Holding On to the Rocks, Slipping Through the Cracks: Anthropogenic Threats to Sumatran Ranids Bearing Gastromyzophorous Tadpoles

An organism resembling a loricariid catfish crawled from the rapids, foraging to satisfy its nutritional needs by scraping organic overgrowth off the surface of a large boulder in the Curug Embun Waterfall area, Kepahiang Regency, Bengkulu Province, Sumatra, Indonesia. As it detected that it was being closely monitored by two observers, the catfish-like organism reduced the suction power of its abdominal sucker and plunged back into the torrential stream. My fellow surveyor, Bambang Prayogo, and I exchanged looks. There was no doubt! We had successfully located a gastromyzophorous tadpole of the genus Wijayarana within the fast-flowing river system of Curug Embun.

My initial encounter with a tadpole exhibiting a unique ecomorphological adaptation for fast-flowing river habitats (an abdominal suction apparatus) took place at another waterfall site within North Bengkulu Regency. On that occasion, Bambang Prayogo managed to collect a Wijayarana tadpole using a dipnet from the torrential rapids of the Palak Siring Waterfall. Tadpoles of the genus Wijayarana are highly distinctive and easily recognizable. Beyond having a specialized abdominal suction device, they are characterized by a dorsoventrally depressed, oval body and an M-shaped upper jaw sheath (1, 2). In contrast, Sumaterana tadpoles—which similarly present an abdominal sucker—exhibit a more rectangular body profile and a thick, arcuate upper jaw sheath that thins medially (3). This specialized abdominal apparatus enables the tadpoles to anchor themselves to rocky substrates against high-velocity currents (3–6).

In Sumatra, the ranid frog genera with gastromyzophorous tadpoles (Sumaterana and Wijayarana) were established by Umilaela Arifin, my research supervisor, who first introduced those two genera to me during a training workshop in 2024. While introducing the diversity of Sumatran anuran species, she mentioned that many fascinating morphological traits of Sumatran fauna remain hidden, including forms that represent unique ecomorphological adaptations, such as those exhibited by frogs of the genera Sumaterana and Wijayarana.

Numerous geological and ecological pathways may account for the ecomorphological evolution of the tadpoles belonging to these two genera in Sumatra, as noted by my supervisor, Umilaela Arifin, in the conclusion of her 2022 paper in Scientific Reports (7). Among the areas necessitating more profound investigation is larval ecology. This specific dimension motivated me to collaborate with her on studying these uniquely adapted frogs. Beyond the fact that ecological aspects are fundamental to deciphering the evolution of these endemic Sumatran ranids, we maintain that a comprehensive grasp of the entire biology throughout all life history phases of Sumaterana and Wijayarana can inform the development of effective conservation strategies for these endemic taxa and their habitats. This remains critical because both genera possess vital ecological value for Sumatra—an interdependent bond shaped by millions of years of evolutionary history. The extinction of these two genera would inevitably compromise the integrity and health of numerous fast-flowing river ecosystems throughout Sumatra.

This reality prompted me to accept the baton of researching Sumaterana and Wijayarana when invited by their established authority. I am profoundly intrigued not only by the fact that the larvae of these two taxa exhibit an ecomorphological adaptation inherently rare among anurans (the presence of an abdominal sucker), but also by their vital importance to Sumatra, my place of birth.

"We are going to examine the dietary composition and gastrointestinal tract morphology of Sumaterana or Wijayarana tadpoles in Bengkulu Province," Umilaela Arifin remarked to me during one of our past discussion sessions. Although documentation regarding the diet composition of gastromyzophorous tadpoles of endemic Bornean ranid species (Huia cavitympanum and Meristogenys) was provided by Alexander Haas (6), detailed accounts for analogous species in Sumatra remain unavailable. Consequently, my study aims to address this data deficit: the feeding ecology of gastromyzophorous tadpoles belonging to endemic ranids in Sumatra. This assessment constitutes a highly significant undertaking.

The ecosystems harboring Sumaterana and Wijayarana frogs and their larvae in Sumatra have undergone extensive conversion into waterfall-based nature tourism destinations. This intensive anthropogenic pressure jeopardizes the persistence of both genera alongside their associated taxa, including Sumatran human populations. Conspicuous among these threats is plastic pollution. Notably, my examination of the intestinal contents of a Wijayarana tadpole sampled from Curug Embun Waterfall detected microplastic fibers (unpubl. data). Impaired growth and development, mutagenesis, and mortality represent the severe consequences endured by tadpoles exposed to microplastics (8). While the full ecological ramifications of losing these two genera in Sumatra remain unpredictable, the outcome will unequivocally be devastating. Driven by the detection of microplastic fibers within the gastrointestinal tract of the Wijayarana tadpole at Curug Embun, a conservation status reassessment for species belonging to Sumaterana and Wijayarana is highly warranted. Such an initiative is critical to evaluate how profoundly intensive anthropogenic disturbances impact the long-term survival of these two genera in Sumatra. Moreover, given that the most recent evaluation of these taxa dates back roughly a decade, it raises an urgent alarm to reassess the conservation status of these morphologically unique anurans (9, 10).

Through investigating the feeding ecology of Wijayarana tadpoles at Curug Embun Waterfall, I have now both come to know the nutritional requirements that must be sustained within the ecosystem for the persistence of this species and its habitats, and identified the specific environmental threats manifested as traces within the larvae's digestive tracts. These insights are critical to maintaining the ecological equilibrium of the world's sixth-largest island. If my supervisor established that larval morphology possesses critical taxonomic and evolutionary significance among Asian ranid frogs with gastromyzophorous tadpoles, I have demonstrated that elucidating the feeding ecology of these gastromyzophorous larvae carries paramount value for conservation initiatives. In alignment with my supervisor's perspective, I conclude that empirical data encompassing all biological facets of tadpoles are highly significant for the preservation of the world's most threatened vertebrates (e.g., amphibians). I selected the ecomorphologically unique gastromyzophorous tadpoles of the genera Sumaterana and Wijayarana in Sumatran fast-flowing river systems to coexist alongside our species and enjoy this world—which anuran within which ecosystem do you intend to safeguard?

Acknowledgments

I am deeply indebted to Umilaela Arifin for introducing me to the fascinating world of Sumatran ranids with gastromyzophorous tadpoles, and for her invaluable guidance and exceptional mentorship throughout this study. I also express my sincere gratitude to Stiftung Artenschutz for providing the research grant awarded to Umilaela Arifin, which generously funded and supported my field surveys. Finally, I extend my heartfelt thanks to Bambang Prayogo, Ahmad Yusup, Adzkia Amalia Putri, Deni Parlindungan, and Ariyoga Pratama for their outstanding support and dedication as a highly dependable field research team.

References

1. Yang, D. T. (1991). Phylogenetic systematics of the Amolops Group of Ranid Frogs of Southeastern Asia and the Greater Sunda Islands. Fieldiana: Zoology, New Series, 63. Chicago: Field Museum of Natural History.
2. Arifin, U., Chan, K. O., Smart, U., Hertwig, S. T., Smith, E. N., Iskandar, D. T., & Haas, A. (2021). Revisiting the phylogenetic predicament of the genus Huia (Amphibia: Ranidae) using molecular data and tadpole morphology. Zoological Journal of the Linnean Society, 193(2), 673–699.
3. Arifin, U., Smart, U., Hertwig, S. T., Smith, E. N., Iskandar, D. T., & Haas, A. (2018). Molecular phylogenetic analysis of a taxonomically unstable ranid from Sumatra, Indonesia, reveals a new genus with gastromyzophorous tadpoles and two new species. Zoosystematics and Evolution, 94(1), 163–193.
4. Inger, R. F. (1966). The systematics and zoogeography of the Amphibia of Borneo. Fieldiana: Zoology, 52, 1–402.
5. Gan, L. L., Hertwig, S. T., Das, I., & Haas, A. (2015). The anatomy and structural connectivity of the abdominal sucker in the tadpoles of Huia cavitympanum, with comparisons to Meristogenys jerboa (Lissamphibia: Anura: Ranidae). Journal of Zoological Systematics and Evolutionary Research, 54(1), 1–16.
6. Haas, A., Das, I., Hertwig, S. T., Bublies, P., & Schulz-Schaeffer, R. (2022). A Guide to the Tadpoles of Borneo. Tredition.
7. Arifin, U., Smart, U., Husemann, M., Hertwig, S. T., Smith, E. N., Iskandar, D. T., & Haas, A. (2022). Phylogeographic inference of Sumatran ranids bearing gastromyzophorous tadpoles with regard to the Pleistocene drainage systems of Sundaland. Scientific Reports, 12, Article 12013.
8. Balestrieri, A., Winkler, A., Scribano, G., Gazzola, A., Lastrico, G., Grioni, A., Pellitteri-Rosa, D., & Tremolada, P. (2022). Differential effects of microplastic exposure on anuran tadpoles: a still underrated threat to amphibian conservation? Environmental Pollution, 303, 119137. https://doi.org/10.1016/j.envpol.2022.119137
9. IUCN SSC Amphibian Specialist Group. (2018). Chalcorana crassiovis. The IUCN Red List of Threatened Species 2018: e.T58580A95581452.
10. IUCN SSC Amphibian Specialist Group. (2018). Huia sumatrana. The IUCN Red List of Threatened Species 2018: e.T58306A114920415.