Andamanium thelxinoe ( De Man, 1908 ) Naruse & Ng, 2020

Naruse, Tohru & Ng, Peter K. L., 2020, Revision of the sesarmid crab genera Labuanium Serène and Soh, 1970, Scandarma Schubart, Liu and Cuesta, 2003 and Namlacium Serène and Soh, 1970 (Crustacea: Decapoda: Brachyura: Sesarmidae), with descriptions of four new genera and two new species, Journal of Natural History (J. Nat. Hist.) 54 (7 - 8), pp. 445-532 : 504-508

Andamanium thelxinoe ( De Man, 1908) , comb. nov.

( Figures 29 , 30 )

Sesarma thelxinoe De Man, 1908, p. 181 , pl. 11 [type locality: Mt. Harriet , South Andaman Islands]; Annandale 1911, p. 1.

Sesarma (Sesarma) thelxinoe: Tesch 1917, p. 207 .

Sesarma (Geosesarma) thelxinoe: Serène 1968b, p. 106 .

Geosesarma thelxinoe: Serène and Soh 1970, p. 403 ; Ng et al. 2008, p. 221.

Labuanium finni: Pretzmann 1984, p. 142 , pl. 2, figs 6–8. [not Labuanium finni ( Alcock, 1900 )] Material examined

ZRC 2018.0057 , 8 males, 10.7 × 12.0–16.9 × 19.7 mm, 2 females, 11.5 × 13.2, 12.9 × 15.3 mm, Hill streams from Mt. Harriet, South Andaman Islands , ca. 300–400 m above sea level (asl), evergreen and semi-evergreen forest, coll. I. Das , 20–30 August 1997; RUMF-ZC-3920, 1 male, 14.7 × 16.8 mm, same data as ZRC 2018.0057 .

Redescription

Carapace ( Figure 29 (a)) squarish, 1.01–1.19 times (mean = 1.14, n = 12) as wide as long; dorsal surface weakly convex longitudinal and transversely, regions slightly defined, H-shaped gastric groove present. Front deflexed at anterior margin of postfrontal lobes, distally recurved, directed anteroventrally; frontal margin straight in anterior view, overhanging onto antennular septum and fossae. Two pairs of postfrontal lobes present, mesial lobes slightly inclined posteriorly towards midline of carapace, lateral lobes anteriorly aligned with lateral part of mesial lobes; mesial lobes about 2 times wider than lateral lobes; anterior edge of all lobes relatively close to, but never reaching, frontal margin in dorsal view. Lateral margins of carapace almost parallel, external orbital angle pointed, directed anteriorly; first epibranchial tooth marked by V-shaped notch, second tooth feeble, only demarcated by small depression; row of setae present just below lateral margin, extending onto orbit anteriorly. Antennular septum wide, short. Orbit, in dorsal view, tilted J-shaped, median part of supraorbital margins oblique; inner orbital tooth triangular, short, directed dorsoanteriorly. Suborbital crista almost straight, lateral end recurved into orbit below external orbital angle. Suborbital, pterygostomial, subhepatic regions with reticulate mat of setae. No longitudinal ridge on ventral surface of external orbital angle.

Epistome long, posterior margin with 3 triangular lobes, lateral lobes distally sharp, directed anteroventrally, median lobe directed ventrally.

Eye ( Figure 29 (a)) with cornea as wide as base of peduncle; anterior surface of peduncle with oblique ridge. Antennule with wide, laterally long basal article. Antenna with wide, ellipsoidal basal article, distolateral lobe expanded laterally; antenna entering orbit through wide gap between inner orbital tooth and front.

Mxp3 ( Figure 29 (b)) with mesial margins of ischia-merus leaving wide rhomboidal gape; ischium subtriangular, lateral half depressed; merus ovoid, with oblique row of setae. Exopod reaching distal third of merus, with long flagellum.

Chelipeds symmetrical in both male and female ( Figure 29 (a)), markedly larger in male than in female. Male cheliped merus trigonal in cross section; upper, lower-inner and lowerouter margins randomly granulated, upper margin with small, subdistal tooth; outer surface covered with short, transverse rows of granules; mesial surface with longitudinal rows of long setae near lower margin, upper half of mesial surface scattered with short setae. Carpus rhomboidal, upper surface granulated, mesial margin only slightly produced, not pointed. Male palm ( Figure 29 (c,d)) large, as long as high, thick; outer surface ( Figure 29 (c)) smooth; upper surface ( Figure 29 (d)) with low ridge on proximal half extending from hinge with carpus; inner surface weakly granulated medially and subinferiorly; thick rim extending along occlusal margin of immovable finger to dactylar articulation on both outer and inner surfaces not interrupted near dactylar articulation, thick rims become indistinct in large male with swollen palm. Male fingers ( Figure 29 (c)) long, leaving ovoid gap when closed; occlusal margins of both fingers with largest teeth subdistally, distally recurved and forming V-shaped corneous ends; movable finger as long as palm, continuously curved downwards, no ornamentations on upper surface of movable finger, even in small males.

Ambulatory legs (P2–5) ( Figure 29 (a)) long, slender, P4 longest. Meri with anterior and posterior margins only slightly convex, distal anterior corner angled, followed proximally by low convexity. Setae on propodi and dactyli longer than those on meri and carpi; distal half of propodi and entire dactyli with mat of dense setae on inner margins in P2–4, P5 with mat of setae only on inner margin of dactylus.

Male thoracic sternum ( Figure 29 (b)) transversely wide, sternites 1–4 fused, but sternites 3/4 demarcated superficially by posteriorly convex shallow groove and row of setae ( Figure 29 (b)). Male sternopleonal cavity reaching distal two-fifths of bases of cheliped coxae; margin of sternopleonal cavity on sternite 4 rimmed except for posterior end, posterior end of this rim thick; lateral slope of sternopleonal cavity posterior to nonrimmed part of sternite 4 depressed, accommodating distal end of G 1 in situ. No sternal button for locking mechanism on sternite 5. Penis sternal.

Male pleonal somites 1 and 2 short, somite 3 widest, somite 4 to proximal two-thirds of somite 6 continuously narrowed; telson longer than wide, proximal margin slightly sunken in distal margin of somite 6 ( Figure 30 (a)).

G1 ( Figure 30 (b–d)) slender, almost straight proximally, distally bent ca. 45°, distal end sharply pointed in ventral view, bilobate in mesiodorsal view, subterminal opening on dorsal side. G2 ( Figure 30 (e)) short.

Vulvae ( Figure 30 (f)) rounded, located on distal two-fifths of sternite 6, anterior margin adjacent to thoracic sternal suture 5/6, with rounded sternal vulval cover developed from lateral margin.

Variation

Gap between fingers is not developed in smaller males (<13.5 × 15.4 mm) or in females.

Colouration

Live colouration not known.

Distribution

Known only from the type locality, Mt. Harriet, South Andaman Islands .

Remarks

De Man (1908) described Sesarma thelxinoe from one male (12.8 × 14.4 mm) and two females (11.5 × 13.5, 11.6 × 13. 8 mm). From what we have ascertained, the types are not in Europe, and are probably still in the Indian Museum (present Zoological Survey of India in Kolkata). These specimens were brought by Nelson Annandale, who obtained them from Mt. Harriet, near Port Blair, Andamans, 700 ft. asl. Annandale (1911) later described a rhizocephalan Sesarmaxenos monticola Annandale, 1911 , that was parasitising one of the female syntypes. Serène and Soh (1970) later transferred Se. thelxinoe to Geosesarma De Man, 1892 . The species is here referred to its own genus, Andamanium gen. nov.

Our material collected from the type locality of An. thelxinoe exhibits a remarkable change in the development of a gape between chelal fingers in males, which could be attributed to ontogenetic variation; smaller males (e.g. <13.5 × 15.4 mm) and females lack such a prominent gape between the fingers, but larger males (e.g.> CW 15.5 mm) have a prominent gape between them. In the male syntype (12.8 × 14.4 mm), the gape between the fingers is not yet developed ( De Man 1908, pl. 11, fig. 6), which is considered to represent the immature condition.

Pretzmann (1984, 142, pl. 2, figs 6–8) recorded ‘ Labuanium finni ’ from Bambooflat, South Andaman Islands, but his specimen shows symmetrical chelipeds with a large gape between the fingers ( Pretzmann 1984, p. pl. 2, fig. 8); it is clearly conspecific with An. thelxinoe . When the figure of the holotype of Sc. finni ( Alcock and McArdle 1903, pl. 66, fig. 1) and our female spcimens of An. thelxinoe , being similar in size to the holotype of Sc. finni , are compared, they clearly differ in the shape of the frontal margin (medially concave vs straight), size of chela (much larger in An. thelxinoe ), shape of the carapace (subtrapezoidal with lateral margins divergent posteriorly vs squarish) and subdistal low convexity of the anterior margin of ambulatory meri (not pointed vs pointed). It is clear that Pretzmann’s (1984) specimen is An. thelxinoe , not Labuanium finni .

In addition to An. thelxinoe , there is a second species of freshwater sesarmid on the South Andaman Islands, Geosesarma starmuehlneri Pretzmann, 1984 . This taxon seems to be a valid species of Geosesarma judging from the description and figures by Pretzmann (1984), and the authors have examined photographs of the holotype male in the Natural History Museum, Vienna, courtesy of the curator, Peter Dworschak.

Ecological note

The type specimens of Andamanium thelxinoe were collected from ‘about 700 feet above sea level in a stream running through thick jungle on Mt. Harriet, near Port Blair’ ( Annandale 1911, p. 4). One of the two female syntypes was parasitised by rhizocephalan Sesarmaxenos monticola Annandale, 1911 . Rhizocephalans are known to parasitise marine and a few freshwater crustaceans ( HØeg and Lützen 1995), which suggests a strong association of the host with aquatic habitats. Pretzmann (1984, p. 142) also noted that the species (as Labuanium finni ) was collected from primaeval forest brook running into Dhinkari barrage-lake.

Figures

Figure 29. Andamanium thelxinoe (De Man, 1908), comb. nov. ZRC 2018.0057, male, 16.9 × 19.7 mm (South Andaman Islands). a, habitus, dorsal view; b, cephalothorax, ventral view; c, right chela, outer view; d, right chela, upper view.

Figure 30. Andamanium thelxinoe (De Man, 1908), comb. nov. a–e, ZRC 2018.0057, male, 16.9 × 19.7 mm (South Andaman Islands); f, ZRC 2018.0057, female, 12.9 × 15.3 mm (South Andaman Islands). a, pleon and telson; b, left G1, ventral view; c, left G1, distal part, lateroventral view; d, left G1, distal part, mesiodorsal view; e, G2, ventral view; f, right vulva, mesial part of sternites 6 and 7 and median suture. Scale bars: a= 3 mm; b–f = 1 mm.