Indawmon thanayei Ng & Mar & Shih 2020, new species
Creators: name: Ng, Peter K. L.
name: Mar, Win
name: Shih, Hsi-Te
Publisher: name: Zenodo
Year: 2020
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Potamidae
Indawmon
Indawmon thanayei

description: Indawmon thanayei, new species
(Figs. 1–5)
Material examined. Holotype male (27.8 × 22.9 mm) (ZRC 2019.1032), Nant Tae and Wai Lone streams, flowing into southern entrance of Indawgyi Lake, 24°56′52.8″N 96°20′09.6″E, Mohnyin Township, Kachin State, Myanmar, coll. Than Aye, 2018. Paratypes: 4 males (27.7 × 22.3 mm, 27.6 × 21.9 mm, 23.0 × 18.3 mm, 19.7 × 16.5 mm), 4 females (32.7 × 25.4 mm, 32.2 × 24.6 mm, 25.3 × 20.3 mm, 23.2 × 19.2 mm) (ZRC 2019.1033), same data as holotype.
Diagnosis. As for genus.
Description of male holotype. Carapace transversely ovate in adults, distinctly wider than long (width-to-length ratio 1.21); dorsal surface gently convex from frontal view, glabrous, regions not prominently inflated, appearing almost flat in frontal view, appears glabrous (Figs. 2A, 3A, 4A). Frontal regions with flattened granules, postorbital regions smoother; lateral parts of anterolateral and branchial regions covered with numerous prominent, flattened to uneven granules; mesogastric, urogastric, cardiac and intestinal regions weakly rugose but not granular; suborbital, subhepatic and sub-branchial regions almost smooth; pterygostomial region with low, flattened granules (Fig. 4A). Epigastric cristae prominent, strongly rugose, not distinctly cristate, separated by narrow, median Y-shaped furrow; epigastric cristae anterior to postorbital cristae, separated from it by short furrow; postorbital cristae distinct, margin uneven, prominently raised, lined with striae but not distinctly cristate, subparallel to frontal margin, outer edge low, with granules less prominent (Fig. 3A). Cervical grooves distinct, not deep, relatively narrow, not reaching lateral margins, connected to H-shaped median gastric groove (Fig. 3A). Frontal margin almost straight, appearing gently sinuous in frontal and dorsal views (Figs. 3A, 4A). External orbital tooth distinct, triangular, outer margin almost twice length of inner margin, demarcated from rest of anterolateral margin by small but distinct V-shaped cleft; epibranchial tooth small, sharp (Figs. 2A, 3A). Anterolateral margins convex, cristate, lined with low but distinct granules, appearing gently serrated (Fig. 3A). Posterolateral margin gently sinuous, converging towards gently convex posterior carapace margin (Fig. 3A). Orbits subovate; eye filling orbital space; eye peduncle relatively short, stout; cornea large, round, pigmented (Figs. 3A, 4A). Supraorbital margin gently sinuous, entire (Fig. 3A). Suborbital margin concave, complete, lined with low granules; inner angle with low tooth (Fig. 4A). Antennae short, stretching across base of eyes; antennules short, folding transversely in rectangular fossa (Fig. 4A). Posterior margin of epistome with distinct median triangle, lateral margin sinuous (Fig. 4A).
Third maxillipeds covering most of buccal cavity when closed; ischium subrectangular, with distinct oblique median groove, surface mostly smooth with scattered pits and short setae; merus subquadrate, slightly wider than long, surface rugose, margins cristate, anteroexternal angle rounded; exopod slender, reaching to about one-third length of merus, with elongate flagellum that reaches across width of merus (Fig. 4B).
Chelipeds asymmetrical, right larger (Fig. 2A). Anterior margin of basis-ischium lined with several small low granules; ventral margins of merus lined with prominent low sharp granules, with tubercle on subdistal surface, dorsal margin uneven, appearing weakly serrated (Fig. 3E). Outer surface of carpus rugose, inner distal angle with large sharp tooth and small sub-basal tubercle (Fig. 2A). Outer surfaces of chelae rugose, dorsal surface appears squamose with larger, strongly flattened granules (Fig. 4C). Fingers of major chela short, stout, gently curved, subequal to length of palm, outer surface of both fingers lined with two rows of pits; dorsal margin of dactylus with 5–7 low tubercles, proximal one largest, sharp; cutting edges of both fingers with variously sized sharp teeth and denticles; dorsal margin of dactylus with low tubercles and granules (Fig. 4C). Fingers of minor chela similar to major chela in form (Fig. 4C).
Ambulatory legs short, articles relatively stout; second pair longest, last pair shortest (Fig. 2A). Merus short, stout, outer surface gently rugose, dorsal margin uneven, subcristate, without subdistal spine or tooth; carpus rugose, dorsal margin cristate, outer surface with low submedian crista on first to third legs, that on fourth leg smooth; dorsal margin of propodus with crista, outer surface with low, submedian crista; dactylus relatively short, gently curved, quadrate in cross section, margins with short, sharp pectinate spines with dense, very short setae that do not obscure margins (Fig. 2A).
Thoracic sternum, notably sternites 3 and 4, as long as broad, surface almost smooth (Fig. 3E). Sternites 1, 2 completely fused to form broadly triangular plate; separated from sternite 3 by distinct, gently concave suture (towards buccal cavity); sternites 3, 4 completely fused without discernible groove or suture (Fig. 3E). Sutures 4/5, 5/6, 6/7, and 7/8 medially interrupted; median longitudinal groove on sternites 7 and 8 (Fig. 5A). Penis coxal, on condyle of coxa of fourth ambulatory leg (Fig. 5B). Sternopleonal cavity deep, reaching imaginary line connecting posterior edges of cheliped coxae (Fig. 3E). Sternite 8 not visible when pleon closed. Male press-button pleonal locking mechanism composed of distinct laterally flattened tubercle with distal part distinctly chitinised, positioned on posterior third of sternite 5 (Fig. 5A).
Pleon broadly triangular, all somites and telson free; telson triangular, lateral margins gently concave; somite 6 transversely trapezoidal, much wider than long, lateral margin gently concave; somites 3–5 trapezoidal, gradually decreasing in width, increasing in length; somite 3 widest; somites 1 and 2 subrectangular, very wide, reaching to bases of coxae of fourth ambulatory legs (Figs. 3E, 4D).
G1 relatively slender; subterminal segment widest basally, gradually tapering towards terminal segment, groove for G2 ventral, with prominent subdistal angle on gently concave outer margin, just before terminal segment; subterminal segment not distinctly separated from terminal segment by groove or other structure in ventral view, in dorsal view, poorly defined area of weaker chitinisation marks demarcation between segments; terminal segment long, subcylindrical, no dorsal flap, gently tapering, tip sharp, bent at angle of about about 45° from the longitudinal axis (Fig. 6A–C). G2 elongate, longer than G1; basal segment longer than distal segment (Fig. 6D).
Females. Ambulatory meri of female specimens (Fig. 2D) proportionately shorter than those of males. Pleon longitudinally ovate in adults, covering most of thoracic sternum with all somites and telson free (Fig. 5C). Vulva large, positioned on anterior margin of sternite 6, directed obliquely and anteriorly; posterior part with weakly calcified rim (Fig. 5D).
Variation. The carapace form varies slightly in this species. Adult specimens have a transversely ovate carapace with the anterolateral surfaces prominently granulated, the posterolateral surfaces with distinct striae, and the epigastric and postorbital cristae lined with strong striae and granules. The degree of this granulation, however, varies slightly (Fig. 3A, B). Smaller specimens have a slightly more quadrate carapace with the granules and striae lower and less prominent (Fig. 3C). The granules lining the anterolateral margins are also relatively smaller in smaller specimens (Fig. 3C). In the large specimens (e.g., the females exceeding 30 mm in carapace width), the carapace is proportionately wider, with the granules and striae also relatively weaker (Fig. 3D).
Most of the adult G1 terminal segments, including that of the holotype, are bent at an angle of about 45° from the longitudinal axis (Fig. 6A–C). One adult paratype male (27.6 × 21.9 mm, ZRC 2019.1033), however, is unusual in that the G1 terminal segment (both left and right sides) is bent at an angle of almost 90° (Fig. 6E–G). This male specimen is also unusual in that the male pleon (especially somites 3 and 5) is also slightly wider proportionately (Fig. 4E). These differences must be considered as variation as it agrees with the other paratypes in almost all other aspects, and genetically, they all share the same haplotype (Fig. 7). Subadult males (e.g., smallest paratype male 19.7 × 16.5 mm, ZRC 2019.1033) have proportionately narrower pleons (Fig. 4F) and the G1 is only weakly chitinised, not strongly setose, and its terminal segment is less bent, being at an angle of only 30° (Fig. 6H).
Colour. In life, the carapace is yellowish-brown overall, with the ventral surfaces pale yellow (Fig. 1).
Etymology. The species is named after the collector, Mr. Than Aye, an Assistant Lecturer in the Department of Zoology, Yadanabon University, Myanmar.
Remarks. The variation in the G1 structure observed in one of the larger males (27.6 × 21.9 mm, ZRC 2019.1033) is noteworthy as it is significant, and if locality data were absent and genetic data not available, one would easily have treated it as a separate species. Not only were both left and right G1s of the same form, the pleon was also slightly wider. There is no doubt, however, that they are all conspecific, with all other morphological characters agreeing and the genetic data confirms this (Fig. 7). It is becoming increasingly clear that there is more variation in the G1 morphology of many Asian potamids than previously recorded, and the use of this character to differentiate species must be more carefully scrutinised, especially if specimens are from the same drainage and/or are of different sizes (e.g., see Indochinamon khinpyae; cf. Ng & Win Mar, 2018).
Habitat. The specimens were collected from two highland streams, about 16 km upstream from their entrance into the southern portion of Indawgyi Lake. The substrate of the streams is rocky, with the water shallow but fast-flowing (Than Aye, pers. comm.).
Conservation. A large number of freshwater crab species in the Indo-Burma area are still listed in the IUCN Red List as “data deficient” (Cumberlidge et al., 2009, 2012), and that of Indawmon thanayei, new species, is not very different. As far as recent collections are concerned and from the observations of the collector, the crab is not rare in Indawgyi, and as such we suggest the category of Least Concern for the time being.
Distribution. Known only from Indawgyi Lake, Kachin State, Myanmar.
Molecular analyses. In the phylogenetic tree (Fig. 7), Indawmon, new genus, comes out in a highly supported clade together with the genera from Myanmar, Thailand, Laos, and Yunnan in southwestern China, which belong to the “Indochina–SW China ” group (Shih et al., 2009). Those Myanmar genera include Indawmon, Inlethelphusa, Shanphusa, and Doimon. Noteworthy in this genetic tree is that Shanphusa ywarngan Ng & Whitten, 2017, from Myanmar comes out in a separate clade with Doimon maehongsonense (Naiyanetr, 1992) from Thailand with high support.
description: Published as part of Ng, Peter K. L., Mar, Win & Shih, Hsi-Te, 2020, Indawmon thanayei, a new genus and new species of freshwater crab (Crustacea: Brachyura: Potamidae) from Indawgyi Lake, Kachin State, Myanmar, pp. 529-538 in Raffles Bulletin of Zoology 68 on pages 531-537, DOI: 10.26107/RBZ-2020-0069, http://zenodo.org/record/5344754