Two species of snail have evolved shells that can bat away beetles
There are times when it makes sense to hang tough and endure, and there are times when you have to fight back. Scientists are now trying to figure out exactly when the tipping point came for member of the genus Karaftohelix, as the small creatures have seemingly repurposed what is typically thought of as a defensive structure into a weapon capable of repelling their usual predators. If these were claws or fangs, we probably wouldn’t notice, but Karaftohelix are snails, and they’ve started using their shells as clubs.
The path to swingable shells has been tied to their size. To fend off predators like the carabid beetle, most snails would rely on a shell with a small opening to make it hard for a beetle to get at the snail inside. Some members of Karaftohelix gainesi and Karaftohelix selskii are evolving proportions in the other direction, developing larger openings in their shells, which in turn allows for more muscle tissue to reach the shell. While they’re not about to seriously harm any hungry beetles, the snails can rotate and swing their shells deliberately enough to startle their foes into leaving them alone. And it looks weird.
What sparked the new shells?
As unusual as weaponized shells are, the mechanics of how they work aren’t really the question now. What’s somehow more baffling is figuring out how this all got started, much less twice. The two species of snails live on two separate islands, one in Russia, the other in Japan. Somehow, both lineages came to similar solutions independently in a case of convergent evolution. However, the exact pressures to produce these features aren’t clear— other snails on the same islands, with similar predators and diets, somehow do just fine with more traditional shells. With no intermediate shell forms for comparison, a leading hypothesis is that larger snails have been mating only with each other, segregating themselves into a specialized, offensively-shelled population.
Source: Asian Snails Can Club Predators with Their Shells by Jennifer Frazer, The Artful Amoeba