Natural selection has equipped predators with an amazing armoury – including teeth, claws and venom – and, in turn, prey can defend themselves with toxins, shells and spikes. Of all the weapons displayed by predators on the attack, and the defensive counter-measures evolved by prey, a snail's shell might not look very impressive. But as revealed in a recent paper by Yuta Morii, from the Hokkaido University in Japan, and an international team of collaborators, we may have underestimated this piece of armour and, more importantly, what it can tell us about speciation.

Karaftohelix land snail shells vary in their size, colour and shape. Because they look so different, some Karaftohelix species were assigned to different genera until researchers realised that these snails are anatomically and genetically indistinguishable. Given that these snails live in almost identical microhabitats and can coexist in some regions, how did such diverse shell forms evolve? Morri and colleagues suspected that predation may play a vital role.

To find out whether predation promotes phenotypic radiation in these snails, the team studied several Karaftohelix species from populations in Japan and Russia. The team observed the behaviour of snails when they were nudged with a pair of fine tweezers or housed with a predatory beetle. Two very different defensive strategies emerged. Some species behaved just like many snails in our gardens, retreating into their shells and using them like shields to keep marauding beetles at bay. But other species did something surprising: they fought back, rhythmically swinging their shells to-and-fro, bashing the beetles onto their backs.

If predator–prey interactions helped drive the phenotypic radiation we see in these snails, then we would expect these differences in behaviour to correlate with morphological variation in shells. To see whether this is the case, Morii and colleagues characterised several shell traits, such as height and diameter, and then related these traits to defensive behaviour. The team found that beetle-bashing snails have wider relative shell apertures. This makes sense given that a narrow shell entrance may be great for keeping predators out, but wider apertures allow for a muscular body capable of swinging a shell like a battle club. Additionally, shells used like shields tended to have a smaller diameter overall, suggesting that a narrow shell offers a more secure retreat.

So far, this study tells us that shells can serve as shields or clubs and that snails can go on the attack. But the key advance made by this paper is in showing how predator–prey interactions might drive speciation. Phylogenetic analyses showed that the passive (retreating) and active (beetle-bashing) strategies of defence, and the associated shell forms, evolved independently in island and continental populations of snail. This parallel evolution of similar shell shapes and behaviours suggests that selection for anti-predation defence is responsible for these behavioural and morphological differences. While researchers have long suggested that predation pressure could drive speciation, it has been unclear how this might work. Morii and colleagues show us how anti-predator adaptations can promote phenotypic radiation, which may be a key step towards speciation.

Parallel evolution of passive and active defence in land snails
Sci. Rep.