Minervarya charlesdarwini (Das, 1998) Garg & Chandrakasan & Gokulakrishnan & Gopika & Das & S. D. Biju, 2022

Garg, Sonali, Chandrakasan, Sivaperuman, Gokulakrishnan, G., Gopika, C., Das, Indraneil & S. D. Biju,, 2022, The curious case of Charles Darwin's frog, Rana charlesdarwini Das, 1998: Phylogenetic position and generic placement, with taxonomic insights on other minervaryan frogs (Dicroglossidae: Minervarya) in the Andaman and Nicobar Archipelago, Vertebrate Zoology 72, pp. 169-199 : 169

Minervarya charlesdarwini (Das, 1998) comb. nov.

Charles Darwin’s minervaryan frog Figs 1 , 2 , 3 , 4

Note.

Das (1998) described Rana charlesdarwini based on three adult and five larval specimens from Mount Harriet National Park in the South Andaman Island. Until recently, new vouchered reports of this species were lacking and the species was known only from its type series. Chandramouli (2017) reported rediscovery of this species along with a redescription based on new collections and some old museum specimens reidentified as Ingerana charlesdarwini , while also discussing morphological variations among the studied individuals. However, owing to the confounding taxonomic history and generic placement of this taxon (see Introduction), clarity on the systematics relationships of Rana charlesdarwini remained lacking. Our first ever molecular assessment of this taxon combined with morphological studies based on topotypic collections has confirmed its placement in the dicroglossid genus Minervarya . We further confirmed the prevalence of high morphological variation among individuals of this species, particularly with regard to dorsal colouration, markings, and body size (Figs 3 , 4 ). However, due to the uncertainty of its systematic position, a morphological comparison with relevant taxa has been lacking. Below, we provide a revised morphological diagnosis for the species as well as comparisons with the closely related members of the M. andamanensis species group. We also elaborate on and provide detailed illustrations of the morphological variations observed in our study, considering that this species is likely to have been confused with other dicroglossids found in the regions for several years, before and after its formal description (see taxonomic remarks for M. andamanensis , Limnonectes doriae and L. hascheanus ).

Morphological diagnosis.

Minervarya charlesdarwini can be morphologically diagnosed by the following suite of characters: small to medium-sized adults (male SVL 24.8-30.1 mm, female SVL 30.8-36.6 mm); rather elongate body; dorsal skin shagreened to granular, or with prominently glandular warts; presence or absence of a weakly to well developed interrupted inverse V-shaped ridge (chevron mark) at the centre of dorsum; upper ⅔rd of tympanum and inner margin of tympanic fold dark brown; groin and thighs without prominent reticulations; finger and toe tips rounded with slightly swollen discs, without circum-marginal grooves; foot webbing relatively reduced, up to or just above the second subarticular tubercle but not beyond on either side of toe IV; elongate inner metatarsal tubercles; small and rounded outer metatarsal tubercle; presence of fejervaryan lines on abdomen; vomerine ridge with weakly developed teeth; absence of lingual papilla; omosternum unforked.

Redescription (all measurements in mm).

A small to medium-sized species (males: SVL 24.8-30.1, 26.6 ± 1.8, N =6; females: SVL 30.8-36.6, 33.5 ± 1.9, N =7), body rather elongate; head longer (males: HL 10.0-12.0, 10.6 ± 0.7; females: HL 11.1-13.8, 12.7 ± 1.0, N =7) than wide (males: HW 9.0-10.9, 9.6 ± 0.7; females: HW 10.4-12.4, 11.6 ± 0.7, N =7); snout rounded in dorsal and lateral view; snout length (males: SL 4.2-4.4, 4.3 ± 0.1, N =6; females: SL 4.7-5.7, 5.2 ± 0.4, N =7) longer than horizontal diameter of eye (males: EL 3.1-3.9, 3.4 ± 0.3, N =6; females: EL 3.6-4.5, 4.0 ± 0.3, N =7); loreal region obtuse; canthus rostralis rounded; interorbital space flat; tympanum diameter (males: TYD 1.9-2.3, 2.1 ± 0.2, N =6; females: TYD 2.1-2.8, 2.5 ± 0.3, N =7) nearly ⅗th of the eye diameter (males: EL 3.1-3.9, 3.4 ± 0.3, N =6; females: EL 3.6-4.5, 4.0 ± 0.3, N =7); pineal ocellus present; supratympanic fold well developed, extends from posterior corner of the eye up to nearly the shoulder; vomerine ridge present, bearing small teeth; tongue moderately long, emarginated; 1-4 glands present at labial commissure (Figs 3 , 4 ).

Forearm length (males: FAL 4.8-6.0, 5.3 ± 0.5, N =6; females: FAL 6.0-6.9, 6.6 ± 0.3, N =7) shorter than hand length (males: HAL 6.1-7.8, 6.7 ± 0.6, N =6; females: HAL 7.4-8.7, 8.1 ± 0.6, N =7); subarticular tubercles prominent, single, circular, all present; prepollex oval, prominent; two rounded palmar tubercles; supernumerary tubercles absent; relative length of fingers II < I=IV < III; tip of fingers rounded, not enlarged into discs. Hind limbs short in comparison to the body length with tibiotarsal articulation reaching up to the anterior end of eye when hind limb is stretched along the body; thigh (males: TL 13.1-15.4, 13.6 ± 0.9, N =6; females: TL 16.2-17.0, 16.6 ± 0.4, N =7) shorter than shank (males: SHL 14.1-15.8, 14.5 ± 0.6, N =6; females: SHL 17.5-18.9, 18.0 ± 0.7, N =7) and nearly equal to foot (males: FOL 12.8-15.3, 13.4 ± 1.0, N =6; females: FOL 15.9-17.7, 16.8 ± 0.7, N =7); total foot length (males: TFOL 19.1-22.7, 19.7 ± 1.6, N =6; females: TFOL 23.0-26.1, 24.7 ± 1.2, N =7); toe tips rounded, slightly swollen without discs, toes without dermal fringes, foot webbing moderate: I2--2II 1½ -3-III2--3+IV3--2-V; subarticular tubercles prominent, all present; inner metatarsal tubercle prominent, elongate; outer metatarsal tubercle small, rounded; supernumerary tubercles absent.

Skin of dorsum highly variable from shagreened to prominently granular or with glandular warts; an interrupted inverse V-shaped ridge (chevron mark) at the centre of dorsum weakly to well developed or absent. Ventral surfaces of throat, chest, belly, and limbs smooth; and posterior parts of thigh and region surrounding the vent sparsely granular (Fig. 4 ). Dorsal and ventral skin colouration is extremely variable. Dorsal surface: uniform grey, brownish-grey, yellowish-brown, light to dark brown, blackish-brown, reddish-brown, and occasionally (but not rarely) with a broad median band extending from the upper eyelids or anterior border of eyes to vent, and a thin or broad middorsal line extending from the tip of the snout to vent (Figs 3 , 4 ); presence or absence of reddish-brown or orange colouration on snout, lateral surfaces of dorsum, and fore and hind limbs (Figs 3 , 4 ); presence or absence of dark blackish-brown lining on dorsal tubercles; faint or prominent crossbands on lips; upper ⅔rd of tympanum and inner margin of tympanic fold dark brown; anterior and posterior parts of thighs without prominent reticulations. Ventral surface: throat and chest light to dark brown or dark grey, with light or dark patches; belly yellowish-white, with or without orange tinge; margins of limbs usually with blackish-brown colouration; hand and foot light or dark brown.

Comparison (only with males, N=6).

Minervarya charlesdarwini cannot be confused with other known species of the genus Minervarya , except three members of the M. andamanensis group ( M. andamanensis , M. nicobariensis , and M. muangkanensis ). Minervarya charlesdarwini can be distinguished from M. andamanensis and M. nicobariensis by its relatively smaller adult size, male SVL 24.8-30.1 mm (vs. larger, SVL 36.2-42.2 mm and SVL 40.0-49.8 mm, respectively); and elongate body (vs. stout and robust in both the species). It specifically also differs from M. andamanensis by the absence of forearm tubercles (vs. present); canthus rostralis rounded (vs. indistinct); posterior half of thigh without reticulations, usually brown or orangish-brown (vs. light to dark brown with yellowish reticulations); upper eyelid width nearly equal to inter upper eyelid width, UEW 2.4 ± 0.2 vs. IUE 2.4 ± 0.2 (vs. wider, UEW 3.6 ± 0.3 vs. IUE 2.8 ± 0.1); and thigh nearly equal to foot length, TL 13.6 ± 0.9 vs. FOL 13.4 ± 1.0 (vs. shorter, TL 19.9 ± 1.3 vs. FOL 21.7 ± 1.5). Minervarya charlesdarwini also differs from M. nicobariensis by its head being longer than wide, HL 10.6 ± 0.7 vs. HW 9.6 ± 0.7 (vs. wider, HW 17.5 ± 1.0 vs. HL 16.5 ± 1.1); upper eyelid width nearly equal to inter upper eyelid width, UEW 2.4 ± 0.2 vs. IUE 2.4 ± 0.2 (vs. wider, UEW 4.5 ± 0.4 vs IUE 3.2 ± 0.4); thigh nearly equal to foot length, TL 13.6 ± 0.9 vs. FOL 13.4 ± 1.0 (vs. shorter, TL 20.7 ± 1.4 vs. FOL 22.1 ± 1.5); presence of outer metatarsal (vs. absent); and posterior part of thighs without prominent reticulations, usually brown or orangish-brown (vs. light to dark red with thin black reticulations). Further, M. charlesdarwini differs from M. muangkanensis , a species endemic to Thailand and Myanmar, in possessing a distinct supratympanic fold that extends from posterior corner of upper eyelid, along the upper margin of tympanum, up to the shoulder (vs. indistinct supratympanic fold, and not extending up to posterior corner of upper eyelid and down to the shoulder); webbing between toes relatively reduced, up to the second subarticular tubercle on either side of toe IV (vs. above); and posterior part of thighs without reticulations, usually brown or orangish-brown (vs. light to dark brown with yellowish reticulations).

Distribution.

Minervarya charlesdarwini is endemic to the Andaman Archipelago of India, where we find it to be widely distributed in all the major groups of islands: North and Middle Andamans (North Andaman Is., Interview Is., Middle Andaman Is., Baratang Is., and Long Is.), South Andamans (South Andaman Is., Neil Is., Havelock Is., Boat Is., Red Skin Is., Alexandra Is., Rutland Is., and Tarmugli Is.), up to the Little Andaman Island. This species has been observed between elevations of nearly sea level up to 600 m asl (Fig. 2 ; Table 2 ).

Figures

Figure 1. Phylogenetic position and genetic structure of species in the Minervarya andamanensis species group. A. Maximum likelihood phylogram based on a multi-gene dataset (2,101 bp of mitochondrial and nuclear DNA), depicting the phylogenetic position of Minervarya charlesdarwini comb. nov. in the genus Minervarya and relationships among three members of the M. andamanensis species group. Voucher numbers at terminal nodes are referenced in Table 1. Values above and below the branches represent Ultrafast Bootstrap Support (UFB,> 50 %) and Bayesian Posterior Probabilities (BPP,> 0.50), respectively. Vertical bars indicate the recovery of three M. andamanensis group members as distinct species in multi-gene bPTP species delimitation analysis. B. Median-Joining network based on 513 bp of the mitochondrial 16 S gene depicting the genetic structure among three species. Circle colours indicate different species; circle size is proportional to the frequency of haplotypes; black circles indicate median vectors; values on circles indicate frequency of haplotypes; values on connecting branches indicate number of mutation steps.

Figure 2. Distribution of Minervarya species in the Andaman and Nicobar Archipelago. A Location of the Archipelago in the Bay of Bengal. B Expanded view of the Andaman and Nicobar groups of islands. C Minervarya nicobariensis in the Nicobar Islands. D Minervarya charlesdarwini in the Andaman Islands. E Minervarya andamanensis in the Andaman Islands. F Minervarya agricola in the Andaman Islands.

Figure 3. Morphological variation in skin colouration and markings observed among individuals of Minervarya charlesdarwini in the Andaman Islands. A-N Dorsolateral views. A SDBDU 2021.4212 (♂). B-C SDBDU 2019.4059 (♀). D Not preserved. E SDBDU 2019.4006 (♂). F SDBDU 2019.3975 (♂). G SDBDU 2021.4212 (♂). H SDBDU 2019.4005 (♀). I SDBDU 2019.3968 (♀). J SDBDU 2019.4004 (♂). K SDBDU 2021.4213 (♀). L SDBDU 2020.4165 (♀). M SDBDU 2019.3946 (♀). N SDBDU 2021.4214 (♀). Photographs: S. D. Biju, G. Gokulakrishnan & Sonali Garg.

Figure 4. Morphological variation in skin texture, colouration, and markings observed among individuals of Minervarya charlesdarwini in the Andaman Islands. A-H Dorsal views. A-D Variation in skin texture and enlarged view of dorsal skin (demarcated with white square) above. A SDBDU 2019.4059 (♀). B SDBDU 2020.4162 (♂). C SDBDU 2021.4212 (♂). D SDBDU 2019.3977 (♂). E-G Not preserved (♂). H SDBDU 2019.4004 (♂). I SDBDU 2019.4005 (♀). J Ventral view, SDBDU 2021.4212 (♂). K Lateral view, SDBDU 2021.4218 (♂). L Dorsal view of thigh, SDBDU 2021.4212 (♂). M Posterior view of thigh, SDBDU 2021.4212 (♂). N Ventral view of hand, SDBDU 2021.4212 (♂). O Ventral view of foot, SDBDU 2021.4212 (♂). P Schematic illustration of foot webbing, SDBDU 2021.4212 (♂). Photographs: S. D. Biju.