Generally, a shell is an evolutionary trait that allows a creature to retreat from predators. This is the case for almost all snails. But at least two species of snails use their shell in a much more active form of defence -- knocking the crap out of predators.
Scientists recently studied the Karaftohelix gainesi of Japan and the Karaftohelix selskii of Russia in an effort to better understand the ways that predator-prey interactions affect the evolution of the prey.
The carabid beetle is the mortal enemy of snails and these two species have evolved in a way that allows the snail to simply bat the beetle away, knocking it on its back until it gets so bothered that it just gives up.
Yuta Morii, the lead author of the study writes, "The difference in their defensive behaviours is also reflected in their shell morphology, indicating that their behaviours and shell shapes are interrelated to optimise the preferred defence strategy."
The study points out that there are other snail species on Hokkaido Island, a focal point of their research. But, "It is likely that these carabid beetles prey on K. editha and K. gainesi because there are a few other large snail species on Hokkaido, but no species other than K. editha and K. gainesi are distributed throughout all of Hokkaido." That distribution could indicate an evolutionary correlation between the predator-prey interactions.
DNA analysis of the two species also showed that they evolved independently.
Check out the swinging shell action in the video below. If you'd like to see the sad state of National Geographic these days, you can watch the same clip with kung fu chop sound effects here.