Plasticity in salt tolerance can be crucial for successful biological invasions of novel habitats by marine gastropods. The intertidal snail Batillaria attramentaria, which is native to East Asia but invaded the western shores of North America from Japan eighty years ago, provides an opportunity to examine how environmental salinity may shape behavioral and morphological traits. In this study, we compared the movement distance of four B. attramentaria populations from native (Korea and Japan) and introduced (USA) habitats under various salinity levels (13, 23, 33, and 43 PSU) during 30 days of exposure in the lab. We sequenced a partial mitochondrial CO1 gene to infer phylogenetic relationships among populations and confirmed two divergent mitochondrial lineages constituting our sample sets. Using a statistic model-selection approach, we investigated the effects of geographic distribution and genetic composition on locomotor performance in response to salt stress. Snails exposed to acute low salinity (13 PSU) reduced their locomotion and were unable to perform at their normal level (the moving pace of snails exposed to 33 PSU). We did not detect any meaningful differences in locomotor response to salt stress between the two genetic lineages or between the native snails (Japan versus Korea populations), but we found significant locomotor differences between the native and introduced groups (Japan or Korea versus the USA). We suggest that the greater magnitude of tidal salinity fluctuation at the USA location may have influenced locomotor responses to salt stress in introduced snails.

The purplish bifurcate mussel Mytilisepta virgata is widely distributed and represents one of the major components of the intertidal community in the northwestern Pacific (NWP). Here, we characterized population genetic structure of NWP populations throughout their whole distribution range using both mitochondrial (mtDNA cox1) and nuclear (ITS1) markers. Population genetic analyses for mtDNA cox 1 sequences revealed two monophyletic lineages (i.e., southern and northern lineages) geographically distributed according to the two different surface water temperature zones in the NWP. The timing of the lineage split is estimated at the Pliocene- mid-Pleistocene (5.49-1.61 Mya), which is consistent with the timing of the historical isolation of the East Sea/Sea of Japan from the South and East China Seas caused by sea level decline during glacial cycles. Historical sea level fluctuation during the Pliocene-Pleistocene and subsequent adaptation of mussels to different surface water temperature zones may have contributed to shaping the contemporary genetic diversity and deep divergence of the two mitochondrial lineages. Unlike mtDNA sequences, a clear lineage splitting between the two mitochondrial lineages was not found in ITS1 sequences, showing a star-like structure that is composed of a mixture of southern and northern mitochondrial lineages. Possible scenarios are proposed to explain this type of mito-nuclear discordance: stochastic divergence in the coalescent processes of the two molecular markers, or balancing selection under different marine environments. Future work is required to address whether the thermal physiology of these mussels correlates with the deep divergence of their mitochondrial genes.