Potamonautes amosae Cumberlidge & Johnson & Clark & Genner 2021, sp. nov.

Potamonautes amosae sp. nov.

urn:lsid:zoobank.org:act: 1B3001B9-7101-4551-AE46-2BEFCAD3598A

Figs 3, 4C–D, 5B, 6C–F, 7C, 9, 11B–F, 14, Table 1

Potamon (Potamonautes) lirrangensis – Balss 1936: 188 (partim, nec 189, fig. 24, Lirranga, Stanleyville; Kituri uberer Lualaba. Katanga-region). — Chace 1942: 188–189, fig. 1. – Capart 1952: 62–64, figs 12, 14c–d.

Potamonautes (Lirrangopotamonautes) lirrangensis – Bott 1955: 268–270, pl. XVI figs 2a–d, 38–39, 83. — Coulter 1991: 253, 255, tabs 9.XX, 9.XXI.

Potamonautes lirrangensis – Cumberlidge 1998: 201. — Reed & Cumberlidge 2006: figs 41–51, 153 – 154, 177 pl. V (partim). — Cumberlidge & Meyer 2011: 1845–1848 (partim, nec Malawi: Lake Malawi).

Diagnosis

Exorbital tooth large forward-pointing spine; lateral margin of exorbital tooth lined by granules before meeting postfrontal crest; epibranchial tooth small, granular, followed by small granules lining anterolateral margin (Fig. 4C–D). Anterolateral margin posterior to epibranchial tooth curving strongly outward (Fig. 4C–D); postfrontal crest distinct, completely traversing carapace between epibranchial teeth; posterior surface of carapace with deep urogastric grooves; third maxilliped ischium smooth (either lacking vertical sulcus or with faint sulcus); thoracic sternal sulcus S3/4 faint, shallow (Fig. 9B); major chela fixed finger with 3 large molars proximally, fused in older specimens into flat surface (Figs 6A–B, 8A); cheliped carpus inner margin with two large, subequal, forward-pointing spines (Fig. 7C); cheliped merus inner lower margin with spine-like tooth distally; P5 carpus, propodus, and dactylus not elongated (Fig. 9A–B); G1 TA (Fig. 11C–F) slightly widened by slim dorsal lobe (⅓ TA width at TA-SA junction); tip straight, only slightly curved upwards.

Etymology

The new species is named to honour the memory of Marilyn Suzanne Amos, of Mobile, Alabama, USA, who passed away during these studies. She was the mother of the second author (EJ). The specific epithet is used as a Latin noun in apposition. The vernacular name is Amos’s crab.

Material examined

Holotype DEMOCRATIC REPUBLIC OF THE CONGO • ♂ subadult; Idjwi Island, Lake Kivu; 2.082854° S, 29.071167° E; Feb. 1939; A. Loveridge leg.; MCZ CRU-11224.

Other material

DEMOCRATIC REPUBLIC OF THE CONGO • 1 ♂ subadult (CW 46.5 mm); Lake Kivu; donated by Royal Belgian Institute of Natural Sciences, Brussels; NHMUK 2020.3 • 1 ♂ subadult (CW 44.5 mm); Goma, Lake Kivu; 30 Nov. 1952; I. Gordon leg.; wide coast; NHMUK 2020.4.

RWANDA • 1 ♂ juv. (CW 30.0, CL 23.7, CH 11.2, FW 9.8 mm); Gisenye, Lake Kivu; Mar. 1936; J.C. Bequaert leg.; MCZ CRU-9177 • 1 ♀ juv. (CW 26.9 mm); Gisenye, Lake Kivu; 12 May 1955; Smithsonian-Bredin Congo Exped., W.L. Schmitt leg.; in water at shoreline; USNM 98937 • 1 ♀ adult (CW 62 mm); Kalemie (formerly Albertville), Lake Tanganyika; 8 Mar. 1919; M. Dhont de Bie leg.; NHMUK 1919.3.8.1-3.

TANZANIA • 1 ♂ subadult (CW 39.5 mm); Mungonya River, Mwandiga, near Kigoma; 4.828819° S, 29.666191° E; Apr. 1971; T.R. Williams leg.; NMU TRW 1971.05 • 1 ♀ adult (CW 80.1 mm); Malagarasi River, Uvinza, Kigoma District; 5.115673° S, 30.380144° E; Apr. 1971; T.R. Williams leg.; NMU TRW 1971.15.

Description

Carapace height equal to front width (CH /FW 1.0); carapace length 2.4 × front width (CL/FW 2.5); carapace width ~3× front width (CW/FW 3.1); posterior region of carapace with deep urogastric grooves; exorbital tooth large forward-pointing spine; lateral margin of exorbital tooth lined by small granules; epibranchial tooth small, granular, followed by large granules lining anterolateral margin (Fig. 4C–D); anterolateral margin posterior to epibranchial tooth curving strongly outward (Fig. 4C–D); postfrontal crest distinct, completely traversing carapace between epibranchial teeth; posterior surface of carapace with deep urogastric grooves; carapace branchiostegal wall divided by pleural (vertical) suture into suborbital region (with granules on surface), subhepatic region (with granules, crinae on surface); divided by epimeral (longitudinal) suture; pterygostomial region with granules on surface (Fig. 5B). 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; third maxilliped ischium smooth (either lacking vertical sulcus or with faint sulcus). Thoracic sternal sulcus S3/4 faint, shallow; episternal sulci S4/E4, S5/E5, S6/E6, and S7/E7 faint.

Major chela dactylus (moveable finger) and pollex of propodus (fixed finger) thick, broad, leaving long thin interspace between fingers when closed; both fingers with 3 large teeth proximally, other teeth small unfused distally; major chela fixed finger proximal molars fused into flat surface in older specimens from the Malagarasi River (CW 80.1 mm) (Fig. 6E–F); cheliped carpus inner margin with two large subequal forward-pointing spines (Fig. 7C); cheliped merus lower margins heavily granulated, inner lower margin with spine-like tooth distally; P3 longest, P5 shortest (carpus, propodus, and dactylus not elongated); P2–5 dactyli tapering to pointed tip, each bearing 4 rows of downward-pointing, short, sharp spines.

Male pleon 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 60° angle to longitudinal axis of G1 SA; G1 TA (Fig. 11B–E) widened by low dorsal lobe (⅓ TA width at TA-SA junction); tip straight, only slightly upcurved. 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. 11F). Margins of G1 TA, SA lined by setae.

Size

Large species, adult size range between CW 50 to 80 mm.

Colour

Preserved specimens are uniformly light brown like the holotype, but the large adult female from the Malagarasi River in Tanzania has black pigmentation on both fingers of the chelae (Fig. 6E–F).

Distribution

Potamonautes amosae sp. nov. was collected from rocky areas of Lake Kivu in the D.R. Congo and Rwanda (Fig. 3). Lake Kivu is a relatively small (100 km long by 50 km wide), deep lake (depth 480 m) situated in the Albertine Rift of the Western Rift Valley. This lake is divided by the border between the D.R. Congo and Rwanda, with the large Idjwi Island lying in the D.R. Congo. The Ruzizi River drains south out of Lake Kivu and links it to the northern part of Lake Tanganyika in Burundi, but this species has not been recorded from this river. Potamonautes amosae sp. nov. is found along the eastern shores of Lake Tanganyika in localities associated with the Malagarasi River in western Tanzania (Capart 1952; Reed & Cumberlidge 2006; M. Mbalassa & S. Marijnissen pers. com.) where it flows through the Kigoma District, and on the western shores of Lake Tanganyika at Kalemie in the D.R. Congo (Capart 1952).

Ecology

Little is known about the habitat and ecology of P. amosae sp. nov. In the region of Lake Tanganyika this species was often captured in marshes and wetlands near the lake, but never in the lake itself (Capart 1952). In Lake Kivu this species is found on islands in the lake as well as in the lake (Chace 1942). The range of P. amosae sp. nov. includes part of the Lake Victoria Basin Freshwater Ecoregion (FEOW #521) (Thieme et al. 2005; Abell et al. 2008).

Conservation status

An IUCN extinction risk assessment of P. amosae sp. nov. has not yet been carried out. This species has a wide distributional range (with an estimated extent of occurrence (EOO) of almost 46 600 km 2) and has been recorded from seven localities (Fig. 3) in three different countries. Given that there are no known immediate threats to this species, it would probably be assessed as Least Concern.

Remarks

There are a number of characters that distinguish P. amosae sp. nov. from P. orbitospinus in Lake Malawi. For example, the male thoracic sternal sulcus S3/4 of P. amosae sp. nov. is faint and shallow (vs deep and complete in P. orbitospinus); the low dorsal lobe of the G1 TA of P. amosae sp. nov. means that it is only slightly widened (vs a G1 TA dorsal margin that is conspicuously widened by a high dorsal lobe in P. orbitospinus); the anterolateral margin of P. amosae sp. nov. is lined by small granules (vs lined by a row of small distinct teeth in P. orbitospinus); the merus, propodus, and dactylus of P5 of P. amosae sp. nov. are all short (vs all elongated and slender in P. orbitospinus); and the third maxilliped ischium of P. amosae sp. nov. is smooth (vs with a third maxilliped ischium that has a deep vertical sulcus in P. orbitospinus).

In the past, P. amosae sp. nov. from Lake Kivu has been identified as P. lirrangensis s. lat. by a number of authors (Chace 1942; Bott 1955; Reed & Cumberlidge 2006; Cumberlidge & Meyer 2011). These identifications were made based on characters shared with the type of P. lirrangensis s. str. from Liranga (such as denticles or granules lining the anterolateral margin, 2 large pointed spines on the cheliped carpus inner margin, and a large pointed spine on the cheliped merus inner margin). There are a number of illustrations of P. amosae sp. nov. available, but most of these specimens have been identified as P. lirrangensis s. lat. For example, Chace (1942) illustrated the carapace and G1 of a specimen from Lake Kivu (MCZ CRU-11224), and Capart (1952: fig. 12) figured an entire specimen from Kalemie (formerly Albertville) on the western shores of Lake Tanganyika and remarked on its similarity to the species found in Lake Kivu. Later, Reed & Cumberlidge (2006: figs 41–51, 153–154, 177 pl. V) described in detail an adult female (CW 81 mm) and male (CW 56.5 mm) of P. lirrangensis s. lat. (NMU TRW1971.15) from the Malagarasi River at Uvinza in the Kigoma District of Tanzania near Lake Tanganyika.

DNA sequence data are available from specimens formerly assigned to P. lirrangensis s. lat. from Lakes Kivu, Tanganyika, and Malawi (Marijnissen et al. 2006; Daniels et al. 2015; Kochey et al. 2017). Marijnissen et al. (2006) used 2 mitochondrial DNA sequence markers (12S rRNA and 16S rRNA) to investigate relationships between specimens identified morphologically as P. lirrangensis s. lat. from Ruzizi in Lake Kivu in the D.R. Congo (GenBank DQ203210, DQ203236), from Uazua in the Zambian part of Lake Tanganyika (DQ203211, DQ203237), and from Thumbi West Island near Cape Maclear in southern Lake Malawi (GenBank DQ203209, DQ203235). Marijnissen et al. (2006: fig. 1) found that the specimen from Lake Kivu (here recognised as P. amosae sp. nov.) formed a separate basal lineage from the clade formed by the other 2 specimens from Lake Malawi (here recognised as P. orbitospinus).

Daniels et al. (2015) sequenced four DNA markers (GenBank AY803494, AY803534, AY803568, AY803682) for a specimen (ZMA.Crust.De.204681) held in the NBL that was identified in that work as P. lirrangensis s. lat. and incorrectly listed as being from Lake Malawi. In fact, specimen ZMA.Crust. De.204681 was collected from Lake Kivu (site 13, E. major; 23 Aug. 2002; Pascal Isumbisa leg.) and is therefore properly identified as P. amosae sp. nov.

There is molecular support for the recognition of P. amosae sp. nov. as a valid species from mitochondrial 16S rRNA and the nuclear coding gene Histone H3 sequences (Fig. 14). Across the 2 genes, 5 specimens are assigned to P. amosae sp. nov.: 3 from Uvinza, Kigoma District, Tanzania (2016-07-08 -UV1; 2016- 07-08 -UV2; 2016-07-08 -UV3), and two from Lake Kivu. The first specimen from Lake Kivu is ZMA. Crust.De.204681 represented by AY803534 and AY803682 (Daniels et al. 2015); the second specimen is from Ruzizi, Lake Kivu represented by DQ203236 (Marijnissen et al. 2006).