Arcopotamonautes unisulcatus ( Rathbun, 1933 )

Arcopotamonautes unisulcatus ( Rathbun, 1933) View in CoL

( Figures 1–5 View FIGURE 1 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 , 10 View FIGURE 10 )

Potamon (Potamonautes) johnstoni unisulcatus Rathbun, 1933: 255 , pl. 2, figs. 2–4.

Potamon unisulcatus — Chace 1942: 223.

Potamonautes (Lirrangopotamonautes) johnstoni johnstoni View in CoL — Bott 1955: 265–267, pl. XV, fig. 2a–d, fig. 36a, b.

Potamonautes unisulcatus View in CoL — Reed & Cumberlidge 2006: 3–5 View Cited Treatment , 11, 15, 23, 37, 38, 58–60, pl. XIII, figs. A–D, table 1–4, figs. 124–133, 169,170, 185, appendix.—Ng et al. 2008: 171.

Arcopotamonautes unisulcatus View in CoL — Cumberlidge & Daniels 2022: 1274, 1294.

Type material. Holotype: MCZ CRU-7678 , adult female (CW 52.0 mm, CL 35.0 mm), Tanzania, Uluguru Mountains, Bagilo (-7.105079, 37.654157, 1,935m ASL), coll. A. Loveridge, September 1926 . Paratypes: MCZ CRU-7678a , adult male (CW 33.0 mm, CL 21.0 mm), same data as holotype . – MCZ CRU-7678b , 3 adult males (no measurements available), 2 adult females (no measurements available), same data as holotype .

Other material examined: NMU TRWEA62.45 , subadult male ( CW 31.6 mm), subadult female ( CW 41.1 mm), 2 juvs. (CWs 17.1, 16.3 mm), Tanzania, Uluguru Mountains , Bunduki , Kitange-Tange River, tributary of Mgeta River 2.4–3.0 m wide, near Hululu Waterfalls Mgeta (-7.049021, 37.642489, 1,603 m ASL), riverbed with large boulders, gravel, stones and sand, coll . T. R. Williams , 23 February 1962. – NMU TRWEA62.47 , 2 subadult males (CWs 29.8, 27.0 mm), 4 juvs. (CWs 15.6 mm, 15.9 mm, 16.2 mm, 17.8 mm), Tanzania, Uluguru Mountains , Bunduki rest house, Kitange-Tange River, 1859–1890 m ASL, river falling rapidly over large boulders, with gravel flats and occasional large stones, coll. T. R. Williams , 23 February 1962. – NMU TRWEA62.49 (illustrated), adult male ( CW 33.6 mm, CL 23.4 mm, CH 11.1 mm, FW 10.6 mm), adult female ovigerous ( CW 28.3 mm), Tanzania, Bunduki , Uluguru Mountains, from small, shaded stream near forest margin, coll . T. R. Williams , 23 February 1962 .

Diagnosis. Postfrontal crest sharp-edged, completely crossing carapace; exorbital tooth low but distinct, epibranchial tooth reduced to small granule; ( Fig. 1A, B View FIGURE 1 ); anterolateral, lateral carapace margins distinctly granulated; carapace branchiostegite smooth; third maxilliped ischium lacking vertical sulcus; S3/4 incomplete, deep at sides, obscure in middle; thoracic episternal sulci S4/E4, S5/E5 deep, complete, S6/E6, S7/E7 obscure ( Fig. 2B, C View FIGURE 2 ). Male right (major) chela dactylus straight, not arched, broad, cutting edge lined by large teeth, largest proximally, becoming smaller distally; cheliped merus stout, anterior inferior margin lined by small teeth, distal meral tooth large, posterior inferior margin smooth ( Fig. 3C, D View FIGURE 3 ); cheliped carpus inner margin distal tooth medium sized pointed, proximal tooth small, pointed, followed by two granules ( Fig. 3E View FIGURE 3 ); G1TA about 1/3 G1SA length (G1TA/G1SA 0.4), basal half straight, distal half curved outward at 45° to longitudinal axis of G1SA; G1TA tapering to pointed straight tip, mid-section widened by additional rounded crest on dorsal lobe with distinct setae on ventral lobe; G1TA dorsal, ventral lobes separated in middle by long groove ( Figs. 4A–C View FIGURE 4 , 5B, C View FIGURE 5 ).

Description. Carapace surface smooth, widest in anterior third (CW/FW 3.1), medium height ( CH /FW 1.1) ( Figs. 1A, B View FIGURE 1 , 2A View FIGURE 2 ), semi-circular, urogastric grooves deep; cardiac region weakly marked, cervical grooves short, faint; transverse branchial grooves faint ( Fig. 1A, B View FIGURE 1 ). Front about 1/3 carapace width (FW/CW 0.3); frontal margin straight ( Figs. 1A, B View FIGURE 1 , 2A View FIGURE 2 ); exorbital tooth small, blunt; epibranchial tooth reduced to granule; postfrontal crest sharply defined, complete, traversing entire carapace; lateral carapace margin posterior to epibranchial tooth granulated ( Fig. 1A, B View FIGURE 1 ). Branchiostegite with two sutures, one longitudinal (epimeral), one vertical, dividing carapace sidewall into suborbital, subhepatic, pterygostomial regions, all smooth ( Fig. 3A View FIGURE 3 ).

Third maxillipeds filling entire oral field, except for transversely oval efferent respiratory openings at superior lateral corners, exopod with long flagellum, endopod ischium smooth ( Fig. 2D View FIGURE 2 ). Mandibular palp consisting of basis plus two articles; terminal article simple, undivided, lacking lobe or ridge at junction between articles ( Fig. 2E View FIGURE 2 ). Anterior, posterior male thoracic sternum surface completely smooth; sternal sulci S1/2, S2/3 completely traversing sternum; S3/4 incomplete, deep at sides, obscure in middle; thoracic episternal sulci S4/E4, S5/E5 deep, complete, S6/E6, S7/E7 obscure ( Fig. 2B, C View FIGURE 2 ).

Male right (major) chela dactylus straight, not arched, broad, cutting edge lined by large teeth, largest proximally, becoming smaller distally; immovable finger (propodus pollex) broad, cutting edge lined by large teeth, largest proximally, becoming smaller distally; tips of both fingers touching when chela closed, enclosing long narrow interspace; major chela propodus palm enlarged, swollen, lower margin curving downward, distinctly convex, cutting edge toothed, largest teeth proximally, becoming smaller distally ( Fig. 3A View FIGURE 3 ). Male left (minor) chela dactylus slim, gently curved, cutting edge with large teeth proximally, small teeth distally; propodus pollex cutting edge lined by large teeth, largest proximally, becoming smaller distally; tips of both fingers touching when chela closed, enclosing long narrow interspace; minor chela propodus palm subequal to that of major chela; lower margin gently curved, convex ( Fig. 3B View FIGURE 3 ). Cheliped merus stout, anterior inferior margin lined by small teeth, distal meral tooth large, posterior inferior margin smooth ( Fig. 3C, D View FIGURE 3 ); cheliped carpus inner margin distal tooth medium sized, pointed; proximal tooth small, pointed, followed by two granules ( Fig. 3E View FIGURE 3 ); ambulatory legs P2–5 stout, distal limb articles (merus, carpus, propodus, dactylus) not elongated; dactyli of P2–5 tapering to point, each bearing four rows of downward-pointing sharp bristles ( Fig. 1A View FIGURE 1 ). 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. 2B, C View FIGURE 2 ).

G1TA about 1/3 G1SA length (G1TA/G1SA 0.4), basal half straight, distal half curved outward at 45° to longitudinal axis of G1SA; G1TA with setae on lateral margin, rounded crest in midsection, distal third tapering to pointed straight tip; G1TA dorsal lobe low, ventral lobe low widened by additional rounded crest in mid-section which has long setae; lobes separated in middle by thin groove ( Fig. 4A–C View FIGURE 4 , 5B, C View FIGURE 5 ). G1SA widest at base, narrowest at G1TA-G1SA junction; G1SA mesial, lateral margins lined by long setae ( Fig. 4A View FIGURE 4 , 5B View FIGURE 5 ). G2SA ( Fig. 5A View FIGURE 5 ) 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 0.5) ( Fig. 5A View FIGURE 5 ).

Size. Medium-sized species, the adult size range is between CW 33.0–52.0 mm.

Type locality. Tanzania, Uluguru Mountains , Bagilo .

Colour. Living male specimens have a purple-pink carapace and paler walking legs. Carapace of living female distinctly mottled with darker areas. Specimens preserved in ethanol are light brown.

Habitat. At Bunduki in the Uluguru Mountains this species is most abundant either near, or within, higher altitude forest and occurs only rarely at lower altitudes. Crabs were collected from a small shaded stream near the forest margin.

Distribution. Endemic to the Uluguru Mountains, Tanzania, East Africa ( Fig. 10 View FIGURE 10 ).

Conservation status. Arcopotamonautes unisulcatus is currently assessed as vulnerable because it is only known from two locations, has a restricted extent of occurrence, less than 20,000 km ²), and a small area of occupancy that are both below the thresholds for vulnerable (IUCN 2004). In addition, its distribution is fragmented, and it is threatened by a continuing decline in the extent and quality of its habitat due to degradation driven by human population increases and industrial and agrarian development, and A. unisulcatus is not found in a protected area.

Remarks. Bott (1955) treated Potamon (Potamonautes) unisulcatus Rathbun, 1933 as a junior synonym of A. johnstoni ( Miers, 1885) . The present study, however, agrees with the opinion of Reed & Cumberlidge (2006) that both of these taxa are each valid species. Arcopotamonautes unisulcatus is redescribed here because the earlier redescription of this species by Reed & Cumberlidge (2006) provided only a brief account and did not make comparisons with other species. In addition, the earlier descriptions have been expanded by adding new taxonomic characters such as mandibles and gonopods that were not previously described. Furthermore, this species is also compared with similar newly-described taxa from the region.

Comparisons. Arcopotamonautes ( Bott, 1955) currently comprises 18 species from the D.R. Congo, Kenya, Malawi, Rwanda, Tanzania, and Zambia ( Reed & Cumberlidge 2006; Cumberlidge & Daniels 2022; Cumberlidge & Jonas 2024; Cumberlidge & Conners 2025). Arcopotamonautes unisulcatus can be distinguished from A. suprasulcatus ( Hilgendorf, 1898) , 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. unisulcatus ( Figs. 4A–C View FIGURE 4 , 5A, C View FIGURE 5 , 9A–C View FIGURE 9 , 10B, C View FIGURE 10 ) (versus slim, curved, and needle-like in A. suprasulcatus and A. bellarussus ( Daniels et al. 2014: fig, 5A, B), or cone-shaped, not widened, and tapering to a pointed tip in both A. xiphoidus ( Fig. 9A, B View FIGURE 9 ) and A. infravallatus ; Cumberlidge & Conners 2025: figs. 1A, B, D, 6E). Arcopotamonautes unisulcatus 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 granular in A. unisulcatus ( Figs. 1A, B View FIGURE 1 , 6A, B View FIGURE 6 ) (versus a lateral margin that has several teeth behind the epibranchial tooth in A. orbitospinus ( Reed & Cumberlidge 2006: pl. V, A, B; fig. 42 as Potamonautes lirrangensis ), and A. platynotus ; Reed & Cumberlidge 2006: fig. 94). Arcopotamonautes unisulcatus 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. unisulcatus ( Figs. 3C View FIGURE 3 , 8E View FIGURE 8 ) (versus extremely broad, flat and blunt in A. platycentron ; Reed & Cumberlidge 2006: figs. pl. IX, A).

Arcopotamonautes unisulcatus View in CoL can be distinguished from A. amosae ( Cumberlidge, Johnson, Clark & Genner, 2021) View in CoL , A. caputanatis Cumberlidge, Clark & Fastiggi, 2019 View in CoL ), A. johnstoni ( Miers, 1885) View in CoL , A. raybouldi ( Cumberlidge & Vannini, 2004) View in CoL , A. gerdalensis ( Bott, 1955) View in CoL and A. montivagus ( Chace, 1953) View in CoL by the form of the G1TA, which has a rounded low crest arising out of the dorsal lobe in A. unisulcatus View in CoL ( Figs. 4A–C View FIGURE 4 , 5A, C View FIGURE 5 , 9A–C View FIGURE 9 , 10B, C View FIGURE 10 ) (versus a G1TA, where a high thin crest arises from the dorsal lobe in A. amosae View in CoL , A. caputanatis View in CoL , A. johnstoni View in CoL ( Reed & Cumberlidge 2006: figs. 151, 152), A. raybouldi View in CoL ( Reed & Cumberlidge 2006: figs. 165, 166), A. gerdalensis View in CoL ( Reed & Cumberlidge 2006: figs. 147, 148), and A. montivagus View in CoL ; Chace 1953: fig. 3e–g, j). Arcopotamonautes unisulcatus View in CoL can be distinguished from A. parekeeae Cumberlidge & Jonas, 1924 View in CoL and A. ngae Cumberlidge & Jonas, 1924 View in CoL by the sulci on the anterior thoracic sternum, where the S3/4 is missing except for two short notches at the margins in A. unisulcatus View in CoL (versus deep and completely traversing the thoracic sternum in A. parekeeae View in CoL and A. ngae View in CoL (Cumberlidge & Jonas, 1924: figs. 2B, C, 7B, C)). Finally, A. unisulcatus View in CoL can be distinguished from A. picus Cumberlidge & Conners, 2025 View in CoL by the form of the G1TA, which is distinctly widened in the midsection in A. unisulcatus View in CoL ( Figs. 4A– C View FIGURE 4 , 5A, C View FIGURE 5 , 9A–C View FIGURE 9 , 10B, C View FIGURE 10 ) (versus slim and tapering to a slightly upcurved tip in A. picus View in CoL ; Cumberlidge & Conners 2025: fig. 4A, B, D–F).