Potamonautes langi ( 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 : 120-126

Potamonautes langi ( Rathbun, 1921)

Lang’s Freshwater Crab

( Figs. 1–4 )

Potamon (Potamonautes) langi Rathbun, 1921: 430 –433, pl. 32, fig. 14.

Potamonautes langi— Balss 1936: 189, 190, fig. 25.

Potamon langi— Chace 1942: 215.

Potamonautes (Obesopotamonautes) langi— Bott 1955: 256–257, figs. 17, 18, 79a, b, pl. 12, figs. l’a–d. Potamonautes langi – Cumberlidge 1998: 200; Ng et al. 2008: 171; Cumberlidge et al. 2009: appendix item 942.

Type material examined. D.R. CONGO. Congo River at Kisangani (0.52° N, 25.20° E), adult male holotype (CW 45, CL 34.5, CH 16, FW 15), 21 Apr. 1915 (J. Chapin and H. Lang) ( AMNH 3353). Congo River at Kisangani, 1 male, 10 females (4 ovigerous) paratypes ( AMNH 3336); 3 females (2 ovigerous), paratypes ( AMNH 3469); Congo River at Kisangani, 3 males, 8 females (5 ovigerous), 1 juvenile ( AMNH 3363); Congo River at Kisangani, 1 male, 7 females (4 ovigerous) (5 males, 10 females (2 ovigerous) sent to USNM), paratypes ( AMNH 3332); Congo River at Kisangani, 2 males, 11 females (5 ovigerous) paratypes ( AMNH 3362); Congo River at Kisangani, 3 males, 16 females (8 ovigerous), 3 juveniles, paratypes ( AMNH 3343); Congo River at Kisangani, 3 males, 5 females (3 ovigerous), paratypes; Congo River at Kisangani, 2 females (both ovigerous) paratypes ( AMNH 3418); 2 females (both ovigerous) paratypes ( AMNH 3393); Congo River at Kisangani, 1 female, paratype, caught on the shore; coll. 14 Aug. 1909; Congo River at Kisangani, 1 female, paratype, coll. 15 Aug. 1909; Congo River at Kisangani, 4 females, paratypes, common under logs in the water, coll. 17 Aug. 1909; Congo River at Kisangani, 1 female, paratypes, coll. 18 Aug. 1909; Congo River at Kisangani, 1 male, paratype, coll. 20 Aug. 1909; Congo River at Kisangani, 1 female, paratype, ovigerous, coll. 21 Aug. 1909; Congo River at Kisangani, 1 female, paratype, coll. 28 Aug. 1909; Congo River at Kisangani, 2 females, paratypes, 1 female, paratype, ovigerous, coll. Feb. 1915; Congo River at Kisangani, 1 male, 13 females (8 ovigerous), paratypes, coll. Apr. 1915; Kisangani, 4 paratypes, (1–30 Apr. 1915) (J. Chapin and H. Lang) ( MCZ CRU- 10614). Other material. D.R. CONGO. Bukama, Katanga (9.15° S, 25.83° E), 2 males, 12 females, Jun. 1935 (P. Brien) ( MRAC 23.937-23.953); Congo River at Kisangani (0.52 N, 25.20 E) ( MRAC 32.872-32.875, 32.879-32.883, 32.895-32.897); Kaleke, Lake Kisale (8.26° S, 26.49° E), 2 males, 1 female ( MRAC 32.010-32.012); Lualaba, Kindu, Maniema (2.93° S, 25.94° E), 1 male, 1 female ( MRAC 32.898-32.900); 9 females, 1 juvenile male; Mobi River, 30 km south of Kisangani (0.22 S, 25.32 E), 3 females ( MRAC 32.901-32.903); Lake Upemba, Haut-Lomami District (8.59° S, 26.51° E), 19 adults, l female subadult, CW 37, 01–07 May 1925 (G. F. de Witte) ( ZSM 1193/1, formerly ZSM 1935, don. MRAC).

Diagnosis. Postfrontal crest complete; exorbital, epibranchial teeth both large forward-pointing spines; anterolateral margin of carapace behind epibranchial tooth with 3 long, pointed teeth ( Fig. 1 B); ischium of third maxilliped lacking visible sulcus ( Fig. 2 C); thoracic sternal sulcus s3/s4 incomplete, deep at sides absent in middle ( Fig. 1 C); carpus of cheliped with 2 subequal long, pointed teeth ( Fig. 1 F,G); G1 terminal article slim, directed outward at a 60° angle to vertical, tapering to up-curved tip ( Fig. 2 D,E,F).

Redescription. Postfrontal crest complete, epigastric lobes continuous with postorbital crests, lateral ends of postorbital crests meeting anterolateral margins. Exorbital, epibranchial teeth both large forward-pointing spines; anterolateral margin between exorbital, epibranchial teeth smooth, curving slightly outward, lacking intermediate tooth; anterolateral margin posterior to epibranchial tooth with 3 long, pointed teeth ( Fig. 1 B). Suborbital margin granular ( Fig. 1 A). Carapace flat ( CH /FW 1.07); front broad, measuring one-third CW (FW/CW 0.33) ( Fig. 1 A). Semi-circular, urogastric, cardiac, posterior, cervical carapace grooves all distinct. Epigastric crests clear, median sulcus between crests short, forked posteriorly ( Fig. 1 B). Carapace sidewall with distinct granular vertical sulcus, ending at longitudinal sulcus, dividing sidewall into 3 parts ( Fig. 1 A). Exopod of third maxilliped with long flagellum, third maxilliped ischium lacking visible sulcus ( Fig. 2 C). Epistomial tooth large, triangular, edges lined by large, round granules ( Fig. 1 A). Mandibular palp two-segmented; terminal segment simple ( Fig. 2 A, B). Thoracic sternal sulcus s2/s3 deep, running horizontally across sternum; thoracic sternal sulcus s3/s4 incomplete, deep at sides absent in middle, sides slanted inward & backward toward anterior margin of sternoabdominal cavity ( Fig. 1 C). Thoracic episternal sulci s4/e4, s5/e5, s6/e6, s7/e7 all visible grooves ( Fig. 1 C). Chelipeds of adult male unequal; fingers of major (right) cheliped long, slender, widely gaping, teeth irregular, those in proximal half bigger than distal half; fingers of minor cheliped long, slender, almost meeting. Propodus of major right cheliped of adult male with swollen palm, fixed finger long, straight; movable finger (dactylus) long, curved Fig. 1 D, E). First, second carpal teeth of inner margin of carpus of cheliped subequal long, pointed teeth. Lateral, medial inferior margins of merus of cheliped granular, with single large pointed distal meral tooth at distal end; superior surface of merus granular Fig. 1 F, G). Abdomen slim, triangular, edges straight ( Fig. 1 I). G1 terminal article about one-third as long as subterminal segment, longitudinal groove visible on dorsal, superior sides (but not on ventral side); G1 terminal article slim, directed outward at a 60° angle to vertical, tapering to up-curved tip ( Fig. 2 D,E,F). G2 terminal article long, flagellum-like ( Fig. 2 G).

Size. Large-size species, adult CW 45 mm and higher.

Color. Dorsal carapace varying from dark brown/green to dark brown/yellow tinged with red, carapace of newly molted animals much lighter, sternum gray/yellow, abdomen white/grey, chelipeds brown/purple. Cryptic color pattern may be protective.

Type locality. Congo River at Kisangani, D.R. Congo.

Habitat. This is a strictly aquatic species that prefers the quieter parts of the Congo River where floating material accumulates and sunken logs offer protection. Potamonautes langi rapidly seeks cover when disturbed, including digging into mud. This species was captured in traditional traps baited with putrid or baked manioc set in deep water with a muddy substratum. These traps also catch fishes, including small silurids and mormyrids.

Conservation status. Potamonautes langi was listed as Least Concern ( IUCN 2003; Cumberlidge et al. 2009) because it is known to occur in six localities and has a wide distributional range of about 140,000 km 2. Furthermore, there is no evidence of a decline in the extent and quality of its habitat and there are no known longterm threats to its habitat from disturbance and pollution ( Cumberlidge 2008a).

Distribution. This species is found in the tributaries of the Upper Congo River at Kisangani and south into the Katanga district of the D.R. Congo ( Fig. 4 ). Most of Bott’s (1955) material listed under P. langi has been included here, despite the fact that he relied on a non-type adult male specimen of P. l angi, which he photographed and illustrated (CW 42 mm, MRAC 23.937, plate XII, fig. 1a–d). Bott (1955) incorrectly recorded this specimen as from “Bukoba, Victoria See” (which is in Tanzania), but the museum label actually reads “Bukama” a locality in the Katanga region of the D.R. Congo. There are also doubts as to the identity of two other geographically isolated specimens from the lower Congo basin and these have not been included here. As an example, Rathbun’s (1921) record of P. langi from Kinshasa (formerly Léopoldville) in the Lower Congo basin based on a female specimen collected on 11 Aug 1909 is doubtful due to the lack of male gonopod, cheliped, and sternum characters. Bott’s (1955) identification of specimens from the Congo River at Matadi in the D.R. Congo (MRAC 21.829-21.831) is also doubtful.

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 (see above). Moreover, Bott’s (1955) classification of the large African genus Potamonautes is also questionable, because he placed P. langi in the subgenus P. ( Obesopotamonautes ), Bott, 1955, as the type species of a subgenus that also included P. obesus (A. Milne-Edwards, 1868) , and P. calcaratus (Gordon, 1929) . However, the redescription of P. ob e s u s and P. calcaratus (Reed & Cumberlidge 2004) casts doubt on a close relationship between P. langi and these two species given the significant morphological differences in the gonopods and somatic characters of these two taxa and P. langi . The most recent molecular phylogeny of the Afrotropical potamonautids ( Daniels et al. 2015) groups P. obesus and P. calcaratus together in a separate clade from P. langi and does not support the recognition of the subgenus P. ( Obesopotamonautes ). Cumberlidge (1997, 1998), Ng et al. (2008), and Cumberlidge et al. (2009) all recognized P. langi as a valid species of Potamonautes and did not recognize the subgenus assignment, but they all made this taxonomic change without explanation.

Comparisons. Potamonautes langi can be recognized by the characters listed above in its diagnosis and by the illustrations and photographs of the holotype ( Figs. 1–3 ), including those by Rathbun (1921: 430–433, pl. 32, fig. 14) as Potamon (Acanthopotamonautes) langi . Illustrations and photographs of another specimen from the D.R. Congo can be found in Bott (1955: pl. XII, fig. 1 a–d; fig. 17, 18, 79a) as Potamonautes (Obesopotamonautes) langi .

This species is superficially similar to several other species of Potamonautes with which it shares a number of taxonomic characters. These include a strong and complete postfrontal crest; strong, sharp exorbital and epibranchial teeth; several distinct spines on the anterolateral margins immediately behind each epibranchial tooth ( Fig. 1 B); a third maxilliped ischium that lacks a visible sulcus ( Fig. 2 C); a first carpal tooth on the carpus of the cheliped that is strong and pointed; and a strong and pointed distal meral tooth ( Fig. 1 F, G). Other African freshwater crabs with this suite of characters include P. niloticus (H. Milne Edwards, 1837) (Nile River basin), P. paecilei (A. Milne-Edwards, 1886) (Lower Congo River basin), P. w ar re n i (Calman, 1918), and P. dentatus, Stewart, Coke & Cook, 1995 . The latter are found in river basins in southern Africa.

Potamonautes langi differs from P. niloticus in that the second carpal tooth on the carpus of the cheliped is as large as the first carpal tooth in P. langi ( Fig. 1 F,G) but weak, low and blunt in P. niloticus , and the G1 terminal article of P. langi is clearly directed outward at 45° to the longitudinal axis of G1 ( Fig. 2 D,E), whereas that of P. niloticus is only slightly angled outward at 25° to the longitudinal axis of G1 ( Bott 1955; Cumberlidge 2009; Cumberlidge & Clark 2010). Potamonautes langi differs from P. paecilei in that P. l a ng i has three large pointed teeth on the anterolateral margin behind the epibranchial tooth ( Fig. 1 B), whereas P. paecilei has two large teeth behind the epibranchial tooth on the anterolateral margin, and the sternal sulcus s3/s4 of P. l a ng i is deep at the sides and absent in the middle ( Fig. 1 C), whereas the sternal sulcus s3/s4 of P. paecilei is completely absent ( Capart 1954; Cumberlidge 1999).

Potamonautes langi differs from P. dentatus in that the carapace margin between the exorbital and epibranchial teeth is straight in P. langi and there are three large pointed teeth on the anterolateral margin behind the epibranchial tooth ( Fig. 1 B), whereas in P. dentatus there is a deep sinus posterior to the exorbital tooth and the pointed teeth on the anterolateral margin behind the epibranchial tooth are all small. The sternal sulcus s3/s4 of P. langi is deep at the sides and absent in the middle ( Fig. 1 C) whereas sternal sulcus s3/s4 of P. dentatus is deep and complete and forms a distinct V-shape with the bottom of the V meeting the sternoabdominal cavity ( Stewart et al. 1995). Potamonautes langi differs from P. w arreni in that P. l a ng i has a slim G1 terminal article that is strongly directed outward at a 45° angle ( Fig. 2 D,E), whereas the G1 terminal article of P. w a r ren i is only slightly directed outward at a 30° angle ( Bott 1955; Daniels et al. 2001).

Four other Afrotropical genera include species that have strong, sharp exorbital and epibranchial teeth and several pointed teeth on the anterolateral margin immediately behind the epibranchial tooth. These are Platythelphusa armata (A. Milne-Edwards, 1887) (Lake Tanganyika), Erimetopus brazzae (A. Milne-Edwards, 1886) (Lower Congo River basin), Liberonautes chaperi (A. Milne-Edwards, 1887) (West Africa), and Sudanonautes faradjensis ( Rathbun, 1921) (Central Africa) but they can all be distinguished from species of Potamonautes at the genus level ( Cumberlidge 1985, 1995, 1999; Cumberlidge et al. 1999; Cumberlidge & Reed 2004). In contrast to Potamonautes , Liberonautes and Sudanonautes have an intermediate tooth on the anterolateral margin between the exorbital and epibranchial teeth (Cumberlidge 1999); Erimetopus has spines on the margins of the merus of P2-P5 (Cumberlidge & Reed 2004); and Platythelphusa has a frontal margin that projects forward and is not downturned over the antennular fossae (Cumberlidge et al. 1999).

Figures

FIGURE 1. Potamonautes langi (Rathbun, 1921). Holotype, adult male, CW 45 mm, from the Congo River at Kisangani, D. R. Congo (AMNH 3353). 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). Scale bar: 10.8 mm (A – I).

FIGURE 2. Potamonautes langi (Rathbun, 1921). Holotype, adult male, CW 45 mm, from the Congo River at Kisangani, D. R. Congo (AMNH 3353). A, mandible frontal view; B, mandibular palp superior view; C, third maxilliped frontal view; D, G 1 ventral view; E, G 1 dorsal view; F, superior view of terminal article; G, G 2 ventral view. Scale bar: 2.5 mm (A), 1.5 mm (B), 12.5 mm C, 4.5 mm (D – G).

FIGURE 3. Potamonautes langi (Rathbun, 1921). Holotype, adult male, CW 45 mm, from the Congo River at Kisangani, D. R. Congo (AMNH 3353). Whole animal A, frontal view; B, dorsal view; C, ventral view. Scale bar: 21.4 mm (A – C).

FIGURE 4. Map of the D. R. Congo in Central Africa showing the known localities of Potamonautes langi (black circles, not shown to scale). See text for exact locality coordinates.