Abstract
The larval stages of many Southeast Asian ranid frogs remain poorly documented, limiting comprehensive studies of their taxonomy, ecology, and distribution. In this study, we provide the first detailed description of the tadpole of Hylarana luctuosa from southern Thailand, based on both morphological and molecular evidence. Specimens collected from Hala-Bala Wildlife Sanctuary (Yala Province) were identified using mitochondrial 16S rRNA gene sequences (~1,300 bp). Phylogenetic reconstructions using Maximum Parsimony (MP) and Bayesian Inference (BI) produced congruent topologies, with all Thai tadpole specimens (YC03952.1 and YC03952.2) forming a strongly supported monophyletic clade that clustered with adult H. luctuosa sequences (MP-BS = 100%, BPP = 1.00). Low uncorrected pairwise genetic distances (p-distance = 0.0–3.0%) confirmed the conspecific status of the larvae and adults, while demonstrating clear genetic differentiation from closely related sister taxa (H. decorata and H. nazgul). Morphologically, the tadpoles are medium to large in size and characterized by an anteroventrally directed oral disc with multiple rows of papillae and a variable labial tooth row formula (LTRF: 1:4+4/1+1:3, 1:3+3/1+1:3, 1:3+3/2+2:2, 5+5/1+1:3, and 1:4+4/3+3:2). The lateral line system is well developed but variable, with neuromast counts increasing ontogenetically and exhibiting bilateral asymmetry across the supraorbital, canthoinfraorbital, rostroinfraorbital, ventrolateral, dorsal, dorsal fin, oral-pouch, lateral, and medial lines. Distinctive buccopharyngeal structures are also documented. In life, coloration differs from Bornean populations, with Thai specimens exhibiting darker, more uniform dark brown to blackish pigmentation with dense pale speckles. This study fills an important gap in larval knowledge of H. luctuosa and extends its confirmed distribution in Thailand to the southernmost region. Further field surveys are needed to better understand amphibian diversity in southern Thailand, particularly in the context of ongoing biodiversity loss.