As the heavy rains lashed and flooded Chennai last week, the city saw an infestation of a unique kind. Over the last week, residents have spotted scores of snails on the walls, gardens, rooftops and waterlogged roads of the city. This mollusk invasion continues to persist in several neighbourhoods, from Madhavaram in the north and Choolaimedu in central Chennai, all the way to the Pallikaranai marshlands in the south, the snails have been spotted everywhere.
Where did they come from? Why are there so many of them? Why are they most visible during the rains? And what makes them so interesting to scientists? TNM spoke to experts to understand more about this infestation. The snail in question is the Giant African Snail. It made its way to Chennai hundreds of years ago, hitching a ride on trade ships during the colonial period. Native to countries of East Africa i.e Burundi, Ethiopia, Kenya, Tanzania, etc – the Giant African Snail (scientific name: Achatina Fulica) was accidentally brought to cities in India through commercial activities. But once they touched new soil, they quite literally conquered the land, eating all kinds of plants and crops.
These mollusks are even known on occasion to consume construction material including paint and stucco. Not only that, according to researchers the shells of these snails offer information on the rainfall rate of a particular region.
Today, more than seven decades after colonial rule ended, the Giant African Snail continues to thrive not just in Chennai and the rest of Tamil Nadu, but also in 12 out of 14 districts in Kerala, parts of Karnataka and the North East. With no natural predators in the region, this highly invasive species has multiplied beyond measure in the region. According to the International Union for Conservation of Nature (IUCN), the Giant African Snail is among the 100 most invasive species in the world.
“This is not a new phenomenon. The Giant African Snail has been found in plenty even back in the 90s and early 2000s. I remember the walls of my school in south Chennai being filled with these snails, especially when it rained,” recalls Yuvan M, a member of the Madras Naturalist Society.
According to him, species such as the Giant African Snail and Hunter’s Slug end up in places with no predators and then explode in numbers, becoming an invasive species. “Plants and animal species native to a region co-evolve over millions of years. They grow and negotiate with each other and the plants learn to adapt and develop defenses against these creatures which prey on them. But when taken to a new region, the ecosystem in that region has no defenses against this species. The local vegetation and wildlife are taken by surprise, leaving the invasive species with zero threat and the freedom to populate without limit,” he adds.
Giant African Snail on a road in Injambakkam after rains. Photo credit: Twitter/Mohan Ranganathan
While the Giant African Snail population has posed several challenges, most dangerously, it has turned into a formidable agricultural pest, feeding on over 500 different varieties of plants. According to the New York Invasive Species Information, the Great African Snail resides in “plant habitats and preferentially consumes beans, peas, cucumbers and peanuts”. These snails can destroy whole farms. Thereby, causing large-scale crop destruction resulting in losses. This has led farmers to take precautions during the rainy season, such as sprinkling salt on their crops to ward off a snail invasion.
African snails on a compound wall. Photo credit: Twitter/Kamal Ganesan
Human beings should also avoid consuming these snails. According to research, wild populations of the Giant African Snails carry Angiostrongylus cantonensis or rat lung worm, a parasite which causes meningitis in humans.
During the dry months, these snails are dormant, taking cover under the soil or inside tree trunks and crevices and hibernate, says Geetha Jaikumar, a member of the Madras Naturalist Society. However, it is during the rainy season that they become active, even engineering their mating season to coincide with the rains. “Research has also found that the shells of these snails grow faster during the rains,” Yuvan adds. The Giant African Snails are much larger than even the largest variety of native snails found in India. In some cases, their shells can grow up to 20 centimeters. But studies have found that the snails and rains are connected in more ways than one.
In 2017, new research discovered a fascinating connection between the shells of the snails and the sub-seasonal rainfall received in the region where these mollusks lived. Scrutiny of these snail shells, which grow faster during rains, can offer extremely fine-grained records of the rainfall received in the region. The researchers were able to record week-to-week rainfall rates of past monsoon seasons by studying the shells of snails from as far back as 1918.
Snail on a tree Photo credit: Twitter/Varunkumar Nagarajan
According to Prosenjit Ghosh who is the lead author of the paper, the snails were observed to grow at extraordinary rates during the monsoons and it is this feature that made this snail species the “ideal candidate to record high-frequency changes in monsoon precipitation”. The rainfall patterns can be reconstructed by measuring the isotope ratios in these shells. As per the research paper, the rapid growth rate of snails during monsoons helps preserve week-to-week rainfall rates through the moisture-sensitive oxygen isotope ratios in the growth bands of the shells of these animals. According to its authors, the snail shells preserve more incremental changes than even data taken from caves, tree trunks, or other mollusks which are also used for climate research.
Like most snails, the Giant African Snails too are hermaphrodites – having both male and female reproductive parts. “This means that 100% of the snail population have reproductive capabilities. And they reproduce a lot, helping them grow in population. Each individual snail can lay up to 500-900 eggs, twice a year. They lay eggs during the rainy season and the bury them inside the soil” says TS Sajeev, Principal Scientist at the Kerala Forest Research Institute or KFRI. An adult snail lives for 5-7 years and can start laying eggs within one year.
The mating ducts of two snails fuse together to transfer the sperm into the female. Usually, the bigger snail is the ‘male’ and the smaller one the 'female’. However, if a snail cannot find a mate, it can self-mate, Sajeev adds. They then bury their eggs under the soil where they remain until the next spell of rain. These eggs then turn into hatchlings. During the months where there is no rain, the snails crawl into the soil as they need more moisture than what is available on the surface.
“Once the rains end, the snails go back into the soil, tricking residents into thinking that the snail menace is over. But it is far from over. The next rains, they are back and ready to conquer every corner of the region,” Sajeev adds.