Arcopotamonautes xiphoidus ( Reed & Cumberlidge, 2006 )

Cumberlidge, Neil, Huiskens, Alyssa J. & Jonas, Gabrielle I., 2025, Redescription of two species of freshwater crabs from Tanzania, East Africa: Arcopotamonautes unisulcatus (Rathbun, 1933) and A. xiphoidus (Reed & Cumberlidge, 2006) (Brachyura: Potamoidea: Potamonautidae), Zootaxa 5659 (4), pp. 565-580 : 573-577

Arcopotamonautes xiphoidus ( Reed & Cumberlidge, 2006)

( Figures 6–10 )

Type material. Holotype: NMU TRW1966.06 b, adult male ( CW 38.3 mm, CL 26.1 mm, CH 15.6 mm, FW 11.4 mm), Tanzania, West Usambara Mountains, Herkulu Estate, Lushoto District , Tanga region, coll. J.N. Raybould, November 1964 . Paratype: NMU TRW1966.06 b, adult male ( CW 27.2 mm, CL 19.3 mm, CH 11.7 mm, FW 8.4 mm) , same data as holotype .

Other material examined. NMU TRW1966.06 a, 5 subadult & adult males (CWs 16.1–33 mm), 2 adult females (CWs 30.9 mm, 33.1 mm), 6 subadult & adult females (CWs 16.0– 25.3 mm) Tanzania, West Usambara Mountains, streams and dams near Herkulu Estate, Lushoto District , Tanga region, coll. J.N. Raybould, November 1964 . – USNM 64108 , female (damaged), male (damaged), Tanzania, East Usambara Mountains , Amani, coll. A. Loveridge, 8 December 1926 .

Diagnosis. Exorbital tooth small, blunt; epibranchial tooth reduced to granule; postfrontal crest sharply defined, complete, traversing entire carapace; carapace lateral margin posterior to epibranchial tooth smooth ( Fig. 6A, B ). Branchiostegite suborbital, subhepatic, pterygostomial regions, all smooth ( Fig. 7A, B ). Ischium of third maxilliped with deep vertical sulcus ( Fig. 8E ). Dactylus of except for several small granule-like teeth distally; both finger tips touching when chela closed, enclosing long oval interspace; major chela propodus palm enlarged, swollen, lower margin distinctly concave ( Fig. 8A ). G1TA smooth, tapering sharply to pointed straight tip; G1TA widened basally; ventral, dorsal lobes low ( Fig. 7A, B ).

Redescription. Carapace surface smooth, widest in anterior third (CW/FW 3.4), medium height ( CH /FW 1.4) ( Figs. 6A, B , 7A ), semi-circular groove deep, urogastric, cardiac, cervical, transverse branchial grooves faint ( Fig. 6B ). Front about 1/3 carapace width (FW/CW 0.3); frontal margin slightly indented ( Figs. 6B , 7A ); exorbital tooth small, blunt; epibranchial tooth reduced to granule; postfrontal crest sharply defined, complete, traversing entire carapace; carapace lateral margin posterior to epibranchial tooth smooth ( Figs. 6A, B , 7A ). Branchiostegite with two sutures, one longitudinal (epimeral), one vertical, dividing carapace sidewall into suborbital, subhepatic, pterygostomial regions, all smooth ( Fig. 7A ).

Third maxillipeds filling entire oral field, except for transversely oval efferent respiratory openings at superior lateral corners ( Fig. 7A ), exopod with long flagellum maxilliped, endopod ischium with deep vertical sulcus ( Fig. 8E ). Mandibular palp consisting of basis plus two articles; terminal article undivided, lacking ridge at junction between articles ( Fig. 7D–F ). Thoracic sternum S4 outer margin thickened and raised, sternal sulci S1/2, S2/3 completely traversing sternum; S3/4 deep, V-shaped, completely traversing sternum; thoracic episternal sulci S4/ E4, S5/E5, S6/E6, S7/E7 present ( Fig. 7A, B ).

Dactylus of male right (major) chela distinctly arched, slim, cutting edge lacking teeth except for several small granule-like teeth distally; immovable finger (propodus pollex) slim, cutting edge with two medium teeth proximally; both finger tips touching when chela closed, enclosing long oval interspace; palm of propodus of major chela enlarged, swollen, lower margin distinctly concave ( Fig. 7A ). Dactylus of male left (minor) chela long, slim, gently curved, cutting edge lined by series of small teeth; immovable finger (propodus pollex) with four medium teeth interspersed by small teeth; both fingers touching at tip when chela closed, enclosing long oval interspace; palm of propodus of minor chela subequal to that of major chela, 0.8× propodus height of major chela; lower margin slightly concave ( Fig. 7B ). Cheliped merus elongated, almost as long as CW; inferior margin proximally with three granule-like teeth and larger one at midpoint, otherwise smooth, distal meral tooth lacking ( Fig. 8C, D ); cheliped carpus inner margin distal tooth medium sized, blunt; proximal tooth small, blunt, margin behind it granulated ( Fig. 8C ); ambulatory legs P2–5 long, distal limb articles (merus, carpus, propodus, dactylus) elongated; dactyli of P2–5 tapering to point, each bearing four rows of downward-pointing sharp bristles ( Fig. 6A ). Male pleon together with telson forming slim triangle; telson triangular, apex rounded, base broadest, sides slightly indented inwardly 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 smooth, lined by short setae, tapering sharply to pointed straight tip; G1TA widened basally; ventral, dorsal lobes low ( Fig. 9A, B ). G1SA widest at base, narrowest at G1TA-G1SA junction; basal G1SA mesial margin lined by sparse short setae; G1SA lateral margin mostly smooth ( Fig. 9A, B ). G2SA ( Fig. 9C ) long, slim, subequal to G1SA; G2SA widest at base, tapering sharply inward about one-third along length, last 2/3rds forming long, thin, tapering, upright process supporting long flagellum-like G2TA (G2TA/G2SA 0.6) ( Fig. 9C ).

Size. Medium-sized species, adult size range between CWs 24–38 mm.

Type locality. The Herkulu Estate is a tea plantation in the Western Usambara Mountains in Lushoto District of the Tanga region of Tanzania ( Fig. 10 ). This locality is situated in the montane forest zone (1,666 m ASL) where much of the forest has been cleared, but patches of forest have been retained to prevent soil erosion .

Distribution. This species is endemic to the East and West Usambara Mountains in Tanzania ( Fig. 10 ). The rivers of the western slopes of the Usambaras drain into the Pangani River, while the eastern slopes of these mountains are drained by the Sigi River and its tributaries.

Conservation status. Arcopotamonautes xiphoidus is assessed as vulnerable because it is still known only from three localities, has a narrow extent of occurrence, and a restricted area of occupancy that are all below the thresholds for vulnerable (IUCN 2004). This species is threatened by increasing habitat disturbance and destruction by deforestation associated with growing human populations in the region. In addition, A. xiphoidus is not found in a protected area.

Remarks. Arcopotamonautes xiphoidus is morphologically close to A. unisulcatus , but the two taxa can be recognized by differences in the G1TA, which is long, straight, smooth, evenly-tapering, and spear-like in A. xiphoidus ( Fig. 9A, B ) (versus curved and tapering to a pointed straight tip, with a mid-section widened by an additional rounded crest, and distinct setae on the ventral lobe in A. unisulcatus ; Figs. 4A, B , 5A–C ).

Figures

FIGURE 4. Arcopotamonautes unisulcatus (Rathbun, 1933), adult male (CW 33.6 mm) (NMU TRWEA62.49), from Bunduki, Uluguru Mountains, Tanzania. A, left G1, ventral view; B, left G1, dorsal view; C, left G2, ventral view. Scale bar: A, B = 1.0 mm; C = 1.3 mm.

FIGURE 5. Arcopotamonautes unisulcatus (Rathbun, 1933), adult male (CW 33.6 mm) (NMU TRWEA62.49), from Bunduki, Uluguru Mountains, Tanzania. A, left G1TA, ventral view; B, left G1TA, turned to show detail; C, left G1TA, superior view. Scale bar: A = 0.5 mm; B = 0.7 mm; C = 0.6.

FIGURE 6. Arcopotamonautes xiphoidus (Reed & Cumberlidge, 2006) adult male holotype (CW 38.3 mm) (NMU TRW1966.06b), from West Usambara Mountains, Herkulu Estate, Lushoto District, Tanga region, Tanzania. A, whole animal, dorsal view; B, carapace dorsal view. Scale bar: A = 13.3 mm; B = 5.1 mm.

FIGURE 7. Arcopotamonautes xiphoidus (Reed & Cumberlidge, 2006) adult male holotype (CW 38.3 mm) (NMU TRW1966.06b), from West Usambara Mountains, Herkulu Estate, Lushoto District, Tanga region, Tanzania. A, carapace, frontal view; B, cephalothorax showing anterior thoracic sternum, ventral view; C, cephalothorax showing pleon and posterior thoracic sternum, ventral view; D, mandible, frontal view; E, mandible, turned slightly to show ridge on terminal article of mandibular palp; F, mandibular palp, showing ridge on terminal article. Scale bar: A = 6.1 mm; B = 13.3 mm; C = 8.0 mm; D, E = 2.0 mm; F = 1.7 mm.

FIGURE 8. Arcopotamonautes xiphoidus (Reed & Cumberlidge, 2006) adult male holotype (CW 38.3 mm) (NMU TRW1966.06b), from West Usambara Mountains, Herkulu Estate, Lushoto District, Tanga region, Tanzania. A, right chela, frontal view; B, left chela, frontal view; C, left cheliped, carpus, merus, ischium, dorsal view; D, left cheliped, merus, ischium, ventral view; E, third maxilliped. Scale bar: A, B = 16.7 mm; C, D = 6.0 mm; E = 4.2 mm.

FIGURE 9. Arcopotamonautes xiphoidus (Reed & Cumberlidge, 2006) adult male holotype (CW 38.3 mm) (NMU TRW1966.06b), from West Usambara Mountains, Herkulu Estate, Lushoto District, Tanga region, Tanzania. A, left G1, ventral view; B, left G1, dorsal view; C, left G2, ventral view. Scale bar: A, B = 1.0 mm; C = 1.1 mm.

FIGURE 10. Map of south eastern Kenya and eastern Tanzania, East Africa, showing the geographic distribution of Arcopotamonautes unisulcatus (black triangles) and A. xiphoidus (black circles). Scale bar = 140 km. See text for exact localities.