Monday, July 8, 2019

[Crustacea • 2019] Geosesarma mirum • A New Species of Semi-terrestrial Sesarmid Crab (Decapoda, Brachyura) from central Taiwan

Geosesarma mirum
Shy & Ng, 2019

A new species of semi-terrestrial sesarmid crab of the genus Geosesarma De Man, 1892, is described from central Taiwan. Geosesarma mirum sp. nov. is distinct in possessing a strong transverse crest on the inner surface of the male chela and a diagnostic male first gonopod which is relatively long and stout, with the distal chitinous part broad and spatuliform. Like most Geosesarma species, G. mirum sp. nov. has large eggs and direct development, contrasting with the only other species known from Taiwan, G. hednon Ng, Liu and Schubart, 2004, which has small eggs and planktotrophic larvae.

Keywords: Direct development, East Asia, freshwater, large eggs, Sesarmidae, taxonomy

Figure 1. Geosesarma mirum sp. nov. A, B male (9.9 × 8.6 mm) (NTOU F10396) C ovigerous female with eggs (specimen not collected) D brooding female with young crabs (specimen not collected).
 Photographs A–C J-Y Shy, D Peter Wang.

Figure 2. Geosesarma mirum sp. nov. holotype male (11.9 × 10.8 mm) (NTOU F10395), Taiwan
A overall dorsal view B dorsal view of carapace C frontal view of cephalothorax D anterior thoracic sternum and pleon.

Figure 6. Habitat of Geosesarma mirum sp. nov.
A Chilan River, Jhongpu, Chiayi County, Taiwan, B Lunziding Canal, Jhongpu, Chiayi County, Taiwan.

Family Sesarmidae Dana, 1851
Genus Geosesarma De Man, 1892

Type species: Sesarma (Geosesarma) nodulifera De Man, 1892, 
subsequent designation by Serène and Soh (1970).

Geosesarma mirum sp. nov.

Diagnosis: Carapace quadrate, slightly wider than long or subequal, adult width to length ratio 1.10–1.21, lateral margins gently sinuous, gently diverging posteriorly (Fig. 2A, B); dorsal surface with regions visible, anterior regions with small rounded granules on gastric regions, branchial regions with numerous striae (Fig. 2A, B); front distinctly deflexed, frontal lobes broad, with subtruncate margins in dorsal view; postfrontal, postorbital cristae prominent, rugose (Fig. 2A–C); external orbital tooth triangular to subtruncate, directed obliquely laterally, outer margin convex, shorter than inner margin, tip reaching lateral margin; second lateral tooth low, rounded, separated from external orbital tooth by deep notch (Fig. 2A, B). Merus of third maxilliped subovate, subequal to ischium; exopod slender, reaching to just before edge of merus, with long flagellum (Fig. 4A). Merus of cheliped with low ventral lobe with serrated margin, upper lobe relatively lower. Outer surface of palm of adult male covered with small rounded granules and striae; inner surface granulated, with distinct, high transverse granulated ridge; dorsal margin of dactylus with 11 or 12 low, non-chitinous tubercles on proximal two-thirds (Fig. 3). Ambulatory legs with relatively stout, short merus, with sharp subdistal spine on dorsal margin, surfaces rugose (Fig. 1A). Part of male thoracic sternite 8 exposed when pleon closed. Male pleonal locking mechanism formed by expanded posterior edge of thoracic sternite 4. Male pleon triangular; somite 6 wide, with convex lateral margins; telson triangular, not recessed into distal margin of somite 6, margins convex (Figs 2D, 4B). G1 relatively long, stout, gently curved outwards; outer margin of subdistal part of subterminal segment with subangular shelf-like structure (Fig. 4C, D), distal chitinous part broad, tip spatuliform, margin uneven (Fig. 4C–K). G2 short, ca. a third length of G1 (Fig. 4L).

Etymology: The name is derived from the Latin for “surprise”, alluding to the unexpected discovery of this species in Taiwan.

 Jhy-Yun Shy and Peter K.L. Ng. 2019. Geosesarma mirum, A New Species of Semi-terrestrial Sesarmid Crab (Crustacea, Decapoda, Brachyura) from central Taiwan.  ZooKeys. 858: 1-10. DOI: 10.3897/zookeys.858.35198

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