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.