A snail’s shell is both mobile home and mobile defense system. But did you know they can also use them to do this?
That poor beetle looks shaken and stunned.
As it turns out, two related snail species – Karaftohelix gainesi and Karaftohelix selskii -- on the Japanese island of Hokkaido and across the Sea of Japan in far northeastern Russia have the astounding ability to whack their predators with their shells, re-purposing a structure originally evolved as a fortress into a weapon of active smack down. Snail-eating beetles were shoved away or even flipped over by the snail. Beetles really hate that.
A team of Japanese and Russian scientists wondered if predation pressure alone could have lead to the evolution of this behavior. They concluded this was indeed the case in a paper published on Nov. 11 in the journal Scientific Reports. No other obvious ecological factors explained the difference, as beetle-bopping snails live cheek by eyestalk with closely related shell-retreating snails both on the island and the mainland, eating the same foods and sliming over the same duff. Based on their study of the snails’ DNA, the same bludgeoning behavior seems to have evolved twice on Hokkaido and in Russia independently, also strongly implicating natural selection against predation pressure as the cause, the authors write.
Why might this happen? Boppers have slightly bigger shells with much bigger entryways, and retreaters the reverse. A larger opening allows for a stronger shell-swinging muscle and bigger snail to fit through it, while a smaller, thicker opening (and consequently smaller snail) makes it harder for predatory beetles to poke their head into the shells of snails who choose retreat. There’s a tradeoff between the two strategies, the authors write, such that there they found no intermediate type shells either as a unique species or within species. Apparently, once a species chooses one strategy, natural selection herds its shell and body toward one extreme or another because shell-swingers survive better if they’re bigger, while shell-shielders survive better if they’re smaller and can shrink their front doors. This push toward opposite sizes created by predation pressure may in turn encourage reproductive isolation and thus speciation, because these snails prefer mates of similar size, the authors say.
The only other known instance of a similar behavior occurs in yet another Japanese snail, Acusta despecta, when they are attacked by, oddly enough, the larvae of fireflies. Otherwise, the rest of the world is apparently innocent of predator-clubbing snails.
Morii, Yuta, Larisa Prozorova, and Satoshi Chiba. "Parallel evolution of passive and active defence in land snails." Scientific Reports 6 (2016).