Potamonautes stanleyensis ( Rathbun, 1921 )

Cumberlidge, Neil, 2015, Redescriptions of three species of freshwater crabs from the Democratic Republic of Congo, Central Africa (Brachyura: Potamoidea: Potamonautidae), Zootaxa 3973 (1), pp. 119-138 : 126-131

Potamonautes stanleyensis ( Rathbun, 1921)

Kisangani Freshwater Crab

( Figs. 5–8 )

Potamon (Potamonautes) stanleyensis Rathbun, 1921: 415 –418, plate 26, fig. 1–2, fig. 9. Potamonautes dybowskii— Balss 1936: 187, 188 [part].

Potamon stanleyensis— Chace 1942: 187, 222.

Potamonautes (Lobopotamonautes) stanleyensis— Bott 1955: plate 15, fig. 1 a–d, fig. 35, 86. Potamonautes stanleyensis— Cumberlidge 1999; Ng et al. 2008: 171; Cumberlidge et al. 2009: appendix item 942.

Type material examined. D.R. CONGO: Orientale Province: affluent of the Tschopo River near Kisangani (0.56° N, 25.12° E), adult male holotype (CW 34.7, CL 25.2) 21 Aug. 1909 (J. Chapin and H. Lang) ( AMNH 3459). Forest stream near Kisangani (0.52° N, 25.20° E), 1 male, 6 juveniles, paratypes, Apr. 1915 (J. Chapin and H. Lang) ( AMNH 3345); forest stream near Kisangani, 4 males, 1 female, 51 juveniles, paratypes, Apr. 1915, collected together with P. d y bo w s k i i (J. Chapin and H. Lang) ( AMNH 3390); small affluents of the Tshopo River near Kisangani, 11 males, 15 females, 91 juveniles, paratypes, Apr. 1915 (J. Chapin and H. Lang) ( AMNH 3434); Kisangani, 9 males, 5 females, 45 juveniles, paratypes, May 1915 (J. Chapin and H. Lang) ( AMNH 3437); Kisangani, 1 adult female with 10 hatchlings, paratypes, Apr. 1915 (J. Chapin and H. Lang) ( AMNH 3441, 3478, 3493); Kisangani, 4 males, 2 females, paratypes, from stream, Apr. 1915 (J. Chapin and H. Lang) ( AMNH 3509); Orientale Province, Bafwamoko (= Bafwamogo), between Bafwasende and Bafwaboli (0.87° N, 26.43° E), 1 male juvenile, paratype, 14 Sep. 1909 (J. Chapin and H. Lang) ( AMNH 3451).

Other material. Tshopo River, Apr. 1915 (J. Chapin and H. Lang) ( MRAC 1.044, 1.047, 1.049, 1.051, 1.054); Second-Yangambi (0.77° N, 24.44° E), 2 juv. ( MRAC 31.518-31.524); Kisangani (formerly Stanleyville) 1 juvenile ( MRAC 32.883).

Diagnosis. Postfrontal crest complete; exorbital tooth low, blunt, epibranchial tooth small, distinct; anterolateral margin behind epibranchial tooth smooth ( Fig. 5 B); ischium of third maxilliped with deep vertical sulcus ( Fig. 5 J); thoracic sternal sulcus s3/s4 deep at sides, absent in middle, sides slanted inward toward anterior margin of sterno-abdominal cavity ( Fig. 5 C); second carpal tooth on inner margin of carpus of cheliped small, pointed, followed by 2 smaller teeth ( Fig. 5 F,G); G1 terminal article straight for basal third, then strongly bent outward at a 45° angle ( Fig. 6 D,E,F).

Redescription. Postfrontal crest sharp, completely crossing carapace, epigastric crests clear, median sulcus between crests short, forked posteriorly; epigastric crests continuous with postorbital crests, lateral ends of postorbital crests meeting anterolateral margins ( Fig. 5 B). Exorbital, epibranchial teeth small, low; anterolateral margin between exorbital, epibranchial teeth smooth, lacking intermediate tooth; anterolateral margin behind epibranchial tooth smooth ( Fig. 5 B). Suborbital margin smooth ( Fig. 5 A). Carapace high ( CH /FW 1.33); front narrow (one-third CW, FW/CW 0.3) ( Fig. 5 A). Semi-circular, urogastric, cardiac, posterior, cervical carapace grooves all distinct ( Fig. 5 B). Carapace sidewall with distinct, granular vertical sulcus, meeting longitudinal sulcus, dividing sidewall into 3 parts ( Fig. 5 A). Exopod of third maxilliped with long flagellum, ischium of third maxilliped with deep vertical sulcus ( Fig. 5 J). Epistomial tooth large, triangular, edges lined by large round granules. Mandibular palp two-segmented; terminal segment simple ( Fig. 6 A—C). Thoracic sternal sulcus s2/s3 deep, running horizontally across sternum; thoracic sternal sulcus s3/s4 deep at sides, absent in middle, sides slanted inward toward anterior margin of sterno-abdominal cavity ( Fig. 5 C). Thoracic episternal sulci s4/e4, s5/e5, s6/e6, s7/e7 all visible ( Fig. 5 C).

Chelipeds of adult male unequal; fingers of major (right) cheliped long, robust, enclosing narrow space when closed, teeth irregular, those in proximal half bigger than distal part; fingers of minor cheliped long, slender, almost meeting ( Fig. 5 D,E).

Major right cheliped of adult male with swollen palm, long curved dactylus, long straight propodus, fixed finger with 3, 4 teeth increasing in size from palmar end ( Fig. 5 D, E). First carpal tooth on inner margin of carpus of cheliped large, slender, pointed; second carpal tooth small, pointed, followed by 2 smaller teeth ( Fig. 5 F, G). Lateral, medial inferior margins of merus of cheliped faintly granular, with single large distal meral tooth at distal end; superior surface of merus granular ( Fig. 5 F, G). Walking legs (p2–p5) moderate length, slender; merus of p2 3.4 times as long as wide ( Fig. 7 A, B).

Abdomen slim, outline triangular with straight edges; a6 almost as long as telson (a7); a7 subtriangular, broader than long, margins indented, tip rounded ( Fig. 5 I). G1 terminal article straight for basal third, then strongly bent outward at a 45° angle; longitudinal groove visible on dorsal, superior, ventral sides; G1 terminal article about one-third as long as subterminal segment; broad dorsal membrane on dorsal face of G1 between terminal article and subterminal segment ( Fig. 6 D,E,F). G2 terminal article long, flagellum-like ( Fig. 6 G).

Size. Medium-size species, adults beginning at CW 35 mm; largest male (AMNH No. 845) CW 39.4, CL 28 mm; largest female (AMNH No. 841) CW 40, CL 30 mm. Hatchling crabs under mother’s abdomen have a square carapace (CW 3.0, CL 3.0 mm), weak postfrontal crest in three separate parts each side of midpoint, chelipeds equal-size, walking legs (p2-p5) slender. First stage free-living crabs (range CWs 4.3–4.7 mm, largest CW 4.7, CL 3.0 mm) similar to hatchlings; free-living juvenile crabs up to CW 25 mm, have carapace and legs sparsely covered with very short, coarse setae, and the anterolateral margins of the carapace are minutely granular (smooth in adults).

Color. In life, dorsal carapace dark brown/green, upper sides of chelipeds and dorsal walking legs green/gray, abdomen white-gray. Juvenile crabs (up to CW 25 mm) preserved in alcohol are dark brown/green; subadult crabs (> CW 25 mm) preserved in alcohol are light yellow/green ( Rathbun 1921: 417).

Type locality. Kisangani (formerly Stanleyville), from small tributaries of the Tshopo River, a tributary of the Congo River.

Conservation status. Potamonautes stanleyensis was listed as Least Concern ( IUCN 2003; Cumberlidge et al. 2009) because although it is found in only four localities it has an estimated distributional range of about 20,000 km 2. There are no long-term threats from disturbance and pollution, and no evidence of a decline in the extent and quality of its habitat ( Cumberlidge 2008b).

Distribution. Potamonautes stanleyensis is recognized here from four localities, all in the Orientale Province of the Upper Congo region in the D.R. Congo: Kisangani, Tshopo River, Bafwamogo, and Second-Yangambi ( Fig. 8 ). Bott (1955) listed several more lots from the D.R. Congo as belonging to P. stanleyensis that are not included here because that author’s identification of these specimens is doubtful. The specimens in question are from the following localities: Kima, Lowa River (MRAC 17.608-17.609); Lubongo, Lake Kivu (MRAC 25.504-25.506); Tshienda, Mulabata, Bushimaie River (MRAC 31.228); Kampene, Kidzueme, Moluma Stream, Kwilu (MRAC 31.974-31.989); Kampene (MRAC 32.086, 32.090); Kalima, Maniema (MRAC 38.194); Meshe, Walikale (MRAC 39.038-39.039); and Malaysians (MRAC 39.083-39.093).

Habitat. This species lives in shallow forest streams ( Rathbun 1921: 418, pl. LXIV) where it is found in sympatry with Potamonautes dybowskii (Rathbun, 1905) , together with shrimps, small fishes, and water snakes. Potamonautes stanleyensis prefers shaded places along stream edges, where dead branches, leaves, and overhanging vegetation create a semi-nocturnal environment even during the daytime. Adult crabs tend to be inactive in daylight when they are concealed by their protective coloration, while the lighter-colored young crabs in stagnant bodies of water and muddy places rapidly seek protective cover when disturbed.

Remarks. This redescription is necessary because Bott’s (1955) redescription was not based on type material, and instead relied on a series of previously-unidentified museum specimens. Bott’s (1955) classification of the large African genus Potamonautes is also questionable because he placed P. stanleyensis in the subgenus Potamonautes (Lobopotamonautes) Bott, 1955 , that also included P. aloysiisabaudiae (Nobili, 1906) , P. perparvus ( Rathbun, 1921) , P. minor, Bott, 1955 , and P. gonocristatus (Gordon, 1929) . However, the recent redescription of P. perparvus , P. minor , and P. gonocristatus ( Meyer & Cumberlidge 2011) , makes a close relationship between P. stanleyensis and these species unlikely, given the significant morphological differences in the gonopods and somatic characters of these taxa and P. stanleyensis . The most recent molecular phylogeny of the Afrotropical potamonautids ( Daniels et al. 2015) groups P. stanleyensis with P. niloticus , which Bott (1955) assigned to the subgenus P. ( Acanthothelphusa ). Although Cumberlidge (1997, 1998), Ng et al. (2008), and Cumberlidge et al. (2009) all recognized P. stanleyensis as a valid species of Potamonautes they did not recognize the subgenus assignment, and they made this taxonomic change without explanation.

Comparisons. Potamonautes stanleyensis can be recognized by the characters listed in its diagnosis and by illustrations and photographs of the holotype ( Figs. 5–7 ), including those by Rathbun (1921: 430–433, pl. 32, fig. 14) as Potamon (Potamonautes) stanleyensis . Illustrations and photographs of another specimen from the D.R. Congo can be found in Bott (1955: pl. XV, fig. 1 a–d; fig. 35, 86) as Potamonautes (Lobopotamonautes) stanleyensis (MRAC 25.504, CW 48, CL 33, CH 20, FW 15 mm) from Lubongola, Kivu, D.R. Congo (2.58° S, 27.89° E).

Potamonautes stanleyensis is superficially similar to three other species of Potamonautes that occur in the Congo River basin ( Bott 1955; Cumberlidge 2009): P. lueboensis (Rathbun, 1904) , P. lirrangensis (Rathbun, 1904) , and P. dybowskii (Rathbun, 1905) . Potamonautes stanleyensis is similar to P. lueboensis in that both species are of a similar size, both have a distinct postfrontal crest across the carapace that reaches the anterolateral margins, and both possess smooth, untoothed anterolateral margins ( Fig. 5 A). The holotype of Potamon (Potamonautes) lueboensis Rathbun, 1904 (CW 40.6, CL 30, FW 10.9 mm), however, is a female, so gonopod, thoracic sternum, and major cheliped characters cannot be compared. The two species can nevertheless be distinguished by examination of the vertical sulcus on the ischium of the third maxilliped that is deep in P. stanleyensis ( Fig. 5 J) but faint or absent in P. lueboensis . Potamonautes lueboensis is restricted to the southwestern part of the Congo River basin and is not known to occur in Upper Congo basin ( Bott 1955).

Potamonautes stanleyensis is similar to P. lirrangensis in that both species have a distinct postfrontal crest across the carapace that reaches the anterolateral margins, deep grooves on the posterior part of the carapace, a thoracic sternal sulcus s3/s4 that is deep at the sides and shallow in the middle, a first carpal tooth on the cheliped carpus that is in the form of a large sharp spine, and a large pointed distal meral tooth on the chelipeds merus. The epibranchial tooth of Potamonautes stanleyensis is small, low, and blunt in P. stanleyensis ( Fig. 5 B) but a distinct sharp spine in P. lirrangensis , the anterolateral margin behind the epibranchial tooth is smooth in P. stanleyensis ( Fig. 5 B) but lined by either large granules or small teeth in P. lirrangensis , the vertical sulcus on the ischium of the third maxilliped is deep in P. stanleyensis ( Fig. 5 J) but faint or absent in P. lirrangensis , the ventral margins of the merus of P1 are almost smooth in P. stanleyensis ( Fig. 5 G) but heavily granular in P. lirrangensis , the G1 terminal article is bent outward at a 45° angle to the longitudinal axis of G 1 in P. stanleyensis ( Fig. 6 D,E), but is bent sharply outward at 90° angle to the longitudinal axis of G 1 in P. lirrangensis . Finally, P. stanleyensis is a medium-size species with a pubertal molt starting around CW 40 mm, whereas P. lirrangensis is a large species with a pubertal molt starting around CW 52 mm, and growing up to CW 81 mm ( Reed & Cumberlidge 2006).

Potamonautes stanleyensis is similar to P. dybowskii in that both species have a distinct postfrontal crest across the carapace that reaches the anterolateral margins, and both possess smooth, untoothed anterolateral margins behind the epibranchial tooth. Capart’s (1954, figs. 14, 24) figures of the carapace and G1 of the male type of Potamon (Potamonautes) dybowskii Rathbun, 1904 from Bangui, Central African Republic, indicate that these two taxa are not conspecific ( Rathbun 1921). The thoracic sternal sulcus s3/s4, for example, is deep at both sides and shallow in the middle in P. stanleyensis ( Fig. 5 C), but deep and completely crosses the sternum in P. dybowskii , there is a distinct vertical sulcus on the third maxilliped ischium in P. stanleyensis ( Fig. 5 J), but this sulcus is either faint or absent in P. dybowskii ; the second carpal tooth on the chelipeds carpus is a small sharp tooth in P. stanleyensis ( Fig. 5 F,G), but is reduced to a small granule in P. dybowskii , and there is a narrow rectangular interspace between the closed fingers of the major cheliped in P. stanleyensis ( Fig. 5 D), whereas there is a wide oval interspace between the closed fingers of the major cheliped in P. dybowskii ( Capart 1954; Bott 1955; Cumberlidge 1998).

Figures

FIGURE 5. Potamonautes stanleyensis (Rathbun, 1921). Holotype, CW 34.7 mm, from affluent of the Tschopo River near Kisangani, D. R. Congo (AMNH 3459). A, carapace, frontal view; B, dorsal view; C, anterior thoracic sternum; D, right cheliped, frontal view; E, left cheliped, frontal view; F, carpus and merus of right cheliped, dorsal view; G, carpus and merus of right cheliped, ventral view; H, pereiopod 3; I, pleon (abdomen); J, third maxilliped frontal view. Scale bar: 11.0 mm (A – I), 6.25 mm (J).

FIGURE 6. Potamonautes stanleyensis (Rathbun, 1921). Holotype, CW 34.7 mm, from affluent of the Tschopo River near Kisangani, D. R. Congo (AMNH 3459). A, mandible frontal view; B, mandibular palp superior view; C, mandible posterior view; D, G 1 ventral view; E, G 1 dorsal view; F, superior view of terminal article; G, G 2 ventral view. Scale bar: 2.9 mm (A – C), 4.5 mm (D – G).

FIGURE 7. Potamonautes stanleyensis (Rathbun, 1921). Holotype, adult male, CW 34.7 mm, from the Congo River at Kisangani, D. R. Congo (AMNH 3353). Whole animal A, frontal view; B, dorsal view. Scale bar: 14.8 mm (A – B).

FIGURE 8. Map of the D. R. Congo in Central Africa showing the known localities of Potamonautes stanleyensis (black circles, not shown to scale). See text for exact locality coordinates.