INTRODUCTION

Salinity is one of the most critical factors governing the invasion of aquatic environments by introduced species and largely determines the survival, abundance, and distribution of migrants (Carrete Vega & Wiens, 2012; Drouin, Himmelman, & Béland, 1985; Romano & Zeng, 2012; Whitehead, Roach, Zhang, & Galvez, 2011; Zardi, Nicastro, McQuaid, Rius, & Porri, 2006). When faced with novel osmotic conditions, species can respond to salinity stress through phenotypic plasticity in behavioral (Berger & Kharazova, 1997; Ho et al., 2019a; Hoyaux, Gilles, & Jeuniaux, 1976; Michalesc et al., 2010) and physiological traits (Helmuth, 1998; Whitehead et al., 2011; Williams et al., 2011). Over time, invasive populations can also show various evolutionary changes in response to new habitats (Mooney & Cleland, 2001; Sakai et al., 2001; Suarez & Tsutsui, 2008) including adaptive changes in salinity tolerance (Lee, Remfert, & Gelembiuk, 2003).
The intertidal snail Batillaria attramentaria is native to the northwestern Pacific region of Asia along the coastlines of Japan, Korea, and eastern China. In the early 20th century, it spread via oyster aquaculture (i.e., shipments of Crassostrea gigas from Japan) to the bays and estuaries of the northeastern Pacific coast of the USA and Canada (Galtsoff, 1932) and eventually appeared in the Monterey Bay, California (Bonnot, 1935). Its habitat in Monterey Bay differs strikingly from its native habitat and has much greater temporal salinity fluctuation. Tidal salinity fluctuation can impact perivisceral fluid composition and hemolymph composition (Stickle & Ahokas, 1974, 1975), and osmotic and ionic composition of the body fluid of molluscs and echinoderms (Stickle & Denoux, 1976). Despite these presumably intense challenges, B. attramentaria is a common intertidal species in its introduced range and is gradually replacing the native snail Cerithidea californica in several marshes in northern California (Byers, 2000a, 2000b). Plasticity or adaptive evolution in response to salinity stress might be a factor in its success, and in that of marine invaders worldwide. However, very little is known about behavioral responses to osmotic stress in marine invertebrates, especially gastropods (Ho et al., 2019a).
B. attramentaria is well suited for studying phenotypic changes in invasive species because it (1) exhibits direct development and has limited dispersal capacity (Kojima, Hayashi, Kim, Iijima, & Furota, 2004); (2) quickly forms relatively closed local populations after anthropogenic translocation (Bonnot, 1935; Galtsoff, 1932) or natural disasters (Sato & Chiba, 2016); and (3) has been introduced to areas that differ strongly in salinity conditions from its native region. In addition, this species exhibits a geographic subdivision that apparently corresponds to the main trajectories of the Tsushima and Kuroshio seawater currents which flow around the north and south of the Japanese archipelago, resulting in two divergent mitochondrial lineages termed Tsushima and Kuroshio (Ho, Kwan, Kim, & Won, 2015; Kojima et al., 2004). Here, we examine population-level variability and plasticity in locomotor behavior in response to salt stress in B. attramentariacollected from native and introduced locations. We applied a laboratory culturing and recording method (Ho et al., 2019a) to track horizontal crawling distances of snails during 30 days of exposure to five different salinity levels. To assess the impact of genetic composition on locomotor responses, we also sequenced the mitochondrial CO1gene for each snail. We present our results in terms of the effects of salinity, geographic distribution, and genetic composition on snail locomotion.