Cutting out the excess: Reduction of larval structures in Asian tree frogs (Anura: Rhacophoridae) from Vietnam with different modes of reproduction and development

Rhacophorid frogs, with their unique diversity of reproductive modes, are a perfect model group for studying the evolution of different forms of ontogeny. While phylogenetic studies yield controversial results, the morphological basis of the ontogenetic remodeling remains insufficiently explored. In the present research, we compare embryonic and early-larval development in eight species of Rhacophoridae with four different reproductive modes: jelly nests ( Feihyla palpebralis , Theloderma vietnamense ), bubble nests ( Feihyla hansenae ), foam nests ( Chirixalus nongkhorensis , Polypedates megacephalus , Rhacophorus annamensis , R. vampyrus ), and terrestrial direct-developing egg clutches ( Kurixalus gryllus ). The main focus is on the degree of embryonization in each species, as well as on the development of three transient early-larval features: external gills, opercular fold, and adhesive organs. The initial signs of gill reduction were observed in biphasic jelly-nesting species with a slightly prolonged embryonic period. In direct-developing species, the reduction of larval structures progressed significantly. Development varied among foam-nesting species: while early ontogeny in Chirixalus showed no signs of significant remodeling, both species of Rhacophorus exhibited a strikingly modified developmental pattern, with a severe reduction in external gills and accelerated formation of tadpole morphology within the foam nest, whereas the studied species of Polypedates displayed transitional developmental features. The morphological changes discovered showed that, in the evolution of ontogeny in Rhacophoridae, direct development and advanced foam-nesting represent two fundamentally divergent and highly derived reproductive modes.