Potamiscus Alcock, 1909

Ng, Peter K. L. & Yeo, Darren C. J., 2023, Clarifying the identities of the Asian potamid genera Potamiscus Alcock, 1909, Ranguna Bott, 1966, and Dromothelphusa Naiyanetr, 1992 (Crustacea: Brachyura: Potamoidea), Raffles Bulletin of Zoology 71, pp. 632-657 : 633-636

Genus Potamiscus Alcock, 1909

Potamon (Potamiscus) Alcock, 1909: 246 ; 1910a: 257; 1910b: 56; Bott, 1966: 480.

Potamiscus – Bott, 1966: 479 (partim); 1970: 158 (partim); Chuensri, 1974: 23 (partim); Dai, 1999: 186 (partim); Brandis, 2000: 60 (partim); Yeo & Ng, 2004: 1224 (partim), fig. 2E; 2007: 293 (partim), fig. 11; Ng et al., 2008: 165 (partim).

Type species. Potamiscus annandalii Alcock, 1909 , by original designation.

Diagnosis. Carapace broader than long, relatively high, dorsal surface distinctly convex transversely and longitudinally in frontal view; epigastric cristae well developed, slightly anterior to postorbital cristae, more or less confluent with postorbital cristae, faintly separated by short, indistinct groove; postorbital cristae strong, sharp, almost confluent with epibranchial teeth; regions behind epigastric and postorbital cristae almost smooth; frontal and postorbital regions relatively wide ( Figs. 1A, B , 2A, B, D–F ); external orbital angle triangular, outer margin longer than inner margin; epibranchial tooth low but distinct, separated by clear cleft; anterolateral margins cristate, serrated, gently convex; posterolateral margins gently converging posteriorly; branchial region rugose ( Figs. 1A , 2A, B, E ). Antennular fossae longitudinally narrow, slit-like ( Figs. 1B , 2D, F ). Epistome posterior margin with distinctly triangular median tooth, outer part strongly sinuous ( Figs. 1B , 2D, F ). Ischium of third maxilliped relatively short, broadly rectangular, with shallow oblique median sulcus; exopod long, exceeding distal edge of ischium, with flagellum vestigial or absent ( Fig. 2C ). Ambulatory legs not elongate, stout, with long, slender dactyli ( Fig. 2A ). Male anterior thoracic sternites relatively narrow transversely; suture between male thoracic sternites 2 and 3 distinct, complete, reaching edge of sternum; suture between male thoracic sternites 3 and 4 absent or indistinct; male sternopleonal cavity reaching imaginary line joining posterior edge of cheliped bases; tubercle of male press-button locking mechanism small, on posterior third of thoracic sternite 5 ( Figs. 1C, D , 2G , 3E ). Male pleon broadly triangular ( Figs. 1C , 2G , 3B ). G1 subterminal segment relatively stout, terminal segment distinctly bent, curved outwards, conical, distally tapered, without any trace of dorsal flap, groove for G2 marginal ( Figs. 1E, F , 3C , 4A–D ). G2 longer than G1; distal segment longer than half length of basal segment; basal segment relatively wide, with outer margin convex ( Figs. 1G , 3D , 4E ). Vulvae large, occupying ⅔ anterior part of thoracic sternite 6, touching suture between thoracic sternites 5 and 6, operculum present, sternal vulvar cover on posterior margin, opening obliquely inwards ( Fig. 3F ).

Remarks. Alcock (1909) had first described the genus as “ Potamiscus , gen. nov. ” ( Alcock, 1909: 246), and on the same page, recognised only one new species in it, Potamiscus annandalii Alcock, 1909 . Later in his discussion in the same paper, he stated that “The Potamonidae of the Indian fauna are included in three principal genera, Potamon , Paratelphusa , and Gecarcinucus ” ( Alcock, 1909: 249), and noted that Potamon can be split into four subgenera, including Potamiscus ( Alcock, 1909: 249) . While most workers have regarded Potamiscus as a subgenus, for nomenclatural purposes, it should be treated as a genus when first described, with the type species, Potamiscus annandalii , by monotypy (see also discussion in Pati, 2022).

Potamiscus was established by Alcock (1909) for species from India in which the third maxilliped exopod lacks a flagellum or only has a vestigial one (see also Alcock, 1910a). Alcock (1910b) subsequently added two species to Potamiscus (as a subgenus of Potamon ): Potamon (Geothelphusa) sikkimense Rathbun, 1905 and Potamon (Potamiscus) tumidulum Alcock, 1909 . Alcock (1910b: 58) was unsure about the latter species as he said it had a short flagellum and had first treated it under Potamon (Potamon) but noted in the same paper that it may be a Potamon (Potamiscus) instead. Kemp (1913, 1923, 1924) subsequently described five new Potamiscus species: P. (P.) decourcyi Kemp, 1913 , P. (P.) aborense Kemp, 1913 , P. (P.) obliteratum Kemp, 1913 , P. (P.) alcockianum Kemp, 1923 , and P. (P.) yunnanense Kemp, 1924 . Bott (1966, 1970) disregarded Alcock’s (1909, 1910a, b) characters and redefined Potamiscus solely on the structure of the G1, raising it to the genus rank and reassigning three more species to the genus, viz., Potamon (Potamon) tannanti Rathbun, 1904 , Telphusa tumida Wood-Mason, 1871 , and Telphusa pealiana Wood-Mason, 1871 , while consigning Potamon (Potamiscus) tumidulum , Potamon (Potamiscus) aborense , and Potamon (Potamiscus) obliteratum , to the synonymy of Potamiscus sikkimensis ( Rathbun, 1905) .

More Potamiscus species have since been described from southern and southwestern China, as well as from northeastern India and Myanmar: P. loshingensis (Wu, 1934) , P. montosus Dai, Song, He, Cao, Xu & Zhong, 1975 , P. yongshengense Dai & Chen, 1985 , P. elaphrius Dai, Chen, Liu, Luo, Yi, Liu, Gu & Liu, 1990 , P. rongjingense Dai, Chen, Liu, Luo, Yi, Liu, Gu & Liu, 1990 , P. motuoense Dai, 1990 , P. yiwuensis Dai & Cai, 1998 , P. cangyuanense Dai, 1999 , P. crassus Naruse, Chia & Zhou, 2018 , P. fumariatus Naruse, Chia & Zhou, 2018 , P. palelensis Mitra & Waikhom, 2019 , P. takedai Pati, Mitra & Ng, 2020 , P. whitteni Ng, Hla Htoo & Win Mar, 2020 , P. chizami Pati, 2021 , and P. mima Pati, 2021 (cf. Dai & Cai, 1998; Dai, 1999; Naruse et al., 2018; Mitra & Waikhom, 2019; Ng et al., 2020a; Pati et al., 2020; Pati, 2021).

Brandis (2000) revised Potamiscus following the rationale of Bott (1970) and arguing that the absence or presence of a flagellum on the third maxilliped exopod was not a reliable character, relying almost exclusively on the structures of the G1 and G2 instead. As a result, he synonymised Ranguna Bott, 1966 , and Terrapotamon Ng, 1986 , under Potamiscus . Unfortunately, Brandis’ (2000) concept of Potamiscus , which did not consider many non-gonopod characters of taxonomic significance, groups many unrelated taxa together, and subsequent morphological studies (e.g., Yeo & Naiyanetr, 2000; Yeo & Ng, 2004, 2005, 2007) and genetic studies (e.g., Shih et al., 2009; Ng et al., 2020b; Zhang et al., 2020a, b; Pan et al., 2021) have shown that there are many more phylogenetic units among Asian potamids than Brandis’ (2000) system recognised. For example, the preliminary genetic studies by Yeo et al. (2007: 262, 263) showed that while Terrapotamon is related to Potamiscus , the level of divergence between the two genera is too great for them to be considered the same taxon. The challenge is that while the presence or absence of a flagellum is not a unique apomorphic character, it is also not as variable as some authors (e.g., Brandis, 2000) have believed. In adult specimens, it is a consistent and reliable character that can be used to define species and genera, even if the genera that lack flagella are not closely related (see discussion later under Dromothelphusa longipes ).

The present consideration of additional characters in the carapace, male thoracic sternum, ambulatory legs and male pleon, together with the absence or presence of a third maxilliped flagellum and G1 structure allows us to recognise distinct species-groups in Potamiscus , most of which can be recognised as genera. In reality, only P. annandalii remains in Potamiscus based on the characters used here. The most marked and significant character common to Potamiscus and Ranguna is probably the male sternopleonal cavity being shorter, reaching only to the imaginary line connecting the posterior edge of the bases of the chelipeds. In other genera, the male sternopleonal cavity is longer, reaching to the imaginary line between the midpoint of the bases of the chelipeds. Between the two genera, the carapace of Potamiscus s. str. (as defined by P. annandalii ) is relatively high, with the dorsal surface gently convex, the lateral parts of the posterior margin of the epistome are strongly sinuous, and the male pleon is broadly triangular; while in Ranguna s. str. (as defined by the type species), the carapace is relatively lower, with the dorsal surface appearing almost flat, the lateral parts of the posterior margin of the epistome are weakly sinuous, and the male pleon is narrow and subrectangular in shape. Dromothelphusa is quite different from Potamiscus s. str. and Ranguna s. str., with the male sternopleonal cavity reaching an imaginary line joining the median part of the cheliped bases, the carapace being relatively lower, with the dorsal surface gently convex, the lateral parts of the posterior margin of the epistome very weakly sinuous, and the male pleon triangular in shape. Potamiscus decourcyi (from northeastern India) was left in Potamiscus s. str. by Yeo & Ng (2007) and Ng et al. (2008), but it is here referred to Ranguna as most of its characters better fit this genus as well (see later).

As for Bott’s (1970) treatment of Potamiscus , Potamon (Potamiscus) aborense and Potamon (Potamiscus) obliteratum are in fact distinct species rather than synonyms of Potamon (Geothelphusa) sikkimense as he believed, and both have already been transferred to Quadramon Yeo & Ng, 2007 (see Yeo & Ng, 2007), the two species being easily diagnosed by their carapace features. Potamon tumidulum and Potamon sikkimensis are separate species and are retained in the genus for the time being pending a reappraisal. Potamon tumidulum has a relatively lower carapace than Potamiscus species, with a flatter and more strongly areolated dorsal surface (versus carapace higher, with more convex and smoother dorsal surface); low, rugose postorbital cristae (versus postorbital cristae well developed, smooth); distinctly broadly triangular external orbital angle (versus external orbital angle more acutely triangular); and anterior thoracic sternum with distinctly ridged sutures between thoracic sternites 2 and 3, and 3 and 4 (versus suture between thoracic sternites 2 and 3, and 3 and 4 smoother, respectively) (see also Bott, 1970: pl. 51 fig. 51; S.K. Pati, pers. comm.). Potamon sikkimense has a relatively lower and broadly triangular external orbital angle (versus external orbital angle more acutely triangular); and smooth, rounded postorbital cristae (versus postorbital cristae sharp) than typical Potamiscus species (see Rathbun, 1904: pl. 18 fig. 7). Ng et al. (2008: 167) transferred this species to the Chinese genus Trichopotamon Dai & Chen, 1985 (type species Trichopotamon daliense Dai & Chen, 1985 ) without any explanation. Takeda & Sugiyama (2015) nevertheless followed this classification when they redescribed the species based on non-type material from Nepal. Their descriptions and figures appear to match the original description of Potamon sikkimense by Rathbun (1905). The species is certainly not a Trichopotamon , and its G1, although rather setose, has the general shape of that of Potamiscus s. str., i.e., the terminal segment is very short and has a low dorsal fold ( Takeda & Sugiyama, 2015: figs. 3D, 4E–G) (versus terminal segment long in Trichopotamon ; Dai & Chen, 1985: fig. 6(4–6); Dai, 1999: fig. 198(4, 5); Naruse et al., 2008: fig. 15a). The third maxilliped exopod also lacks a flagellum, and the male pleon is broadly triangular ( Takeda & Sugiyama, 2015: figs. 3C, 4D), similar to typical Potamiscus species. While Potamon sikkimense was described based on a female holotype, and the specimens figured in Takeda & Sugiyama (2015) may prove to be a species different from Potamon sikkimense , it is confirmed that the epigastric and postorbital cristae of Potamon sikkimense are low and not prominent ( Takeda & Sugiyama, 2015: fig. 3A, B) compared to the well-developed epigastric and postorbital cristae of Potamiscus s. str. The generic position of Potamon sikkimense is now being reappraised, and the species will probably need to be moved to a new genus at a later date (S.K. Pati, pers. comm.).

Two other species placed previously in Potamiscus by Bott (1970) and Brandis (2000), Potamon tannanti and Potamon alcockianum , have been transferred to Indochinamon Yeo & Ng, 2007 , and Beccumon Yeo & Ng, 2007 , respectively (see Yeo & Ng, 2007).

Finally, Bott (1970) and Brandis (2000) placed two more species in Potamiscus , Telphusa pealiana and Telphusa tumida . Telphusa pealiana has a relatively longer third maxilliped exopod flagellum than Potamiscus species, being subequal to or exceeding half the width of the merus (versus flagellum vestigial or absent), and a subcylindrical G1 terminal segment with truncate tip (versus G1 terminal segment subconical with tapered tip) (see Bott, 1970: pl. 38 fig. 33), and was referred to Badistemon by Mitra et al. (2020). Telphusa tumida also has a much longer third maxilliped exopod flagellum than Potamiscus species, being subequal to or longer than the merus width (versus flagellum vestigial or absent); and stouter ambulatory dactyli (versus dactyli relatively long, slender) (see Alcock, 1910b: fig. 45; unpublished data). Yeo & Ng (2007) provisionally referred this and some Chinese species of Potamon to Eosamon , but Pan et al. (2022) recently transferred them to a new genus Aiyunamon Pan, Ng & Sun, 2022 . Most recently, Pan et al. (2023) showed that Aiyunamon tumidum is a species complex with three separate taxa, one of which they described as new, with Ng & Ngo (2023) reporting a new species from Vietnam.

For the Chinese species of Potamiscus , we have examined specimens and/or photographs of the types and discerned at least three distinct groups based on shared suites of morphological features. One group is for two species, P. yunnanensis and P. yongshengense distinguished by their relatively low, poorly developed epigastric and postorbital cristae (versus epigastric and postorbital cristae well developed in other Chinese Potamiscus species), and very stocky G1, appearing longitudinally twisted, with truncate tip (see Dai, 1999: figs. 101, 102) (versus G1 relatively more slender, not twisted, with tapered tip). Two recently described species, P. fumariatus and P. crassus , have G1 structures that approach that of P. yunnanensis . The G1 structure of P. crassus ( Naruse et al., 2018: fig. 33A–D), however, appears even shorter and stockier, the terminal segment being relatively stouter with a wider tip, and its carapace features suggest it may belong to a separate group. The identity of P. fumariatus is a problem; a re-examination of the holotype suggests that it is probably a junior synonym of Aparapotamon grahami ( Rathbun, 1931) , and the characteristic G1 ( Naruse et al., 2018: fig. 30A–D) is due probably to breakage of the tips of both G1s (Chao Huang, T. Naruse, pers. comm.). This problem is now being addressed using molecular tools to confirm their conspecificity (Shi Boyang, Pan Da, unpublished data). A second possible group of species possess relatively long, upright, subcylindrical G1 terminal segments with truncate tips and includes P. motuoense , P. loshingensis , P. elaphrius , and P. rongjingense (cf. Dai, 1999: figs. 103–106). Even within this group, there are further differences in the shape of their carapaces, form of the external orbital teeth and slenderness of their ambulatory legs on top of geographical differences, and it can be split into two or even three groups, with the G1 of P. motuoense being especially distinct (Chao Huang, pers. comm.). The third group includes P. montosus , P. yiwuensis , and P. cangyuanense , which are species with a slightly bent G1 terminal segment, with the structure relatively more slender, subconical and tapering to a sharp tip (cf. Dai & Cai, 1998: fig. 5.6; Dai, 1999: figs. 99, 100). Potamiscus cangyuanense is only known from a single male specimen from Yunnan and has never been collected again, and we have doubts about its affinities; other than the absence of a flagellum on the third maxillipeds, its general features and gonopods actually closely resemble species of Aiyunamon (Chao Huang, pers. comm.). While the G1s of these species somewhat resemble the G1 structure of Potamiscus s. str., they still cannot be placed there as their male sternopleonal cavities are relatively longer, the male pleon is relatively narrow, the epigastric and postorbital cristae are less obviously cristate, and the ambulatory dactyli are relatively shorter.

The genetic data of most of the Chinese species have been analysed. In their phylogeny of the Asian Potamidae, Shih et al. (2009: 704 , table 1) observed that two groups of Potamiscus species should be referred to their own new genera, one including P. yunnanensis and the other with P. loshingensis but did not describe them (see also Ng et al., 2020b: fig. 7). This appears to support the above observations based on morphology. The phylogenetic trees of Pan et al. (2021), which include most Chinese Potamiscus species (except P. cangyuanense , P. fumariatus , P. yunnanensis , and P. crassus ), however, recognise three main groups. Within the Southwest China clade, they found P. loshingensis , P. elaphrius , P. yongshengense to be related but not congeneric with each other, and P. motuoense to be only distantly related to the former species (see also Zhao et al., 2021). In another major clade, P. montosus and P. yiwuensis are sister to each other. The phylogenetic tree in Chu et al. (2018) further shows that P. yunnanensis (as “New genus 1 yunnanense ”) also belongs in the Southwest China clade. There appears to be morphological support based on differences in their carapace features; ultimately, more work is needed to determine how many genera should be recognised from what is now called “ Potamiscus ” in China. This matter is now being studied in detail by various Chinese colleagues (Pan Da, Shi Boyang et al., pers. comm.). Certainly, none can be referred to Potamiscus s. str. as defined here.

The Indian-Burmese species now placed in Potamiscus seem to contain two other groups in addition to Potamiscus and Ranguna . Most cannot be accommodated in these two genera as their carapace, male thoracic sternal and male pleonal characters do not agree. Importantly, all are not Potamiscus s. str. as the male sternopleonal cavity of these species is longer, reaching just to the imaginary line connecting the midpoint of the bases of the chelipeds. Potamiscus palelensis , P. whitteni , and P. chizami all have the epigastric and postorbital cristae distinct, their external orbital angle is triangular in shape, and the epibranchial tooth visible and usually distinct. Like Potamiscus s. str., their carapace is relatively high with the dorsal surface distinctly convex in frontal view (cf. Mitra & Waikhom, 2019: fig. 1B; Ng et al., 2020a: fig. 1D; Pati, 2021: fig. 1B) and their ambulatory dactyli are long and slender (cf. Mitra & Waikhom, 2019: fig. 1A; Ng et al., 2020a: fig. 1A; Pati, 2021: fig. 2A). In addition to their longer male sternopleonal cavity, their male pleon is triangular but neither as wide as in Potamiscus s. str. or as narrow and subrectangular as in Ranguna (cf. Mitra & Waikhom, 2019: fig. 1C; Ng et al., 2020a: fig. 3A; Pati, 2021: fig. 2F); and their G1 is slender and the terminal segment is sinuous with the dorsal fold distinct (may be low) (cf. Mitra & Waikhom, 2019: fig. 2E, F; Ng et al., 2020a: fig. 3E, F; Pati, 2021: fig. 3A–C). These three species, however, are arguably the taxa closest to P. annandalii , and genetic studies (Shi Boyang et al., unpublished data) suggest they could represent a sister genus.

Potamiscus takedai superficially resembles members of the above group of species but is markedly different from them as its carapace is not high with the dorsal surface much flatter, its male ambulatory dactylus is short, and the G1 terminal segment is sharply bent ( Pati et al., 2020: figs. 1C, D, 2G, 4F–I, 5E–G). It seems to belong to its own group for now. In many aspects, P. takedai resembles species of Indochinamon except for its vestigial exopod flagellum, and initial morphological and genetic studies also indicate it is not a Potamiscus (S.K. Pati and Shi Boyang et al., pers. comm.). Potamiscus mima superficially resembles species of Ranguna ; its carapace is not high, with the dorsal surface relatively flatter, the male thoracic sternum is also relatively less broad ( Pati, 2021: fig. 2G–I), the ambulatory dactylus is long ( Pati, 2021: fig. 2G–I), and the G1 structure somewhat resembles R. rangoonensis ( Pati, 2021: fig. 3E–G). There are, however, some key discrepancies: its male pleon is less narrow and not as subrectangular ( Pati, 2021: fig. 2L) compared to those of R. rangoonensis and R. decourcyi , new combination; the suture between male thoracic sternites 3 and 4 is discernible; the posterior margin of the epistome has the lateral parts strongly sinuous; and the mesial end of the vulva reaches close to the suture between sternites 4 and 5, with the sternal vulvar cover indiscernible (S.K. Pati, pers. comm.). As such, until the Indian taxa can be treated as a whole, we retain it in Potamiscus s. lato for the time being.

A revision of the genus is not attempted here, as it is outside the scope of the present study. We do, however, treat one junior synonym, Ranguna Bott, 1966 , and show that it is a valid genus with a revised diagnosis, and that two Potamiscus species can be referred to it.

Distribution. Northeastern India.

Figures

Fig. 1. Potamiscus annandalii Alcock, 1909, lectotype male (33.0 × 25.0 mm) (ZSIK 6602-3/9), India. A, overall dorsal view; B, frontal view of cephalothorax; C, thoracic sternum, pleonal somites 4–6 and telson; D, sternopleonal cavity showing left G1 in situ; E, right G1 (ventral view); F, right G1 (dorsal view); G, right G2.

Fig. 2. Potamiscus annandalii Alcock, 1909. A–D, G, paralectotype male (30.5 × 25.7 mm) (NHM 1909.9.2.5), India; E, F, paralectotype female (30.0 × 23.1 mm) (NHM 1909.9.2.5), India. A, E, overall dorsal view; B, dorsal view of carapace; C, right third maxilliped; D, F, frontal view of cephalothorax; G, anterior thoracic sternum, pleonal somites 4–6 and telson.

Fig. 3. Potamiscus annandalii Alcock, 1909. A–E, paralectotype male (30.5 × 25.7 mm) (NHM 1909.9.2.5), India; F, paralectotype female (30.0 × 23.1 mm) (NHM 1909.9.2.5), India. A, chelae (outer view); B, pleon and telson; C, left G1 (ventral view); D, left G2; E, sternopleonal cavity showing pleonal press-button locking mechanism; F, sternopleonal cavity showing vulvae.

Fig. 4. Potamiscus annandalii Alcock, 1909, paralectotype male (30.5 × 25.7 mm) (NHM 1909.9.2.5), India. A, left G1 (ventral view); B, distal part of left G1 (ventral view); C, distal part of left G1 (dorsal view); D, tip of left G1 (ventral view); E, left G2. Scales: A, E = 1.0 mm; B–D = 0.5 mm.