Potamonautes gerdalensis Bott, 1955

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

2. Potamonautes gerdalensis Bott, 1955 (Figs. 12–22, 147–148, 174, plate II)

Potamonautes (Gerdalopotamonautes) gerdalensis Bott, 1955: 261–262 , figs. 34, 82, pl. 13, fig. 3a–d.

Potamonautes gerdalensis — Cumberlidge, 1998: 199.

Type material examined: TANZANIA: Girdalo , on the border between Tanzania and Kenya, adult male holotype (cw 34 mm), adult female paratype (cw 29.3 mm), juvenile paratype (cw 22.1 mm), 27.i.1911 (Kattwinkel) ( ZSM 1189 /1) .

Additional material examined: KENYA: Nyanza Province, Kisii, upper reaches of the Kitare River , male (cw 36.3 mm), 13 female subadults and juveniles (cw 10.2 to cw 34 mm), 3 juvenile males (cw 17.9 to cw 20.5 mm) 23.ix.1960 ( R. B. Highton) ( NMU TRW1960.05 ) ; Nyanza Province, Kisu, near Kodera , Sauda , Kitare and Awash Rivers , subadult male adult (cw 31.8 mm), subadult male (cw 26 mm), subadult male (cw 24.6 mm), subadult female (cw 29.5 mm), subadult female (cw 27 mm), 16.xii.1959 (J. McMahon) ( BMNH 1951.10.31.1–5) .

Diagnosis. Postfrontal crest sharp­edged, completely crossing carapace; exorbital tooth low, epibranchial tooth absent; anterolateral margin behind epibranchial tooth faintly granulated, curving inward for short distance over branchial region of carapace; suborbital region of carapace sidewall smooth, subhepatic, pterygostomial regions of carapace sidewalls with fields of granules, short carinae; ischium of third maxilliped with vertical sulcus; sternal sulcus s3/s4 wide, deep, completely crossing sternum, touching anterior margin of sternoabdominal cavity; episternal sulcus s4/e4 missing, s5/e5 incomplete, s6/ e6, s7/e7 complete; first carpal tooth of P1 large, pointed, second carpal tooth granular, with several small granules behind it; merus of cheliped elongated, dactylus arched, enclosing oval interspace; basal part of terminal article of gonopod 1 thickened; lateral, medial folds low, even­sized, separated by broad seminal groove; medial fold wider (but not higher) than lateral fold; tip of terminal article of gonopod 1 short, hook­shaped, inwardly curved; distal margin of subterminal segment highest on lateral side (forming rounded shoulder), lowest on medial side; dorsal membrane on lateral side very broad, clearly separating terminal article from subterminal segment; dorsal membrane on medial side narrow at boundary between segments; groove for gonopod 2 on subterminal segment of gonopod 1 broad.

Size. Medium to large, with a pubertal molt occurring between cws 34–35 mm.

Variation. The chelipeds of P. gerdalensis exhibit allometric growth. Heterochely becomes obvious in adult males as well as adult females, though to a lesser degree. The dactylus of the major cheliped in younger crabs is straight, and when closed its teeth completely meet those of the fixed finger. In older crabs the dactylus does not meet the fixed finger when closed. The degree of arching of the dactylus of the major cheliped of adult males varies and the enclosed interspace varies from narrow to oval. The sulcus of the ischium of the third maxilliped is always present, but varies from deepin subadults (Kisu specimens) to weak (holotype) in adults.

Type locality. Girdalo Tanzania, on the border between Tanzania and Kenya. Unfortunately, when Bott (1955) established the species P. gerdalensis and the subgenus Gerdalopotamonautes he adopted the spelling based on the type locality of ‘Gerdalo’. Today, this location is known in gazetteers as ‘Girdalo’, while the spelling ‘Gerdalo’ is not recoverable.

Distribution. Mara and Arusha regions of Tanzania and the Nyanza Province of Kenya. The NMU collection contains two specimens of P. gerdalensis both from the Nyanza Province of Kenya that represent the only new material known since the original description of this species in 1955. These records extend the range of this species northwest along the border between Tanzania and Kenya.

Remarks. This material was identified following direct examination by one of us (NC) of the holotype (an adult male, cw 34 mm) and paratypes, an adult female (cw 29.3 mm) and a juvenile (cw 22.1 mm), 27.i.1911 (Kattwinkel) (ZSM 1189/1), as well as Bott’s (1955) photographs and illustrations of the holotype. Bott (1955) erected the subgenus P. ( Gerdalopotamonautes ) to contain a single species, P. gerdalensis ; and although Cumberlidge (1998) accepted the validity of the species, he did not recognize the subgenus, and this opinion is followed here. Potamonautes gerdalensis is morphologically close to P. alluaudi ( Bouvier, 1921) from Mount Kenya and the Aberdares in Kenya: both are large species, both have a sharp­edged complete postfrontal crest, a low exorbital tooth, a missing epibranchial tooth and a rounded shoulder on the distal medial margin of the subterminal segment of gonopod 1. However, these two taxa can be clearly distinguished by the form of the terminal article of the first gonopod (the tip of the terminal article is sharply upcurved in P.gerdalensis and straight in P. alluaudi ).

Natural history and conservation status. The conservation status of P. gerdalensis 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). Its population is estimated to be low based on indirect measures such as the lack of recent collections of specimens and its poor representation in museum collections.