Potamonautes bourgaultae, Cumberlidge & Meyer, 2011

Cumberlidge, Neil & Meyer, Kirstin S., 2011, The freshwater crabs of Lake Kivu (Crustacea: Decapoda: Brachyura: Potamonautidae), Journal of Natural History 45 (29 - 30), pp. 1835-1857 : 1848-1855

Potamonautes bourgaultae , sp. nov.

( Figures 9–12 , Table 2)

Potamon emini: Chace 1942: 197 , fig. 7 (not Telphusa emini Hilgendorf, 1892 )

Potamon berardi: Chace 1942: 207 (not Telphusa berardi Audouin, 1825 )

Type locality

Idjwi Island , Lake Kivu, Democratic Republic of Congo. Type material

Democratic Republic of Congo. Holotype. Subadult male (CW 17.3, CL 11.0, CH 6.5, FW 4.8) Idjwi Island, Lake Kivu, February 1939, coll. A. Loveridge ( MCZ 11231) ( Potamon emini of Chace 1942); subadult female (CW 17.3, CL 12.5, CH 7.3, FW 4.9) Idjwi Island, Lake Kivu, February 1939, coll. A. Loveridge ( MCZ 11231) ( Potamon emini of Chace 1942); subadult female (CW 17.3, CL 12.6, CH 6.4, FW 4.7) Idjwi Island, Lake Kivu, February 1939, coll. A. Loveridge ( MCZ 11230) ( Potamon berardi of Chace 1942).

Diagnosis

Carapace surface smooth; highly arched ( CH / FW 1.4); postfrontal crest incomplete, strongest at lateral edges ( Figure 9A,B , 11A,B ); ischium of third maxilliped with faint vertical groove ( Figures 9B , 11I ); sternal sulcus s3 / s4 incomplete, deep at sides but missing in middle ( Figure 11E ); distal meral tooth reduced to large granule; first carpal tooth small, pointed; second carpal tooth reduced to granule, followed by several other granules ( Figures 9A,C ; 11C,D ); dactylus of major cheliped lined by alternating large and small teeth; fixed finger of propodus of major cheliped lined by low, rounded teeth, first of two large teeth in line with large tooth on dactylus ( Figures 9B , 11G,H ); distal margin of GO1 subterminal segment highest on medial side, forming pronounced shoulder; terminal article of GO1 slim, straight, tapering evenly to tip, curving slightly outward ( Figures 10A,D , 12A,D ); lateral, medial folds of GO1 terminal article low, even; dorsal membrane broad, widest at lateral edge, narrowest at medial edge, proximal margin U-shaped ( Figures 10B,D , 12B,E ).

Description

Based on holotype, subadult male ( CW 17.3). Carapace moderately wide ( CW / FW 3.6), high ( CH / FW 1.4); surface texture completely smooth; front strongly deflexed, relatively narrow (FW / CW 0.28); postfrontal crest incomplete, strongest at lateral edges, gap between postorbital crests and epigastric crests; mid-groove between epigastric crests faint ( Figures 9A , 11A ); epibranchial, exorbital teeth low / absent; suborbital margin smooth; anterolateral margin completely smooth, continuous with smooth posterolateral margin; cardiac groove faint to absent, semicircular, urogastric grooves faint, transverse branchial, cervical grooves absent ( Figures 9A,B , 11A,B ); sidewalls completely smooth with distinct longitudinal suture; vertical suture on carapace sidewall faint, not meeting anterolateral margin ( Figures 9B , 11B ). Mandibular palp with simple setose terminal segment; epistomial tooth triangular, margins granulated; third maxilliped exopod with long flagellum; third maxilliped ischium with faint vertical groove ( Figures 9B , 11I ).

Sternal sulcus s1 / s2 short, very faint; s2 / s3 faint, complete; s3 / s4 incomplete, deep at sides but missing in middle; episternal sulci faint ( Figures 9C , 11E ); abdomen slim, triangular, tapered, widest at base; telson outline triangular with rounded apex and slightly concave sides ( Figure 9C , 11F ); s6 / s7 meeting abdomen at a5 / a6.

Distal margin of GO1 subterminal article lowest on lateral side, highest on medial side, forming pronounced shoulder; GO1 terminal article slim, directed straight upward, tapering evenly to tip ( Figures 10A,C , 12A,D ); lateral and medial folds of GO1 terminal article low, even; groove on terminal article not visible on dorsal face; dorsal membrane broad, widest at lateral edge, narrowest at medial edge, proximal margin U-shaped; junction between terminal and subterminal segments of GO1 visible on ventral side ( Figures 10A,C , 12A,D ); GO2 terminal article long, straight, flagellum-like, about as long as subterminal segment ( Figures 10E , 12C ).

Dactylus of major cheliped slightly arched, upper margin smooth, closed fingers enclosing long narrow interspace, lined by alternating large, small teeth; fixed finger of propodus of major cheliped with series of low, rounded teeth, plus two large teeth, first large tooth in line with large tooth on dactylus; lower margin of propodus slightly indented ( Figures 9B , 11G,H ); first carpal tooth small, pointed; second carpal tooth reduced to small granule, followed by other granules; distal meral tooth reduced to large granule; outer face of merus smooth, superior margin with faint carinae; ventral margins of merus of cheliped with small granules ( Figure 11C,D ); ischium of cheliped smooth; walking legs p2–p5 slender, length normal, neither elongated or shortened; dactyli of p2–p5 tapering to point, each bearing four rows of downward-pointing short, sharp spines. Small species; holotype male ( CW 17.8) almost adult (subadult); pubertal moult estimated to be between CW 18 and 20 mm.

Distribution

This species is endemic to Idjwi Island (40 × 12 km, 275 km 2), Lake Kivu, Democratic Republic of Congo.

Etymology

This species is named in honour of the late Dr Louise M. Bourgault of Northern Michigan University, Marquette, MI, USA, in recognition of her outstanding contributions to the study of HIV-AIDS in Africa.

Remarks

Potamonautes bourgaultae is similar to P. idjwiensis and P. mutandensis , which are also found in Lake Kivu. All of these are small or medium-sized crabs with a smooth untoothed carapace with a faint or absent postfrontal crest. However, P. bourgaultae can be easily distinguished from P. idjwiensis as follows. Potamonautes idjwiensis has a deep complete, sternal sulcus s3 / s4, which meets the top of the sternoabdominal cavity, but s3 / s4 is deep at the sides but missing in the middle in P. bourgaultae ; and the dorsal membrane of GO1 of P. idjwiensis has equal lateral and medial margins with a diagonal proximal margin, whereas the medial margin is narrower, and the proximal margin is U-shaped in P. bourgaultae . Potamonautes bourgaultae can be easily distinguished from P. mutandensis as follows. The distal meral tooth is distinct and pointed in P. mutandensis but reduced to a large granule in P. bourgaultae ; the grooves on the posterior carapace are deep in P. mutandensis but absent in P. bourgaultae ; the junction between the subterminal and terminal segments of GO1 lacks a shoulder in P. mutandensis but has a pronounced medial shoulder in P. bourgaultae ; and the terminal article of GO1 curves strongly outward in P. mutandensis but is straight in P. bourgaultae .

Chace (1942) assigned two specimens of P. bourgaultae to P. berardi and another specimen to P. emini , and indeed, P. bourgaultae is similar to these species in that all three have a smooth untoothed carapace. However, P. bourgaultae can be easily distinguished from P. berardi as follows. The ischium of the third maxilliped completely lacks a groove in P. berardi but has a faint but distinct groove in P. bourgaultae ; the ventral margins of the merus of the cheliped of P. berardi are lined with small, pointed teeth whereas these margins are lined by small granules in P. bourgaultae ; the distal meral tooth is large and pointed in P. berardi but reduced to a large granule in P. bourgaultae ; and the first carpal tooth is small and pointed in P. berardi but small and rounded in P. bourgaultae . Potamonautes bourgaultae can be distinguished from P. emini by the postfrontal crest, which is distinct and complete in P. emini but faint and incomplete in P. bourgaultae ; by the distal meral tooth, which is large and surrounded by granules in P. emini but is isolated as a single granule in P. bourgaultae ; by the junction between the subterminal and terminal segments of GO1, which lacks a shoulder in P. emini but has a pronounced medial shoulder in P. bourgaultae ; and by the terminal article of GO1, which curves strongly outward in P. emini but is straight in P. bourgaultae .

Comparisons

Of the four species of freshwater crabs found in Lake Kivu P. idjwiensis and P. mutandensis are the most similar. Both species have a medium body size, a smooth untoothed carapace, and a faint to absent postfrontal crest; both lack an epibranchial tooth; both have a faint groove on the ischium of the third maxilliped; both have a low and blunt first carpal tooth followed by granules; both have a complete sternal sulcus s2 / s3, and both have faint episternal sulci. However, the two species can be easily distinguished by the following characters: the cervical groove is short in P. idjwiensis but long and faint in P. mutandensis ; the distal meral tooth is low and blunt in P. idjwiensis but distinct and pointed in P. mutandensis ; sternal sulcus s3 / s4 is deep in P. idjwiensis , meeting the top of the sternoabdominal cavity, but incomplete, deep at the sides, and missing in the middle in P. mutandensis . The terminal article of gonopod 1 is straight with an upturned tip in P. idjwiensis , but curved strongly outward in P. mutandensis ; the junction between the terminal article and the subterminal article of gonopod 1 is visible on the ventral face in P. mutandensis but not in P. idjwiensis .

Both P. idjwiensis and P. mutandensis are easily distinguished from P. lirrangensis , which is a large species with a large pointed exorbital tooth. The postfrontal crest is absent in both of the former species but distinct, complete and lined with small granules in P. lirrangensis ; the exorbital and epibranchial teeth are both absent in P. idjwiensis and P. mutandensis , whereas the exorbital tooth is a large, forward pointing spine and the epibranchial tooth is a small spine in P. lirrangensis . The anterolateral margin, which is smooth in P. idjwiensis and P. mutandensis , is lined by either large granules or small teeth in P. lirrangensis ; the carapace sidewalls are smooth in P. idjwiensis and P. mutandensis but are roughened by carinae and granules in P. lirrangensis . The first carpal tooth in both P. idjwiensis and P. mutandensis is low and blunt, but a large pointed spine in P. lirrangensis . Finally, both P. idjwiensis and P. mutandensis are medium-sized species with an adult size range between CW 21–25 and CW 31–39 respectively, whereas P. lirrangensis is a large species with an adult size range between CW 45 and 65 ( Table 2).

A few characters are unique in each of these three species. For example, the distal meral tooth is low and blunt in P. idjwiensis , distinct and pointed in P. mutandensis and a sharp spine in P. lirrangensis . The terminal article of gonopod 1 is straight with an upturned tip in P. idjwiensis and curved strongly outward in P. mutandensis ; in P. lirrangensis the first half is straight, the second half is bent strongly outward, tapering to a broad, upcurved tip. Sternal sulcus s3 / s4 is complete and deep, reaching the top of the sternoabdominal cavity in P. idjwiensis , but incomplete in both P. mutandensis and P. lirrangensis . Two characters are shared by all three species: a complete s2 / s3 and a faint groove on the ischium of the third maxilliped.

Biogeography

Two genera of freshwater crabs ( Potamonautes and Platythelphusa ) are found in the African Great Lakes and both belong to the Potamonautidae , a family that is found throughout the African continent and which is endemic to the Afrotropical region ( Daniels et al. 2006; Cumberlidge et al. 2008; Yeo et al. 2008; Cumberlidge 2009). Of these genera, Potamonautes is widespread and is found throughout sub- Saharan Africa and the Nile River basin, while Platythelphusa is endemic to Lake Tanganyika ( Cumberlidge et al. 1999; Marijnissen et al. 2004). Lakes Kivu (with two endemic species of Potamonautes ) and Tanganyika (with nine endemic species of Platythelphusa and one endemic species of Potamonautes ) are the only two African lakes that have endemic species of freshwater crabs ( Table 1). Lake Kivu has one species ( P. mutandensis ) that also occurs in the Nile catchment, one species ( P. lirrangensis ) that occurs in the Congo basin ( Rwanda and Democratic Republic of Congo), and two species ( P. idjwiensis and P. bourgaultae ) that are endemic to the lake ( Table 2). Lake Kivu shares no species with Lakes Edward and Albert, and one ( P. lirrangensis ) with Lake Tanganyika’s tributaries. While freshwater crabs are represented in the other African lakes, species richness overall is low and none are lake endemics. For example, even the two largest lakes – Lake Victoria (two species of Potamonautes ) and Lake Malawi (one species of Potamonautes ) – lack endemic crabs and are species poor ( Table 1). The generally low faunal richness and endemism rates of freshwater crabs in the African lakes contrast sharply with the impressive species richness and endemism seen in the cichlid fish in these lakes ( Beadle 1981).

Key to the species of Potamonautes from Lake Kivu

1. Postfrontal crest distinct, complete; anterolateral margin with either small teeth or large granules; exorbital tooth large, forward pointing spine; epibranchial tooth small spine; first carpal tooth large, sharp spine........................................................................ P. lirrangensis

Postfrontal crest faint to absent, incomplete; anterolateral margin smooth; exorbital, epibranchial teeth low or absent; first carpal tooth low.......... 2

2. Sternal sulcus s3 / s4 complete, meeting top of sternoabdominal cavity; telson sides strongly concave......................................... P. idjwiensis

Sternal sulcus s3 / s4 missing in middle, not meeting top of sternoabdominal cavity; telson sides slightly concave or straight............................ 3

3. Distal meral tooth distinct, pointed; distal margin of GO1 low, lacking shoul- der; dorsal membrane of GO1 broad, lateral and medial margins equal, proximal margin straight; GO1 terminal article evenly curved strongly outward.................................................. P. mutandensis

Distal meral tooth granular; distal margin of GO1 highest medially, forming pronounced shoulder; medial margin of dorsal membrane of GO1 narrower than lateral margin, proximal margin U-shaped; GO1 terminal article straight, tapering evenly to tip............................... P. bourgaultae

Figures

Figure 9. Potamonautes bourgaultae, sp. nov. (A) Dorsal view; (B) frontal view; (C) sternal view. Holotype subadult male (MCZ 11231), CW 17.3 mm.

Figure 10. Potamonautes bourgaultae, sp. nov. (A) GO1, ventral view; (B) GO1, dorsal view; (C) detail of GO1 tip, ventral view; (D) detail of GO1 tip, dorsal view; (E) GO2. Holotype subadult male (MCZ 11231), length of terminal article measured along the mid-line of the dorsal face from tip to dorsal membrane 1.4 mm.

Figure 11. Potamonautes bourgaultae, sp. nov. (A) Carapace, dorsal view; (B) carapace, frontal view; (C) carpus and merus of right pereiopod 1 (major cheliped), dorsal view; (D) carpus and merus of right pereiopod 1 (major cheliped), ventral view; (E) male sternum; (F) male abdomen; (G) major (right) cheliped; (H) minor (left) cheliped; (I) left third maxilliped. Scale bars 10 mm, A–H; 5 mm, I.

Figure 12. Potamonautes bourgaultae, sp. nov. (A) GO1, ventral view; (B) GO1, dorsal view; (C) GO2; (D) detail of GO1 tip, ventral view; (E) detail of GO1 tip, dorsal view. Scale bars: 1 mm, A–C; 0.67 mm, D, E.