Himalayapotamon robertsianum, Klaus & Brandis, 2019

Klaus, Sebastian & Brandis, Dirk, 2019, A new species of Himalayapotamon Pretzmann, 1966 (Crustacea: Brachyura: Potamidae) marking the westernmost distribution of the genus, with notes on its geographical range evolution, Journal of Natural History 52 (47 - 48), pp. 2993-3004 : 2995-2999

Genus Himalayapotamon Pretzmann, 1966a

Himalayapotamon robertsianum sp. nov.

( Figure 2 )

Himalayapotamon koolooense – Pretzmann 1966c, p. 346, fig. 7 (map) (partim)

Material examined

Holotype: adult male (carapace width = 43.2 mm, length = 33.1 mm, height 19.3 mm; width of frontal margin = 14.8 mm), Muree hills, Punjab, Pakistan, 33°50 ʹ N 73°19 ʹ E, 900 m asl, coll. T. J. Roberts, 30 May 1986 ( SMF 24731) . Paratypes: 2 males, 1 female, severely disintegrated ( SMF 24732); 3 males, 2 females ( SMF 24733); 1 female ( SMF 24734), same data as holotype.

Etymology

Named in honour of the collector, Mr T.J. Roberts.

Distribution

Slopes of the Murree hills, Punjab province, Pakistan, drainage of Sohan River . Only confirmed from the type locality, but possibly having a wider distribution, at least within the Sohan River drainage .

Diagnosis

Carapace with numerous scattered setae; anterolateral margin well developed, sharp; postorbital crista sharp, leading nearly from epibranchial tooth to postfrontal crista, with distinct angle at end of branchial region. Male pleon relatively slender, triangular; pleonal somite 6 trapezoidal, distinctly broader than long; male telson triangular, slightly longer than pleonal somite 6, lateral margins gently sinuous, tip rounded. Terminal article of G1 short, stout, conical-triangular shaped, gradually curved outwards, tip blunt; flexible zone distinctly elongated, relatively broad, V-shaped; subterminal part long, c. 0.7× length of G1, relatively broad.

Description of male holotype

Carapace smooth, except for rugose frontal and branchial regions, covered with numerous setae. Front bilobed, gently bent downwards; postfrontal lobe broad, well developed, shifted posteriorly. Postfrontal crista sharp, projecting anteriorly. Postorbital crista sharp, concave, reaching from epibranchial tooth to postfrontal lobe, separated by deep cervical groove at beginning of epibranchial region, strongly curved anteriorly towards epibranchial tooth. Epibranchial tooth distinct but small, situated at level of postorbital margin; anterolateral margin lined by numerous irregular small teeth. Posterolateral margin convex, slightly converging. Central H-groove and cervical grooves distinct but shallow. Cervical groove characteristically S-shaped. Exorbital tooth projecting, sharp, triangular outer margin slightly convex. Supra- and infraorbital areas slightly granular. Epistome with smooth anterior part and posterior margin with prominent median tooth. Supraorbital, subbranchial and pterygostomial regions glabrous, rugose, Sternal and abdominal surfaces glabrous, smooth. Ischium of third maxilliped with deep, oblique, median sulcus, exopod width one third of merus width, with short flagellum. Terminal segment of the mandibular palp simple, not furcate.

Chelipeds slightly unequal, outer surface granulated. Carpus with 2 well developed, strong spines at inner distal margin, one larger and one smaller. Merus mesially with two short spines. Fingers of both chelae as long as palm, without gap when closed, with well-developed teeth along cutting edges.

Pereiopods 2–5 glabrous, smooth. Lateral regions of carpus and propodus with long edges lined by small teeth. Pleon slender, triangular; pleonal somite 1 shortest; pleonal somites 2–5 gradually elongated; pleonal somite 6 longest, trapezoidal, distinctly broader than long; triangular, slightly longer than sixth pleonal somite, lateral margins gently sinuous, tip rounded.

Terminal article of G1 short, stout, conical-triangular in shape, gradually curved outwards, tip blunt; flexible zone distinctly elongated, relatively broad, V-shaped; subterminal part long, c. 0.7× length of G1, relatively broad. Second gonopod with a long, straight, rigid terminal tube, followed by elongated flexible part.

Phylogenetical analysis

The genus Himalayapotamon , including H. robertsianum sp. nov., was monophyletic in the molecular phylogeny ( Figure 3 ), although species sampling was limited and the present results are based only on one single mitochondrial marker gene. Within Himalayapotamon , two clades could be identified, albeit with low support: a clade of H. koolooense and H. robertsianum sp. nov., and a clade of H. emphyseteum , H. atkinsonianum and H. sunkoshiense . Himalayapotamon atkinsonianum and H. sunkoshiense appear as sister species. The uncorrected pairwise genetic distance of H. robertsianum sp. nov. to the closest related species included, H. koolooense , is 6.3%; distance to H. emphyseteum is 6.5%, to H. sunkoshiense 9.0% and to H. atkinsonianum 10.0%. For comparison, genetic distance between Himalayapotamon and its closest related genus Paratelphusula ranges from 11.9% to 13.1%. The sequences from GenBank apparently originate partly from misidentified specimens: ZRC 2006.0143 (‘ H. atkinsonianum ’) and SMF 26069 (‘ H. emphyseteum ’) belong to H. sunkoshiense (morphologically reconfirmed for SMF 26069). For H. emphyseteum we could only include specimens from the Ganges drainage, although its type locality is much further west at the Beas (Hyphases) and/or Satluj Rivers ( Figure 1 ). Therefore, we label these more eastern representatives, including specimen ‘ Himalayapotamon sp. ’ (ZRC 2003.0663), provisionally as ‘ Himalayapotamon cf. emphyseteum’.

Remarks

Himalayapotamon robertsianum sp. nov. differs strongly from most other species of the genus with respect to the shape of the G1. The G1 terminal article is broad and conical in H. robertsianum sp. nov., while it is very narrow and elongated in H. atkinsonianum and H. cf. emphyseteum . The recently described species H. chambaensis is geographically closest to H. robertsianum sp. nov. The G1 terminal article in H. chambaensis is very long (c. 1/3 of total length) and rather spindle-shaped than conical, and characteristically bent outwards (vs. short, conical, and gently curved outwards in H. robertsianum sp. nov.; Figure 2 (c), 2(d); cf. Mitra and Valarmathi 2017, fig. 3a, c). Himalayapotamon koolooense ( Rathbun 1904) from Himachal Pradesh is a morphologically similar and geographically close species. In fact, occurrence of this species was also reported – although without reference to available material – from southern Kashmere ( Pretzmann 1966c), and thus possibly within the range of H. robertsianum sp. nov. However, the large genetic distance between both species (6.3%; Figure 3 ), as well as the much more slender and triangular terminal article, and a relatively narrow flexible zone with a ventral tongue-like extension of the G1 of H. koolooense ’s G1 unequivocally distinguish H. koolooense from H. robertsianum sp. nov. ( Figure 2 (d); cf. Klaus et al. 2019, fig. 16.6.64m).

Three further species were described from the Indus drainage: H. kasaulis , H. ambivium and H. babaulti . Himalayapotamon babaulti has a stouter G1 terminal article with a sharply pointed tip, in contrast to the blunt tip in H. robertsianum sp. nov. ( Figure 2 (c), 2 (d); cf. Klaus et al. 2019, fig. 16.6.64o). Initially, because of the morphological similarity with H. koolooense , H. kasaulis was described as a subspecies of H. koolooense , and H. ambivium as a variety of H. atkinsonianum close to H. koolooense ; but both were treated as separate species by Ng et al. (2008). All three species also co-occur with H. koolooense in the Satluj and Beas drainages ( Figure 1 ), suggesting that the morphological differences to H. koolooense should be re-evaluated, especially in light of potential intraspecific variability. To make things even more complicated, the type localities of H. emphyseteum (Bilaspur and/or Kangra) and H. ambivium (Dharampur near Shimla) are in very close proximity ( Alcock 1909). A subspecies of H. atkinsonianum , H. a. ventriosum ( Alcock 1909), was reported from the western Ganges drainage. It differs, however, in the shape of the G1 as outlined above for H. atkinsonianum , and Alcock (1909) himself raised the possibility that the individual his description was based on was just an aberrant individual. In the phylogeny, at least the nominal species, H. atkinsonianum appears to be genetically very distant (10.0%) to H. robertsianum sp. nov. The shape of the G1’s flexible zone of H. robertsianum sp. nov. also differs distinctly from the flexible zone in H. garhwalense that occurs in the North Indian state Uttarakhand, and thus already in the Ganges drainage. While the flexible zone of the G1 is slightly V-shaped in H. robertsianum sp. nov., it is distinctly bilobed in H. garhwalense ( Figure 2 (d); cf. Pati and Singh 2017, fig. 3a, b).

Figures

Figure 1. Occurrence of the currently recognised species of the genus Himalayapotamon; based on different authors (Alcock 1909, 1910; Pretzmann 1966b; Brandis et al. 2000; Brandis 2001; Brandis and Sharma 2005; Mitra and Valarmathi 2017; Pati and Singh 2017). The two insets enlarge the sampling sites between Ravi and Satluj rivers (left), and between Gandak and Bagmati rivers (right). The dash-dot line indicates the drainage divide between Indus and Ganges; the dashed line the palaeo-divide at 5 Ma according to Clift and Blusztajn (2005); and the dotted line the palaeo-divide under the hypothesis that the range of Himalayapotamon reflects the palaeo-Ganges drainage.

Figure 2. Himalayapotamon. robertsianum sp. nov., holotype male (SMF 24731). (a) Carapace; (b) sternum and pleon (scale = 2 cm); (c) first gonopod, dorsal; (d) first gonopod, ventral (scale = 5 mm).

Figure 3. Phylogenetic position of Himalayapotamon robertsianum sp. nov. as close relative of its geographically near congener, H. koolooense. Maximum clade credibility tree based on a 16S rRNA gene fragment and rooted with two Potamon species from Iran.