The genus Boulenophrys is one of the most diverse and taxonomically tangled groups within the subfamily Megophryinae, with more than 80 recognized species distributed across southern China and northern mainland Southeast Asia. Most of these frogs inhabit montane forest streams with thick topsoil and well-preserved riparian vegetation. Until this discovery, only a single species, Boulenophrys liboensis from the limestone forests and cave systems of Guizhou Province, had been confirmed as a karst dweller. That precedent hinted that other members of the genus might also colonize the harsh, calcium-rich substrates of karst country, but northern Guangdong — despite containing extensive karst formations — had yielded no such records. Given that Guangdong is recognized as one of the centers of Boulenophrys diversity, the absence of records from its karst region stood out as a conspicuous gap.

To close that gap, researchers conducted herpetological surveys in Ruyuan County, in the northern part of the province. They collected seven adult specimens of Boulenophrys: two adult males and five adult females, all taken along roadside areas after rainfall at elevations between roughly 540 and 690 meters. The specimens were fixed in 10 percent buffered formalin and later transferred to 70 percent ethanol, and they are now deposited at three institutions: the Guangdong Polytechnic of Environmental Protection Engineering in Foshan, the Museum of Biology at Sun Yat-sen University in Guangzhou, and the Herpetological Museum at the Chengdu Institute of Biology of the Chinese Academy of Sciences.

The team approached the question of the frogs’ identity with an integrative taxonomy framework, combining detailed morphological measurement with molecular phylogenetics. Externally, every specimen was measured with digital calipers to the nearest 0.1 millimeter, capturing standard amphibian dimensions such as snout-vent length, head length and width, snout length, internasal and interorbital distances, eye diameter, tympanum diameter, hand length, forearm length, foot length and tibiofibula length. Sex was initially determined by the presence or absence of nuptial pads — rough pads on the fingers of breeding males — and then confirmed anatomically by examining testes in males and ovaries and eggs in females. For the comparative groundwork, the authors compiled morphological characters for all 84 recognized species of the genus from the published literature.

On the genetic side, muscle tissue samples from all seven adults were sequenced for two partial mitochondrial genes: 16S ribosomal RNA and cytochrome c oxidase 1, commonly known as COI. These new sequences were analyzed alongside data downloaded from GenBank, for a total of 96 samples, with two species of the related genus Xenophrys serving as outgroups. Sequences were aligned with Clustal X 2.0, and PartitionFinder was used to determine the optimal partitioning schemes — partitioning by gene for 16S and by codon position for COI — with GTR+I+G identified as the best-fit nucleotide substitution model for all partitions. Phylogenetic trees were then built using two independent methods: Maximum Likelihood implemented in RaxmlGUI v.1.3 with branch support evaluated through 1000 rapid bootstrapping replicates, and Bayesian Inference in MrBayes v.3.2.4, run for 10 million generations with two independent runs and the first 25 percent of samples discarded as burn-in. Convergence was confirmed by checking that the average standard deviation of split frequencies fell below 0.01 and that the potential scale reduction factor approached 1.00. Nodes with bootstrap values of at least 70 and Bayesian posterior probabilities of at least 0.90 were considered well supported.

The results were unambiguous. Both analytical methods recovered highly congruent topologies, and all seven samples from the Ruyuan population formed a strongly supported monophyletic lineage, with maximum bootstrap support of 100 and a posterior probability of 1.00. This lineage clustered with B. yangmingensis, B. shimentaina, B. jiulianensis and B. gutu, although the precise relationships among these taxa remained unresolved. Crucially, the minimum genetic distance between the new lineage and other Boulenophrys species — 2.3 percent for 16S and 7.7 percent for COI, measured against B. changyangensis — exceeds the divergence observed between certain already-recognized species pairs. Combined with a suite of stable morphological differences, the evidence pointed to a distinct, previously undescribed species.

Morphologically, Boulenophrys calciphila is a frog of moderate size, with adult males measuring 49.7 to 52.7 millimeters in snout-vent length and adult females 56.3 to 59.2 millimeters. It has a rounded snout, a well-developed canthus rostralis, a distinct tympanum, vomerine ridges but no vomerine teeth, and smooth dorsal skin bearing sparse granules, a discontinuous X-shaped ridge on the center of the back, and discontinuous dorsolateral ridges. A prominent horn-like tubercle projects from the outer margin of each upper eyelid — a hallmark of the horned toads. In life, the dorsal surface is yellowish-brown to dark brown with irregular dark patches and small orange dots, a dark V-shaped marking between the eyes, and dark transverse bands on the limbs and digits. Notably, males lack subgular vocal sacs, a trait that alone separates the new species from the vast majority of its congeners.

The closest look-alike is B. mirabilis, but the new species can be readily differentiated by its smaller body size, the presence of vomerine ridges, a broad and rounded rather than slender and elongated tongue, a tibiotarsal articulation that reaches only the posterior corner of the eye when the leg is stretched forward, the absence of lateral fringes on the toes, and the presence of dorsolateral ridges. Compared with the karst specialist B. liboensis, the new species has a tongue that is not notched distally, lacks vomerine teeth, and lacks lateral fringes on the toes. Against the remaining congeners, the absence of vocal sacs in males — shared only with B. caudoprocta and B. shuichengensis, from which it differs by lacking vomerine teeth and toe fringes — provides a clean diagnostic separation.

The species’ natural history appears tightly bound to its limestone home. All known specimens come from two isolated karst mountain areas in Daqiao Town, Ruyuan County, and every individual was collected along roadsides after rainfall. Males collected in May showed absent or underdeveloped nuptial pads, while mature eggs were observed in all females collected in April and May, offering preliminary clues about the breeding season. Sexual dimorphism is evident: males are smaller and darker than females. The authors note a caveat, however — only two adult male vouchers were obtained, compared with five females, which limits how thoroughly intraspecific variation and sexual dimorphism can be quantified. They recommend targeted field sampling across the karst habitats of northern Guangdong and adjacent southern Hunan Province to gather more males and strengthen analyses of morphological variation and reproductive traits.

Beyond its taxonomic novelty, the discovery carries a broader message about karst ecosystems and their vulnerability. Karst landscapes across southern China are critical hotspots of amphibian and reptile endemism, and multiple new frog species have been described from these habitats in recent years, suggesting that structurally complex karst landforms facilitate species diversification and the aggregation of unique assemblages. Yet karst environments are inherently fragile: shallow soil cover, discontinuous surface drainage, limited permanent water and specialized hydrological systems make them hostile to most amphibians — and easily degraded by human activity. Intensive limestone quarrying, in particular, renders karst amphibian populations highly vulnerable to decline and local extirpation. The researchers argue that the karst terrain of northern Guangdong, still severely under-sampled, calls for precise and targeted conservation strategies, and that protecting these distinctive landforms and their micro-endemic species is essential to safeguarding the evolutionary resources and ecological integrity of the region’s biodiversity.

Subject of Research: Discovery and integrative taxonomic description of a new karst-dwelling horned toad species in northern Guangdong, China

Article Title: A new species of Boulenophrys Fei, Ye & Jiang, 2016 ( Anura , Megophryidae ) from karst landscapes in northern Guangdong, China

Article References: Zeng, Z.-C., Wang, J., Qi, S., Song, H.-M., Cai, Y.-Y., Zhu, Y.-S., Lin, S.-S., Li, Y.-H., Zhou, D.-Y., Yang, H.-C., Lyu, Z.-T., Yang, Y.-P., & Wang, Y.-Y. (2026). A new species of Boulenophrys Fei, Ye & Jiang, 2016 (Anura, Megophryidae) from karst landscapes in northern Guangdong, China. Herpetozoa, 39, 291-302. https://doi.org/10.3897/herpetozoa.39.e202590

Image Credits: AI Generated

DOI: 10.3897/herpetozoa.39.e202590

Keywords: Boulenophrys calciphila, horned toads, karst landscapes, amphibian diversity, new species, Guangdong, China, integrative taxonomy, phylogenetics, mitochondrial DNA, conservation, Megophryidae