Minervarya andamanensis (Stoliczka, 1870)

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

Taxonomic identity of Minervarya andamanensis (Stoliczka, 1870)

Figs 1 , 2 , 5 , 6

Note.

This species was originally described as a variety of Rana gracilis var. andamanensis Stoliczka, 1870. The description was based on four specimens-one "about one-third of an inch long" (~9 mm), "two next above one inch" (~ 25 mm), and "the fourth 2⅓rd inches" (~ 60 mm). Of these, ZSIC 3539 (ZSIC 8539 according to Chanda et al. 2001 “2000”) was designated as the lectotype by Annandale (1917). Furthermore, three of the original syntypes-two from the ZSI collection and one in the NHM collection-were suggested to represent two other dicroglossid species (see detailed taxonomic remarks for Limnonectes doriae and L. hascheanus ). Hence, Minervarya andamanensis was restricted to a single juvenile specimen, which we found to be in an extremely dehydrated condition (Fig. 6 ). Since the lectotype is not reliable for identification, much of what is known of this nomen is based on its original description ( Stoliczka 1870) and a subsequently published illustration ( Annandale 1917). Additional specimens were reported by Annandale (1917), and further, based on tentatively identified records its phylogenetic position and relationships have also been discussed ( Kotaki et al. 2010; Sanchez et al. 2018; Garg and Biju 2021). Recently, Chandramouli et al. (2021) provided a redescription of the species based on new collections. In our study, we further report the prevalence of high morphological variations among individuals of this species. Typically, M. andamanensis has been identified based on its chestnut-brown dorsal colouration and dark brown lateral surfaces (e.g., Annandale 1917; Sarkar 1990; Chandramouli 2017). However, we observe that this character is not constant, and several genetically confirmed individuals with uniform colouration and other colour morphs from our study are conspecific (Figs 5 , 6 ). In addition, we find the Little Andaman population to be divergent from that found in South Andamans (see Phylogenetic Results). Hence, in order to aid further studies, below we provide a revised morphological diagnosis for the species, compare it with other closely related members of the M. andamanensis species group, discuss morphological variations accompanied with detailed illustrations, and also shed light on the possibility of this species having been confused with other dicroglossids found in the regions (see taxonomic remarks for M. charlesdarwini , Limnonectes doriae and L. hascheanus ).

Redescription (all measurements in mm).

A medium-sized species (males: SVL 36.2-42.2, 39.2 ± 2.1, N =6; females: SVL 39.4-57.1, 48.6 ± 7.4, N =6), body stout and robust; head longer in males (HL 14.3-17.0, 15.2 ± 1.0 vs. HW 12.4-15.9, 13.9 ± 1.2, N =6) and subequal in females (HL 14.2-20.9, 17.6 ± 3.1, N =6 vs. HW 13.8-21.6, 17.7 ± 2.9, N =6); snout rounded or subovoid in dorsal and ventral view, rounded or obtuse in lateral view; snout length (males: SL 5.6-7.4, 6.4 ± 0.7; females: SL 5.8-8.6, 7.3 ± 1.1) longer than horizontal diameter of eye (males: EL 4.4-5.4, 4.8 ± 0.5; females: EL 4.0-6.6, 5.2 ± 1.0); loreal region obtuse; indistinct canthus rostralis; interorbital space flat; tympanum diameter (males: TYD 2.4-3.3, 2.8 ± 0.3; females: TYD 2.9-4.4, 3.3 ± 0.6) nearly ⅗th of the eye diameter (males: EL 4.4-5.4, 4.8 ± 0.5; females: EL 4.0-6.6, 5.2 ± 1.0); pineal ocellus present; supratympanic fold well developed, extending from the posterior corner of the eye down to nearly the shoulder; vomerine ridge present, bearing small teeth; tongue moderately long, emarginated (Figs 5 , 6 ). Forearm length (males: FAL 7.2-9.1, 8.0 ± 0.8; females: FAL 8.7-11.8, 9.6 ± 1.4) shorter than hand length (males: HAL 8.6-9.8, 9.2 ± 0.5; females: HAL 10.1-13.2, 11.6 ± 1.2); 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 bluntly rounded, not enlarged into discs. Hind limbs shorter 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 18.4-21.9, 19.9 ± 1.3; females: TL 20.9-30.1, 24.8 ± 3.4) shorter than shank (males: SHL 20.1-24.1, 21.7 ± 1.6; females: SHL 24.5-31.3, 27.4 ± 2.8) and foot (males: FOL 20.1-23.7, 21.7 ± 1.5; females: FOL 23.4-29.6, 25.7 ± 2.9); total foot length (males: TFOL 28.1-34.9, 31.3 ± 2.9; females: TFOL 33.0-41.4, 36.5 ± 2.8); toe tips rounded, slightly swollen without discs, toes without dermal fringes, webbing between toes moderate: I1+-2II1+-2III1+-2IV2-1+V; subarticular tubercles prominent, all present; inner metatarsal tubercle prominent, elongate; outer metatarsal tubercle small, prominent, rounded; supernumerary tubercles absent (Figs 5 , 6 ).

Comparison (only with males).

Minervarya andamanensis cannot be confused with other known members of the genus Minervarya , except three members of the M. andamanensis group ( M. charlesdarwini , M. nicobariensis , and M. muangkanensis ). Minervarya andamanensis can be distinguished from M. nicobariensis by its head longer than wide, HL 15.2 ± 1.0 vs. HW 13.9 ± 1.2 (vs. wider, HW 17.5 ± 1.0 vs. HL 16.5 ± 1.1); shank nearly equal to foot, SHL 21.7 ± 1.6 vs. FOL 21.7 ± 1.5 (vs. shorter, SHL 20.9 ± 1.0 vs. FOL 22.1 ± 1.5); presence of outer metatarsal tubercle (vs. absent); and posterior part of thighs light to dark brown with yellowish reticulations (vs. light to dark red with thin black reticulations). Further, it differs from M. muangkanensis (based on Köhler et al. 2019) in having a larger adult male size, SVL 36.2-42.2, 39.2 ± 2.1, N =6 (vs. SVL 25.8-35.1, 31.2 ± 3.1, N =7); supratympanic fold distinct, extending from posterior corner of upper eyelid, along upper margin of tympanum, down to the shoulder (vs. indistinct supratympanic fold and not down to the shoulder); webbing between the toes relatively reduced, up to the second subarticular tubercle on either side of toe IV (vs. above). For comparison with M. charlesdarwini , see the comparison section of that species.

Distribution.

Minervarya andamanensis 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., Landfall Is., East Is., Paget Is., Interview Is., Smith Is., Long Is., North Passage Is., North Reef Is., Baratang Is., and Middle Andaman Is.), South Andamans (South Andaman Is., Boat Is., Alexandra Is., Tarmugli Is., Rutland Is., Neil Is., and Havelock Is.), down to the Little Andaman Island. This species has been observed between elevations of sea level up to nearly 400 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 5. Morphological variation in skin colouration and markings observed among individuals of Minervarya andamanensis. A-J Dorsolateral views. A SDBDU 2021.4206 (♀). B SDBDU 2021.4207 (♀). C-D Not preserved (♂). E SDBDU 2020.4179 (♀). F SDBDU 2010.4178 a (♀). G SDBDU 2019.4011 (♂). H SDBDU 2020.4155 (♂). I Not preserved (♂). J SDBDU 2019.3956 (♂). Photographs: S. D. Biju, G. Gokulakrishnan & Sonali Garg.

Figure 6. Morphological variation observed among individuals of Minervarya andamanensis. A Lectotype (ZSIC 3539 / ZSIC 8539). B-H Dorsal views. B SDBDU 2020.4181 (♀). C SDBDU 2020.4154 (♂). D SDBDU 2021.4207 (♀). E SDBDU 2020.4171 a (♀). F SDBDU 2020.4179 (♀). G SDBDU 2020.4155 (♂). H SDBDU 2020.4180 (♂). I-K Ventral views. I SDBDU 2020.4180 (♂). J SDBDU 2020.4154 (♂). K SDBDU 2020.4171 a (♀). L Lateral view, SDBDU 2021.4291 (♀). M-N Ventral view of hand, SDBDU 2021.4207 and SDBDU 2000.4179, respectively). O Schematic illustration of foot webbing, SDBDU 2021.4207 (♀). P-Q Ventral view of foot, SDBDU 2020.4179 and SDBDU 2001.4207, respectively. R Posterior view of thigh, SDBDU 2020.4179 (♀). S. Dorsal view of thigh, SDBDU 2021.4207 (♀). Photographs: S. D. Biju.