The habitat use and trophic niche comparisons among closely related land snails in Northeast Asia

The diversification of phenotypes and species has been a major concern in evolutionary biology. Both resource competition and predator-prey interactions have been suggested as mechanisms that drive phenotypic divergence and speciation, but the latter has attracted much less attention than the former. The Karaftohelix land snails in Northeast Asia provide an excellent model to investigate the effects of competition and predation on the divergence process. This is because (1) all species-especially Karaftohelix editha and Karaftohelix gainesi in northern Japan-are closely related and sometimes indistinguishable by genital morphologies and neutral nuclear and mitochondrial DNA markers, despite the distinct phenotypes among species; (2) two different anti-predatory strategies (i.e., passive and active defenses) are observed for each species in this group; and (3) multiple species have generally overlapping distributions and were often observed at exactly the same time and place. In this study, we conducted quantitative quadrat surveys and carbon/nitrogen stable isotope analyses to compare the microhabitat and trophic niches among Karaftohelix land snails and estimated the cause of adaptive radiation in this taxon. All the results indicated that the differences in microhabitats and trophic niches among Karaftohelix land snails-especially between the most closely related sister species, K. editha and K. gainesi in northern Japan-were too small to promote the adaptive radiation in this group. Therefore, it is difficult to explain the extreme divergence of phenotypes and species among Karaftohelix land snails in Northeast Asia based on environmental adaptation through resource competition and/or reproductive interference.