- An endemic freshwater fish species known as the “stone sucker” (Garra ceylonensis) has six genetically different populations spread in different river basins in Sri Lanka despite managing to sport similar looks, a new study shows.
- The research estimates that the ancestor of G. ceylonensis first colonized Sri Lanka around 3 million to 4 million years ago via rivers on the land bridge between India and Sri Lanka and that subsequent climatic events trapped them on the island, causing the fish to evolve as a distinct species along with genetic variances.
- The study highlights the need for innovative efforts in conserving genetically distinct populations, going beyond a mere species-level focus to conserve Sri Lanka’s rich biodiversity.
- G. ceylonensis, also known as the “doctor fish,” is part of a popular group of fishes used in the foot spa industry to nibble off dead skin, driving an increase in wild collections for the aquarium trade.
COLOMBO — Sri Lanka’s fast-flowing freshwater streams are home to a variety of beautiful fish, which, as a rule, swiftly swim away if you dip your feet into the water. But not so the welcoming “stone suckers” (Garra ceylonensis), renowned for their use in “fish spas,” where they nibble away at the dead skin cells of visitors’ feet.
They’re also among the most widely distributed of Sri Lanka’s freshwater fish. Now, a new study into that distribution shows that while G. ceylonensis populations in different river basins around the island are physically identical — in size, body patterns, color — they differ genetically.
“We found six genetically different populations of G. ceylonensis from different locations,” says lead researcher Hiranya Sudasinghe, a molecular biologist at the University of Peradeniya’s Post-Graduate Institute of Science.
Sri Lanka is home to 91 freshwater fish species, 50 of which are found nowhere else on Earth.
But G. ceylonensis is a class apart: It has already adapted to survive in fast-flowing water, feeding on rock-clinging algae with its pouty mouth. The evolutionary history of G. ceylonensis hasn’t been investigated until now, but it’s an ideal organism to reconstruct the evolutionary history of a widespread, yet morphologically specialized, freshwater fish in a tropical-island setting, Sudasinghe says.
The research, for which Sudasinghe and his team collected specimens from 38 locations in 16 river basins across Sri Lanka, also investigates exactly when the species first arrived on the island and looks at its ancestors. The study dates that initial colonization to the late Pliocene era, some 4.3 million to 3.2 million years ago, over the Palk Isthmus, the land bridge that linked Sri Lanka to India when sea levels were much lower than they are today. The arid climate during the Plio-Pleistocene, more than 5 million years ago, would have dried out the river connections between Sri Lanka and India, trapping the ancestral population of Garra in Sri Lanka, allowing them to later evolve into a separate species and form genetically distinct populations across the island.
Similar but different
It’s a scenario that has played out countless times with other species: Indian shrub frogs, for instance, also crossed the land bridge to Sri Lanka and later evolved as endemic species, according to study co-author and evolutionary biologist Madhava Meegaskumbura. He tells Mongabay that in the case of Garra, the species didn’t branch out into several distinct new species, unlike with the shrub frogs, likely because of continuous gene flow via the network of streams and other waterways.
Another reason could be the transportation of larvae long distances along with the monsoonal rains. Together, these two processes effectively ensured that the fish populations weren’t isolated over sufficiently long periods of time to allow a rise in new species, Meegaskumbura says.
Study co-author Rohan Pethiyagoda, a global expert in freshwater fish and former deputy chair of the IUCN’s Species Survival Commission (SSC)., says the findings on G. ceylonensis addressing have implications for conservation policymaking.
“Hiranya’s study looks at the deeper genetics of G. ceylonensis addressing several interesting questions, and most importantly, it gives insights into the direction of conservation of freshwater fish in Sri Lanka,” he tells Mongabay. “It shows that our focus on ‘species’ conservation is perhaps misplaced and what we should focus on instead is the conservation of genetically distinct groups.”
He cites the case of hydropower and irrigation development projects in Sri Lanka’s south, fed by the Kirindi Oya River. To ensure an adequate supply, water is diverted to the Kirindi Oya from the Uma Oya, a tributary of Sri Lanka’s longest river, the Mahaweli. This also brings the fish and larvae from the Mahaweli into the Kirindi Oya, giving rise to what Pethiyagoda calls a genetic mix of Garra from the two rivers.
This highlights the environmental problems that could result from the diversion of water between basins, he adds.
According to Pethiyagoda, this principle applies not only to fish, but also to other organisms, such as parasites, which could lead to diseases being transmitted in this manner and illustrating the importance of carefully assessing such interventions.
Pethiyagoda did pioneering work in 1990s exploring the freshwater fish of Sri Lanka, but calls it “kindergarten work” compared to the current level of sophisticated studies using genetic analysis. These new studies by young scientists like Sudasinghe are bringing Sri Lankan fish studies into the 21st century, casting a new light on our understanding of species, evolution and conservation and helping to find answers to persisting questions, Pethiyagoda says.
A previous study by Sudasinghe also looked at how species from another freshwater genus, Devario, had evolved after making the crossing from India to Sri Lanka. The earliest freshwater fish seem to have arrived on the island around 30 million years ago, but there must have been fish in Sri Lanka before that, Pethiyagoda says, but how and when did they become extinct?
Compared to southern India, where about 30 genera of fish are present, Sri Lanka has low freshwater-fish diversity. The latest study indicates that perhaps it was the changing climate that triggered severe extinctions across Sri Lanka.
Questions like these remain relevant because their answers could guide future conservation policy, Pethiyagoda says, including when we should factor climate change into biodiversity conservation.
Sudasinghe, H., Dahanukar, N., Raghavan, R., Senavirathna, T., Shewale, D. J., Paingankar, M. S., … Meegaskumbura, M. (2021). Island colonization by a ‘rheophilic’ fish: The phylogeography of Garra ceylonensis (Teleostei: Cyprinidae) in Sri Lanka. Biological Journal of the Linnean Society, 132(4), 872-893. doi:10.1093/biolinnean/blaa221
Sudasinghe, H., Pethiyagoda, R., & Meegaskumbura, M. (2020). Evolution of Sri Lanka’s giant danios (Teleostei: Cyprinidae: Devario): Teasing apart species in a recent diversification. Molecular Phylogenetics and Evolution, 149, 106853. doi:10.1016/j.ympev.2020.106853
Banner image of Garra ceylonensis, also known as the stone sucker or log sucker, one of the most widespread endemic freshwater fish species found in Sri Lanka. The species has adapted to survive in fast-moving waters. Image courtesy of Hiranya Sudasinghe.