Rotundopotamonautes rwanda Cumberlidge & Krajenka 2023, n. sp.
Creators: name: Cumberlidge, Neil
name: Krajenka, Grace E.
Publisher: name: Zenodo
Year: 2023
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Potamonautidae
Rotundopotamonautes
Rotundopotamonautes rwanda

description: Rotundopotamonautes rwanda n. sp.
Rwandan Freshwater Crab
(Figs. 4–7)
Holotype. RMCA _ CRUST _40580. Adult male (CW 27, CL 19, CH 10, FW 8.6 mm), Nyanza, Ndugu Province, Rwanda (2.3396578796°S, 29.7505697773°E), February 1953, coll. P. Basilewski.
Paratypes. RMCA _ CRUST _40579, 40581, 40582. Same collection data as the holotype.
Diagnosis. Carapace surface completely smooth; postfrontal crest smooth, distinct, complete, traversing entire carapace to meet epibranchial teeth; cervical grooves deep, long, ending just before reaching postfrontal crest; semicircular, urogastric, cardiac, branchial grooves all deep, distinct (Fig. 4A). Exorbital, epibranchial teeth reduced to granules; lateral margin smooth, lacking granules or teeth; suborbital margin, vertical sulcus on branchiostegite, all granulated (Fig. 4A, B). Thoracic sternal suture S3/4 incomplete comprising two shortened sutures that begin at sides of sternum, fading centrally (Fig. 4B). Third maxilliped exopod with long flagellum, ischium with faint vertical sulcus (Fig. 4B). Cheliped carpus inner margin with distal tooth large, acute, proximal tooth small, blunt, followed by raised margin (Fig. 5E). Cheliped merus lower margin with conspicuous large pointed distal meral tooth followed by raised granules along margin (Fig. 5D). G1 TA short (TA/SA = 0.33), slightly curved outward, tapering evenly to pointed tip; mesial margin with long setae (Fig. 6A, B). G2 TA long, flagellum-like (G2 TA/SA = 0.66) (Fig. 6C). Small-sized species, adult at CW 27 mm.
Description. Carapace moderate height (CH /FW = 1.12), width (CW/FW = 3.1), length (CL/FW = 2.2). Surface of anterior carapace completely smooth; postfrontal crest smooth, distinct, complete, traversing entire carapace to meet epibranchial teeth; cervical grooves deep, long, ending just before reaching postfrontal crest; semicircular, urogastric, cardiac, branchial grooves all deep, distinct; postfrontal crest, suborbital margin, vertical sulcus on branchiostegite, all granulated; epigastric crests separated by clear, short, forked mid-groove (Fig. 4A). Exorbital, epibranchial teeth reduced to granules; suborbital margin, vertical sulcus on branchiostegite both granulated (Fig. 4A, B); carapace anterolateral margin between exorbital, epibranchial teeth granulated, curving slightly outward, lacking intermediate tooth; carapace lateral margin posterior to epibranchial tooth smooth, posterior end curving inward, not continuous with posterolateral margin. Branchiostegite with distinct granulated vertical sulcus, subhepatic, suborbital, pterygostomial regions smooth (Fig. 4A, B, 8A).
Third maxilliped exopod with long flagellum; ischium with faint vertical sulcus (Fig. 4A).
Thoracic sternal sulcus S2/3 deep, completely traversing sternum; thoracic sternal suture S3/4 incomplete comprising two shortened sutures at sides of sternum, faint centrally (Fig. 4B); episternal sulci S4/E4, S5/E5, S6/ E6, S7/E7 all obscure (Fig. 4B).
Male pleon, telson together forming slim triangle, pleon edges only slightly indented; telson apex rounded, base broadest, sides outwardly sloping; pleomeres PL1–6 rectangular, wider than long, PL6 longest, more than 1/2 as long as wide (Fig. 4B); remaining pleomeres short, less than 1/3 as long as wide. G1 TA short (G1 TA/SA = 0.33), slightly curved outward, tapering evenly to a pointed tip; midsection not widened; mesial margin with long setae. G2 TA long, flagellum-like (G2 TA/SA = 0.66) (Fig. 6C). Chela unequal, right (major) longer than left (minor) (Fig. 5B, C). Fixed finger (pollex) of propodus, movable finger (dactylus) each with 2 or 3 large teeth interspersed with smaller teeth; dactylus slim, arched, tips of fingers touching, enclosing board oval interspace when closed (Fig. 5B). Cheliped carpus inner margin with distal tooth large, acute, proximal tooth small, blunt, followed by raised margin (Fig. 5E). Cheliped merus lower margin with conspicuous large pointed distal meral tooth followed by raised granules along margin (Fig. 5D). P2–5 stout, not elongated (Fig. 4A). Small-sized species, adult at CW 27.0 mm.
Type locality. Rwanda. Nyanza, Ndugu Province (2.3396578796010163°S, 29.75056977735007°E) (Fig. 7).
Etymology. The species is named for the country of Rwanda where it was collected. The species epithet is used as a noun in apposition.
Habitat. Rotundopotamonautes rwanda n. sp. is a small species that is endemic to Rwanda and there are no further details available regarding its habitat.
Colour. The colour of living specimens is unknown, but specimens preserved in ethanol are light brown.
Distribution. This species is only known from a single locality in Rwanda in Ndugu Province (Fig. 7). Rwanda lies well within the distributional range of Rotundopotamonautes that centres on Uganda, Kenya and Ethiopia, with the border between Kenya and Tanzania serving as the south-eastern boundary line and Sudan and Egypt the northern boundary (Cumberlidge & Daniels 2022: fig. 10E). The distributional range of Rotundopotamonautes also includes the central and northern parts of the Rift Valley around Lakes Tanganyika, Kivu and Victoria and their drainages [D. R. Congo (Nord-Kivu and Ituri Provinces), Rwanda, Burundi, Tanzania (Kagoro Province)] and South Sudan (Chace 1942; Corace et al. 2001; Cumberlidge 2009; Cumberlidge & Dobson 2008; Cumberlidge & Clark 2010a, b, 2012, 2016, 2017, 2018; Cumberlidge & Meyer 2010, 2011).
Conservation status. The lack of information available on the distribution, population trends, habitat, or threats of this species precludes a conservation assessment using the IUCN’s Red List of Threatened Species protocols, so it would probably be listed as Data Deficient. This species was last collected in 1953 and no further material has come to light since then.
Remarks. Bott (1955) included the specimens identified here as R. rwanda n. sp. in the material he compiled for his description of Potamonautes (Rotundopotamonautes) loashiensis Bott, 1955 that comprised a number of specimens from the eastern Rift Valley in the D. R. Congo (Loashi in Kivu Province (RMCA_CRUST_38202– 38211, 38219–38232), Kibati (RMCA_CRUST_38235–38240), and Bukavu (RMCA_CRUST_17412)), Rwanda (Nyanza) (RMCA_CRUST_40579–40582), and Burundi (Kishubi) (RMCA_CRUST_41896, 41897). We found the specimens from Rwanda to differ morphologically from the adult male holotype of P. (R.) loashiensis (Bott 1955: pl. XXV, fig. 1, a–d (RMCA_CRUST_38218, CW 18, CL 12, CH 9, FW 5 mm) from Luashi, D. R. Congo in a number of characters that led us to conclude that we should recognize the Rwandan specimens as a new species. Differences between these two species are as follows. The adult male body size is CW 27 mm in R. rwanda n. sp. (vs CW 18 mm in P. (R.) loashiensis); the dactylus of the major chela is distinctly arched with a rounded oval interspace when closed in R. rwanda n. sp. (Fig. 5B) (vs a dactylus of the major chela that is only slightly arched and encloses a long narrow interspace when closed in P. (R.) loashiensis (Bott, 1955: pl. XXV, fig. 1c)}; the G1 TA is curved outward in R. rwanda n. sp. (Fig. 6A) (vs G1 TA that is straight along its entire length in P. (R.) loashiensis (Bott 1955: fig. 56)); and the distal tooth on the cheliped carpus is large and acute in R. rwanda n. sp. (Fig. 5E) (vs a cheliped carpus distal tooth that is low and blunt in P. (R.) loashiensis (Bott 1955: pl. XXV, fig. 1a)). We have not examined any of the other specimens included by Bott (1955) under P. (R.) loasahiensis so we cannot comment here on their identity.
Comparisons. The new species corresponds closest to the diagnosis of the genus Rotundopotamonautes as defined by Cumberlidge & Daniels (2022) and is therefore included in this genus. Rotundopotamonautes rwanda n. sp. conforms to the diagnosis by Cumberlidge & Daniels (2022: 1298) in that the third maxilliped ischium is smooth and lacks a vertical sulcus; the thoracic sternal sulcus S3/4 is incomplete and is deep only at the sides and is faint centrally; and the proximal tooth on the cheliped carpus inner margin is reduced to a small granule. Rotundopotamonautes rwanda differs from the genus diagnosis in that the G1 TA is not distinctly widened in the midsection (Fig. 6A, B) (although the tip does curve upward).
The new species shares a number of morphological characters with the other species of Rotundopotamonautes such as a faint, incomplete postfrontal crest that does not traverse the carapace, reduced or absent exorbital and epibranchial teeth, and a smooth carapace lateral margin behind the epibranchial tooth. Rotundopotamonautes rwanda n. sp. differs from similar species found in this part of Central Africa in the following ways. The S3/4 of R. rwanda n. sp. comprises only two short deep grooves at the margins (Fig. 4B) (vs two long deep grooves at the margins in R. bwindi (Cumberlidge & Clark, 2018), and deep, completely traversing the sternum, and meeting the anterior margin of the sternopleonal cavity in R. perparvus (Rathbun, 1921) and in Platythelphusa idjwiensis (Chace, 1942)}; and the distal meral tooth on the cheliped merus is large and triangular in R. rwanda n. sp. (Fig. 5D, E) (vs reduced to a large granule in R. bourgaultae (Cumberlidge & Meyer, 2011)); the third maxilliped ischium has only a faint vertical sulcus in R. rwanda n. sp. (Fig. 4B) (vs a third maxilliped whose ischium has a deep vertical sulcus in R. loveni (Colosi, 1924), R. mutandensis (Chace, 1942), R. rukwanzi (Corace, Cumberlidge & Garms, 2001), and R. williamsi (Cumberlidge & Clark, 2010a)); the distal tooth on the cheliped carpus of R. rwanda n. sp. is large and pointed (Fig. 5E) (vs small, broad, and low in R. bwindi and R. mutandensis); the lateral margin of the carapace of R. rwanda n. sp. is smooth (Fig. 4A) (vs lined by granules in R. gonocristatus (Bott, 1955)); the G1 TA is slim, curved, and tapers to a point in R. rwanda n. sp. (Fig. 6A,B) (vs a G1 TA that is widened in the middle with a lateral fold higher than the medial fold in R. minor (Bott, 1955)); and the distal part of the G1 TA is straight and the tip is not upturned in R. rwanda n. sp. (Fig. 6A,B) (vs a G1 TA that is short, and laterally directed with an upturned tip in R. emini (Hilgendorf, 1892)).
Rotundopotamonautes rwanda n. sp. can be distinguished from species of Arcopotamonautes Bott, 1955 that are also found in the river basins that drain into Lake Kivu as follows. The G1 TA is slim and not widened in the midsection in R. rwanda n. sp. (Fig. 6A, B) (vs a G1 TA that is distinctly widened in the midsection with a dorsal fold higher than the ventral fold in Arcopotamonautes (Cumberlidge & Daniels 2022)); the proximal tooth on the cheliped carpus inner margin is reduced to a small granule in R. rwanda n. sp. (Fig. 5E) (vs a large acute spine in Arcopotamonautes); the distal meral tooth is reduced to a small granule in R. rwanda n. sp. (Fig. 5D,E) (vs a distal meral tooth that is a large spine in Arcopotamonautes); and the exorbital tooth is low and blunt in R. rwanda n. sp. (Fig. 4A) (vs an exorbital tooth that is a large acute spine in Arcopotamonautes).
description: Published as part of Cumberlidge, Neil & Krajenka, Grace E., 2023, Description of two new species of Central African freshwater crabs from the Democratic Republic of the Congo and Rwanda (Brachyura: Potamoidea: Potamonautidae), pp. 379-391 in Zootaxa 5293 (2) on pages 385-390, DOI: 10.11646/zootaxa.5293.2.11, http://zenodo.org/record/7959937