The great majority of the more than 400 families of snails are found only in the sea, while about 5% of them are exclusively freshwater. Very few snail groups are common in both environments and just three marine families have rare freshwater members.
One of these is the Littorinidae (periwinkles), familiar from rocky shores. In the nineteenth century three freshwater periwinkle species (genus Cremnoconchus) were discovered in the mountainous Western Ghats of India, living in fast-flowing streams at altitudes between 300 and 1400 m. These have not been studied for over 100 years.
Figure from the original description of Cremnoconchus (images 1-7) Image courtesy of Biodiversity Heritage Library. http://www.biodiversitylibrary.org
In a collaboration with scientists from the NHM's partner organisation the Ashoka Trust for Research in Ecology and the Environment (ATREE), Bangalore, David Reid revisited the type localities of the three known species to collect new specimens. (The type locality is the place in which the reference specimen was found that was originally used to describe and name the species.) These were studied to find out more about the snails and allowed the relationships between the species to be investigagted in more detail and revised. There are distinctive differences between the species particularly in terms of their radula (the rasping tongue of snails), their reproductive systems and the calcified operculum (the disc that fits into the shell opening when the snail retreats into the shell, providing additional protection from predators and desiccation).
In addition, an unknown radiation of six new Cremnoconchus species was discovered in the central Western Ghats, 500 km south of the previously known range where David and his collaborators looked at the known species.
Cremnoconchus is interesting in evolutionary terms: the current evidence suggests that its closest living relatives are marine snails found only in New Zealand and Australia, suggesting that the ancestral population was split by the breakup of the ancient continent Gondwana during the Cretaceous, between 145 and 65 million years ago. However, more evidence and DNA studies would be needed to confirm this hypothesis.
Each of the six new species was restricted to a single stream system on the steep western escarpment of the Deccan Plateau, with limited overlap in distribution in two places. This suggests that populations of ancestral species were isolated by waterfalls or other features allowing evolutionary divergence over time The habitat of these snails is fragile, being very limited in scale and threatened by tourism, road construction and domestic pollution: all the species are judged to be endangered.
Reid, D.G., Aravind, N.A. & Madhyastha, N.A. (2013) A unique radiation of marine littorinid snails in the freshwater streams of the Western Ghats of India: the genus Cremnoconchus W.T. Blanford, 1869 (Gastropoda: Littorinidae). Zoological Journal of the Linnean Society. 167: 93-135.