Potamonautes kensleyi Cumberlidge & Tavares, 2006

Cumberlidge, Neil, Ahles, Katelyn M. & Daniels, Savel R., 2021, Redescriptions of three species of freshwater crabs from Angola, southern Africa (Brachyura: Potamoidea: Potamonautidae), Zootaxa 5032 (4), pp. 516-532 : 519-521

Potamonautes kensleyi Cumberlidge & Tavares, 2006

( Figs. 2 , 4B , 5C, D, I, J , 6B, E , 7B , 8 )

Potamonautes kensleyi Cumberlidge & Tavares, 2006: 248 , 249, 251–256, table 1, figs. 1, 2; Cumberlidge 2008c: 1–8, fig. 1; Ng et al. 2008: 171.

Holotype. MZUSP 16879 , adult female (CW 40, CL 28.6, CH 8.8, FW 13.2 mm) Angola, Cuanza (Kwanza) River upstream from the Capanda hydroelectric dam (-9.8299, 15.8509), northern Malanje Province, 150 km southeast of Malanje city, from rocky river bed with no ciliary vegetation, water temperature 26.8 °C, pH 7.3, October 2002, coll. Hamilton Garboggini.

Other material. AMG GEN 1887 A, adult male (CW 35.4, CL 25.9, CH 7.6, FW 11.8 mm), Angola, Soxujwa, Kekere, Cuanza River , downstream of Capanda hydroelectric dam (-9.806556, 15.4084722), northern Malanje Province, 150 km southeast of Malanje city, caught in a hand net , 12 Nov. 2006, coll. I. R. Bills .

Rediagnosis. Carapace surface smooth, widest in anterior third ( CW /FW 3.0), extremely flat ( CH /FW 0.66) ( Fig. 4B ), semi-circular, urogastric, transverse branchial grooves deep; cardiac region weakly marked, cervical grooves short, faint ( Fig. 2A ). Front relatively broad, about 1/3 carapace width (FW/ CW 0.33); frontal margin conspicuously bilobed, deeply indented in center ( Fig. 2A, B ); exorbital, epibranchial teeth large, pointed, directed forward; postfrontal crest sharply defined, complete, crossing entire carapace; straight part consisting of fused epigastric, postorbital crests curving backward; ends curving forward to meet anterolateral margins at epibranchial teeth; anterolateral margin posterior to epibranchial tooth smooth ( Fig. 2A, B ). Branchiostegite with 2 sutures, one longitudinal (epimeral), one vertical, dividing sidewall into suborbital, subhepatic, pterygostomial regions, each with smooth surface ( Fig. 4B ).

Third maxillipeds filling entire oral field, except for transversely oval efferent respiratory openings at superior lateral corners; long flagellum on exopod of third maxilliped; ischium of third maxilliped smooth, completely lacking vertical sulcus ( Figs. 2B , 4B ). Mandibular palp 2-articled; terminal article undivided, with setae but no hardened ledge at junction between articles. Male thoracic sternal sulcii S1/2, S2/3 completely traversing sternum; S3/4 absent except for 2 small notches at sides of sternum; thoracic episternal sulci S4/E4, S5/E5, S6/E6, S7/E7 all obscure, either missing or faint ( Fig. 2B ). Dactylus of male right (major) chela long, slim, gently curved, with 4 well-spaced medium teeth; immovable finger (propodus pollex) slim, slender, with small teeth; both fingers touching along distal third of their length, enclosing long slim interspace when closed; palm of propodus of major chela enlarged, swollen, lower margin of propodus distinctly concave ( Figs. 2B , 5C ). Dactylus of male left (minor) chela long, slim, gently curved, with 4 well-spaced medium teeth; immovable finger (propodus pollex) slim, slender, almost smooth, lacking teeth; both fingers touching along distal quarter of their length, enclosing long slim interspace when closed; palm of propodus of minor chela slim, not enlarged, 0.6 × propodus height of major chela; propodus elongated (as long as propodus of major chela), lower margin distinctly concave ( Fig. 5D ). Inferior margins of cheliped merus both completely smooth, distal meral tooth missing ( Fig. 5I, J ); cheliped carpus inner margin smooth, distal, proximal teeth completely reduced (missing) ( Fig. 5J ); ambulatory legs P2–5 slender, distal limb articles (merus, carpus, propodus, dactylus) all elongated; P4 longest, P5 shortest; P4 dactylus extremely long (as long as P4 propodus), P5 dactylus short (1/2 as long as P4 dactylus ( Figs. 2A, B )); dactyli of P2–5 tapering to point, each bearing 4 rows of downward-pointing sharp bristles ( Fig. 2A, B ).

Male pleon, telson together forming slim triangle, pleon edges slightly indented; telson triangular, apex round- ed, base broadest, sides outwardly sloping; pleomeres PL 1–6 rectangular, wider than long, PL 6 longest, more than 1/2 as long as wide ( Fig. 2B ); remaining pleomeres short, less than 1/3 as long as wide. G1 TA very short, about 1/4 G1 SA length (G1 TA / SA 0.26), angled outward at 30° to longitudinal axis of G1 SA; G1 TA slim, not widened, tapering gently to pointed gently upcurved tip ( Figs. 6B, E ). G1 SA widest at base, narrowest at TA / SA junction; basal G1 SA mesial margin lined by conspicuous long setae basally, margin smooth distally; G1 SA lateral margin smooth ( Fig. 6B, E ). G2 ( Fig. 7B ) long, slim, subequal to G1. G2 TA long, flagellum-like (G2 TA / SA 0.62)); G2 SA widest at base, tapering sharply inward about one-third along length, last two-thirds forming long, thin, tapering, upright process supporting long flagellum-like TA.

Size. Medium species, adult at CW 35.0 mm.

Colour. The colour of living specimens is unknown, but specimens preserved in ethanol are light brown.

Type locality. In the Cuanza (Kwanza) River upstream from the Capanda hydroelectric dam (-9.8299, 15.8509), northern Malanje Province, 150 km southeast of Malanje city, Angola ( Fig. 8 ) .

Distribution. Potamonautes kensleyi is known only from two localities 50 km apart in the Cuanza River in northern Cuanza Sul Province in the savanna region of central Angola. Although these two localities lie on the same river they are now separated by the Capanda hydroelectric dam, one downstream of the dam and the other behind the dam in a stream flowing into the artificial lake/reservoir created by the dam ( Fig. 8 ).

Habitat. This species is found in the Cuanza River in central Angola and lives under rocks in the river bed in an area that lacks aquatic vegetation where the water temperatures are warm (26.8° C).

Conservation status. Potamonautes kensleyi was listed in the IUCN Red List of Threatened Species as Data Deficient ( Cumberlidge 2008b) because at that time it was known to occur in just one locality and there was very little information available on its population trends or habitat. The construction of the Capenda hydroelectric dam where this restricted range species occurs has dramatically altered the freshwater habitats associated with the Cuanza River by creating a large reservoir of standing water in the natural path of the river. The new material indicates that this species is found both upstream and downstream of the dam but neither of the two localities ( Fig. 8 ) are in a protected area. The unusual morphological characters that set P. kensleyi apart from nearly all other species of Potamonautes MacLeay, 1838 in continental Africa indicate that it is uniquely adapted for a narrow niche, and that large scale disturbance of its habitat (such as alterations of water flow associated with the hydroelectric dam) would be expected to have a large impact on its long-term survival.

Remarks. This species was originally described from a single female specimen because it’s extremely distinct morphological characters clearly set it apart from other species, even without reference to adult male characters of the gonopods, chelipeds, pleon, and sternum ( Cumberlidge & Tavares 2006). For example, characters that clearly distinguish P. kensleyi from all other species of Potamonautes include the deeply bilobed front ( Figs. 2A, B ), the extremely flattened carapace ( Fig. 4B ), the extremely elongated propodus and dactylus of P4 ( Figs. 2A, B ), the completely smooth inner margins of the cheliped carpus that lacks teeth ( Fig. 5J ), and the completely smooth lower margins of the cheliped merus ( Figs. 5I, J ). To these can be added characters of the male chelipeds, sternum, and gonopods of P. kensleyi that are reported here for the first time ( Figs. 2A,B , 4B , 5C, D, I, J , 6B, E , 7B ) that also help distinguish this species.

Potamonautes kensleyi is similar in some respects to P. macrobrachii , another medium-sized species that is endemic to Angola, because both have a smooth anterolateral carapace margin ( Figs. 2A , 3A ) and both lack a vertical sulcus on the third maxilliped ischium ( Figs. 2B , 3B ). However, a number of characters of the carapace and chelipeds of P. kensleyi help distinguish this species from P. macrobrachii . These characters are the deeply indented and bilobed frontal margin in P. kensleyi ( Figs. 2A, B , 4B ) (vs the straight and not bilobed frontal margin in P. macrobrachii ( Figs. 3A, B , 4C ); the forward-projecting front in P. kensleyi ( Figs. 2A, B , 4B ) (vs the sharply deflexed front in P. macrobrachii ( Fig. 4C ); the large spine-like exorbital and epibranchial teeth in P. kensleyi ( Figs. 2A , 4B ) (vs very low and inconspicuous exorbital and epibranchial teeth in P. macrobrachii ( Figs. 3A, B , 4C )); the sharpedged postfrontal crest whose ends curve backward at the sides in P. kensleyi ( Figs. 2A , 4B ) (vs the low and straight postfrontal crest whose ends do not curve backward in P. macrobrachii ( Figs. 3A , 4C ); and the cheliped carpus that is completely missing both proximal and distal teeth in P. kensleyi ( Figs. 2A , 5J ) (vs the cheliped carpus with small but visible granule-sized teeth in P. macrobrachii ( Figs. 3A , 5L )). In addition, P. kensleyi has slim, slender, straight forceps-like major chela fingers that touch along the distal third of their length, enclosing long slim interspace ( Figs. 2B , 5C, D ) (vs a broad distinctly arched dactylus of the major chela that encloses a broad oval interspace between the fingers that touch only at their tips when closed in P. macrobrachii ( Fig. 5E )); and a male thoracic sternum with an S3/4 that is obscure except for two small side notches, and an S4 whose outer margins are neither thickened nor raised in P. kensleyi ( Fig. 2B ) (vs a deep and v-shaped S3/4 that completely traverses the thoracic sternum, and an S4 whose outer margins are thickened and raised in P. macrobrachii ( Fig. 3B )). Finally, characters of the G1 TA of P. kensleyi include its short length (about one-quarter as long as the SA (G1 TA/SA 0.26)), its outward angle (at 30° to the longitudinal axis of the G1 SA), and its slim unwidened shape that tapers gently to the tip, and lacks a raised mesial fold ( Figs. 6B, E ) (vs a G1 TA that is about one-third as long as the SA (G1 TA/SA 0.33), angled more sharply outward at 50° to the longitudinal axis of the G1 SA, and distinctly widened in the midsection by a raised, rounded mesial fold in P. macrobrachii ( Figs. 6C, F )).

Characters that distinguish P. kensleyi from P. anchietae include the smooth anterolateral margin of the carapace in the former species ( Figs. 2A , 4B ) (vs a granulated anterolateral margin in P. anchietae (cf. Bott 1955: fig. 1a)); the third maxilliped ischium that lacks a vertical sulcus in P. kensleyi ( Figs. 2B , 4B ) (vs an ischium with a distinct vertical sulcus in P. anchietae (cf. Bott 1955: fig. 1b, d)); the cheliped carpus that lacks a distal tooth on the inner margin in P. kensleyi ( Figs. 2A , 5J ) (vs a large and pointed distal tooth in P. anchietae (cf. Bott 1955: fig. 1a)); and the P4 dactylus that is slender and elongated in P. kensleyi ( Figs. 2A, B ) (vs a short P4 dactylus in P. anchietae ( Bott 1955: fig. 1a)). Other characters that distinguish P. kensleyi from P. anchetiae include a deeply indented and bilobed frontal margin in P. kensleyi ( Figs. 2A, B , 4B ) (vs a straight frontal margin in P. anchetiae (cf. Bott 1955: fig. 1a)); a front that projects straight out without curving downward in P. kensleyi ( Figs. 2A, B , 4B ) (vs a front that is distinctly deflexed in P. anchietae (cf. Bott 1955: fig. 1d)); exorbital and epibranchial teeth that are large forward pointing spines in P. kensleyi ( Figs. 2A, B , 4B ) (vs a small granule and a low and broad tooth, respectively in P. anchietae (cf. Bott 1955: fig. 1a)); a postfrontal crest that is sharp-edged with ends that curve backward at the sides in P. kensleyi ( Figs. 2A , 4B ) (vs a faint, low and straight postfrontal crest whose ends do not curve backward in P. anchietae (cf. Bott 1955: fig. 1a)); and the cheliped carpus that lacks distal and proximal teeth on the inner margin in P. kensleyi ( Fig. 5J ) (vs a cheliped carpus inner margin with two large, pointed teeth in P. anchietae (cf. Bott 1955: fig. 1a)).

It is interesting that some of the unusual characters of P. kensleyi resemble those of Erimetopus brazzae (A. Milne-Edwards, 1886) from the lower Congo River basin in Central Africa ( Cumberlidge & Reed 2004). For example, both species share a conspicuously bilobed deeply indented frontal margin that projects forward rather than curves downward, and large, sharp, forward-pointing epibranchial teeth ( P. kensleyi , Fig. 2A, B; E . brazzae, Bott 1955: pl. 3: figs. 1a, b; Cumberlidge & Reed 2004: figs. 1, 13, 14). However, these two taxa are not closely related, and can be easily distinguished as follows: P. kensleyi has a larger adult body size above CW 35 mm (vs a smaller adult size range beginning at CW 23.5 mm in E. brazzae ); a transversely oval carapace outline ( Figs. 2A, B ) (vs a subhexagonal/rounded carapace outline in E. brazzae ); a curved broad, upper orbital margin (1/4 CW) (vs a semicircular, narrow upper orbital margin (1/7 CW) in E. brazzae (cf. Bott 1955: pl. 3: figs. 1a, b; Cumberlidge & Reed 2004: figs. 1, 13, 14)); the lack of distal and proximal teeth on the inner margin of the cheliped carpus (vs large, slender, and pointed teeth on the inner margin of the cheliped carpus in E. brazzae ); a smooth outer margin of the cheliped carpus (vs a series of pointed teeth on the outer margin in E. brazzae ); and carpi and propodi of P2–4 that have smooth anterior margins (vs P2–4 anterior margins lined by spines in E. brazzae (cf. Bott 1955: pl. 3: figs. 1a; Cumberlidge & Reed 2004: figs. 4, 13)).

Figures

FIGURE 2. Potamonautes kensleyi Cumberlidge & Tavares, 2006, adult male CW 35.4 mm, from the Cuanza River, Soxujwa, Kekere, Malanje Province, Angola (AMG GEN 1887A). Whole animal, dorsal view (A), whole animal, ventral view (B). Photographs by T. Busschau, University of Stellenbosch, South Africa. Scale bars: A = 12.0 mm, B = 12.2 mm.

FIGURE 3. Potamonautes macrobrachii Bott, 1953, holotype, adult ♂ CW 32.9 mm, from Huamba Province, Angola (SMF 1892). Whole animal, dorsal view (A), whole animal, ventral view (B). Scale bars: A = 12.5 mm, B = 5.38 mm.

FIGURE 4. Frontal view of carapace. (A) Potamonautes adeleae Bott, 1968, paratype, adult ♂ CW 55.3 mm, from Lunda Norte Province, Angola (SMF 4444); (B) Potamonautes kensleyi Cumberlidge & Tavares, 2006, adult male CW 35.4 mm, from the Cuanza River, Soxujwa, Kekere, Malanje Province, Angola (AMG GEN 1887A); (C) Potamonautes macrobrachii Bott, 1953, paratype, adult ♂ CW 32.9 mm, from Huamba Province, Angola (SMF 1892). Photograph B by Theo Busschau, University of Stellenbosch, South Africa. Scale bar: A = 10.4 mm; B = 5.7 mm; C = 4.7 mm.

FIGURE 5. (A–F) Frontal view of right and left chela; (G–L) cheliped merus and carpus. (A, B) Potamonautes adeleae Bott, 1968, paratype, adult ♂ CW 55.3 mm, from Lunda Norte Province, Angola (SMF 4444); (C, D) Potamonautes kensleyi Cumberlidge & Tavares, 2006, adult male CW 35.4 mm, from the Cuanza River, Soxujwa, Kekere, Malanje Province, Angola (AMG GEN 1887A); (E, F) Potamonautes macrobrachii Bott, 1953, holotype, adult ♂ CW 32.9 mm, from Huamba Province, Angola (SMF 1892); (G, H) Potamonautes adeleae Bott, 1968, paratype, adult ♂ CW 55.3 mm, from Lunda Norte Province, Angola (SMF 4444); (G) ventral view of right chela merus; (H) dorsal view of right chela carpus and merus; (I, J) Potamonautes kensleyi Cumberlidge & Tavares, 2006, adult male CW 35.4 mm, from the Cuanza River, Soxujwa, Kekere, Malanje Province,Angola (AMG GEN 1887A) (I) ventral view of right chela merus; (J) dorsal view of right chela merus and carpus; (K, L) Potamonautes macrobrachii Bott, 1953, holotype, adult ♂ CW 32.9 mm, from Huamba Province, Angola (SMF 1892): (K) ventral view of right chela merus; (L) dorsal view of right chela merus and carpus;. Photographs C, D, I, J by Theo Busschau, University of Stellenbosch, South Africa. Scale bar: A = 18.4 mm; B = 17.9 mm; C, D = 8.9 mm; E = 8.33 mm; F = 5.81 mm; G = 17.9 mm; H = 7.3 mm; I = 8.9 mm; J = 5.9 mm; K = 5.81 mm; L = 5.6 mm.

FIGURE 6. Right G1, ventral view (A–C) and dorsal view (D, F). (A, D) Potamonautes adeleae Bott, 1968, paratype, adult ♂ CW 55.3 mm, from Lunda Norte Province, Angola (SMF 4444); (B, E) Potamonautes kensleyi Cumberlidge & Tavares, 2006, adult male CW 35.4 mm, from the Cuanza River, Soxujwa, Kekere, Malanje Province, Angola (AMG GEN 1887A); (C, F) Potamonautes macrobrachii Bott, 1953, holotype, adult ♂ CW 32.9 mm, from Huamba Province, Angola (SMF 1892). Photo- graphs B and E by Theo Busschau, University of Stellenbosch, South Africa. Scale bar: A= 6.7 mm; B = 2.3 mm; C = 1.13 mm; D = 2.7 mm; E = 2.3 mm; F = 1.2.

FIGURE 7. Right G2 ventral view (A–C); (A) Potamonautes adeleae Bott, 1968, paratype, adult ♂ CW 55.3 mm, from Lunda Norte Province, Angola (SMF 4444); (B) Potamonautes kensleyi Cumberlidge & Tavares, 2006, adult male CW 35.4 mm, from the Cuanza River, Soxujwa, Kekere, Malanje Province, Angola (AMG GEN 1887A); (C) Potamonautes macrobrachii Bott, 1953, holotype, adult ♂ CW 32.9 mm, from Huamba Province, Angola (SMF 1892). Photograph C by Theo Busschau, University of Stellenbosch, South Africa. Scale bar: A = 5.6 mm; B = 2.3 mm; C = 1.2 mm.

FIGURE 8. Map of the Angola showing the known localities of Potamonautes adeleae (black square, not shown to scale), P. kensleyi (black circles, not shown to scale), and P. macrobrachii (black diamond, not shown to scale). Scale bar = 520 km. See text for exact locality coordinates.