DISCUSSION

Compared to the control groups (exposed to a salinity of 33 PSU, a mean salinity of seawater), we found that all four B. attramentariapopulations substantially reduced their movement distance when being exposed to low salinity (13 PSU) and did not significantly alter their locomotion in response to moderate increases or decreases in salinity (43 and 23 PSU; Figure 3A). Reduced activity seems to be a general gastropod response to unfavorable environmental conditions (Elfwing & Tedengren, 2002; Hughes, Chapman, & Kitching, 1987; Kitching, Chapman, & Hughes, 1987). Marine invertebrates commonly reduce their activity when experimentally exposed to salt stress (Berger & Kharazova, 1997; De Lange, Noordoven, Murk, Lürling, & Peeters, 2006; Felten et al., 2008; Lawrence & Poulter, 2001; Piscart, Webb, & Beisel, 2007), presumably to conserve energy for ionic-osmotic regulation (discussed in Ho et al., 2019a). Thus, the reduction in movement observed at 13 PSU validates our experimental approach as an appropriate method for measuring stress responses in snails under varying salinity exposures. A lack of differences in locomotion among the 23, 33, and 43 PSU groups suggests that B. attramentaria can successfully acclimate to moderate changes in salinity. The universal failure to thrive among the groups exposed to 3 PSU may indicate that this approximates the lower threshold of this species’ salinity tolerance.
Based on the LMM, ANOVA, and post-hoc tests, we identified significant differences in locomotor responses between native and introduced populations, but no significant differences in locomotor responses among native populations, regardless of lineage. In addition, snails belonged to the Tsushima or Kuroshio lineages (the li factor) did not have any meaningful impact on their locomotor performance (Model xv, Table 3), even though the relative importance of the li factor was high (85%) and ranked just after the es, o, and es × o factors (Table 1). Taken together, the Matsushima, Japan population and the Elkhorn Slough, USA population responded to salt stress quite differently, despite being closely related. Similarly, when the USA population was compared against other countries, the locomotor response also varied: the USA snails exposed to 13 PSU exhibited the shortest movement distance, and this location also had lower performance at 33 PSU compared with snails from other countries (Figure 2C). The Elkhorn Slough site, where B. attramentaria invaded at most 80 years ago, experiences a much wider range of salinity levels due to tidal fluctuation than the Korean and Japanese locations. Differences in responses to salinity stress in the introduced population could be due to local adaptation, phenotypic plasticity, or both. The salinity fluctuation records for Elkhorn Slough, as well as the fact that we recorded a point salinity of 4 PSU at the time of collection, suggest that this particular invasive population is frequently exposed to the lower limit of its salinity tolerance, and is presumably under strong selective pressure. Exposure to an extended period of 13 PSU would be unusual for the native snails from Korea and Japan but not for the introduced snails in the USA. Additional population sampling from other regions of the USA could be helpful in elucidating whether the differences we observed in salt-stress responses are common to the snail’s whole introduced range, or specific to the Elkhorn Slough.
Our examination of shell length indicated that introduced snails were significantly longer than native ones by about 31% (Appendix 1: Table A3B and Figure A2). This is comparable to a previous study reporting a size increase of about 14% in this species (see Figure 1, Grosholz & Ruiz, 2003). We also found significant differences in size among native populations (Appendix 1: Table A3 and Figure A2). Anatomical and morphological changes in marine gastropods after introduction to a new region are not uncommon (for example: changes in the excretory system of the littorinid Cenchristis muricatus (Emson, Morritt, Andrews, & Young, 2002); shell color polymorphisms in White Sea Littorina saxatilis (Sokolova & Berger, 2000), and increases in body size inIlynassa obsoleta and Urosalpinx cenerea (Grosholz & Ruiz, 2003)). Increases in body size in introduced populations might be due to life history selection, more abundant resources, or absence of key predators or parasites (Mitchell & Power, 2003; Torchin, Lafferty, & Kuris, 2001). Alternatively, significant variations in size might be due to age structure; for instance, larger and older snails might be regularly harvested by humans at Hacheon and Matsushima bay.
Is a larger body size responsible for the reduced locomotor performance that we observed in these invasive snails? We suggest that it is not, but caution the reader that our study was not designed to test this question directly. Body size would be expected to be positively rather than negatively correlated with locomotion speed in marine invertebrates: for example, in the jellyfish Aurelia aurita(McHenry & Jed, 2003), sea urchin Paracentrotus lividus(Domenici, González-Calderón, & Ferrari, 2003) and sea starArchaster typicus (Mueller, Bos, Graf, & Gumanao, 2011), although not in other sea star species including Linckia laevigata , Protoreaster nodosus , and Acanthaster planci(Mueller et al., 2011) and the bat star Patiria miniata(Montgomery & Palmer, 2012). Currently, there is no strong evidence for a relationship between body size and speed in gastropods except for one study of the terrestrial Cornu aspersum, which displayed a positive correlation between foot length (but not body mass) and speed (Hemmert & Baltzley, 2016). In contrast, we observed the shortest movement distances in the population with the largest average body size. Furthermore, we did not find any significant difference in movement distance among the native populations or between the two CO1lineages, all of which had significant size differences (Appendix 1: Table A4). Thus, our results do not support a link between size and locomotion in B. attramentaria . However, an additional research specifically designed to test whether size contributes to salt tolerance and locomotion is needed.
In conclusion, this paper was successful to investigate locomotor responses to salt stress in the intertidal snail Batillaria attramentaria from different geographic locations and having different genetic composition. We observed that snails living in native habitats (Korea and Japan) and belonging to different genetic groups (Hacheon/Nemuro versus Matsushima) did not significantly differ in their responses to salinity stress. However, we found that a population of introduced snails (in the USA) exhibited shorter movement distance than snails from native habitats when exposed to salinity stress. This study demonstrates intraspecific variation in salt tolerance in snails, and suggests a correlation between locomotor performance and tidal salinity fluctuations. We speculate that plasticity and adaptive evolution may have contributed to the ability of B . attramentaria to successfully invade a novel osmotic niche.