Teretamon spelaeum, Absar & Mitra & Kharkongor, 2017

Absar, Parveen Farzana, Mitra, Santanu & Kharkongor, Ilona Jacinta, 2017, Teretamon spelaeum, a new species of freshwater crab (Crustacea: Decapoda: Potamidae) from a limestone cave in Meghalaya, India, Zootaxa 4365 (3), pp. 302-310 : 303-309

Teretamon spelaeum n. sp.

( Figs. 2–4 )

Type material. HOLOTYPE: ZSI Reg. No.IV /CRU/ERS-312, adult male (cw 14.2 mm, cl 11.0 mm, ch 6.3 mm, fw 4.8 mm), Krem Khung Cave, 25°23’21.9”N, 92° 34’48.8”E, Larket Village , East Jaintia hills, District, Meghalaya, India, coll. P.F.Absar, 29 March 2017.

PARATYPE: ZSI Reg. No.IV/CRU/ERS-313, 1 female (cw 19.4 mm, cl 15.0 mm, ch 8.3 mm, fw 6.0 mm), same data as holotype.

Diagnosis. Carapace transversely ovate, cw 1.28 – 1.29 cl, cl 1.75 – 1.81 ch, posterior part narrow ( Fig. 2A , 4A ), surface pitted; frontal margin slightly bilobed, post frontal region slightly deflexed (2E); epigastric cristae relatively high, obliquely positioned, lower side of epigastric cristae not distinctly marked, upper border of cristae irregularly serrated, indistinct; epigastric and postorbital cristae separated by indistinct oblique groove, postorbital cristae long but not prominent throughout length, reaching to just below external orbital tooth, postorbital cristae indistinct, granular; cervical groove not prominent throughout length ( Fig. 2A , 4A ), visible as crescentic mark in mesogastric region, lateral part of groove indistinct, present only as pitted line. Chelipeds strongly unequal in male, subequal in female, external surface with irregular squamiform ridges ( Fig. 2D ); carpus with small tooth on inner distal angle; major palm relatively short, very stout; dactylus stout. Ambulatory legs relatively short, covered with short setae, setae denser in propodus and carpus. G1 subterminal segment about 2.44 times longer than terminal segment; base of terminal segment slightly curved outward; broad, low, dorsal flap prominent, tip up curved, blunt. G2 flagellum as long as narrowed part of subterminal portion, proximal half of narrowed part almost straight, flat, distal half straight cylindrical, tip funnel-shaped.

Description of holotype male. Carapace transversely ovate, cw 1.28 cl, cl 1.75 ch ( Fig. 2A ); front broad, fw 0.34 cw ( Fig.2B ), gently deflexed, frontal margin slightly bilobed ( Figs. 2A, E ); epibranchial cristae high, rugose; post orbital cristae indistinct; orbital margin cristate, supraorbital margin obliquely straight; infraorbital margin broad, outer terminal part of joints of both margin narrowly pointed. External orbital tooth not prominent, outer border relatively broad, crenulate with small granules, inner border almost straight. Epibranchial tooth very small, just separated from rest of margin by small notch; cervical groove not distinct, only cardiac groove prominent, mesogastric groove long, forming inverted V-shape, anterolateral margin cristate, epibranchial region with small striae and granules; posterolateral margin convergent posteriorly, almost straight, longer than anterolateral margin, mesobranchial region with oblique striae ( Fig. 2A ).

Eyes with proximally broad stalk, tapering distally, cornea pigmented, very small. Antennular fossae almost quadrangular, antennules long. Antennae stout, long. Epistome posterior margin with broadly triangular median lobe, tip blunt ( Fig. 2B, E ).

Third maxilliped exopod reaching proximal half of merus, flagellum longer than half of the merus width, ischium with prominent submedian sulcus, merus wider than long, quadrilateral ( Fig. 2E ).

Chelipeds unequal, left chela larger ( Fig. 2A ), inner distal carpal spine small, sharp; outer surfaces of carpus and merus with short striae and tufts of long golden brown setae ( Fig. 2D ). Length of fingers subequal to palm, relatively short; dactylus and pollex flattened laterally, pitted, fluted, tips of fingers overlapping when closed; cutting edges with 5 or 6 broadly triangular teeth, proximally forming broad gape when closed ( Figs.2A, D ).

Ambulatory legs slender; first ambulatory leg almost as long as chelipeds; third and fourth legs longer than chelipeds; fourth leg longest. Merus of third and fourth legs slightly longer than 2/3 of carapace length. Width of propodus of second and third legs about 1/3 to 1/4 of length. Upper and lower border of merus, carpus and upper, lower and inner margins of propodus with long brownish bristles. Dactylus of ambulatory legs almost as long as or slightly longer than respective propodus, all margins of dactylus spinose and setose on all; tips of dactylus with sharp brown spines slightly bent inwards ( Fig. 2 A, D ).

Sterno-pleonal cavity almost reaching sutures between sternite 2 and 3, beyond anterior bases of cheliped coxae ( Fig. 2C ); sternite 1 and 2 completely fused, broadly triangular; sternite 3 very narrow, compressed, barely discernible from sternite 4, separated by faint groove ( Fig. 2C ). Pleon broadly triangular, longer than greatest width; sixth somite with lateral margins straight, converging distally, length almost half greatest width; base of fifth and sixth somites slightly concave ( Fig. 2C ).

G1 subterminal segment broad, groove for G2 visible submedially; terminal part relatively broad, cylindrical, slightly bent outwards, almost straight from dorsal view, with low broad triangular median dorsal flap ( Fig. 3 A– D ), subterminal segment of G1 1.90 × length of terminal part ( Fig. 4 B, C ); flexible joints U-shaped, 0.33 mm long in holotype; G2 flagellum 1.60 mm long with subterminal part 2.62 mm in holotype; proximally bent, laterally flattened, subterminal segment straight, broad, distal end of subterminal segment slightly bent outwards, base broad ( Fig. 3E ).

Live colour. Carapace and ambulatory legs are almost white or colourless owing to loss of pigmentation; cornea present only as a small black pigmented dot ( Figs. 2. 4 A, B ).

Paratype female. Female paratype almost identical to male holotype in non-sexual characters ( Fig. 4 A, B ). Carapace slightly wider than male holotype (1.29 vs 1.28). Female pleon very broad, widely ovate, and almost covering all sternites ( Fig. 4B ); vulvae large, rounded, without operculum, openings in laterally oblique direction, with sternal plate just below opening ( Fig. 4C ).

Etymology. The name refers to the habitat of this species, derived from the Latin, spelaeum , meaning cave. The name is used as a noun in apposition.

Type locality. Krem Khung cave (( 25°23’21.9”N, 92°34’48.8”E), Larket village , East Jaintia hills district, Meghalaya, India.

Remarks. Potamon (Geotelphusa) adiatretum Alcock, 1909 , described from Myanmar and recorded from Arunachal Pradesh of India, was made the type species of the monotypic Teretamon by Yeo & Ng (2007). Recently, T. indicum Mitra, 2017 was added to this genus. Teretamon spelaeum n. sp. agrees well with the generic characters, such as the indistinct or absent epibranchial tooth and external orbital angle, rounded appearance of the carapace front, epigastric cristae slightly anterior to the post orbital cristae, small cornea of the eyes, male pleonal cavity reaching anteriorly to the level of the anterior edge of the chelipeds bases, and the G1 terminal segment with a dorsal flap. The new species, however, differs from its congeners by many carapace and gonopod characters.

Teretamon spelaeum differs from the T. adiatretum (Alcock, 1909) by only slightly bilobed frontal margin ( Fig. 2E ) (vs. distinctly bilobed frontal margin in T. adiatretum ; Yeo & Ng 2007: fig. 13 B); the cheliped carpus has a slender sharp inner tooth ( Fig. 2D ) (vs. sharp and broad inner tooth in T. adiatretum ; Yeo & Ng 2007: fig. 13A, C); very slender dactylus of the walking legs ( Figs. 2A , 4A, B ) (vs. dactylus moderately slender in T. adiatretum ) and the G1 with a long, gently curved and convex dorsal flap along the dorsal side of terminal segment ( Fig. 3A–D ) (vs. G1 with a distinct triangular dorsal flap on the top of the terminal segment in T. adiatretum ; Yeo & Ng 2007: fig. 13 D).

Teretamon spelaeum is superficially similar to T. indicum in having an ovate carapace, smooth dorsal surface of the carapace, a low and broad carpal spine of the cheliped, and prominently setose walking legs; Teretamon spelaeum n. sp., however, differs in having a relatively low carapace ( Fig. 2A, B , 4A ) (vs. more rounded and high carapace in T. indicum ; Mitra 2017: figs. 58, 61); the anterolateral margin has only a trace of an epibranchial tooth ( Figs 2A , 4A ) (vs. no epibranchial tooth present in T indicum ; Mitra 2017: figs 58, 61); the eyestalk is stouter, more or less conical with the cornea poorly developed ( Fig.2B ) (vs. cornea large, eyestalk slender and cylindrical in T. indicum ; Mitra 2017: fig. 59); and the ambulatory legs are prominently setose and the outer surface of the chela has long setae ( Figs. 2A, D ) (vs. ambulatory legs with scattered short setae and the chela glabrous in T indicum ; Mitra 2017: figs. 58, 61).

The absence of carapace and pereiopod pigmentation and the reduction of eye structures in Teretomon spelaeum indicates that it is a troglobitic species, adapted to life in perpetual darkness. It has not yet, however, evolved long ambulatory legs and chelipeds, troglomorphic features often seen in completely cave dwelling species such as Sundathelphusa sottae Ng & Sket, 1996 , from Ughab Cave in the Phillipines, and Stygothelphusa antu Ng & Grinang, 2014 , from caves in Sarawak, Malaysia (Ng & Sket 1996; Ng & Grinang 2014; Meyer-Rochow 2004). The absence of longer pereiopods may be partially compensated by the more setose chelipeds and ambulatory legs of the new species, which presumably aid in sensing its environment. These features, however, are not unique to cave species, as several riverine and marine species also have setose legs.

Habitat. The cave system of Krem Khung is more than 7000 m long and is located in the East Jaintia Hills district in the State of Meghalaya, India ( Fig. 1 ). The cave lies at the valley level at an altitude of 879 m above mean sea level. The 1.4 m × 1.6 m triangular shaped entrance of the cave ( Fig. 5A ) is through a sinkhole lying below a cliff. Beyond the entrance, the cave floor is covered with dry mud and flood debris for about 50–60 m, after which numerous pools with soft mud and knee- deep water are present the main cave system is beyond this.

The cave system is directed towards the Kopili River and is a major hydrological sink for the area, taking a lot of water during the monsoon. A characteristic feature of Krem Khung is its meandering nature which almost obliterates the transitional zone so the dark zone starts within less than 100 m of the entrance. The main track of the cave leads into a large chamber with a diameter of about 50 m and height of about 8 m. This chamber, named the Paradise Chamber, is spectacular from the point of view of speleological features. Its base is strewn with fallen boulders and it has development of multi-coloured stalactites, stalagmites and columns ( Fig. 5B ); some of the stalactites are bright yellow colour owing to presence of sulfur. The cave is estimated to be of Eocene age as indicated by the presence of index fossils of foraminiferan species of Nummulites.

The crabs were collected during the return trip from the cave, about 50 m into the meandering main trend of the cave after crossing the Paradise Chamber or 50 m before reaching Paradise Chamber on the way in. The collection site is approximately 400 m from the entrance. The collection site is a wet, gravelly and muddy area by the side of the cave passage, located between two meanders, devoid of any flowing stream tributary or stagnant pool.

Geographical distribution. Teretamon spelaeum is known only from the type locality, the Krem khung cave of Meghalaya state of India.

Figures

FIGURE 1. Localities of Teretamon spelaeum n. sp. in Krem Khung caves (red circles) in Meghalaya.

FIGURE 2. Teretamon spelaeum n. sp., holotype male (cw 14.2 mm, cl 11.0 mm) (ZSI reg. No. IV/CRU/FRS-312), India: A, overall dorsal view; B, anterior view of cephalothorax; C, male thoracic sternum and pleon; D, dorsal view of major cheliped and first ambulatoryleg showing setose surface; E, third maxilliped.

FIGURE 3. Teretamon spelaeum n. sp., holotype male (cw 14.2 mm, cl 11.0 mm) (ZSI reg.No. IV/CRU/FRS-312), India: A, dorsal view of right G1; B, ventral view of right G1; C, dorsal view of distal part of right G1; D, ventral view of distal part of right G1; E, right G2.

FIGURE4. Teretamonspelaeum n.sp., paratype female (cw 19.4 mm,cl 15.0 mm) (ZSI reg.No.IV/CRU/FRS-313), India:A, overall dorsal view; B, ventral view of cephalothorax; C, thoracic sternum showing vulvae.

FIGURE 5. A, cave entrance.B, limestone structure of cave.