Potamonautes calcaratus ( Gordon, 1929 )

Reed, Sadie K. & Cumberlidge, Neil, 2004, Notes on the taxonomy of Potamonautes obesus (A Milne-Edwards, 1868) and Potamonautes calcaratus (Gordon, 1929) (Brachyura: Potamoidea: Potamonautidae) from eastern and southern Africa, Zootaxa 418 (1), pp. 1-20 : 11-18

2. Potamonautes calcaratus ( Gordon, 1929) ( Figs. 9–16 , 26–29 , 30).

Potamon (Potamonautes) calcaratus Gordon, 1929: 405–411 , figs. 1–5.

Potamon calcaratus: Chace, 1942: 208 .

Potamon (Potamonautes) calcaratus: Barnard, 1950: 193 , fig. 34h, I, 35d.

Potamon (Obesopotamonautes) obesus calcaratus: Bott, 1955: 259–260 , fig. 81.

Potamonautes calcaratus: Daniels et al., 2002a: 172 .

Type Material. MOZAMBIQUE: Lectotype (designated here) Charre , north of Sena (= Vila de Sena) on the Zambezi river, adult male (form I) (cw 35.8, cl 25.4, ch 13.1, fw 11.6 mm), 3.v.1929 (Hugh B. Cott) ( NHML 1929.3 .5.9–13) . Paratypes, Charre , north of Sena (= Vila de Sena) on the Zambezi river, 3 subadult males (cw 31.7, cl 22.9, ch 11.7, fw 10.1 mm; cw 23.5, cl 17.2, ch 9, fw 7.6 mm; cw 21.8, cl 15.7, ch 8.1, fw 7.6 mm), 3.v.1929 (Hugh B. Cott) ( NHML 1929.3 .5.9–13), subadult female (cw 25.5 mm) (Hugh B. Cott) ( ZSM 1518 /6) .

Additional material. MOZAMBIQUE: adult male (form II) (cw 42.9 mm), adult female (cw 40.3 mm) ( ZSM 1518 /5) ; Macequece , juvenile male, soft shell, 1929 ( NHMW 4375 ) ; Quelimane, Zambezia District, subadult male (cw 34.2 mm) ( Peters ) ( ZSM 1517 / 1) . SOUTH AFRICA: (not examined; localities provided by S.R. Daniels, pers comm.) Kruger National Park, north of Kwaggaspan , 25°08.73'S 31°33.47'E ; north of Kwaggaspan, 25°06.59'S 31°32.62'E; south of Kwaggaspan , 25°10.62'S 31°33.86'E ; Muhlambarnadvube pan, 25°11.73'S 31°37.22'E; unnamed pan west of Renosterkoppies , 25°07.03'S 31°35.29'E ; north of Jones Dam , 24°50.22'S 31°42.77'E ; unnamed pan south of Lugman , 24°43.718.S 31°40.266'E; between Nklanguleni and Lugman , 24°42.961'S 31°39.518'E ; south of Talamati , 24°35.261'S 31°38.976'E ; north of Kumana , 24°35.465'S 31°47.285'E ; Girivana Pan, 24°21.600'S 31°41 493'E; Gudzani, 24°16.98'S 31°48.56'E.

Diagnosis. Carapace medium height (ch/fw 1.1); epibranchial tooth large forming deep epibranchial sinus, positioned well behind postfrontal crest; carapace sidewall in three parts (ventral extension of vertical groove across pterygostomial region either faint or absent); anterolateral margin behind epibranchial tooth raised, smooth, curving inward over carapace at posterior end, not continuous with posterolateral margin; episternal sulci s4/e4, s5/e5, s6/e6, s7/e7 not visible; large, pointed distal tooth on both anterior­inferior and posterior­inferior margins of merus of cheliped; dactylus of major cheliped of adult male either flat, broad, enclosing narrow interspace (form I), or highly curved, slim, elongated enclosing wide interspace (form II); propodus of cheliped either shorter than cw (form I) or longer than cw (form II); lower margin of propodus of cheliped either convex throughout (form I) or concave in middle (form II); carpus of second and third pereiopods with small, pointed tooth on outer margin; terminal article of gonopod 1 directed outward at 60° angle to vertical, slim, tapering, ending in slightly upcurved tip; lateral fold on terminal article wider, higher than medial fold; base of subterminal segment of gonopod 1 broad, tapering to slim apex that curves slightly outward.

Redescription. The following is based on the lectotype, a form I adult male (cw 35.8 mm, NHML 1929.3.5.9–13) from Mozambique. Carapace relatively wide (cw/fw 3.1), medium height (ch/fw 1.1); front slightly indented, slightly deflexed, about one third of the carapace width (fw/cw 0.3); postfrontal crest present but faint, epigastric lobes continuous with postorbital crests, ends curving back towards epibranchial teeth, but not quite meeting; surface of carapace smooth; semi­circular, urogastric, cardiac, cervical grooves faint; mid­groove of postfrontal crest short, faint. Exorbital, epibranchial teeth small, pointed; epibranchial tooth forming deep sinus, positioned well behind postfrontal crest; anterolateral margin behind epibranchial tooth raised, smooth, curving inward over carapace at posterior end, not continuous with posterolateral margin; posterior margin of carapace about one third as wide as carapace. Suborbital, subhepatic, pterygostomial regions of carapace sidewall smooth; vertical sulcus on sidewall beginning on anterolateral margin close to exorbital tooth, dividing suborbital from hepatic region, ventral extension of vertical groove in pterygostomial region either faint or absent. Ischium of third maxilliped smooth, lacking vertical groove, exopod of third maxilliped with long flagellum. Second thoracic sternal sulcus s2/s3 deep, completely crossing sternum, third thoracic sternal sulcus s3/s4 incomplete, sides deep, angled inward, middle shallow, distinct bulges on s4 where chelipeds articulate. Episternal sulci s4/e4, s5/e5, s6/e6, s7/e7 not visible. Male abdomen outline forming broad­based triangle, sides of telson straight.

Chelipeds highly unequal, propodus of left cheliped of form I adult male lectotype higher, longer, than propodus of left cheliped (left cheliped missing in lectotype, present in other paratypes); dactylus of major cheliped of form I adult males (cw 35–42 mm) flat, broad, palm very high; dactylus of major cheliped of form II adult male (cw 43 mm, ZSM 1518/5) highly curved, slim, elongated, enclosing long oval interspace; propodus of major cheliped longer than cw, palm very high; teeth along inner margin of dactylus weak, blunt; finger of propodus long (extending beyond dactylus), lined by small teeth interspersed with three larger pointed molars; dactylus of minor (left) cheliped straight, extending beyond finger of propodus; fingers of dactylus, propodus of left cheliped with short pointed teeth along inner margins, touching when closed. Palms of both chelipeds smooth, except for carinae on superior margin. Inner margin of carpus of cheliped with two teeth, first large, pointed, directed forward, second tooth pointed, approximately half size of first; dorsal surface of carpus with distinct carinae. Merus of cheliped with smooth oval surface (meral tympanum) on inner surface; anterior­inferior and posterior­inferior margins of merus of cheliped smooth, each with small, pointed distal tooth; superior surface of merus with rows of prominent, rough grains. Carpus of P2, P3 each with small, pointed tooth on outer margin; merus of P5 longer than fw; propodius of P4, P5 short (shorter than fw), wide; anterior, posterior margins of propodi of P5 clearly serrated; dactylus of P4 long, dactylus of P5 very short; P5 shortest walking leg.

Terminal article of gonopod 1 about one third as long as subterminal segment; entire terminal article directed outward at 60° angle to vertical, slim, tapering, ending in slightly upcurved tip; lateral fold on terminal article wide, higher than medial fold; longitudinal groove visible on superior side of terminal article, ventral side of subterminal segment; medial, lateral margins of subterminal segment fringed by short setae; base of subterminal segment broad, tapering to slim apex that curves slightly outward; dorsal membrane on subterminal segment narrow. Gonopod 2 longer than gonopod 1; terminal article of gonopod 2 long, flagellum­like, over one half as long as subterminal segment.

Size. The adult size range is from cws 35–42 mm. The carapace proportions are given in table 2.

Variation. The chelipeds of juvenile P. calcaratus are of equal size and shape. However, the chelipeds undergo allometric growth that results in a marked heterochely in older crabs. For example, the palm of the propodus of the major cheliped of subadult males (cw 20–34 mm) and adult males (cw> 35 mm) is both high and wide, and the basal margin of the propodus is either straight or curves downward, appearing convex. The fixed finger of the propodus is very broad, tapering sharply at the tip. In subadult and form I adult males the dactylus is flat and broad, and its teeth meet those of the fixed finger. The dactylus of the major cheliped of form II adult males is slim, elongated, and strongly arched, enclosing a wide oval interspace.

Type locality. Charre , Mozambique .

Distribution. Mozambique, northeast South Africa (only in Mpumalanga Province). Barnard (1950) reported P. calcaratus to occur in the Chirinda forest, in the lower Zambezi valley in Zimbabwe.

Natural history. Potamonautes calcaratus is the most terrestrial of the freshwater crab species found in South Africa, and lives in burrows it digs near waterholes or pans; it is the only South African freshwater crab to dig a burrow that reaches a depth of 70 cm ( Daniels et al. 2002b). Most of the localities from South Africa shown in Fig. 30 are in the Kruger National Park and were kindly provided to us by S. R. Daniels. Other localities from South Africa and elsewhere in the region in Fig. 30 are taken from Gordon (1929) and Barnard (1950). Although data regarding the current population trends of P. calcaratus are unavailable, this species has a relatively narrow distribution, is not well represented in museum collections, and has not been collected in the past ten years. For these reasons we would judge its conservation status to be in the Vulnerable category (Red List Criteria B1a; C) of the Red List Assessment ( IUCN 2001).

Comments. Potamonautes calcaratus is closest to P. obesus . The two taxa share the following characters: both have a smooth rounded carapace, both lack a vertical groove on the ischium of the third maxilliped, and both have a male abdomen outline that forms a broad­based triangle. In addition, the major cheliped of adult males of both species exhibits a very unusual metamorphosis that shows two different forms – termed here form I and form II. Gordon (1929) described P. (P.) calcaratus from a form I adult male whose major cheliped has a broad flat dactylus. The form II male major cheliped of P. calcaratus is described here for the first time from a specimen from Mozambique (ZSM 1518/5). The palm of the propodus of the major cheliped of form I adult males is very high, both fingers are broad and flat and they almost touch, enclosing a long, narrow interspace. This configuration changes with subsequent molts, and in form II adult males the major dactylus of the cheliped is slim, elongated, strongly arched, and encloses a wide, oval­shaped interspace.

Potamonautes calcaratus can be distinguished from P. obesus as follows. The carapace of P. calcaratus is flatter (ch/fw 1.1) that of P.obesus (ch/fw 1.5); the terminal article of gonopod 1 in P. calcaratus is directed outward at an angle of 45° to the vertical, whereas in P. obesus it forms an angle of 60°; the apex of the subterminal segment of gonopod 1 of P. calcaratus curves distinctly outward, whereas in P. obesus this segment is straight along its length; the dorsal membrane on the subterminal segment of gonopod 1 is narrow in P. calcaratus but is broad and distinct in P. obesus ; and the thoracic sternum of P. calcaratus lacks episternal sulci s4/e4 and s7/e7, which are both present in P. obesus . Furthermore, the subhepatic and suborbital regions of the carapace sidewall of P. calcaratus are both smooth, and the pterygostomial region is not divided, whereas the subhepatic and suborbital regions of the carapace sidewall of P. obesus are both granular, and the pterygostomial region is divided by a granular sulcus. Finally, the carpi of the second and third pereiopods of P. calcaratus have a small, pointed tooth on the outer margin, which is lacking in P. obesus . For these reasons, P. obesus and P. calcaratus are considered here to belong to two valid species.

Bott (1955) established the subgenus Obesopotamonautes Bott, 1955 , to accommodate P. (O.) o. obesus , P. (O.) o. calcaratus , and P. (O.) langi ( Rathbun, 1921) . However, this subgenus as defined by Bott (1955) is considered here to be doubtful. For example, the holotype of Potamon (Potamonautes) langi Rathbun, 1921 (AMNH 3353), was examined in the present study. The grouping of Potamonautes langi ( Rathbun, 1921) with the taxa under consideration here is questionable, because P. langi from the rivers of the Congo basin in Central Africa bears little resemblance to either P. obesus or P. calcaratus ( Rathbun 1921; Bott 1955). For example, P. langi can be distinguished from P. obesus and P. calacratus by the following characters: the anterolateral margin of the carapace has several long and pointed teeth, the postfrontal crest is sharp­edged in adults and meets the anterolateral margins, the exorbital tooth is large, sharp and pointed, and the teeth on the carpus of the cheliped are of equal size, and both are long and pointed. Furthermore, Daniels et al. (2002a) investigated the phylogenetic relationships of the southern African freshwater crab fauna and found no support for some of the subgenera proposed by Bott (1955).

The distributions of P. obesus and P. calcaratus have some areas of overlap in Mozambique ( Figure 28 ), but it is unknown if sympatric populations do occur. Daniels et al. (2002b) used allozyme analysis and the direct sequencing of mtDNA to study the population genetics of P. calcaratus from South Africa. It would be useful to apply a similar approach to the two taxa that are the focus of the present study to investigate the possibility of hybridization within sympatric populations.

Figures

FIGURES 9–16. Potamonautes calcaratus (Gordon, 1929), lectotype, adult male, form I (cw 35.8 mm, NHML 1929.3.5.9–13). 9, carapace and eyes, dorsal view; 10, cephalothorax, carapace and eyes, frontal view; 11, right cheliped, frontal view; 12, left third maxilliped; 13, carpus and merus of right cheliped, dorsal view; 14, merus of right cheliped, inferior view; 15, sternum and abdomen; 16, right fifth pereiopod (modified from Gordon 1929). Scale bar = 3 cm, 9–11, 13–16; 6 cm, 12.

FIGURES 22–29. Potamonautes obesus (A. Milne­Edwards, 1868), adult male, form II (cw 49.1 mm, NMU TRW 1968.06) from Mlali, near Morogoro, Uluguru Mountains, Tanzania. 22, left gonopod 1, dorsal aspect; 23, left gonopod 1, terminal article, superior aspect; 24, left gonopod 1, ventral aspect. Potamonautes calcaratus (Gordon, 1929), lectotype, adult male, form I (cw 35.8 mm, NHML 1929.3.5.9–13). 25, left gonopod 1, dorsal aspect; 26, left gonopod 1, terminal article, superior aspect; 27, left gonopod 1, ventral aspect.