Minervarya charlesdarwini (Das)

Gopika, C., Garg, Sonali, Sivaperuman, Chandrakasan, Gokulakrishnan, G. & Biju, S. D., 2023, Larval morphology and natural history in two Minervarya species from Andaman Islands, with comments on a new phytotelmonous larval morphotype for the genus, Zootaxa 5353 (6), pp. 567-581 : 572-575

Minervarya charlesdarwini (Das)

Breeding and early development ( Fig. 3 ). As a prolonged breeder, Minervarya charlesdarwini continues breeding activities for nearly two months during the monsoon season (June to July). Males emit advertisement calls from water-filled tree holes up to four feet high or rarely from the ground close to tree holes. Calling activity takes place from 18:00 hrs until 02:00 hrs. Calling males inflate a single externally visible subgular vocal sac ( Fig. 3A ). The vocal sac is greyish-white in colour. Female approaches a calling male, and male mounts the female in axillary amplexus ( Fig. 3B ).

Eggs are laid on inner walls of tree holes, just above the surface of the water column. Clutch size varies from 30– 60 eggs. Freshly laid eggs are half creamy-white and half light-brown to black. Individual eggs are covered with a jelly cover and measure 3.4 ± 0.3 mm in diameter ( Fig. 3C,D ). Embryonic development was not studied in detail; however, cleavage started within an hour of spawning ( Fig. 3D ). Further organogenesis was completed within two days ( Fig. 3E ). Subsequently, in about 20 hours the jelly layers of individual eggs dissolved to form a single jelly mass containing the developing embryos ( Fig. 3F ). Within two to three days, young hatchlings dropped into the water below and completed further development ( Fig. 3G–J ).

Larval morphology (Gosner Stage 36, n = 1; Fig. 4 ). In dorsal view, body is broader than tail (TMW 42% of BW), elliptical-shaped, and has maximum body width at the gill region. The body is depressed with body height 85% of body width. Body length constitutes 29% of total length. Snout shape in the lateral view is roughly rounded from plane of the eyes to the base of upper labium; a slight bulge is present on the dorsal surface of nares. A steep slope runs along the upper labium that ends in the dorsal gap of the oral disc. The narial opening is dorsolaterally oriented, moderately sized, round in shape, and the margin has a prominent ridge on the dorsal surface. The nares are closer to snout than the eyes. The eyes are of moderate size and dorsolaterally positioned. Internarial distance is 61% of the interorbital distance. The spiracle opens sinistrally at the level of the hindlimbs and is located near the mid-point of the body (SS 70% of BL). The spiracular aperture is of moderate size and opens in the dorsoposterior direction. The inner wall of spiracular aperture is fused with the body wall through a small ridge. The vent tube is tubular and opens dextrally at the base of the lower tail fin. The dorsal wall of vent tube is medially attached to the lower fin. Tail is long (TAL 71% of TL). Tail musculature is well developed (TMH 74% of MTH and 68% of BH and TMW 42% of BW). Upper fin originates before the body-tail junction, and lower fin starts near the ventral terminus of body. The upper and lower fins gradually increase in height, reach maximum height at the anterior one-third of the tail, and narrow into the tail tip. The tail muscle shows maximum height at the body-tail junction, gradually tapering and ending before the tail tip. Upper fin is nearly twice the height of lower fin (UFH 25% of MTH and LFH 13% of MTH). The fins terminate into a broadly rounded tail tip ( Fig. 4A–L ).

The oral disc is positioned terminally on the anterior tip of the body, frontally oriented, and completely visible only in the frontal view. The oral disc is elliptical in shape horizontally, and relatively small in size (ODW 36% BW). The upper and lower labia are of similar size, and lateral emarginations are not evident. The margin of upper labium is demarcated by one to two irregular rows of medium-sized, blunt marginal papillae laterally, with a wide dorsal gap in the middle. The lower labium has an irregular row of medium-sized, short marginal papillae without gap. A few submarginal papillae are present near the commissures. The labial tooth row formula is 1/1. P1 is longer than A1 and labial tooth ridges are indistinct. Labial teeth are feebly keratinised and have a curved spatulate apical tip with three to four marginal cusps. Jaw sheaths are well keratinised and the lower sheath is larger than upper sheath. The medial portion of the upper sheath is broadly arched, while that of the lower sheath is U-shaped. Sheaths have long, acutely pointed, and narrow-based serrations. Serrations on the medial portion of the lower sheath are longer than those on the lateral ends, and also longer than those on the upper sheath ( Fig. 4M–P ).

Colour in life. The overall colour of the tadpole is beige with a darker shade throughout the body and tail muscle. Dark melanocytes are dusted throughout the body, except on the posterior half of the ventral body surface that is mostly unpigmented. In the tail region, dark melanocytes cover the tail muscle and adjacent parts of the tail fin. Dark pigments are also present on lateral surfaces of the limb buds. The gills are visible through the cheeks (posteroventral to the eyes) and ventral surface of the body. The gut coil is visible in the ventral view. The dorsal and lateral body surfaces are studded with golden iridocytes, especially concentrated on the lateral abdominal regions. A few golden iridocytes are dusted on the anterior part of the tail, and the lateral surfaces of the limb buds and spiracle. Myosepta are not distinct in the tail musculature. Eyes are black in colour with a few golden iridocytes scattered in the sclera. The oral disc is unpigmented.

Measurements (in mm). TL 21.15, BL 6.15, SU 5.72, BH 3.35, BW 3.94, IOD 1.85, NE 0.92, SN 0.53, IND 1.12, SS 4.29, TAL 15.0, MTH 3.58, TMH 2.2, TMW 1.64, ODW 1.41, LFH 0.48, UFH 0.88. Additional morphometric measurements for different stages of tadpoles are provided in Table 2 .

Ontogenetic variation. At Stage 24, keratinised structures are already formed, jaw sheath are well developed, while only a few labial teeth are randomly formed. At Stage 26, both the tooth rows and the jaw sheaths are well developed and remain intact until Stage 40. At Stage 41, the tooth rows start degenerating. All the keratinised structures are lost by Stage 43; however, the margin of the mouth is still partially outlined by the degenerating papillae. Occasionally, two to three scattered labial teeth are seen laterally on the upper jaw sheath.

Comparative larval morphology. The tadpoles of Minervarya andamanensis closely resemble those of other previously studied congeners, such as M. keralensis (Annandale 1918) , M. nicobariensis ( Chandramouli & Prasad 2020) , and M. syhadrensis ( Khan 1996; 2003), that also have pond-dwelling tadpoles. Their morphological similarities include an elliptical body shape, dorsoventrally depressed body, dorsolaterally positioned eyes, sinistral spiracle, anteroventrally positioned oral disc, wide medial gaps in the marginal papillae along the upper labium, keratinised labial teeth, serrated jaw sheaths with upper sheath broadly arched and lower sheath V-shaped, and LTRF 2(1-2)/2-3(1-2). In addition, we find the labial teeth of M. andamanensis to have a curved and cusped apical tip that was earlier reported in M. syhadrensis tadpoles ( Khan 1996). A further comparison of larval characters with other minervaryan species tadpoles was not possible due to the availability of limited information in previous literature.

The phytotelmonous tadpoles of Minervarya charlesdarwini share some morphological similarities with the pond-dwelling tadpoles of other congeners, such as an elliptical-shaped and dorsoventrally depressed body, dorsolaterally positioned eyes, and sinistral spiracle. In comparison to M. andamanensis , M. charlesdarwini tadpoles have additional similarities such as moderately high upper and lower fins, moderately developed tail musculature, and labial teeth with a curved and cusped apical tip. However, the phytotelmonous tadpoles of M. charlesdarwini also differ considerably from the pond-dwelling tadpoles of M. andamanensis and the three previously studied species. The most obvious differences are in the placement and morphology of the oral disc. In M. charlesdarwini , the oral disc is terminal (vs. anteroventral in all others), and the tooth rows are reduced to a single, undivided row (vs. LTRF 2(1-2)/2-3(1-2) in others). The jaw sheaths in M. charlesdarwini are also unique in possessing long, acutely pointed, and narrow-based serrations (vs. short and broad-based serrations in all others), and the medial region of the upper jaw sheath arched and that of the lower jaw sheath broadly U-shaped (vs. arched upper sheath and V-shaped lower sheath, in all others). The tadpoles of M. charlesdarwini further differ from those of M. andamanensis in having a higher tail-to-body ratio and more feebly keratinised labial teeth. In terms of the marginal papillae, although M. charlesdarwini tadpoles possess the wide dorsal gap present in M. andamanensis tadpoles, the narrow ventral gap, as observed in M. andamanensis is absent. Further, M. andamanensis tadpoles possess smaller-sized marginal and submarginal papillae in the oral disc, and in greater numbers, while those of M. charlesdarwini have comparatively larger-sized papillae in lesser numbers. In conclusion, our study found the larval morphology of pond-dwelling M. andamanensis to be more congruent with previously published pond-dwelling tadpoles of M. nicobariensis , M. keralensis , and M. syhadrensis . At the same time it is evident from our study that the phytotelm-inhabiting tadpoles of M. charlesdarwini are morphologically distinct from those of all other previously known congeneric species, as well as from the tadpoles of M. andamanensis despite the two being sister species, owing to their strikingly different habitat associations.

Figures

FIGURE 3. Breeding sequence and development in Minervarya charlesdarwini. A. Calling male with a single subgular vocal sac; B. amplectant pair; C. eggs adhered to the inner vertical walls of the tree hole, above the water surface; D. freshly laid pigmented eggs after one hour; E. neurula stage after 40 hours; F. Gosner Stage 20 embryos after 50 hours covered in dissolved jelly layers; G. tadpoles in a water-filled tree hole cavity (Gosner Stage 31); H. lateral view of a tadpole at Gosner Stage 31; I. dorsal view of a tadpole at Gosner Stage 42; J. a newly metamorphosed froglet outside water. Photographs: SDB and GG.

FIGURE 4. Tadpole of Minervarya charlesdarwini, in life. Gosner Stage 33 (SDBDU 2021.4194): A. lateral view; B. dorsal view; C. ventral view. Gosner Stage 36 (SDBDU 2019.4056): D. lateral view; E. pigmentation on lateral surface of body; F. pigmentation on lateral surface of tail; G. dorsal view of body; H. ventral view of body. Gosner Stage 31 (SDBDU 2020.4166): I. lateral view; J. dorsal view; K. ventral view; L. enlarged lateral view of anterior body; M. oral disc in anteroventral view; N, oral disc in frontal view. Gosner Stage 36 (SDBDU 2019.4056): O. oral disc in frontal view (in preservation); P. illustration of mouthparts. Photographs: SDB and SG.