Clistocoeloma lanatum (Alcock, 1900)

(Figs. 1–5)

Sesarma lanatum Alcock, 1900: 418, 419.—Alcock 1903: pl. 65 fig. 4, 4a.

Sesarma (Sesarma s.s.) lanata. — Tesch 1917: 168, 209, 230 (list), 239 (key), 241.

Sesarma (Sesarma) lanata. —Tweedie 1936: 68 (footnote).

Sesarma (Sesarma) lanatum. — Rahayu & Takeda 2000: 38, 40.

? Sesarma (Sesarma) lanatum. —Tirmizi & Ghani 1996: 156–158, fig. 60.

Clistocoeloma lanatum.— Serène 1968 (in list).— Komai et al. 2004 (list).— Ng et al. 2008: 220 (list).— Trivedi et al. 2018: 71 (list).— Abraham & Prakasan 2020: 17156 (list).

Cleistocoeloma lanatum. —Tan & Ng 1994: 82 (list).

Type material examined. Lectotype (herein designated): female (CW 10.1 mm, CL 8.5 mm) (ZSI 3409-11/7), Karachi, Pakistan. Other material examined. 2 males (CW 5.8 mm, CL 4.9 mm; CW 11.6 mm, CL 10.0 mm), 4 females (CW 6.4 mm, CL 5.3 mm to CW 10.6 mm, CL 8.7 mm) (LFSC.ZRC-217), Jakhau, 23°13’58”N 68°36’42”E, Gulf of Kachchh, Gujarat, India, mangrove muddy shore, coll. Maheta N. P., 9 January, 2020; 1 male (CW 9.5 mm, CL 8.1 mm) (USNM205921), Pakistan, coll. anonymous, November 1985; 1 juvenile (CW 4.5 mm, CL 3.6 mm) (ZRC 1973.11.2.495), Seletar River, Singapore, coll. anonymous, 17 September, 1968.

Diagnosis. Carapace subquadrate, nearly as wide as long, almost square, with covering of short setae interspersed with small tufts of setae resembling tubercles; anterolateral margin with 3 triangular teeth, inclusive of profound external orbital angle; posterolateral margin slightly convex (Figs. 1A, 2A, D, 3A). Cheliped manus dorsal surface with single longitudinal pectinated ridge with 48–51 chitinous comb-like tubercles, row of 18–20 granules behind pectinated ridge on ventral side of palm (Fig. 4C); dactylus dorsal surface with 5 or 6 evenly spaced tubercles with proximal and distal most tubercles smaller in size, medial tubercles comparatively larger (Fig. 4C), each tubercle on dorsal surface with small median chitinous granules. P2–P5 covered with dense coat of setae, small tufts of setae resembling tubercles, smooth when denuded (Fig. 4D–G). G1 slightly curved, distal region slightly wide; chitinous tip narrow; tip surrounded with dense tufts of setae, tip slightly longer than surrounding setae (Fig. 5B, C).

Redescription. Carapace subquadrate, nearly as wide as long; covered with short setae, interspersed with small tufts of setae resembling tubercles, surface smooth when denuded. Postfrontal lobes distinct, divided by longitudinal grooves, median pair broad, prominent, distal pair less distinct, not divided (Figs. 1A, 2A, D); regions relatively apparent. Front obliquely deflexed. Inner orbital angle distinct, rounded, with convex outer margin, no gap present between tip of inner orbital angle and frontal margin (Figs. 1C, 2C). Lateral carapace margins almost parallel; anterolateral margin with 3 triangular teeth, inclusive of external orbital angle; external orbital angle pronounced, second and third teeth less distinct; posterolateral margin slightly convex (Figs. 1A, 2A, D, 3A). Entire margin of carapace fringed with dense, short setae.

Chelipeds equal, large, robust in males (Figs. 1A, 3A, B, 4A, B); small, slender in females (Fig. 2A, D). Merus mesial surface flat with fine setae covering small tubercles; 2 longitudinal rows of long setae on mesial surface; margins lined with fine tubercles. Carpus dorsal surface covered with tufts of small setae resembling small tubercles, revealing tubercles when denuded; dorsal margin with tuft of setae proximally (Fig. 1A). Manus outer surface smooth (Fig. 4A); inner surface of manus with single median vertical row of 7 or 8 large granules in adult males (Fig. 4B), absent in females and juveniles; dorsal surface with single longitudinal pectinated ridge with 48–51 chitinous comb-like tubercles (Fig. 4C), row of 18–20 granules behind pectinated ridge on ventral side of palm (Fig. 4C). Dactylus dorsal surface with 5 or 6 evenly spaced tubercles with proximal and distal most tubercles smaller in size; medial tubercles comparatively larger (Fig. 4C), each tubercle on dorsal surface with small median chitinous granules; absent or weak tubercles in female and juveniles.

P2–P5 covered with dense coat of short setae and small tufts of setae resembling tubercles, smooth when denuded; longer setae on dorsal and ventral margins of entire leg extending to sub-distal part of dactylus; dactylus short, with sharp tip; merus with deep groove subdistally. P2 shortest, P4 longest. P4 merus approximately twice longer than wide, P5 merus relatively short, 2.1 times longer than broad (0.7 times length of P4 merus) (Fig. 4D–G).

Thoracic sternum covered with short setae, smooth when denuded (Fig. 1B). Male pleon narrow, margin fringed with setae; somite 3 widest; somite 6 with curved edge; telson as long as wide, linguiform; male pleonal locking mechanism absent, without tubercle on sternite 5 (Figs. 1B, 5A).

G1 slightly curved, distal region slightly wide; chitinous tip narrow, long, surrounded by dense tufts of setae, tip slightly longer than the surrounding setae, single row of setae on exterior lateral margin (Fig. 5B, C). G2 shorter than G1, slightly curved, tip rounded.

Female pleon rounded, relatively wider than long; telson wider than long, semicircular, embedded in sixth somite (Fig. 2B, E). Vulvae with central operculum crescent shaped, slightly protruded; sternal vulvar cover slightly raised above operculum (Fig. 2F, G).

Remarks. The specimens examined in the present study agree with the original description given by Alcock (1900). Clistocoeloma lanatum was first described as Sesarma lanatum Alcock, 1900, on the basis of four syntypes collected from the Mumbai coast (Maharashtra, India) and Karachi (Pakistan). Only one female syntype (CW 10.1 mm, CL 8.5 mm) could be found in the ZSI; the whereabouts of the other specimens is not known. The measurements of the remaining female syntype match those of the largest specimen mentioned by Alcock (1900). However, despite the mention of a male specimen in Alcock’s (1900) descriptions, no male specimen could be located. In his synopsis of sesarmid crabs, Tesch (1917) commented that S. lanatum shows affinities with different species of Clistocoeloma in having following characters: carapace and the walking legs being clothed by a dense fur of short hairs, among which isolated tufts of somewhat longer hairs are scattered, the front is not vertically but only obliquely deflexed, the free margin being nearly straight, and the postfrontal lobes more developed. However, Tesch (1917) did not formally transfer S. lanatum to Clistocoeloma. Later, Serène (1968) placed S. lanatum under Clistocoeloma in his checklist of Asian crabs without discussion, and this has been followed by all subsequent workers, including Komai et al. (2004), Ng et al. (2008) and Trivedi et al. (2018).

The species of Clistocoeloma can be broadly divided into two groups: group “a” with anterolateral teeth and group “b” without anterolateral teeth. Based on this character, Clistocoeloma lanatum falls into group “a” along with C. balansae A. Milne-Edwards, 1873, C. merguiense De Man, 1888, C. sinense Shen, 1933, C. amamaparense Rahayu & Takeda, 2000, C. melanesicum Lee, Ng & Ng, 2013, and C. nobile Lee, Ng & Ng, 2023. However, C. lanatum can be distinguished from these congeners on the basis of the various morphological characters (see Table 1).

Colouration. Carapace muddy brown with chelipeds pale to dark purple (Fig. 3A, B). In life, entire crab usually covered in a thin layer of mud.

Ecological notes. This species is found in the muddy or mangrove habitats of the intertidal zone.

Distribution. Western Indian Ocean from Maharashtra (Alcock 1900), Karnataka (Dineshbabu et al. 2011), Kerala (Shet et al. 2016) and Gujarat (present study), India (Alcock 1900); and Pakistan (Alcock 1900).