Gliders of the waterfalls

The old adage ‘slow and steady wins the race’ might not hold water if a snail instead of a tortoise had entered the competition. Because of their bulky shells and their use of just one strong foot to propel themselves forward, snails have earned a reputation for being painfully slow.

Snails come in various sizes—weighing just a few milligrammes and small enough to fit on pin heads to the African giant snail, which may grow 25 cm long and weigh up to 25 kg. Snails have a spiral-shaped calcium carbonate shell that surrounds and protects their delicate body and organs as they develop into adults. Snails’ familiar physical features—large feet, tentacles, and spiral shells—classify them as Gastropod molluscs from the phylum Mollusca and the class Gastropods.

Snails can adapt to a wide variety of environments, including both marine and freshwater environments. However, their habitats have been taken over by the growing human population.

Intending to investigate snail habitats in different areas of the Western Ghats, a group of scientists from Ashoka Trust for Research in Ecology and the Environment (ATREE) and the Centre for Ecological Sciences, India, undertook an extensive study of cliff snails or periwinkles, a group of freshwater snail restricted to the waterfalls in the Western Ghats.

The team studied a unique freshwater mollusc genus called Cremnoconchus from the family Littorinidae. It is only found in the Western Ghats of India, in the spray zone of waterfalls. This species stands out among the twenty genera of Littorinidae -- almost all live in saltwater or brackish environments, while Cremnoconchus is adapted to freshwater habitats, making it an iconic taxon of the Ghats.

“Molluscs are the second largest group of invertebrates globally., India has more than 200 species of freshwater and over 1,200 land snails. The taxonomic research on these molluscs from India is more than a century old, and I expect more than 500 new species awaiting discovery if one undertakes an extensive survey,” says  Aravind N A, Senior Fellow at ATREE. 

The Western Ghats on the southwestern coast of peninsular India are well-known for the rich diversity of unique species. Numerous waterfalls in the area make it a good place to live for a wide range of wildlife, including freshwater snail Cremnoconchus. It is possible to discover new and indigenous species if additional waterfalls are surveyed.

“So far, only nine species of Cremnoconchus have been recorded. We do not have records on how they evolved in waterfall habitats,” says Aravind.

With this view, the research team wanted to uncover the details of the genera Cremnoconchus and their adaptations. Several questions begged for answers. Could Cremnoconchus and other similar genera have descended from a common ancestor? How many species are still waiting to be found? When did Cremnoconchus transition from being a species that lived in saltwater to one that lived in freshwater?

Ancestry study 

Cremnoconchus and other sister genera of the family Littorinidae may have originated from a common ancestor in the marine environment. They share similar appearances, habitats, and genetic characteristics. However, Cremnoconchus prefer freshwater, particularly waterfalls.

The study shows that the separation of Cremnoconchus from its other genus likely occurred around 90 million years ago. Researchers used marine fossils of Littorinid to understand the separation between marine and freshwater species, but the evidence was inadequate.

Many marine organisms, including fish, prawns, and molluscs may have migrated into freshwater habitats as a result of a sea level increase and decline lasting between 100 and 50 million years, during which the sea level rose to a height of 300 meters before falling back to its present level. Endemism, the phenomenon in which a species exists only in a few places, or its “type location” is extremely common due to the isolation offered to many species via this “habitat island” phenomenon.

New species discovered 

Although most of the waterfalls are yet to be explored, an idea of how many new species live in the Western Ghats can be obtained by looking at the two pieces of information: differently patterned snails that have already been recorded and the geographic distribution of snails.

For this analysis, scientists used Automatic Barcode Gap Discovery (ABGD), which bases its results on the idea that individuals from the same species are more closely related than individuals from different species. The analysis focused on the differences in the pieces of DNA from the mitochondrial region. The ABGD analysis showed twenty-one species split across different geographic locations in the Western Ghats.

The study found nine species in the Northern Western Ghats (NWG), but only three have been identified. On the other hand, the Central Western Ghats (CWG) have twelve species, of which six have been named and six have not. Now, experts think this split happened around 56 million years ago, during the Cretaceous period when the Northern Western Ghats were first formed. It additionally influenced the shell patterns of the snails in the northern and central western ghats.

Analysis of the shell shape and pattern, geographic distribution, and DNA sequencing revealed that Cremnoconchus of the Western Ghats may be grouped into two. NWG snails have smooth ribs on their shells, while snails in central parts have no such pattern. This suggests that ribbed-smooth NWG shells lost their patterning as they adapted to the different CWG environments. As a result, CWG shells are completely random.

“We need to undertake integrated taxonomic methods not only molecular data, but also morphology, anatomy, and radula (snail teeth) to confirm the new species status,” says Aravind.

The study team discovered a few things about this group of gastropod molluscs by looking at their DNA. These results show a big difference between species of sea and freshwater types. Just one migration into freshwater caused the population to become more diverse in its new home. Additionally, it seems Cremnoconchus have an open lifestyle, which allows them to cross over into different ecosystems more easily.