Introduction
The northwestern Pacific (NWP) is a vast marine realm characterized by unique tectonic and hydrologic features (Wang, 1999), providing a natural setting in which to investigate mechanisms and processes leading to marine diversification and speciation (Kong, Matsukuma, Hayashi, Takada, & Li, 2012; Ni, Li, Kong, & Yu, 2014; Ho, Kwan, Kim, & Won, 2015; Wang, Tsang, & Dong, 2015; Qiu, Li, Lin, Ding, & Miyamoto, 2016). Three continuous marginal seas are distributed from north to south in the NWP, including the East Sea (ES)/Sea of Japan (SJ), East China Sea (ECS) and South China Sea (SCS) (Tamaki & Honza, 1991). The size and connectivity of these seas probably dramatically changed throughout the Pleistocene (Wang et al., 1997). During past glacial epochs, sea levels dropped more than 120 m below the present level, resulting in the exposure of sills and continental shelves in the NWP (Wang & Sun, 1994; Voris, 2000) and effectively closing the sea passage (the Korean Strait, depth <130 m) between the ES/SJ and ECS, resulting in their geographic separation (Kitamura, Takano, Takata, & Omote, 2001). The ECS experienced pronounced seawards migration (~1200 km), and was reduced to an elongated and closed trough (the Okinawa basin) (Xie, Jian, & Zhao, 1995), while the SCS became a semi-closed gulf with an area that was half its present size, connected to the Pacific through the Bashi Strait (Wang, 1999).
The dramatic paleohydrogeological changes in the NWP probably had a profound impact on marine species’ distribution and evolution (Kong & Li, 2009; Liu, Li, Kong, & Zheng, 2011). An evolutionary paradigm emerging from marginal sea separation is that each sea basin served as an independent refugium for surviving populations during the glacial periods, and enhanced vicariant divergence among separated populations (Liu, Gao, Wu, & Zhang, 2007; Xu, Chan, Tsang, & Chu, 2009; Ni et al., 2014; Qiu et al., 2016). This pattern has been demonstrated in various marine faunas, including fishes (Shen, Jamandre, Hsu, Tzeng, & Durand, 2011; Song, Ma, Zhang, Gao, & Sun, 2014; Qiu et al., 2016), molluscs (Kong & Li, 2009; Liu et al., 2011; Ni, Li, Kong, & Zheng, 2012), barnacles (Chan, Tsang & Chu, 2007; Tsang et al. 2012) and mitten crab (Xu et al., 2009; Xu & Chu, 2012), all of which show substantial intraspecific evolutionary partitions among the seas. However, the paradigm is not applicable to all cases and is often complicated by biotic and/or abiotic factors, such as surface water temperate gradients, ocean currents, and diverse species-specific life-history traits (Dong, Han, Ganmanee, & Wang, 2015; He, Mukai, Chu, Ma, & Zhang, 2015; Ni, Li, Ni, Kong, & Yu, 2015; Li et al., 2017; Ni et al., 2020).
The surface water temperature gradient along the main coastline of the NWP is substantial. This coastline spans a broad latitudinal range roughly from 15 to 45° N and represents a transition zone connecting the tropical Indo-West Pacific and the cold North Pacific (Chen & Jiao, 1997; Briggs & Bowen, 2012). A biogeographic line, which begins at the Yangtze Estuary in China and extends northeastwards to Jeju Island (Korea) and southern Japan (Fig. 1), separates the North Pacific Temperate Biotic Region (characterized by cold-temperate fauna) and Indo-West Pacific Warm-water Biotic Region (characterized by tropical/subtropical fauna) (Liu, 2013; Ni, Kern, Dong, Li, & Park, 2017). Species with narrow thermal tolerances are restricted to one side of the boundary because of the steep temperature gradient across the Yangtze Estuary (Zhang, Qi, Zhang, & Ma, 1963; Liu, 2013): the annual sea surface temperature is >20°C on the south side of the estuary, and rapidly decreases to <15°C on the north side (Johnson & Boyer, 2015). The surface water temperature gradient in this region has been reported as an influential driver that accelerates genetic differentiation (e.g., seagrass Zostera japonica , Zhang, Zhou, Xue, & Liu, 2016) or cryptic speciation between neighboring populations (e.g., fish Mugil cephalus , Shen et al., 2011). However, whether this temperature gradient is a general driver of genetic differentiation in other marine species, especially coastal invertebrate species, is still largely unknown.
Ocean currents can also impact species’ genetic patterns in this region (Dong et al., 2012; Han, Yanagimoto, Zhang, & Gao, 2012; He et al., 2015; Li et al., 2017). The main current systems in the NWP include the cyclonic circulation of the Kuroshio Current (KC) and coastal currents (Su & Yuan, 2005; Wei, Yu, Ran, & Zang, 2011). An extension of the North Equatorial Current, the KC is a major current that dramatically affects this region’s environment and ecology (Kao, Wu, Hsin, & Dai, 2006; Fujikura, Lindsay, Kitazato, Nishida, & Shirayama, 2010). It originates east of the Philippines and flows north-eastwards through Taiwan and the Ryukyu Islands to the Pacific coast of southern mainland Japan (Su, 2004). The KC and its branch currents transport large volumes of water, salt, and nutrients from low latitude tropics to the northern reaches of the marginal seas (Chen, 1997; Heath, Zenitani, Watanabe, Kimura, & Ishida, 1998). This region is also characterized by diverse coastal currents (Su & Yuan, 2005; Wei et al., 2011), such as the Subei Coastal Current and the China Coastal Current along the coast of China, the Korean Coastal Current around the Korean Peninsula, and the Oyashio Current (a cold subarctic ocean current that originates in the Bering Sea flowing southwest off the Kuril Islands and meet the KS east of northern Japan). Oceanographic currents may potentially enhance population connectivity, often producing different outcomes from sea surface temperature gradient. Lack of genetic structuring among distant populations can be maintained by continuous gene flow of current-driven larval dispersal (Guo et al., 2015) and rafting events (Nikula, Fraser, Spencer, & Waters, 2010).
The purplish bifurcate mussel Mytilisepta virgata (Wiegmann, 1837), a member of the family Mytilidae, is widely distributed in the NWP including Korea, Japan, mainland China and Taiwan and forms large mussel beds, constituting one of the major intertidal rocky shore communities (Iwasaki, 1994; Liu & Morton, 1994; Morton, 1995; Okutani, 2000; Min, Lee, Koh, & Je, 2004). These mussel beds contribute to marine biodiversity in the intertidal zone by providing habitat and shelter for diverse marine invertebrate species (August, 1985; Seed, 1996). This species spawns twice a year, in spring (from February to March) and autumn (from September to October) (Morton, 1995). The larval duration of this species’ planktonic stage is 4 days (Sasaki, 1984), which can facilitate gene flow over a certain distance. These traits (distribution range and larval ecology) make this species ideal for testing the impacts and their relative strength of historical sea level fluctuations, surface water temperature, and ocean currents on marine phylogeography. In this study, based on comprehensive sampling throughout the mussel’s entire distribution in the NWP, we assessed multiple lines of evidence (including mitochondrial and nuclear genes as well as morphological data) to (1) test whether there exist evolutionarily divergent linages in the NWP and (2) investigate driving factors that may have prompted cryptic divergence.