Thursday, March 9, 2023

[Crustacea • 2023] Isolapotamon seowi • A New Species of potamid Crab (Brachyura: Potamidae: Potamiscinae) from Kokol Hill in western Sabah, Malaysia, Borneo

Isolapotamon seowi 
Ng, 2023

A new species of freshwater crab of the genus Isolapotamon Bott, 1968 (Potamidae) is described from western Sabah in Malaysian Borneo. Isolapotamon seowi new species, is the sixth species of Isolapotamon known from Sabah, and differs from its closest congener, I. buntyae Ng, Low & Clark, 2022, from eastern Borneo, in possessing a relatively lower carapace, a more rugose dorsal carapace surface, the postorbital cristae being sharper and subparallel with the frontal margin, relatively shorter fourth ambulatory legs, and the terminal segment of the male first gonopod is gently curved with the subdistal process slightly longer than the distal projection and angled at 90° to the longitudinal axis.

Keywords: Potamidae; Potamiscinae; freshwater crab; taxonomy; new species; Southeast Asia; Sundaland 

Family Potamidae Ortmann, 1896 
Subfamily Potamiscinae Bott, 1970 sensu Yeo & Ng (2003) 

Genus Isolapotamon Bott, 1968

Isolapotamon seowi new species, holotype male (40.7 × 31.6 mm) (ZRC 2022.778), Kokol Hill, Sabah.
A, Overall dorsal view; B, dorsal view of carapace; C, frontal view of cephalothorax; D, chelae; E, left third maxilliped; F, right P5; G, anterior thoracic sternum and pleon; H, pleon.

Colour in life, Isolapotamon seowi new species.
A, Holotype male (40.7 × 31.6 mm) (ZRC 2022.778), Kokol Hill, Sabah;
B, C, paratype male (33.8 × 26.5 mm) (ZRC 2022.779), Kokol Hill, Sabah.
A, B, overall dorsal view; C, frontal view.
Photographs: Paul Y. C. Ng.

Isolapotamon seowi new species

Diagnosis.— Carapace wider than long, relatively high, dorsal surface of carapace gently convex; epigastric cristae rugose, not sharp, anterior of postorbital cristae; postorbital cristae sharp, subparallel with frontal margin; median lobe of posterior margin of epistome acutely triangular (fig. 2A-C). P5 merus relatively short, dactylus long (fig. 2A, F). Press button of male pleonal locking mechanism on submedian part of sternite 5 (fig. 3A). Male pleon distinctly triangular, lateral margins of somites 3-6 gently concave (fig. 2H). G1 gently sinuous; subterminal segment relatively long, with basal part broad, forming subrectangular structure, tapering suddenly distally, forming shelf-like feature on outer margin; terminal segment gently sinuous, curved, distal part bifurcated, subdistal projection positioned at 90° to longitudinal axis of terminal segment, slightly longer than distal projection, lobiform with rounded tip, lateral margins subparallel, laterally flattened (figs. 3B-E, 4A-F).

Etymology.— The species is named after an old friend, Francis Seow-Choen, noted gastroenterologist and an expert on phasmid insects who has published several important books on the stick insect fauna of Malaysia. An intrepid explorer, he came across the present specimens during one of his many surveys and kindly arranged for them to be passed to the author for this study.

 Peter K. L. Ng. 2023. A New Species of potamid Crab, Isolapotamon seowi (Brachyura, Potamoidea) from Kokol Hill in western Sabah, Malaysia, Borneo. Crustaceana. 96(2); 131–138. DOI:  10.1163/15685403-bja10264