Potamonautes obesus (A Milne­Edwards, 1868 )

Reed, Sadie K. & Cumberlidge, Neil, 2006, Brachyura: Potamoidea: Potamonautidae, Platythelphusidae, Deckeniidae (), Zootaxa 1262 (1), pp. 1-139 : 1-139

7. Potamonautes obesus (A Milne­Edwards, 1868) (Figs. 62–71, 157–158, 179, plate VII)

Thelphusa obesa A. Milne­Edwards, 1868: 86 , pl. 20, fig. 1–4; 1869: 178; 1887: 146; Pfeffer, 1889:

33. Telphusa obesa — Hilgendorf, 1878: 801; 1891: 20; 1898: 16. Potamon (Potamonautes) obesum — Ortmann, 1897: 303, 305. Potamon obesum — de Man, 1898: 434, 437. Potamon (Potamonautes) Bottegoi — de Man, 1898: 262–270, fig. 3. Potamon (Potamonautes) obesus — Rathbun, 1904: pl. 15, fig. 8, 9; 1905: 180, fig. 45; Sendler,

1912: 199; Bouvier, 1921: 49; Chace, 1942: 1907; Barnard, 1950: 192; Capart, 1954: 841, fig.

36, 17. Potamon (Potamonautes) Bottegoi Rathbun, 1905: 180 ; 1933: 258; 1935: 26; Colosi, 1925: 2;

Parisi, 1925: 98; Barnard, 1950:192, fig. 34 f, g. Potamonautes obesus — Balss, 1929: 348; Barnard, 1935: 484; Cumberlidge, 1997: 580; 1998:

202–203; Reed & Cumberlidge, 2004: 418–423, figs. 1–8, 17–25. 30. Potamon bottegoi — Chace, 1942: 208. Potamonautes (Obesopotamonautes) obesus obesus — Bott, 1955: 257–259, pl. XXII fig. 2a–d, fig.

19, 80; Pretzmann, 1977: 238, figs 7–12. Potamonautes bottegoi — Cumberlidge, 1997: 581–582; 1998: 198.

Type material examined: TANZANIA: Thelphusa obesa A. Milne­Edwards, 1868 , dried, adult male, form II, holotype (cw 50.6, cl 39.5 mm), Zanzibar (Grandidier) ( MNHN­B 4632). SOMALIA: de Man (1898) described Potamon (Potamonautes) bottegoi de Man, 1898 based on four sub­adult males from Matagoi Bool (=Bohol), between Brava and Lugh (Captain Bottego), x.1895; one of these males, a paratype (cw 27 mm) ( ZMA 102868) was examined in the present study. Bott (1955) listed the ‘type’ of Potamon (Potamonautes) bottegoi as the specimen with the following dimensions: cw 31, cl 24, ch 13, fw 10 mm. Pretzmann (1977) referred to all four specimens from Matagoi Bool as the ‘holotype’, but did not specify an individual specimen.

Additional material examined: for a complete account of other material from Tanzania, Kenya, Somalia and Malawi see Reed & Cumberlidge (2004).

Diagnosis. Carapace very high (ch/fw 1.5), smooth rounded; exorbital tooth small, low, epibranchial tooth small, pointed, positioned well behind postfrontal crest; carapace sidewall clearly divided into four parts; episternal sulci s4/e4 s7/e7 incomplete, s5/e5 s6/e6 complete; dactylus of major cheliped of form I adult males (cw 39–42 mm) flat broad, palm very high; dactylus of major cheliped of form II adult males (cw 43–59.6 mm) highly curved, slim, elongated (propodus of cheliped longer than cw), palm very high; terminal article of gonopod 1 directed outward at 45° angle to vertical; terminal article of gonopod 1 slim, tapering to slightly upcurved tip; lateral fold on terminal article of gonopod 1 wider higher than medial fold; subterminal segment of gonopod 1 columnar (broad from base to distal margin); basal margin of terminal article at dorsal membrane almost horizontal; distal margin of subterminal segment slightly curved at dorsal membrane with medial side raised slightly higher than lateral side; dorsal membrane equally wide on medial, lateral margins.

Size. The adult size range is from cw 33 to cw 59.6 mm.

Variation. Reed & Cumberlidge (2004) provide a detailed description of the series of changes the major cheliped of the male undergoes from juvenile to subadult stages and in the subsequent molts of the adult stage.

Type locality. Potamonautes obesus : Zanzibar, Tanzania . Potamon (Potamonautes) bottegoi de Man, 1898 : Matagoi Bool (Bohol), Somalia.

Distribution. Somalia, Kenya, Tanzania (the coastal region, plus Zanzibar and Pemba islands) and Malawi ( Reed & Cumberlidge 2004). Barnard (1950) reported this species from Harare, Zimbabwe.

Remarks. Photographs of the male holotype and of an ovigerous female from Nyassa (cw 49.5, cl 35.5 mm) are provided by Rathbun (1904, 180–182, plate VII, figs. 8, 9), and the holotype was illustrated by Capart (1954). Reed & Cumberlidge (2004) redescribed Potamonautes obesus and considered Potamon (Potamonautes) bottegoi de Man, 1898 to be a junior subjective synonym of P. obesus and Potamonautes calcaratus ( Gordon, 1929) to be a valid species, and this opinion is accepted here.

Natural history and conservation status. Potamonautes obesus is a semi­terrestrial crab that occurs in a wide variety of habitats ranging from the banks of rivers to rice fields, where it digs burrows down to reach ground water (S. Marijnissen, pers. comm.). The conservation status of P. obesus is categorized as least concern (LC) ( Table 4) because it has a range of occurrence and area of occupancy in excess of the thresholds for vulnerable (VU) ( IUCN 2004). Its population is estimated to be stable based on indirect measures such as the fact that a lot of material has been collected recently from more than one locality ( Reed & Cumberlidge 2004; S. Marijnissen, pers. comm.) and that it is well represented in museum collections.