Potamonautes xiphoidus, Reed & Cumberlidge, 2006

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

14. Potamonautes xiphoidus n. sp. (Figs. 134–144, 171–172, 186, Plate XIV)

Type material examined: TANZANIA: West Usambara Mountains, Herkulu Estate, Lushoto District , Tanga region, adult male holotype, here designated (cw 38.3, cl 26.1, ch 15.6, fw 11.4 mm), xi.1964 (J. N. Raybould) ( NMU TRW1966.06 b) . West Usambara Mountains, Herkulu Estate, Lushoto District , Tanga region, subadult male paratype, here designated (cw 27.2, cl 19.3, ch 11.7, fw 8.4 mm), xi.1964 (J. N. Raybould) ( NMU TRW1966.06 b) .

Additional material examined: TANZANIA: West Usambara Mountains, streams and dams near Herkulu Estate, Lushoto District , Tanga region, 5 males (cw 16.1 to cw 33 mm), 2 adult females (cws 30.9, 33.1 mm), 4 juvenile females (cw 21.6 to cw 25.3 mm), 2 subadult females (cws 16, 16.2 mm), xi.1964 (J. N. Raybould) ( NMU TRW1966.06 a) ; West Usambara Mountains, Mkolo River, 3 km NNE of Mayo, Lushoto District , 11.iii.1975 (J. N. Raybould) ( NMU TRW1975.02 ) ; East Usambara Mountains, Amani , female (damaged), male (damaged), 8.xii.1926 (A. Loveridge) ( USNM 64108 ) ; East Usambara Mountains, Amani , found away from water in a cultivated area, 25.v.1966 (J. N. Raybould) ( NMU TRW1966.10 ) ; East Usambara Mountains, Amani , 2 juveniles (cws 10.9, 11.6 mm), juvenile female (cw 13 mm), 3 subadult females (cw 18.7 to cw 20 mm), 7 males (cw 17.6 to cw 25.6 mm), 2 adult females (cws 24.2, 27 mm), 1970 ( NMU TRW1970.09 ) .

Diagnosis. Carapace medium height (ch/fw 1.3), anterolateral margin smooth, continuous with posterolateral margin, postfrontal crest distinct, sides curving down, not completely meeting anterolateral margins, midgroove deep, epibranchial tooth absent, exorbital angle smooth, blunt; carapace sidewalls smooth, vertical (pleural) groove weak, not meeting anterolateral margin; sternal sulcus s1/s2 visible, s2/s3 deep, straight, s3/s4 reduced to two deep notches, with sternal bulges on s4; episternal sulci s4/e4, s5/e5, s6/e6, s7/e7 complete, but weak; ischium of third maxilliped with very weak groove; first carpal tooth of cheliped sharp spine, second carpal tooth of cheliped small, sharp; merus of major cheliped very slim and long (2/3 cw) superior margin of merus of cheliped with smooth carinae, medial inferior margin with small granules and small pointed tooth, lateral inferior margin weakly granulated; major cheliped longer than carapace width (ratio 1.1), dactylus of major cheliped narrow, highly arched, enclosing large oval interspace, fixed finger of propodus with two small pointed teeth, lower margin of propodus of cheliped concave in middle; terminal article of gonopod 1 straight, slightly directed outwards, small hairs cover the terminal article, lateral, medial folds equal, small setae covering terminal article, base of subterminal segment broad, tapering to terminal article.

Size. Small, with a pubertal molt from cw 19 to cw 26 mm. The largest adult specimen is a male, cw 38.3 mm.

Variation. The chelipeds of P. xiphoidus exhibit allometric growth, which results in obvious heterochely in older male crabs, and to a lesser degree in older females. The dactylus of the major cheliped in juveniles is straight with teeth meshing with those of the fixed finger. The dactylus of adults is highly arched, forming an oval gap between it and the fixed finger when closed. The smallest male specimen exhibiting a high degree of heterochely was cw 16.1 mm. However, some male specimens as large as cw 27.4 mm showed equality in the size of the chelipeds.

Type locality. The Herkulu Estate , a tea plantation in the Western Usambara Mountains in Lushoto District of the Tanga region of Tanzania. This locality is 1,666 m asl and is situated in the montane forest zone, where areas of forest have been retained to prevent soil erosion .

Distribution. This species is endemic to the Usambara Mountains in Tanzania. The rivers of the western slopes of the Usambaras drain into the Pangani River, while the eastern slopes are drained by the Sigi River and its tributaries.

Remarks. This species is morphologically close to P. infravallatus , but can be recognized by differences in the terminal article of the first gonopod, which is short with bristles in P. infravallatus (Figs. 169–170) and a long, straight, smooth evenly­tapering spear­like process in P. xiphoidus (Figs. 171–172). The conservation status of P. xiphoidus is categorized here 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 factors such as the lack of recent specimens, its poor representation in museum collections and increasing habitat disturbance from deforestation associated with growing human populations in the region.

Etymology. The name xiphoidus is derived from the Greek word ‘xiphoid’, a combination of ‘xiphos’ (straight sword) and ‘eidos’ (like), meaning ‘like a straight sword’. This is a reference to the distinctive sword­like shape of the terminal article of the first gonopod of this species.