Erimetopus brazzae ( A. Milne-Edwards, 1886 )

Cumberlidge, Neil, 2017, Potamonautes nheenae, a new species of freshwater crab from Gabon, Central Africa, with a description of the first known male of Erimetopus brazzae (A. Milne-Edwards, 1886) from the Democratic Republic of the Congo, Central Africa (Brachyura: Potamoidea: Potamonautidae), Zootaxa 4286 (2), pp. 228-240 : 236-238

Erimetopus brazzae ( A. Milne-Edwards, 1886)

( Fig. 6 )

Material examined. D. R. CONGO: several kilometers northeast of Kinganga, Bas Congo, on the Congo River (- 5.260628 S, 13.767800 E), subadult ♂, CW 11, CL 9 mm (collector and date unknown) ( MCZ 67421) .

Diagnosis (emended from Cumberlidge & Reed 2004). Carapace outline subhexagonal/rounded; epibranchial tooth large, sharp, pointing forwards, positioned well behind postfrontal crest; orbit narrow (1/7 CW), upper orbital margin semi-circular; anterolateral margin between exorbital, epibranchial teeth very long, curving slightly outward, with several small pointed teeth, without visible intermediate tooth ( Fig. 6 A). Exopod of third maxilliped with long flagellum, ischium smooth lacking vertical sulcus. Distal tooth on inner margin of cheliped carpus large, slender, pointed, curving forwards; outer margin of cheliped carpus with series of pointed teeth ( Fig. 6 F); superior margins of meri of p2–p5 with 2 large, pointed distal teeth; anterior margins of carpi of p2–p4 spiny, posterior margins smooth; anterior margins of propodi of p2–p4 spiny, posterior margins smooth; propodus of p5 with spines on anterior, posterior margins. S1/s2 either faint or absent; s2/s3 shallow, completely crossing thoracic sternum; s3/ s4 reduced to 2 small side notches; s4 completely smooth; s4/e4, s5/e5, s6/e6, s7/e7 each marked by visible sulcus ( Fig. 6 B). Pleon slim, outline triangular, tapered, widest at base, telson outline forming straight-sided triangle with broad base, rounded apex; s6/s7 meeting abdomen at a5/a6; s5/s6 meeting a6 one half of somite length from a6/a5 ( Fig. 6 B). G1 terminal article one-third as long as subterminal segment, terminal article slim throughout length, tapering to pointed tip ( Fig. 6 C, D), G1 terminal article straight basally turning sharply outwards at 45° angle at halfway point ( Fig. 6 C, D), G1 dorsal membrane narrow, rectangular, sides parallel ( Fig. 6 D), G2 subterminal segment long, slender, G2 terminal article long, slender, flagellum-like ( Fig. 6 E).

Size. Erimetopus brazzae is a small species with an adult size range between CW 23.5–33.5 mm ( Cumberlidge & Reed 2004).

Type locality. Republic of the Congo, Ngabe (= Ngancin) (- 3.310963 S, 16.121973 E), in the Pool District, on the right bank of the Congo River opposite Kwamouth in the D. R . Congo, adult female lectotype of Thelphusa brazzae A. Milne-Edwards, 1886 , CW 23.5, CL 18.5 , CH 8.0, FW 7.0 mm, Apr. 1884 (M. de Brazza) ( MNHN B5069).

Habitat. Erimetopus brazzae is found on both sides of the lower Congo River in the Republic of the Congo and in the D. R. Congo where the river is about 1.5 km wide and the water is deep and fast flowing because the Congo River is joined by the Ubangi, Sangha, and Kwa rivers. This species is also found in the Malebo Pool, a large, lake-like widened stretch of the Congo River (about 25 km by 30 km) close to Kinshasa, D. R. Congo. All of the adult egg-bearing females of E. brazzae so far have been found either in the swampy areas of the lower Congo River and its tributaries or on the banks of the Malebo Pool, rather than in the river itself ( Cumberlidge & Reed 2004). The breeding season for E. brazzae is between April and August based on when egg-bearing females have been collected, and this species appears to favor the muddy ground near river banks where it lives either under stones, under boardwalks, or tangled in water plants. ( Cumberlidge & Reed 2004). At Malebo Pool E. brazzae occurs together with P. paecilei in the mud under boards and timbers.

Distribution. Erimetopus brazzae is a Central African species associated with the lower reaches of the Congo River at Ngabe (= Ngancin), Pool District, Republic of the Congo (- 4.910339 S, 14.688858 E) across the Congo River from Gombe-Matadi, D. R. Congo. This species is also found in the Kwa, Kwilu, and Kwango Rivers that are all southeastern tributaries of the Congo River in the D. R. Congo ( Cumberlidge & Reed 2004: fig. 45). The report by Rathbun (1894) that this species occurs in Gabon remains unconfirmed and it is not included here in the species list for that country for the moment.

Remarks. Erimetopus brazzae was redescribed by Cumberlidge & Reed (2004), who referred to all relevant type material plus the descriptions and illustrations of the dorsal carapace of the female lectotype from Ngabé, Republic of the Congo by Rathbun (1894, 1921) and Capart (1954: fig. 43). The description of the species is enhanced here by the addition of characters of the sternum and pleon of the male specimen of E. brazzae ( Fig. 6 B) that was judged to be a subadult based on the size of adult females of this species that measure between CW 23.5 and 33.5 mm ( Cumberlidge & Reed 2004). Many of the morphological characters commonly used for taxonomy (such as those of the third maxilliped, thoracic sternum, mandible, and the merus and carpus of the cheliped) undergo isometric growth and are not likely to be significantly different in subadults and adults. Because the G1, G2 and the major cheliped of male freshwater crabs grow allometrically, however, the subadult male of E. brazzae has incompletely developed gonopods and chelipeds ( Fig. 6 A). Despite this, it is still possible to make the following comparisons with the adult gonopods of E. vandenbrandeni (Balss, 1936) (see Cumberlidge & Reed 2004: figs. 31–33, 42–44). The G1 of E. brazzae ( Fig. 6 C, D) is similar in most respects to that of the adult male of E. vandenbrandeni ( Cumberlidge & Reed 2004: figs. 27, 29, 41), but the G1 of E. brazzae ( Fig. 6 C, D) differs from that of E. vandenbrandeni ( Cumberlidge & Reed 2004: figs. 31–33, 42–44) as follows: the G1 terminal article of E. brazzae is slim and tapers to a pointed tip ( Fig. 6 C, D) (tubular with parallel sides and ends in a broad tip with a distinctly upturned triangular corner in E. vandenbrandeni ), the G1 terminal article of E. brazzae is straight basally and then turns sharply outward at a 45° angle to the longitudinal axis of the gonopod (straight along its entire length in E. vandenbrandeni ), and the G1 dorsal membrane of E. brazzae is a narrow rectangle with parallel sides (broad membrane with a straight superior margin and medially-indented inferior margin in E. vandenbrandeni ) ( Cumberlidge & Reed 2004: figs. 32–33).

The two species of Erimetopus are found in the Lower Congo River basin in the Republic of the Congo and the D. R . Congo ( Cumberlidge & Reed 2004). DNA recently extracted from an egg of an adult female syntype of E. brazzae ( MCZ 4255 ) from Malebo Pool in the Congo River in the D. R . Congo allowed this taxon to be included in a molecular phylogenetic analysis of the entire Afrotropical freshwater crab fauna ( Daniels et al. 2015: table 1, fig. 2), a study that positioned Erimetopus as a sister group to P. ballayi from the Lower Congo River basin in a clade that also included P. ecorssei ( Marchand, 1902) , from Mali , West Africa , and another species from São Tomé (‘ Potamonautes sp. 1’ in Daniels et al. 2015: table 1, fig. 2). This discovery raises questions about the stability of Potamonautes because this genus is rendered paraphyletic if Erimetopus is nested within it. This is a noteworthy finding that requires further taxonomic studies to elucidate.

Figures

FIGURE 6. Erimetopus brazzae (A. Milne-Edwards, 1886). Subadult male, CW 11 mm, from Kinganga, D. R. Congo (MCZ 67421). A. Carapace, dorsal view. B. thoracic sternum and pleon. C. G 1 ventral view. D. G 1 dorsal view. E. G 2 ventral view. F. merus, carpus of right cheliped, dorsal view. Scale bar: 3.0 mm (A, B, F), 1.4 mm (C – E).