Potamonautes orbitospinus
Creators: name: Cumberlidge, Neil
name: Johnson, Emily
name: Clark, Paul F.
name: Genner, Martin J.
Publisher: name: Zenodo
Year: 2021
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Potamonautidae
Potamonautes
Potamonautes orbitospinus

description: Potamonautes orbitospinus (Cunnington, 1907)
Figs 2, 4E, 5C, 6G–H, 7D, 10, 12–14, Table 1
Potamon (Potamonautes) orbitospinus Cunnington, 1907: 259–261, pl. 16 fig 1.
Potamon (Potamonautes) orbitospinus – Balss 1929: 349 (partim, nec D.R. Congo: Lake Kivu). Potamon orbitospinus – Chace 1942: 218.
Common name
The Malawi blue crab.
Diagnosis
Exorbital tooth large forward-pointing spine; lateral margin of exorbital tooth not angled, in line with midline axis of carapace curving slightly inward (concave) before meeting postfrontal crest; epibranchial tooth pointed, as large as other teeth lining anterolateral margin (Fig. 4E). Anterolateral margin posterior to epibranchial tooth curving strongly outward (Fig. 4E); postfrontal crest distinct, completely traversing carapace between epibranchial teeth; posterior surface of carapace with deep urogastric grooves; third maxilliped ischium with thin, deep vertical sulcus; thoracic sternal sulcus S3/4 deep, V-shaped, completely traversing sternum (Fig. 10B); cheliped carpus inner margin with two large, subequal, forward-pointing spines (Fig. 7D); cheliped merus inner lower margin with spine-like tooth distally; P5 carpus, propodus, and dactylus all slender, distinctly elongated; G1 TA (Fig. 12A–C, E–G,) conspicuously widened by high, rounded dorsal lobe (as wide as TA width at TA-SA junction); tip distinctly curved upwards.
Material examined
Lectotype (here designated)
MALAWI • ♂ adult (CW 56.9, CL 38.4, FW 13.8 mm); western shore of Lake Malawi; 31 Jan. 1908; Tanganyika Exped., J.E.S. Moore leg.; NHMUK 1908.1.31.27.
Paralectotypes
MALAWI • 3 juvs (including CW 33.2, CL 22.8, CH 11.6, FW 10.0 mm); Lake Malawi; 19 Dec. 1891; M. Woodward leg.; NHMUK 1891.12.19.1 to NHMUK 1891.12.19.3 • 1 ♂ subadult (CW 27.5 mm); Universities Mission, Likoma, Lake Nyassa (now Lake Malawi); 14 Jan. 1893; J.A Williams leg.; NHMUK 1893.1.14 • 1 ♀ adult (CW 52.8 mm); west coast of Lake Malawi from Nkhata Bay to Ruarwe; Jun. 1896; A. Whyte leg.; NHMUK 1897.4.29.1 • several subadults; Nkhata Bay, Lake Malawi; 23 Jun. 1904; Third Tanganyika Exped., local fishermen and Dr W.A. Cunnington leg.; NHMUK 1897.4.29.23.
Additional material
MALAWI • 2 ♀♀ adults (CW 61.1, 65.8 mm), 2 ♀♀ subadults (CW 36.7, 45.1 mm), 1 ♂ subadult (CW 36.6 mm), 9 juvs; Lake Malawi, N of Hudzi; 20 Oct. 1926; Cristy leg.; NHMUK 1926.10.20.1 to NHMUK 1926.10.20.5 • 1 ♂ subadult (CW 31.1 mm); NW coast of Lake Malawi, near Nkhata Bay; 31 Jan. 1908; NHMUK 1908.1.31.16-18 • 1 ♀ adult (CW 55.6 mm); Lake Malawi, Monkey Bay; 20 Oct. 1926; NHMUK 1926.10.20.6 • 1 ♂ subadult (CW 49 mm), 1 ♀ adult with hatchlings (CW 64.8 mm); NW coast of Lake Malawi, near Nkhata Bay; 26 Jul. 1954; Miers leg.; NHMUK 1954.7.26.5, NHMUK 1954.7.26.6 • 1 ♀ adult (CW 61.1 mm), 1 ♂ adult (CW 55.4 mm); Lake Malawi; 5 Jun. 1956; G. Fryer leg.; NHMUK 1956.6.5.10, NHMUK 1956.6.5.11 • 1 ♀ adult (CW 57 mm), 1 ♂ subadult (CW 40 mm), 2 juvs (CW 32.5, 33.4 mm); Lake Malawi; 26 Jul. 1954; W.A. Cunnington leg.; NHMUK 1954.7.26.3, NHMUK 1954.7.26.4 • 1 ♂ subadult (CW 51.1 mm); Lake Malawi, Monkey Bay; Mar. 1968; D.H. Eccles leg.; among rocks in sand; NMU TRW 1972.04 • 1 ♂ subadult (CW 46.2 mm); Lake Malawi, 2 km ENE of Monkey Bay; May 1968; D.H. Eccles leg.; NMU TRW 1972.05 • 1 ♀ subadult (CW 51.1 mm); Lake Malawi; Sep. 1988; Irv. Kornfield leg.; NMU 09.1988k.1 • 1 ♀ (damaged); Lake Malawi, N of Monkey Bay; 5 Apr. 1972; D.H. Eccles leg.; depth 91 m; NMU TRW1972.02 • 1 ♂ (CW 51.4 mm); Lake Malawi, Monkey Bay; 24 Mar. 1968; D.H. Eccles leg.; among rock in sand with little vegetation; NMU TRW1972.04 • 1 ♂ (CW 46.5 mm); Lake Malawi, ENE of Monkey Bay; 23 May 1968; D.H. Eccles leg.; NMU TRW1972.05 • 1 ♂ subadult (CW 46.5 mm); Lake Malawi, Cape Maclear; M. Genner leg.; NHMUK 2010-06-CM-CM6 • 1 ♀ subadult (CW 50.15 mm); same collection data as for preceding; NHMUK 2010-06-CM-CM8 • 1 ♀ adult (CW 52.2 mm); same collection data as for preceding; Jun. 2010; NHMUK CM13 • 1 ♀ adult (CW 54.4 mm); same collection data as for preceding; NHMUK CM14 • 1 ♀ subadult (CW 44.1 mm); same collection data as for preceding; NHMUK CM21 • 1 ♀ adult (CW 54.8 mm); Lake Malawi, NW coast near Nkhata Bay; 1961; Sweeney leg.; NHMUK 2011.1509 • 1 ♀ subadult (CW 32.7 mm); Lake Malawi, Cape Maclear; 17 Jun. 2010; M. Genner leg.; UB CM17 • 1 ♀ subadult (CW 37.4 mm), 1 ♀ adult (CW 57.6 mm); 2 ♂♂ adults (CW 55.1, 54.9 mm); same collection data as for preceding; UB CM22, UB CM11, UB CM20, UB CM12 • 1 ♀ adult (CW 57.1 mm), 1 ♀ subadult (CW 33.1 mm); same collection data as for preceding; 26 Jun. 2010; UB CM10, UB CM5 • 1 ♂ subadult (CW 49.2 mm); same collection data as for preceding; 21 Jun. 2010; UB CM4 • 1 ♀ subadult (CW 48.3 mm); same collection data as for preceding; UB CM9 • 1 ♀ subadult (CW 46.7 mm); same collection data as for preceding; UB CM15 • 1 ♀ subadult (CW 35.4 mm); same collection data as for preceding; UB CM16 • 1 ♀ subadult (CW 45.4 mm); same collection data as for preceding; UB CM24 • 1 ♂ adult (CW 59.4 mm); same collection data as for preceding; UB CM7 • 2 ♀♀ adults (CW 55.4, 54.4 mm), 2 ♀♀ subadults (CW 44.9, 45.9 mm); same collection data as for preceding; UB CM7 • 1 ♂ adult; same collection data as for preceding; 26 Jun. 2010; R. Bills leg.; AMG CAW 467A.
Redescription
Carapace height equal to front width (CH /FW 1.0); carapace length 2.5 × front width (CL/FW 2.5); carapace width 3.5 × front width (CW/FW 3.5); exorbital tooth large forward-pointing spine; exorbital tooth lateral margin not angled, in line with midline axis of carapace, curving slightly inward (concave) before meeting postfrontal crest; epibranchial tooth pointed, as large as other teeth lining anterolateral margin (Figs 4E, 10A). Anterolateral margin posterior to epibranchial tooth curving strongly outward (Fig. 4E); postfrontal crest distinct, completely traversing carapace between epibranchial teeth; posterior surface of carapace with deep urogastric grooves; carapace branchiostegal wall divided by vertical pleural suture into suborbital and subhepatic regions, both smooth with sparse granules, pterygostomial region smooth (Fig. 10B); epistomial tooth prominent, granulated, V-shaped. Mandible palp comprising 2 articles; terminal article single, undivided, with setae (but no hard flap) at junction between articles. Third maxillipeds filling entire oral field, except for transversely ovate respiratory openings at superior lateral corners; exopod with long flagellum; ischium with deep vertical sulcus. Thoracic sternal sulcus S3/4 deep, completely traversing sternum; episternal sulci S4/E4, S5/E5, S6/ E6, and S7/E7 distinct.
Major chela dactylus (moveable finger) and pollex of propodus (fixed finger) thick, broad, leaving long interspace between fingers when closed; both fingers with 3 large teeth unfused proximally, several medium-sized teeth distally (Fig. 6G–H); cheliped carpus inner margin with two large subequal forwardpointing spines (Fig. 7D); cheliped merus inner lower margin with spine-like tooth distally; P5 carpus, propodus, and dactylus all slender, distinctly elongated; P2–5 dactyli elongated, tapering to pointed tip, each bearing 4 rows of downward-pointing, short, sharp spines.
Pleon of male slim, triangular, telson narrow triangle with rounded apex, pleomeres Al–6 quadrate. G1 TA proximal third straight, not widened, margins parallel, at midpoint bent sharply outward at 90° angle to longitudinal axis of G1 SA; G1 TA (Fig. 12A–C, E–G) conspicuously widened by high, rounded dorsal lobe (as wide as TA width at TA-SA junction); tip distinctly curved upwards; G1 SA at junction with G1
TA with horizontal margin on ventral side, U-shaped indentation filled by conspicuous dorsal membrane on dorsal side. G2 TA: long, flagellum-like (Fig. 12D, H). Margins of G1 TA, SA lined by setae.
Size
Large species, pubertal molt starting around CW 53 mm (largest adult male CW 56.9 mm, largest adult female CW 53.9 mm).
Colour
The carapace surface and branchiostegal walls of living specimens are deep blue, and are especially bright in newly-hardened specimens (Fig. 13). There are distinct white outlines marking the postfrontal crest, anterolateral margins, frontal margin, orbital margins, exorbital teeth, epistome, and the third maxilliped ischium and merus. The thoracic sternum is pinkish blue/grey and cream, and the arthrodial membranes on the inner side of the joints between the coxae and the basis of the chelipeds and P2–5 are cream.
Distribution
Potamonautes orbitospinus is abundant and widely distributed throughout Lake Malawi (Fig. 2) and has not been recorded from outside of the lake.
Ecology
Lake Malawi is the southernmost Great Lake in the East African Rift system and lies in 3 countries: Malawi, Mozambique, and Tanzania. The Ruhuhu River in Tanzania flows west into the northeastern part of Lake Malawi while the Shire River drains south out of the lake and is a tributary of the Zambezi River. In 1904, Cunnington and his assistants collected the first known specimens of P. orbitospinus from the waters of Lake Malawi itself, noting that some specimens were found on the beach (Cunnington 1907). The specimens reported on here are all restricted to Lake Malawi, and this species is a lake specialist that has never been collected in the rivers of the drainage basin that flow into the lake.
Conservation status
An IUCN conservation assessment of P. orbitospinus has not yet been carried out. The species is known from a large number of specimens from 16 localities all in Lake Malawi (29 600 km 2). Given that its estimated extent of occurrence (EOO) is more than 21 100 km 2, and that no specific threats are known, it would probably be assessed as Least Concern. It is significant that the population levels of P. orbitospinus are sufficient to be regularly caught as bycatch in local fisheries in Lake Malawi, and this species is also captured to supply a steady demand by the global aquarium trade.
Remarks
The recognition of P. orbitospinus and P. lirrangensis s. str. as valid species returns to the original taxonomic situation over 110 years ago when they were first described from two widely separated locations (Rathbun 1904; Cunnington 1907). Chace (1942) also treated P. lirrangensis and P. orbitospinus as valid species, but Bott (1955), Reed & Cumberlidge (2006) and Cumberlidge & Meyer (2011) considered Potamon (Potamonautes) orbitospinus to be a junior synonym of Potamonautes lirrangensis s. lat. The result has been that the available descriptions and distribution maps of Potamonautes lirrangensis s. lat. (Reed & Cumberlidge 2006: fig. 177) incorrectly combine characters and localities of P. lirrangensis s. str. from the Congo River with those of P. orbitospinus from Lake Malawi, and P. amosae sp. nov. from Lake Kivu and Kigoma District near Lake Tanganyika.
Potamonautes orbitospinus is recognised here based on characters of the lectotype described by Cunnington (1907) from Lake Malawi as well as other comparable material from this lake. The redescription includes new taxonomically important characters because although the description by Cunnington (1907) of P. orbitospinus was based on an adult male, he did not illustrate the first gonopod or sternal characters of the type specimen. See concluding remarks below for comparisons with other superficially similar species.
The combined phylogeny based on mitochondrial 16S rRNA and the nuclear coding gene Histone H3 (Fig. 14) includes a specimen from Thumbi West Island near Cape Maclear in southern Lake Malawi (GenBank DQ203209, DQ203235), alongside eight other specimens from Cape Maclear and Chiofu on the east coast of Lake Malawi. The phylogeny suggests a monophyletic clade for P. orbitospinus from Lake Malawi, separate from the clade for P. amosae sp. nov. (Fig. 14).
A specimen identified as P. lirrangensis s. lat. from ‘Uazua’ in the Zambian part of Lake Tanganyika (POlirrangensisZAM31; Marijnissen et al. 2006) has a partial 16S sequence (DQ203237) with high similarity (99%) to a specimen of P. orbitospinus from Lake Malawi (POlirrangensisMAL27; DQ203235; Marijnissen et al. 2006). This same specimen (POlirrangensisZAM31), however, has a partial 12S sequence (DQ203211) which has only 97% similarity to POlirrangensisMAL27 (DQ203209). This may indicate that P. orbitospinus shares a close evolutionary affinity to specimens within Lake Tanganyika, but further sampling is required to determine the evolutionary relationships of these two groups.
Kochey et al. (2017) carried out a molecular study of the Malawi blue crab (which they identified as P. lirrangensis s. lat.) that found the morphologically similar populations in Lake Malawi to be equally close genetically, and confirmed that the lake hosts only a single species of freshwater crab (here identified as P. orbitospinus). Those authors also found that the blue crab populations in Lake Malawi had only moderate haplotype diversity and low levels of nucleotide diversity for two mitochondrial loci (NADH dehydrogenase subunit 1 (ND1) and cytochrome b (CytB) (Kochey et al. 2017). The lack of divergence of blue crab populations in Lake Malawi and the morphological similarity of specimens found in different parts of the lake suggests a recent colonisation (Kochey et al. 2017).
description: Published as part of Cumberlidge, Neil, Johnson, Emily, Clark, Paul F. & Genner, Martin J., 2021, Unravelling the Potamonautes lirrangensis (Rathbun, 1904) species complex (Potamoidea: Potamonautidae), with the description of two new species, pp. 145-178 in European Journal of Taxonomy 744 (1) on pages 167-173, DOI: 10.5852/ejt.2021.744.1309, http://zenodo.org/record/4690300