Farmers and gardeners are locked in an eternal battle with pests. One of those pests is the humble snail. The sluggish and slimy creatures love to munch on anything green and moist, and leave their feces all over the place.

To ward off snails, farmers sometimes employ the help of microscopic parasitic nematodes – tiny worms. When applied to fields in a watery mixture, the nematodes seek out and infest snails, killing them over the ensuing weeks.

While the parasites are usually quite effective, the snails are far from helpless. Turns out, they have an incredible and alien defense mechanism.

When parasitic worms creep inside a snail's shell, cells on the shell's inner layer adhere to the nematodes, swarm over their bodies, then enase them in the shell!

Robbie Rae, a researcher at Liverpool John Moores University, originally observed the bizarre mechanism back in 2015. He recently examined it in-depth in the lab, publishing his findings in the journal Scientific Reports.

Rae infected grove snails, a common species of land snail in Europe, with nematodes and observed what happened. Shell cells began swarming the wormy invaders in less than a day. After a week, between six and ten of the parasites were fused within each snail shell. After a month, each snail had encased between forty and ninety nematodes on average. (Figure below: A step-by-step view of the encasing process.)

Outside of the lab, Rae found encased parasites in hundreds of wild snail shells he collected in northern Scotland and northwest England. He also found nematodes within snail shells stored at Liverpool and Manchester museums. Some of the these shells were more than 500 years old!

"The shell seems to be a formidable defence system that is able to quickly trap hundreds of nematodes," Rae writes. "It is unknown how cells of the shell recognise and attach to the nematode cuticle, but they could respond to lectins, mucins, glycoproteins or collagens that are present on the nematode surface coat and cuticle."

"This is the only example of an exoskeleton that has been co-opted as an immune system," he says.

Source: R. Rae. The gastropod shell has been co-opted to kill parasitic nematodes. Scientific Reports 7, Article number: 4745 (2017) doi:10.1038/s41598-017-04695-5