Flightless beetles evolved distinct snail-hunting shapes on Japan's largest island

Beetles on Japan's largest island, Honshu, have evolved distinct body shapes to adapt to snail hunting, according to a new study. This beetle, Carabus blaptoides, feeds primarily on terrestrial snails and has reduced hind wings, making it unable to fly. Instead, it disperses by walking, which makes it particularly suitable for studying how geographic factors influence gene flow and population differentiation.

The beetles exhibit body shape variation, with slender forms having long, narrow heads and thoraxes to insert their heads deep into large snail shells, and stout forms with short, wide heads and powerful mandibles for crushing smaller snails' shells. Previous research indicated that regional differences in snail sizes contributed to the beetle's body shape diversification.

A research team, led by Associate Professor Junji Konuma of Toho University, combined genetic data from 170 individuals collected at 70 sites with morphological data from 50 populations to reconstruct the evolutionary and geographic history of the species. Their findings, published in the Biological Journal of the Linnean Society, suggest that body shape adaptation occurred not only among islands, including Hokkaido and Sado Island, but also within regions within Honshu.

The study indicates that adaptive divergence may be promoted by geographic distance and environmental variation within large continental islands, not just isolated islands. This process might be particularly significant for flightless animals due to their limited dispersal, which reduces gene flow among populations.