Arcithelphusa Pati & Sudha Devi 2015
Creators: name: Pati, S. K.
name: Sujila, P. S.
name: Devi, A. R. Sudha
Publisher: name: Zenodo
Year: 2019
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Gecarcinucidae
Arcithelphusa

description: Arcithelphusa Pati & Sudha Devi, 2015
(Figs. 2 A–F, 3B, C, D, E–K, N–P, 4A–F)
Arcithelphusa Pati & Sudha Devi 2015a: 35; Pati et al. 2017: 1297 (list); Rajesh et al. 2017: 134 (list); Pati & Thackeray 2018: 10 (list).
Type species. Arcithelphusa cochleariformis Pati & Sudha Devi, 2015, by original designation; gender feminine.
Diagnosis [emended from Pati & Sudha Devi (2015a)]. Carapace ovate, broader than long (cw/cl = 1.2–1.4), relatively deep (ch/cw = 0.6–0.7); dorsal surface arched, almost smooth; anterolateral margin lacking distinct serrations; front strongly deflexed, with narrow frontal margin (fw/cw = 0.25); epigastric cristae only visible as 2 low, broad, oval protuberances; postorbital cristae indistinct; external orbital angle indistinct; epibranchial tooth indistinct; branchial regions highly inflated; cervical grooves relatively shallow, not reaching beyond level of postorbital cristae; frontal median triangle incomplete, lateral margins indiscernible; epistome posterior margin with triangular median lobe and sinuous lateral lobes (Figs. 2A, B, D, E, 4A, B, D, E). Third maxilliped exopod longer than ischium, lacking flagellum or with relatively short flagellum, ca. 0.5× width of merus (Fig. 3B, N). Chelipeds relatively smooth (Figs. 2 A–F, 4A–F). Ambulatory legs slender, long (Figs. 2 A–F, 4A–F). Suture between male thoracic sternites s2/s3 shallow or deep, broad, not reaching edge of sternum; suture between male thoracic sternites s3/s4 deep, broad, reaching edge of sternum (Figs. 2C, F, 3C, 4C). Pleonal locking mechanism with low, anteriorly directed tubercles on each antero-submedial part of sternite 5 (Fig. 3I). Male sternopleonal cavity long, reaching imaginary line joining medial part of cheliped coxae (Figs. 2C, F, 4C). Male pleon narrowly triangular, with concave lateral margins; pleonal somite 6 trapezoidal, broader than long, with gently convex lateral margins (Figs. 2C, F, 3D, 4C). Male telson narrow, with strongly concave lateral margins; apex rounded (Figs. 2C, F, 3D, 4C). G1 relatively stout, with large flexible zone, tip not reaching pleonal locking structure; terminal segment relatively short, ca. 0.3–0.4× length of subterminal segment, medial portion distinctly curved outwards, dorsal flap absent or low, broad; subterminal segment relatively stout, with strongly convex outer margin at basal half (Fig. 3 E–G, I, J, O). G2 short, ca. 0.6–0.7× length of G1; distal segment very short, ca. 0.2× length of basal segment (Fig. 3H, K, P).
Remarks. Among the Indian genera of gecarcinucid crabs, Arcithelphusa is most close to Cylindrotelphusa in carapace morphology and gonopod structures (see Pati & Sudha Devi 2015a; Pati et al. 2017). Both the genera can be distinguished from other Indian gecarcinucid genera mainly by the presence of a relatively deep carapace (ch/cw = 0.5–0.7), a narrow frontal margin (ca. 0.25 times the carapace width), an incomplete frontal median triangle, and a short G2 distal segment (ca. 0.2–0.3 times the length of the basal segment) (Figs. 2B, E, 3H, K, P, 4B, E; see Bahir & Yeo 2007: figs. 4D, 5B; Pati et al. 2017: figs. 11b, 12d, 13b, 14d). Other major characters common to both are their strongly convex dorsal surface, the highly inflated branchial regions, the prominent and triangular median lobe on the epistome posterior margin, a narrowly triangular male pleon, and a short G2 (Figs. 2 A–F, 3D, E, H, J, K, O, P, 4A–E; see Bahir & Yeo 2007: figs. 4A, D, 5A–C; Pati et al. 2017: figs. 11a–c, h, 12a, d, 13a–c, h, 14a, d). While they share many features of the carapace and gonopods, the flagellum on the exopod of the third maxilliped shows considerable variations. The flagellum on the third maxilliped exopod is altogether absent in A. cochleariformis (Fig. 3N) whereas it is distinct but short in A. tumpikkai sp. nov., reaching nearly half the merus width (Fig. 3B). On the other hand, all the species of Cylindrotelphusa have a well-developed and relatively long flagellum on the third maxilliped exopod, which reaches up to 0.6–0.7 times the merus width (see Pati et al. 2017: figs. 11i, 13i).
The variations in the length of the flagellum on the third maxilliped exopod could be associated with the ecological requirements of each species of Arcithelphusa and Cylindrotelphusa. According to Cumberlidge (1999), the presence of a long flagellum on the third maxilliped exopod of a crab is a plesiomorphic state, and the absence of the said flagellum is an apomorphic state, which is common in air-breathing crabs and mostly associated with the adaptations of the respiratory system by the relatively terrestrial species of freshwater crabs (see Ng & Shokita 1995). Cumberlidge (1999) also noted that this character is useful to distinguish some genera of the West African crabs. The presence or absence of a flagellum on the third maxilliped exopod, however, is not a reliable character to exclusively define Arcithelphusa and Cylindrotelphusa because the condition of the flagellum varies considerably among their species depending upon the habitat preferences. Huang (2018) recently found that some crab genera are strikingly similar in external morphology, and the use of the G1 characters alone could be problematic to define them. In these situations, he suggested using a combination of characters that are apparently under less environmental selective pressure, such as male pleon, G1, and female vulvae. Furthermore, a molecular study has now become necessary to support morphological data (see Shih et al. 2016; Huang 2018).
Currently, no molecular data is available for Arcithelphusa and Cylindrotelphusa, and the female vulvae were not described for all the species. Their male pleon is quite similar in morphology. On the contrary, the G1 structure is a reliable character to set them apart. While the generic treatment of Arcithelphusa needs investigation based on morphological and molecular data, we still continue to recognize the genus and provide here an emended diagnosis to accommodate both A. cochleariformis and A. tumpikkai sp. nov.
Arcithelphusa can be now distinguished from Cylindrotelphusa by the following suite of characters: the anterolateral margins of the carapace lack distinct serrations (Figs. 2A, D, 4A, D) (vs. with distinct serrations; see Bahir & Yeo 2007: fig. 5A; Pati et al. 2017: figs. 11a, 13a); the third maxilliped exopod lacks a flagellum (Fig. 3N) or with a short flagellum that reaches half the merus width (Fig. 3B) (vs. with a well-developed flagellum that reaches beyond half the merus width; see Pati et al. 2017: figs. 11i, 13i); the chelipeds are relatively smooth (Figs. 2 A–F, 4A–F) (vs. rugose; see Bahir & Yeo 2007: fig. 5A–C; Pati et al. 2017: figs. 11a, c, d, 13a, c, d); the G1 is relatively stout with a large flexible zone (Fig. 3 E–G, J, O) (vs. relatively slender with a reduced flexible zone; see Bahir & Yeo 2007: fig. 4A–C; Pati et al. 2017: figs. 12a–c, 14a–c); the G1 terminal segment is relatively short, ca. 0.3–0.4 times the length of the subterminal segment (Fig. 3E, J, O) (vs. relatively long, ca. 0.4–0.6 times the length of the subterminal segment; see Bahir & Yeo 2007: fig. 4A; Pati et al. 2017: figs. 12a, 14a); the G1 subterminal segment is relatively stout, with a strongly convex outer margin at the basal half (Fig. 3E, F, J, O) (vs. relatively slender, with a straight to gently convex outer margin at the basal half; see Bahir & Yeo 2007: fig. 4A, B; Pati et al. 2017: figs. 12a, c, 14a, c).
Distribution. The genus is endemic to the Western Ghats of India and known only from Wayanad district of Kerala (Fig. 1).
description: Published as part of Pati, S. K., Sujila, P. S. & Devi, A. R. Sudha, 2019, Description of a new species of freshwater crab of the genus Arcithelphusa Pati & Sudha Devi, 2015 (Decapoda: Brachyura: Gecarcinucidae) from the Western Ghats, Kerala, India, pp. 203-214 in Zootaxa 4674 (2) on pages 205-207, DOI: 10.11646/zootaxa.4674.2.2, http://zenodo.org/record/3458925