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.