Blyth's River Frog

Limnonectes blythii is a large stream frog with individuals having a snout vent length of 230 – 260 mm (Boulenger 1920; Taylor 1962). The oval head of L. blythii is narrow, longer than it is wide. The nostrils are located closer to the snout tip than eyes and the canthus rostralis is indistinct. The loreal region is slightly concave and oblique. The interorbital distance is shorter than the upper eyelid length. The distinct, sunken tympanum is set back from the eye at a greater distance than the tympanum diameter (Boulenger 1920; Taylor 1962). The tympanum is 2/5-3/5 the size of the eye (Boulenger 1920). This species has a supratympanic fold that initiates at the anterior edge of the eye margin, curves around the tympanum, and terminates above the arm near the anterior edge of the mouth (Taylor 1962). The jaw is slightly countersunk. The limbs are moderately long. The palms have three tubercles with the two flat outer tubercles being somewhat fused together. The subarticular tubercles are moderately large. The fingers are slightly swollen at the tips. The first finger is much longer than the second and the third finger has a distinct lateral skin fold (Boulenger 1920; Taylor 1962). The tibia is between 3 and 4 times as long as it is broad (Boulenger 1920). When adpressed to the body, the tibiotarsal articulation reaches the nostrils. The inner metatarsal tubercle is elongated, compressed, and more than half the length of the first toe. There is no outer metatarsal tubercle. The toes have large subarticular tubercles and complete webbing – with the fifth toe having a distinct lateral skin fold. The tips of the toes are dilated (Taylor 1962). The skin of L. blythii appears generally smooth but upon close inspection may appear to have micro-corrugations, warts, or tubercles (Boulenger 1920; Taylor 1962). Males and females are sexually dimorphic. Males lack vocal sacs, nuptial pads, and forearm flexors (Emerson 1996). Larvae have tails that are at least twice as long as the body and end in an acute point (Boulenger 1920). Members of genus Limnonectes can be easily distinguished by disproportionately large heads and the presence of “fangs”: odontoid processes that protrude upwards from the dentary (Emerson and Inger 1992). As with other Limnonectes species, fangs of L. blythii are larger in males and reduced in females. The eyes are smaller relative to similar Limnonectes species. Males generally have hypertrophied jaw muscles and are also distinguished by having no vocal sac (Boulenger 1920). More specifically, L. blythii can be differentiate from L. macrodon by the former having a head that is longer than wide and is narrower than its body, and a broad, light-colored median band. The lack of vocal slits and large size in L. blythii differentiates it from L. hascheanus . A distinct tympanum differentiates L. blythii from L. kuhlii and L. laticeps (Taylor 1962). In life, this species may show considerable color variation, especially when considering that L. blythii is a complex of Limnonectes sp. across its considerable range. That said, forma typica for L. blythii are brown, blackish-brown, or olive brown above, sometimes rusty or reddish, and often with a cream-colored stripe that extends from the snout to the coccyx. Some specimens, namely those without the ventral bar, may also have a light-colored band or chevron between the eyes . The upper and lower lip has gray or black spots, with those on the lower lip being bordered by yellow or cream color. Limbs are brown with usually prominent reticulations, shadowy bars, or dark banding, though may be uniformly brown in some. Dark spotting or mottling may be present on the anterior of the hind limbs, especially in specimens that are lighter and more uniform in color, or those lacking bars across the legs. The cream or yellow color on the venter extends the length of the body, and shadowy lavender blotches may be present on the breast, matching the shadowy lavender coloration that is always present on the underside of the palms and feet (Boulenger 1920; Taylor 1962).Live juveniles have two distinct black bars or spots running from the bottom of the eye to the lower margin on the lip (Boulenger 1920; Taylor 1962). Preserved specimens are mostly uniform brown with banding across the legs. The chin, breast, and venter are smoky or dirty white in coloration, with bars or spots on the lip retaining their prominence. The shadowy lavender color on the underside of hands and feet is retained in preserved specimens (Taylor 1962)Larvae are reddish-brown. Its beak is black along the margins with several rows of upper and lower labial teeth. The eye is bisected by a dark line and banding may appear to radiate from the eye. The tail is also banded, allowing the larvae to remain conspicuous against the substrate background (Boulenger 1920).There are variations in the coloration and patterning with some having faint bands on their hind limbs (Taylor 1962). Additionally, the species exhibits sexual dimorphism. Males lack vocal sacs, nuptial pads, and forearm flexors (Emerson 1996). As with other Limnonectes species, fangs of L. blythii are larger in males and reduced in females. Males also generally have hypertrophied jaw muscles (Boulenger 1920).The species authority is: Boulenger, G. A. (1920). “A monograph of the South Asian, Papuan, Melanesian and Australian frogs of the genus Rana”. Records of the Indian Museum 20: 1–226.This species definitively represents a complex of several frogs in genus Limnonectes and has been historically synonymous with L. macrodon (Boulenger 1920; Emerson et al. 2000; Inger 1966; van Dijk and Iskandar 2004). Maximum Parsimony, Maximum Likelihood, and Bayesian methods of phylogenetic analysis on mitochondrial DNA sequences indicate multiple colonization and diversification events in the islands of the Philippines and Wallacea, resulting in the species that is known as L. blythii (as of 2019) to in fact be a complex of multiple undescribed species (Evans et al. 2003). As cryptic species are likely present, further phylogenetic research is needed to both clarify the taxonomy of this species and enable formal assessments of its conservation status. Phylogenetic studies suggests species demarcation for (at least) populations in Vietnam, west and east peninsular Malaysia, Thailand, Sumatra, and Borneo respectively (Emerson and Ward 1998; Emerson et al. 2000).Limnonectes blythii was originally described by Boulenger (1920), as Rana macrodon (Var. blythii). Zoologists of the time noticed R. macrodon had two forms: a broad-headed, and a narrow-headed variety. Zoologist and curator of the Asiatic Society Museum of India, Edward Blyth, called the narrow-headed variety Rana fusca in 1855: a moniker that Boulenger nullified and replaced with R. macrodon Var. blythii (later Limnonectes blythii). Dubois (1986) is responsible for the recognition of over 48 species of fanged Ranid frogs that would serve as a monophyletic group (Emerson 1996). They were subsequently removed from genus Rana and placed in genus Limnonectes. To elucidate the hormonal mechanisms for the unique male secondary sex characteristics (voicelessness, lack of nuptial pads or vocal sac, and parental care) among Limnonectes, Sharon Emerson compared androgen levels in several species, including L. blythii. Interestingly, male L. blythii had markedly lower (< 30 - 50%) testosterone and dihydrotestosterone (DHT) levels than other frogs (Emerson et al. 1993). When she then treated breeding male L. blythii with DHT, thumb pad size and thickness significantly increased – reverting to the ancestral condition (Emerson 1996).

Limnonectes blythii occurs throughout mainland Southeast Asia including: Myanmar, Thailand, Laos, Vietnam, and Malaysia. It is native to Indonesia (Sumatra, Java, Natuna Islands, Anambas Archipelago, Lombok, Flores, and Borneo) and Singapore (Boulenger 1920; van Dijk and Iskandar 2004). It occurs from sea level to 1,200m asl in rainforest streams, especially in areas abundant in gravel and boulder substrates (Emerson and Boyd 1999; van Dijk and Iskandar 2004). Adults are often found perched on rocks in the stream, just above the water’s edge (Taylor 1962).

Male L. blythii show notable secondary sex characteristics including the lack of a vocal sac. They are “voiceless”, lacking advertisement calls all together (Emerson 1992; Emerson 1993; Emerson 1996). Most interestingly, females of this species do exhibit advertisement calls to solicit male initiation of amplexus. Males are extremely territorial and it has been posited that the function of the female call is to signal that a frog entering a male’s territory is a gravid female rather than a male opponent (Emerson and Boyd 1999). These frogs are stream obligates, and breeding occurs year round (Inger and Bacon 1968). Males construct cup-like nests in gravel bars along shallow parts of the stream and aggressively fight to defend territories (Emerson and Inger 1992). Females choose a resident male or nest site, and subsequent to egg deposition males attend the nest and defend the site against invaders (Emerson and Inger 1992). Larval L. blythii mature in calm, still side branches or pools with sandy substrate proximate to egg deposition sites on gravel bars (Boulenger 1920; Emerson and Inger 1992). Limnonectes blythii are opportunistic feeders and prey include various arthropods, as well as other frogs (Emerson and Inger 1992). Although sexually dimorphic, there appears to be no specialization between the sexes on prey size, type, or proportion of prey types in the overall diet of L. blythii (Emerson 1994). Several distinct antipredator behaviors have been recorded in L. blythii from Malaysia. When attempting to capture L. blythii along streams, investigators repeatedly noted a feigning death behavior, whereby the frog would invert, lying motionless on its back with eyes open, its hind limbs tucked-in against the body and forelimbs raised above the body. When attempting to flee, L. blythii also exhibits a crouching behavior similar to water conservation posture, wherein a threatened animal will leap away from the risk, take refuge under leaf litter, and assume the motionless crouching posture with arms and legs tucked in close to the body and the head, chin, and venter pressed against the substrate. Finally, when prodded with a stick or other implement, investigators have recorded an elevated-inflation defensive behavior. Rather than fleeing, some threatened L. blythii will maintain an elevated posture with head and snout pointed upwards, the raised body inflated and not making contact with the substrate, and arms fully erect. Where this defensive behavior was observed, the elevated-inflation posture was also always coupled with a defensive call (Shahrudin 2016).

As a result of habitat loss and harvesting for human consumption, this species is listed as “Near Threatened” on the IUCN Redlist of Threatened species. The most recent IUCN assessment estimated this species was in significant decline; however, rates were less than 30% in the last decade (van Dijk and Iskandar 2004).

This species is widely harvested for consumption (van Dijk and Iskandar 2004).

Blyth's river frog (Limnonectes blythii), also known as Blyth's frog, giant Asian river frog or (ambiguously) giant frog, is a species of frog in the family Dicroglossidae found from Myanmar through western Thailand and the Malay Peninsula (Malaysia, Singapore) to Sumatra and Borneo (Indonesia). Earlier records from Laos and Vietnam are considered misidentifications.