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.