonautes platycentron exhibits heterochely where the major cheliped of males is larger than the minor cheliped, while the overall shape and appearance of the major cheliped remains unchanged.

Type locality. Lake Chala spanning the border between Kenya and Tanzania .

Distribution. Potamonautes platycentron is endemic to Lake Chala (3 19’S, 37 41’E). The border between Kenya and Tanzania crosses the slopes of Kilimanjaro and passes through this crater lake. The locality information lists P. platycentron from Kenya but it is highly likely that this species is also found on the Tanzanian side of Lake Chala, so it is included in the list of species for Tanzania for completeness.

Remarks. Bott (1955) included this taxon as a subspecies of P. johnstoni and the two taxa share a large body size and characters of the postfrontal crest and chelipeds. However, the two taxa can be distinguished as follows. The dactylus of the major cheliped of P. platycentron is very broad with a rounded tip (whereas that of P. johnstoni is slim and pointed); the first carpal tooth of P. platycentron is thick and broad (whereas that of P. johnstoni is slim and pointed); the medial inferior margin of the merus of the cheliped of P. platycentron is lined by sharp teeth (whereas that of P. johnstoni is granulated or lined by small teeth); the dorsal membrane of gonopod 1 of P. platycentron is diagonally sloped at the distal margin (whereas that of P. johnstoni is rounded); and the lateral fold of the terminal article of gonopod 1 of P. platycentron is low (whereas that of P. johnstoni is high). For these reasons, these two taxa are treated in the present work as two valid species.

Natural history and conservation status. The conservation status of P. platycentron is categorized as vulnerable (VU) ( Table 4) because it has a narrow range of occurrence and 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 the lack of recent collections of specimens, a poor representation in museum collections and increasing habitat disturbance associated with growing human populations in the region.