Arcopotamonautes ngae Cumberlidge & Jonas, 2024, n. sp.
Creators: name: Cumberlidge, Neil
name: Jonas, Gabrielle I.
Publisher: name: Zenodo
Year: 2024
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Potamonautidae
Arcopotamonautes
Arcopotamonautes ngae

description: Arcopotamonautes ngae n. sp.
(Figs. 6–11)
Type material. Holotype: NMU 1966.08a, adult male (CW 27.5, CL 18.7, CH 10.7, FW 8.5 mm), Tanzania, Eastern Arc Mountains, Kwebuyu River, Nguu Mountains (863 m ASL), near Tamota, Handeni District, Tanga Region (5.583224, 37.507317), 1965, coll. J. N. Raybould. Paratypes: NMU 1966.08b, adult male (CW 26.6, CL 19.5, CH 10.1, FW 8.45 mm), subadult male (CW 16.2, CL 12.6, CH 7.4, FW 6.0 mm), same details as holotype.
Diagnosis. Exorbital tooth small, pointed (Fig. 6A, B); anterior, posterior male thoracic sternum surface roughened by small raised granules (Fig. 7B); cheliped merus medial inferior margin lined by small teeth, lateral inferior margin lined by granules; distal meral tooth small (Fig. 3C, D); cheliped carpus inner margin with large pointed distal tooth, extremely small pointed proximal tooth, otherwise smooth (Fig. 8C); dactylus of male right (major) chela short, robust, straight, not curved, with three large teeth proximally (Fig. 8A), G1TA widened basally in three parts, ventral lobe low, dorsal lobe with two parts, thin crest medially, low lobe laterally, with wide gutter in between ventral, dorsal lobes running almost to tip (Fig. 9A–C, 10A, C).
Description. Carapace surface smooth, widest in anterior third (CW/FW = 3.2), medium height (CH/FW = 1.3) (Figs. 6A, B, 7A), semi-circular, urogastric grooves deep; cardiac region weakly marked, cervical grooves short, faint, transverse branchial grooves faint (Fig. 6A, B). Front about 1/3 carapace width (FW/CW = 0.3); frontal margin straight (Fig. 6A, B); exorbital tooth small, pointed; epibranchial tooth reduced to granule; postfrontal crest sharply defined, complete, crossing entire carapace; lateral margin posterior to epibranchial tooth smooth (Fig. 6A, B). Branchiostegite with two granulated sutures, one longitudinal (epimeral), one vertical, dividing carapace sidewall into suborbital, subhepatic, pterygostomial regions, each with granulated surface (Fig. 7A).
Third maxillipeds filling entire oral field, except for transversely oval efferent respiratory openings at superior lateral corners; long flagellum on exopod of third maxilliped; ischium of third maxilliped with deep vertical sulcus (Fig. 7F). Mandibular palp consisting of basis and two articles; terminal article undivided, with short hardened ridge along superior margin originating at article junction (Fig. 7D, E). Anterior, posterior male thoracic sternum surface roughened with small raised granules; sternal sulci S1/2, S2/3 completely traversing sternum; S3/4 deep, U-shaped, completely traversing sternum; thoracic episternal sulci S4/E4 distinct, S5/E5, S6/E6, S7/E7 all obscure (Fig. 7B, C).
Chelipeds unequal. Dactylus of male right (major) chela short, robust, straight, not curved, with three well-spaced medium teeth interspersed by small teeth; immovable finger (propodus pollex) broad, cutting edge with four medium teeth proximally, several small teeth distally; both fingers touching at tips, enclosing long slim interspace when closed; major chela propodus palm enlarged, swollen, lower margin of propodus convex (Fig. 8A). Dactylus of male left (minor) chela long, slim, gently curved, lined by small teeth; immovable finger (propodus pollex) slim, slender, with four medium teeth proximately, rest lined by small teeth; both fingers enclosing long slim interspace when closed; minor chela propodus palm slim, not enlarged, 0.6 × propodus height of major chela; propodus elongated (as long as propodus of major chela), lower margin slightly concave (Fig. 8B). Cheliped merus stout, distinctly shorter than CW; medial inferior margin lined by small teeth, lateral inferior margin lined by granules; distal meral tooth small (Fig. 8C, D); cheliped carpus inner margin with large pointed distal tooth, extremely small pointed proximal tooth, otherwise smooth (Fig. 8C); ambulatory legs P2–5 distal limb articles (merus, carpus, propodus, dactylus) stout, not elongated; dactyli of P2–5 tapering to point, each bearing 4 rows of downward-pointing sharp bristles (Fig. 6A). Male pleon, telson together forming slim triangle, pleon edges slightly indented; telson triangular, apex rounded, base broadest, sides outwardly sloping; pleomeres PL1–6 rectangular, wider than long, PL 6 longest, more than 1/2 as long as wide; remaining pleomeres short, less than 1/3 as long as wide (Fig. 7B, C).
G1TA about 1/3 G1SA length (G1TA/G1SA = 0.3), angled outward at 45° to longitudinal axis of G1SA; G1TA slim, smooth, lacking setae, tapering to pointed upcurved tip; G1TA widened basally in three parts, ventral lobe low, dorsal lobe with two parts, thin crest medially, low lobe laterally, with wide gutter in between ventral, dorsal lobes running almost to tip (Fig. 9A–C, 10A, C). G1SA widest at base, narrowest at TA-SA junction; basal G1SA mesial margin lined by long setae; G1SA lateral margin smooth (Fig. 9A, B). G2SA (Fig. 10B) long, slim, subequal to G1SA; G2SA widest at base, tapering sharply inward about one-third along length, last two-thirds forming long, thin, tapering, upright process supporting long flagellum-like G2TA (G2TA/G2SA = 0.62) (Fig. 10B). Small species, adult at CW 26.0 mm.
Colour. The colour of living specimens is unknown, but specimens preserved in ethanol are light brown.
Type locality. Tanzania, Kwebuyu River, Tamota, Nguu Mountains, Tanga Region (-5.583224°, 37.507317°) (Fig. 11).
Etymology. The species name, ngae, is derived from “Ng” (the first two letters of Nguu, a reference to the Nguu Mountains in the Eastern Arc Mountain range of Tanzania where the specimens were collected, as well as “Ng”, the family name of Dr Ngan Kee Ng, that is given a feminine ending. The specific epithet is used as a Latin noun in the genitive singular.
Distribution. Arcopotamonautes ngae n. sp. is known only from one locality in the Kwebuyu River in the Nguu Mountains (863 m ASL) in the Tanga Region of northeastern Tanzania (Fig. 11). The relatively small Nguu Mountains (area 1,591 km 2 that reach up to 1,550 m ASL) are part of a series of forested highlands in the central Eastern Arc mountains that also includes the nearby Mkungwe, Nguru, Ukaguru, Rubeho, and Uvidunda Mountains.
Habitat. This species is found in the Kwebuyu River that drains the forested Nguu Mountains in Handeni District in Tanzania. The collection locality in the Kwebuyu River in the forested Nguu Mountains at 863 m ASL is a tributary in the watershed of the Wami River that drains east to the Indian Ocean. The Nguu Mountains are a protected site listed as a Key Biodiversity Area of international significance because they meet established criteria for recognition as a site of biodiversity importance.
Remarks. The genus Arcopotamonautes Bott, 1955 currently comprises 15 species from the D. R. Congo, Kenya, Malawi, Rwanda, Tanzania, and Zambia (Cumberlidge & Daniels 2022). Arcopotamonautes parekeeae n. sp. and A. ngae n. sp. are assigned to this genus because they conform to the diagnosis provided by Cumberlidge & Daniels (2022: 1291). For example, in both species the postfrontal crest is distinct and completely traverses the carapace, the epibranchial tooth is reduced to a small granule, the posterior carapace sulci are distinct (Figs. 1A, B, 6A, B); and the G1TA is distinctly widened in the midsection and ends in an upturned tip (Figs. 4A–C, 5B, C, 9A–C, 10B, C).
Comparisons. Arcopotamonautes parekeeae n. sp. and A. ngae n. sp. can be distinguished from A. suprasulcatus, A. xiphoidus (Reed & Cumberlidge, 2006) A. infravallatus (Hilgendorf, 1898), and A. bellarussus (Daniels, Phiri & Bayliss, 2014) by the form of the G1TA, which is distinctly widened in the midsection in A. parekeeae n. sp. (Figs. 4A–C, 5B, C) and A. ngae n. sp. (Figs. 9A–C, 10B, C) (vs slim, curved, and needle-like in A. suprasulcatus and A. bellarussus, cf. Daniels et al. 2014: fig, 5A, B; or cone-shaped, not widened, and tapering to a pointed tip in A. xiphoidus, cf. Reed & Cumberlidge 2006: figs. 171, 172, and A. infravallatus, cf. Reed & Cumberlidge 2006: figs. 149, 150). Arcopotamonautes parekeeae n. sp. and A. ngae n. sp. can be distinguished from A. orbitospinus (Cunnington, 1907) from Lake Malawi and A. platynotus (Cunnington, 1907) from Lake Tanganyika by the form of the carapace lateral margin which is smooth and lacks teeth in A. parekeeae n. sp. (Fig. 1A, B) and A. ngae n. sp. (Fig. 6A, B) (vs a lateral margin that has several teeth behind the epibranchial tooth in A. orbitospinus, cf. Reed & Cumberlidge 2006: pl. V, A, B; fig. 42 (as Potamonautes lirrangensis), and A. platynotus, cf. Reed & Cumberlidge 2006: fig. 94). Arcopotamonautes parekeeae n. sp. and A. ngae n. sp. can be distinguished from A. platycentron (Hilgendorf, 1897) from Lake Chala (Kenya and Tanzania) by the form of the cheliped carpus distal tooth, which is slim and pointed in A. parekeeae n. sp. (Fig. 3C) and A. ngae n. sp. (Fig. 8E) (vs extremely broad and blunt in A. platycentron, cf. Reed & Cumberlidge 2006: pl. IX, A). Arcopotamonautes parekeeae n. sp. and A. ngae n. sp. can be distinguished from A. unisulcatus (Rathbun, 1921) by the sulci on the anterior thoracic sternum, where the S3/4 is deep and completely traverses the thoracic sternum in A. parekeeae n. sp. (Fig. 2B, C) and A. ngae n. sp. (Fig. 7B, C) (vs S3/4 is missing except for two short notches at the margins in A. unisulcatus, cf. Reed & Cumberlidge 2006: fig. 130). Arcopotamonautes ngae n. sp. can be distinguished from A. amosae (Cumberlidge, Johnson, Clark & Genner, 2021), A. caputanatis Cumberlidge, Clark & Fastiggi, 2019), A. johnstoni (Miers, 1885), A. raybouldi (Cumberlidge & Vannini, 2004), A. gerdalensis (Bott, 1955) and A. montivagus (Chace, 1953) by the form of the G1TA, which comprises rounded dorsal and ventral lobes separated by either a wide gutter or a high thin crest in A. parekeeae n. sp. and A. ngae n. sp. respectively (Figs. 4A–C, 5B, C, 9A–C, 10B, C) (vs a G1TA, where the dorsal and ventral lobes are close together and not separated by a wide gutter in A. amosae, A. caputanatis, A. johnstoni, cf. Reed & Cumberlidge 2006: figs. 151, 152; A. raybouldi, cf. Reed & Cumberlidge 2006: figs. 165, 166; A. gerdalensis, cf. Reed & Cumberlidge 2006: figs. 147, 148; and A. montvagus, cf. Chace 1953: fig. 3e–g, j).
Arcopotamonautes ngae n. sp. can be distinguished from A. parekeeae n. sp. by the exorbital tooth which is small and pointed (Fig. 1A, B) (vs small and blunt in A. parekeeae n. sp.; Fig. 6A, B); by the anterior and posterior male thoracic sternum surface that is roughened by small raised granules (Fig. 2B, C) (vs completely smooth in A. parekeeae n. sp.; Fig. 7B, C); by the inferior margins of the cheliped merus which are each lined by small granules and the distal meral tooth is small (Fig. 3C, D) (vs cheliped merus inferior margins with four large well-separated teeth and a large distal meral tooth in A. parekeeae n. sp.; Fig. 8C, D); by the cheliped carpus inner margin which has a large pointed distal tooth, and an extremely small pointed proximal tooth (Fig. 3C) (vs a cheliped carpus inner margin that has a medium blunt distal tooth and a small pointed proximal tooth in A. parekeeae n. sp.; Fig. 8E); by the dactylus of the male right (major) chela which is short, robust, and straight, with three large teeth proximally (Fig. 3A) (vs a male right (major) chela dactylus that is distinctly arched with three well-spaced small teeth interspersed by granule-like teeth in A. parekeeae n. sp.; Fig. 8A); and by the G1TA where the dorsal lobe has a thin low crest medially and a low lobe laterally, and the ventral lobe lacks setae (Figs. 4A–C, 5B, C) (vs a G1TA where the dorsal lobe has a high rounded crest medially and a low lobe laterally, and the ventral lobe is lined by long setae in A. parekeeae n. sp.; Fig. 9A–C, 10B, C).
description: Published as part of Cumberlidge, Neil & Jonas, Gabrielle I., 2024, Two new freshwater crab species from the Eastern Arc Mountains, Tanzania, East Africa (Brachyura: Potamoidea: Potamonautidae), pp. 51-65 in Zootaxa 5476 (1) on pages 57-64, DOI: 10.11646/zootaxa.5476.1.8, http://zenodo.org/record/12682014