Temperature-driven mitochondrial lineage splitting
Phylogeographic patterns of marine organisms in the NWP are shaped by the interplay of historical and/or contemporary oceanography, and present-day gene flow, which is often correlated with species-specific life-history features such as larval traits (Cheang et al., 2012). The timing of lineage splitting event between the two M. virgatalineages is estimated to be 5.49-1.61 Mya. Although there is substantial deviation between estimates using the lowest and highest calibration bounds of the molecular clock, the estimated divergence time coincides with the glacial periods from the Pliocene to Pleistocene epochs. Sea level fluctuation during this time affected population fragmentation and demographic changes for many coastal species, resulting in population structuring and/or lineage splitting in the NWP (Shen et al., 2011; Ludt & Rocha, 2014).Cyclic alternations of glacial-interglacial periods dramatically affected the seafloor topography and sea level dropped up to 120 m below its present level during the glacial periods (i.e., early Pleiostocene), resulting in historical isolation in the NW Pacific marginal seas: Pleiostocene isolation of the ES/JS, ECS, and SCS (Wang, 1999; Voris, 2000). The ES/JS was semi-enclosed and prevailed by cold surface water flows from the northern sea during the Northern Hemisphere glaciation and mid-Pleiostocene transition (Tada, 1994; Kitamura et al., 2001; Matsuzaki, Itaki, Tada, & Kamikuri, 2018). Meanwhile, the SCS was separated from the ECS (Ota, 1998; see Fig. 7 for details) by a large land bridge extending between eastern China and Taiwan Island (Kimura, 2000). Peng et al. (2019) found that there is a post-LGM expansion ofM. virgatus in Zhejiang, China (belonging to the southern lineage in the present study), based on population genetic analysis of mtDNAcox1 and 16S rDNA sequences.
In addition to historical geography in the NWP, some environmental factors (e.g., ocean current, surface water temperature and salinity) might act in combination with stochastic processes such as local selection and/or lineage sorting through time in shaping contemporary population structure (Palumbi, 1994; Miglietta, Faucci, & Santini, 2011). In our results, the geographic distribution of the two mitochondrial lineages agrees with two biogeographic regions with different surface water temperature zones: the northern lineage occupies the North Pacific Temperate Biotic Region which is affected by the cold-water Liman Current flowing southward from the Sea of Okhotsk into in the East Sea (the Sea of Japan) and the Oyashio Current (also known as the Kurile Current) that flows south and meets the Kuroshio Current off the eastern shore of Japan. In contrast, the southern lineages are occupied by Chinese populations, Taiwan population and the southern Japan populations, all belonging to the Indo-West Pacific warm-water Biotic Region which is affected by the warm, northeasterly flowing Kuroshio Current. It is interesting to note that some Japanese populations where the two lineages co-occur (JSH, JSS, JKS, JOI, JKG, JNG; Table 1 and Fig. 1) agree well with the region where two currents with different surface water temperatures meet: JOI (16), JKG (17), and JNG (18) are found on the western side of Japanese archipelago where the cold-water carrying Liman Current (LC) and the warm-water carrying Tsushima Warm Current (TSWC) meet in the North Pacific Temperate Biotic Region, whereas JSH (8), JSS (9), and JKS (11) are found in southeastern side of Japan where the Kuroshio Current (KC) mixes with the southeasterly flowing, cold-water carrying Oyashio Current (OC) in the Indo-West Pacific warm-water Biotic Region.
Present-day oceanic currents are among the most influential factors that contribute to continuous gene flow over a wide geographic range in many marine invertebrates, including molluscan species (Dong et al., 2012; Guo et al., 2015; Li et al., 2017). M. virgata has a planktonic larval stage for 4 days, allowing them to disperse considerable distances via northeasterly flowing ocean currents (Sasaki, 1984). The present-day geographic distribution of the northern and southern lineages is assumed to be due to current-driven long-distance dispersal of planktonic larva from the northern and southern glacial refugia. Deep lineage splitting and geographic distribution of the two mitochondrial lineages uncovered in this study suggest that along with ocean currents, surface water temperature preference and/or differential selection between the respective ancestral populations of the southern and northern lineages have shaped contemporary phylogeographic patterns: The ancestral population of the northern lineage would have been under selection for adaptation to colder water, while the southern lineage might have adapted to a warmer environment. This thermal adaptation likely restricted gene flow between the two regional populations by reducing migrant fitness, resulting in lineage splitting along a temperature gradient (Teske, Von der Heyden, McQuaid, & Barker, 2011; Teske et al., 2019). Such a vicariant lineage splitting following long-term adaptation to different temperature zones has been reported in some other marine species including mantis shrimp (Cheng & Sha, 2017), mitten crab (Xu et al., 2009), barnacles (Tsang et al. 2012), and flathead mullet (Shen et al., 2011). We cautiously suggest that cryptic mitochondrial lineage splitting in M. virgata might be the result of an adaptive response (i.e., a balancing selection) to different environments, most likely different surface water temperature zones. An extensive genetic survey of nuclear genes, along with in-depth analysis of balancing selection from mitochondrial genome sequences, would be needed to clarify how these two mitochondrial lineages have adapted in response to thermal differences in the NWP.