A key feature of tailless amphibians (Amphibia: Anura) is their complex life cycle, in which the terrestrial adult form is separated from the aquatic larval stage by metamorphosis. The radical morphological changes that occur during catastrophic metamorphosis essentially "zero out" the larval morphology and "re-create" the adult organism with a different body plan. This enables independent evolution of the larval and adult stages, which exist in different environments and participate in different ecosystems, including food chains. It follows that, depending on the conditions of reproduction and development, externally and ecologically similar frogs can have very different tadpoles with different trophic specializations and adaptations.
A good model for studying this phenomenon are frogs of the Dicroglossidae family, which is widespread in the Asian tropics. Researchers from the Severtsov Institute of Ecology and Evolution of the Russian Academy of Sciences (IEE RAS) studied seven species syntopically inhabiting the southern regions of Vietnam: Fejervarya limnocharis, F. moodiei, Hoplobatrachus rugulosus, Limnonectes dabanus, Occidozyga lima, O. martensii and Quasipaa verrucospinosa. These are outwardly quite similar frogs, similar in their near-water or almost aquatic lifestyle and differing mainly in size. A comprehensive study of the external and internal morphology (morphometric parameters, oral apparatus, digestive tract, visceral skeleton) of their tadpoles was conducted. Although adults feed in a similar way, tadpoles belong to different trophic guilds – generalized scrapers-filter feeders (Fejervarya limnocharis, F. moodiei, Limnonectes dabanus, Quasipaa verrucospinosa) or specialized predatory macrophages (Hoplobatrachus rugulosus, Occidozyga lima, O. martensii).
The study showed for the first time that in addition to the features of the external morphology and structure of the oral apparatus (uniform generalized in scrapers and highly specialized in predators), the studied representatives of different guilds also differ significantly in the general morphology of the digestive tract. The most typical scraper tadpoles, common to many Anura groups, are characterized by a generalized spiral intestine of great length and the absence of a formed stomach (Fig. 1A). In contrast, predatory tadpoles are characterized by a shortened intestine, a reduced number of loops, and the presence of a more or less formed stomach (Fig. 1B,C).
The first detailed study of the morphology of the larval skeleton of tadpoles – the skull (chondrocranium) and the hyoid-branchial apparatus (hyobranchia) – showed that with “predatory” specialization, the adaptive transformation of the skeleton can proceed in two different ways: by “robustization” of the generalized form (Fig. 2A, B), as in Hoplobatrachus rugulosus (Fig. 2C, D), without radical changes, and by strong modification and creation of a unique, extremely specialized morphology, as in tadpoles of the genus Occidozyga (Fig. 2E, F).
"At the same time, if the generalized form of the skeleton and digestive tract is typical for omnivorous tadpoles, then in the predatory tadpole H. rugulosus these structures have only individual features associated with macrophagy, and the "predatory" specialization affects mainly the keratin oral apparatus; field observations and laboratory experiments show that the larvae of this species are rather omnivorous opportunists, and not obligate predators, as was previously thought. On the contrary, in tadpoles of the genus Occidozyga, the morphology is so specialized that they lose the ability to any type of nutrition, except for obligate predation, which includes a narrow range of prey," said Anna Vasilyeva, PhD, senior researcher at the IEE RAS.
Thus, independent evolution of the larval stage in similar species of Dicroglossidae leads to the formation of completely different morphological forms of tadpoles, capable of mastering different food resources and occupying different niches within the same reservoir. In general, this example clearly shows how Anura achieves significantly greater ecomorphological diversity at the larval level than in adult forms.
The study was carried out at the Joint Russian-Vietnamese Research and Technology Center.
The study was supported by the Megagrant International Cooperation Program, project 075-15-2022-1134.
Publication imprint: Vassilieva A.B., Trung Duc Nguyen, Sorokin P.A. (2025). Morphological diversity of tadpoles of fork-tongued frogs (Anura: Dicroglossidae) with different trophic specializations. Vertebrate Zoology, 75, 31-57. DOI 10.3897/vz.75.e139103