Wednesday, June 10, 2020

[Crustacea • 2020] On the Taxonomy of the Endemic Inle Lake Crab, Inlethelphusa acanthica (Kemp, 1918) (Brachyura: Potamidae) of Myanmar

Inlethelphusa acanthica (Kemp, 1918)

in Ng, Mar & Yeo, 2020. 

 The potamid freshwater crab, Inlethelphusa acanthica (Kemp, 1918), is endemic to Inle Lake, in Shan state, Myanmar, and was hitherto known only from the types and several small specimens collected in the 1930s. The species, the type of Inlethelphusa Yeo & Ng, 2007, is redescribed from a good series of fresh specimens. The taxonomy of the species is discussed and variations in the armature of the carapace anterolateral margin, male pleon, chela, as well as the male first gonopod structure are documented. Significantly, the male first gonopod of smaller specimens has the terminal segment straight and more cylindrical, with the tip truncate or weakly bifurcated, in contrast to that of larger males, which is more elongate, with the distal part gently curved upwards and the tip sharp. 

Key words: Decapoda, freshwater crab, redescription, variation, diagnostic characters, wetland conservation

Fig. 8. Inlethelphusa acanthica (Kemp, 1918), female (40.4 × 28.5 mm) (ZRC 2019.1794). Colour in life. Photograph: P.Y.C. Ng.

Family Potamidae Ortmann, 1896
Subfamily Potamiscinae Bott, 1970 sensu Yeo & Ng, 2004

Inlethelphusa Yeo & Ng, 2007
Type species. Potamon (Potamon) acanthicum Kemp, 1918, 
by monotypy.

Diagnosis. Carapace broader than long, relatively low; dorsal surface gently convex; epigastric cristae low, distinct, not sharp, anterior to postorbital cristae, distinctly separated from postorbital cristae by groove; postorbital cristae low, blunt, not confluent with epibranchial teeth; external orbital angle well developed, separate from rest of anterolateral margin by deep cleft; epibranchial tooth well developed, spiniform; anterolateral margins prominently serrated, with anterior serrations larger; antennular fossae subrectangular, broad; epistome posterior margin with low but distinct median tooth; endostomial ridge absent. Third maxilliped with long exopod, with long flagellum. Ambulatory legs short; merus without subdistal spine on dorsal margin. Suture between male thoracic sternites 2 and 3 distinct; thoracic sternite 8 completely separated by a longitudinal medial line; sternopleonal cavity almost reaching imaginary line joining anterior edges of cheliped bases; male pleon triangular. G1 subterminal segment with distal third bent outward, appears neck-like, being distinctly tapered or constricted; G1 terminal segment slender, elongate, tubular to tapering, tip truncate or sharp, upcurved, dorsal flap absent. G2 longer than G1, distal segment distinctly less than half length of basal segment.

Inlethelphusa acanthica (Kemp, 1918) (Figs. 1–8) 

Potamon (Potamon) acanthicum Kemp, 1918: 85, fig. 2, pl. 24 figs. 3, 4. 
Lobothelphusa acanthia – Bott, 1970: 150; Yeo & Ng, 1999: 640. 
Inlethelphusa acanthica – Yeo & Ng, 2007: 283, fig. 6; Ng et al., 2008: 163.

Ecological notes. Kemp (1918: 89) notes that this “is apparently the only crab that makes its way into the central region of the Inle Lake, on the bottom of which it is occasionally found. It is more abundant among the roots of the floating islands at the edge of the lake, and also frequents small hill-streams.” The series of recent specimens (ZRC 2018.1370) were from the centre of the lake and were collected in the nets of fishermen. Two other specimens (ZRC 2018.0736) were from small streams flowing into Inle Lake. Nothing else is known about its ecology. This potamid species occupies a lacustrine niche in northern Indochina that, in other parts of Southeast Asia, is normally filled by crabs of the family Gecarcinucidae Rathbun, 1904. The chelae of I. acanthica are gaped in large males and females (Fig. 3D, E), similar to those of the analagous lentic gecarcinucids found in swamps and lakes in Thailand and Cambodia, and are not specifically adapted for feeding on molluscs (see Ng & Naiyanetr, 1993; Ng, 1995). In ancient lakes in Sulawesi, molluscivorous species have large molariform teeth at the base of the gaping fingers (see Chia & Ng, 2006; Schubart & Ng, 2008), while there is no evidence of incipient development or evolution of any such molariform teeth in I. acanthica.

 Peter K. L. Ng, Win Mar and Darren C. J. Yeo. 2020. On the Taxonomy of the Endemic Inle Lake Crab, Inlethelphusa acanthica (Kemp, 1918) (Crustacea: Brachyura: Potamidae) of Myanmar. RAFFLES BULLETIN OF ZOOLOGY. 68; 453–463.