Description
Diagnosis: Limnonectes megastomias can be distinguished by the following combination of characters: (1) body size: adult male SVL 40.0–123.7 mm, adult female SVL 53.5–86.3 mm; (2) tympanum not visible; (3) prominent supratympanic fold from posterior superior corner of eye to angle of jaw; (4) lack of vocal sac and vocal slits in males, but advertisement call is made; (5) nuptial pads on Fingers I and II in males; (6) "fangs" in males consisting of thick, elongate odontoid processes; females have reduced odontoid processes; (7) males have enlarged heads (HL 41–56% of SVL; 39–45% in females); (8) sexually dimorphic pigmentation: males have heavily pigmented (mottled) throat, venter, and borders of thigh and leg; females have lightly pigmented throat and moderately pigmented venter and borders of thigh and leg; (9) small, low glandular warts tipped with translucent horny spinules present on flanks of body, on dorsal surfaces of shanks and feet, and around the vent; dorsum moderately rugose except for regions with warts; venter smooth; (10) Finger II longer than Finger I when adpressed; (11) fingers unwebbed, toes fully webbed; (12) larval labial tooth row formula: 2(2)/3(1); (13) narrow gap in ventral row of oral papillae; (14) eggs pigmented; clutches are large; no parental care is given (McLeod 2008).

Description: Limnonectes megastomias is a robust, very large-headed fanged frog. Adult males range from 40-123.7 mm SVL and the females measure 53.5 – 86.3 mm SVL. The head is somewhat longer than wide, and males have larger heads (41-56% of SVL) than females (39-45% of SVL). Males also have thick, elongated odontoid processes, which act as “fangs.” These same processes are present in females but are reduced in size. Males have nuptial pads, consisting of tiny spines on the medial surface of Finger I and the dorsomedial surface of Finger II, above the fringe on the preaxial side. The snout is rounded when viewed from above, obtuse when viewed in profile, and protrudes beyond the lower jaw. The canthus rostralis is rounded and the loreal region is concave. The upper lip is swollen and flares out, extending as far as the post-rictal tubercle. Eyes have a diameter 15% of the head length, and the upper eyelid width is 63% of the interorbital distance. The pupil has a diamond shape. The tympanum is not visible. A moderate supratympanic fold is present, extending from the angle of the jaw to the eye. Vomerine teeth are present on oblique ridges. Choanae are ovoid. A symphysial knob is present. Tongue is oval and has deep notches at the posterior. Fingers have rounded tips that are not expanded but do have a rounded distal pad. Relative finger length is III>IV>II>I. No webbing is present on fingers, but Fingers II and III have a distinct, movable fringe along the pre-axial and post-axial edges, and Finger IV has indistinct fringing. Toes are fully webbed (to the middle of the terminal phalanx) and toe tips are rounded with elevated toe pads, but are not expanded. Relative toe length is IV>III>V>II>I. Toe V has a movable flap of skin on the postaxial side, while Toe I has a movable flap of skin on the preaxial side, continuing as a weak tarsal fold. The inner metatarsal tubercle is an elongated oval, with an elevated border on the postaxial side (outer metatarsal tubercle not mentioned). Skin on all but the venter is moderately to distinctly rugose, with the dorsal surfaces of the lower arm, shank, and foot, and the area around the vent covered with small glandular warts having pearl-colored tips. The venter is smooth. A transverse fold runs between the posterior margins of the orbits (McLeod 2008).

In life L. megastomias has an olive-brown colored dorsum with a dark brown blotch on the shoulders and indistinct, dark transverse bars on the upper hind limbs. The sides of the head are yellowish brown and the chin is a brownish white. A dark brown interorbital band is bordered by yellowish brown. Nuptial pads are white. The iris is brown above and gold below, separated by a horizontal band of dark brown. In preserved specimens, the color is similar to the color in life except that the yellow brown coloration fades to a gray-brown (McLeod 2008).

The tadpoles of L. megastomias are oval in shape, with large eyes and a tail 2x the body length. The body is slightly depressed. The tailfin is slightly higher than the body, with the dorsal fin beginning just behind the caudal musculature's origin, and reaching a maximum height just after mid-length, tapering gradually for most of the length, then abruptly narrowing to a thin, rounded tip. Eyes are positioned dorsolaterally and oriented laterally. The nares are located between the tip of the shout and the eyes, and lack raised rims. The oral disc is located ventrally, near the anterior end of the body; LTRF is 2(2)/3(1), with papillae above the mouth in a single row at the corners of the mouth, and papillae below the mouth in a single staggered row with a narrow median gap. Jaw sheaths are serrated and marginally black. The spiracle is closer to the eye than to the terminus of the body, and set midway up the side of the body. The end of the spiracle is not attached to the body wall (McLeod 2008).

In life the tadpole body, caudal muscle, and dorsal fin are brownish gold with faint pigmented mottling on the dorsal surface, while the ventral fin is mottled and spotted only in the distal half and lacks brown pigment in the proximal half. The venter lacks pigment. In preservation the tadpole’s color fades to a pale yellowish white (McLeod 2008).

Similar species: L. megastomias can be distinguished from L. kuhlii (Javanese specimens) by the presence of nuptial pads in males (absent in L. kuhlii), a longer snout-vent length of up to 123.7 mm in males and 86.3 mm in females, and different relative finger lengths. L. megastomias can be distinguished from L. laticeps by having more extensively webbed toes, nuptial pad arrangement, and a longer snout-vent length. L. megastomias can be distinguished from the similar species L. namiyei (Japan) by the absence of vocal slits in males (vs. present in L. namiyei males) (McLeod 2008).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Thailand

View distribution map in BerkeleyMapper.

Limonectes megastomias is an aquatic frog found in three regions of eastern Thailand near the Khorat Basin: Pang Si Da National Park (90-600 m asl), where the habitat consists of bamboo and evergreen forest; Phu Luang Wildlife Sanctuary (850-1460 m asl), with habitat consisting of a mix of hill evergreen, bamboo mixed with evergreen, and rhododendron forests; and Kok Nok Kraba summit (1460 m asl), with a cool and foggy forest habitat (McLeod 2008).

Life History, Abundance, Activity, and Special Behaviors
The function of the enlarged head in males is likely to be associated with male-male combat, as it is in other species of Limnonectes. Males will defend their territories in order to attract females to areas of optimal mating. Though no male-male competition has been observed, specimens with missing limbs and digits, fresh bite marks, and scars have been collected indicating predation attempts or conspecific combat (McLeod 2008).

Females deposit their eggs in the water and there is no evidence of parental care (McLeod 2008).

Possible reasons for amphibian decline

General habitat alteration and loss
Habitat modification from deforestation, or logging related activities
Intentional mortality (over-harvesting, pet trade or collecting)
Climate change, increased UVB or increased sensitivity to it, etc.

Comments
Species authority: McLeod (2008).

References

McLeod, D. (2008). ''A new species of big-headed, fanged dicroglossine frog (genus Limnonectes) from Thailand.'' Zootaxa, 1807, 26-46.

Species Account Citation: AmphibiaWeb 2012 Limnonectes megastomias: Khorat big-mouthed frog <https://amphibiaweb.org/species/7148> University of California, Berkeley, CA, USA. Accessed Nov 12, 2024.

Citation: AmphibiaWeb. 2024. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 12 Nov 2024.

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