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The accumulation of information on the geographical distribution range, genetic structure, and host utilization of the symbiont is important for studying how the symbiont spreads its distribution. Pseudocladophora conchopheria is a green alga that grows on the shells of intertidal gastropods distributed in the Japanese Archipelago, the Ryukyu Archipelago, and the southern part of Korean Peninsula. However, we discovered algae similar to P. conchopheria on the shells of gastropods from Vietnam. Therefore, species identification of the algae and the host gastropods was performed based on morphological observations and molecular genetic methods. As a result, the morphological characteristics of the algae were consistent with those of P. conchopheria, and the morphological characteristics of the host gastropods were consistent with those of Lunella granulata. Molecular genetic methods further supported these results of morphological identification. Interestingly, their algae were more closely related to P. conchopheria attached to L. correensis from the Sea of Japan rather than those attached to L. granulata from the Ryukyu Archipelago. These results suggest that P. conchopheria has expanded its distribution by utilizing multiple hosts and are explained on a wider scale than previous findings. [ABSTRACT FROM AUTHOR]

Understanding the method and history of introduction in non-native species are essential for conservation biology, especially in regions like Japan that have a long history of human activity. Bradybaena similaris is a circumtropical land snail that is considered a non-native species from Southeast Asia or East Asia to Japan, which was believed to be introduced by human activities such as sugarcane or sweet potato establishment several hundred years ago. Bradybaena pellucida, a sister species of B. similaris, is native to western Japan, but has recently spread throughout the eastern part of the country. We investigated genetic diversification pattern of non-native organisms using the two land snail species. We clarified the phylogenetic relationships of B. similaris and B. pellucida based on genome-wide SNPs and mtDNA. We revealed the population demographic history using ABC analysis with SNP data. Bradybaena similaris of mainland Japan could be distinguished from populations on the continent. Introgressive hybridization between the species occurred approximately 29,700 years ago. The present findings provide two new hypotheses: that agriculture began in Japan about 2800 years ago, Japanese populations of B. similaris were introduced from the continent more than 29,000 years ago by human activity other than agriculture; or B. similaris is native to a part of mainland Japan. The history of a non-native species with human activity is complex, urging us to reconsider previous hypotheses. [ABSTRACT FROM AUTHOR]

Supplementary Information The online version contains supplementary material available at https://doi.org/10.1007/s10530-023-03154-9. Graph: Supplementary file1 (DOCX 42 kb) Publisher's Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. [Extracted from the article]