Movement and local adaptation between ecotypes of the snails
Littorina obtusata on rocky intertidal shores
Introduction:
Rocky shores have some of the most extreme environmental conditions owing
to both abiotic variables such as temperature, salinity, wave action and irradiation;
and biotic variables such as competition, predation and interaction among species
(Johannesson, 2003). In order to cope with the environmental stress, species may
adopt different lifestyle strategies; consequently, ecological adaptation has occurred
in heterogeneous habitats (Johannesson, 2003). The ecological adaptation is
especially important for the motile animals such as gastropods on the rocky shore.
According to Reid (1996), the shell shape and size are very similar in the same habitat
among ecotypes of different species of Littorina. Generally, in the moderate wave,
exposed zone on the rocky shores, the shells of snails are large and thick and the
apertures are small to prevent crab attacks (Boulding and Van Alstyne, 1993 and
Johannesson, 1986). In the “wave-exposed” zone where the crabs were rare but the
dislodgement by wave-action is the main problem, the snails have evolved relatively
thinner and smaller shells with a large foot and aperture to cope with the
dislodgement by waves (Trussell et al., 1993).
Since there are no physical barriers on the rocky shore, which can be referred
to as sympatric habitats, gene flow between organisms in different habitats occurs.
Although gene flow might spread favorable alleles among populations, it can also
hinder local adaptation by mixing of the alleles from parent populations.
(Johannesson, 2003). According to a study on Littorina saxatilis from Rolan-alvarez
(2006), two ecotypes of L. saxatilis from high intertidal zones and low intertidal zones
with distinct characteristics do meet and mate, producing intermediate forms of
hybrids on a rocky shore in Spain. However, the frequency of hybrids was relatively
low and the adaptability of them was poor (Rolan-alvarez, 2007). In order to maintain
the high adaptability, the organisms must evolve some mechanism, or reproductive
barrier, to avoid gene flow. The partial reproductive barrier between the two ecotypes
of L. saxatilis was due to the assortative mating, which was a consequence of male
behavioral differences and non-random distribution over micro-habitats. (Erlandsson
et al., 1998 and Johnanesson et al., 2000). In other words, the males do not mate at
random with females, and the distinct ecotypes exhibited specific micro-habitat
selection. Thus, the migratory behavior of animals and maintenance of zonational
distribution are extremely important with respect to gene flow within sympatric
habitats. Furthermore, for the marine gastropod that doesn’t have a pelagic larval
development, the dispersal pattern was mainly influence by local migrations
(Erlandsson et al. 1998). Lots of factors can influence the movement of gastropods,
such as gravity, the direction of wave movement and light. (Gendron, 1977 and
Petraitis, 1982). The maintenance of a position on the shore is an essential problem
encountered by the motile intertidal animals (Gendron, 1977). When the molluscs
have been dislocated, behavioral mechanisms and environmental cues are important
for the animals return to their preferred habitat (Erlandsson et al. 1998).
Littorina obtusata is a common periwinkle that lacks a pelagic larval
development on the intertidal zone in North Atlantic (Johannesson, 2003). According
to Johnannesson (2003), the L. obtusata shows strong phenotypic variation among
populations inhabiting different types of environments, thus geographical variations
in shell morphology occurred, or ecotypes were produced (Reimchen, 1981).
Particularly, according to Brookes and Rochette (2007), when the L. obtusata snails
were exposed to predation (green crab) cues, the thickness of shells were increased.
Therefore, the shell characteristics of L. obtusata snails from high intertidal zones
were different from the snails from low intertidal zones. The migratory behavior is
extremely important for the maintenance of the local adaptability of the snails.
In this study, the movement of L. obtusata on rocky intertidal shore will be
investigated. The hypothesis tested is that L. obtusata snails show preference towards
their intertidal height of origin. It is expected that after the snails have been released,
the snails from high intertidal zone origins will tend to move upward from the release
line, while the snails from low intertidal zone origins will tend to move downward
from the release line.
Materials and Methods:
Material
L. obtusata were collected from a rocky shore in Letite, New Brunswick on
Oct 31st. In the higher intertidal zone (2.80 m-3.28m above mean water level), 100 L.
obtusata with a size range of 13 mm- 15 mm were collected. 100 snails of the same
size range were sampled between two tide pools (1.72 m- 2.20m above mean water
level). The upper and lower limits of the L. obtusata were 3.28 m and 1.72 m,
respectively.
Methods
Sampled snails were brought to the laboratory. The biofoul on the shell was
removed by cleaning with 95% Ethanol. The snails were exposed to air and dried for
10 minutes. 50 upper-shore snails and 50 lower-shore snails were painted with green
nail polish. The remaining 50 upper-shore snails and 50 lower-shore snails were
painted with blue nail polish. No mortality among snails was observed during the
preparation.
The sampled snails were released on the next day at 10:30 am at the lower
shore. At the fist release line that is 2.26 m above the mean water level, 50 green
high-origin snails and 50 blue low-origin snails were released on top of a macro- algal
mat on a rock with an incline that is parallel to the wave motion. Of the remaining
100, 50 were painted blue for high origin and 50 green for low-origin snails. These
were released at 2.41 m on top of macro-algal mat on a rock with an incline marked
as 2nd release line that is parallel to the coastline. The distance between two release
lines was around 5 m.
Marked snails were recaptured 6 days after release by carefully searching for
surviving snails in each release spot. The color of each snail was recorded and the
minimum distance moved from the release line to the spot of recapture on the ground
was measured using a measuring tape in each site. There were 8 intervals from 0o360oon the protractor. The protractor was placed in the release spot with 0o being the
release line and 90o being on the steep landward side. The direction from the release
line to the recapture spot was estimated with the protractor. The height of the highest
snail found in each site was recorded as well as the lowest point.
the
Statistical analysis
The recovery rate of high and low-origin snails was calculated in each site.
The vertical height and slope of the incline was calculated by trigonometric function.
The height of each snail was then calculated by the slope and distance. The average
moving distances, directions and height were calculated for high-origin and loworigin snails in both sites. Moving distances and heights of high-origin and low-origin
snails were applied to the t-test to determine the statistical significance. The positions
of each snail were plotted in a coordinate system for both sites. The mean vectors and
distribution for high-origin and low-origin snails was calculated and compared for
each site.
Results:
After the snails were recaptured, the distribution of the snails at both release
sites was shown in Figure 1 and Figure 2. In Site 1, the average movement of uppershore snails was 68cm, 75.5o towards land, while the average movement of lowershore snails was 34cm, 51.3o towards land. In Site 2, the average movement of uppershore snails was 176cm, 108.5o towards land, while the average movement of lowershore snails was 28cm, 96.1o towards land. The recapture rates of two experimental
groups at release Site 2 were similar (Table 1). The recapture rate of low intertidal
snails was almost twice the recapture rate of high intertidal snails at release site 1
(Table 1). At release Site 1, the height of the snails recaptured ranged from 38 cm
above the release point (264 cm above mean water level) to 48 cm below the release
point (178 cm above mean water level). At the release site 2, the height of the snails
recaptured ranged from 120 cm above the release point (361 cm above mean water
level) to 67cm below the release point (174 cm above mean water level). ). At the
release site 1, 83% of the snails that were collected from high intertidal zone were
above the release point and 64% of the snails that were collected from low intertidal
zone moved below the release point (Table 2). At the release site 2, 84% of the snails
that were collected from high intertidal zones were above the release point and 62%
of the snails that were collected from low intertidal zone moved below the release
point (Table 2). At both release sites, the average vertical distance and direction of
movement of snails was towards land and higher shore (site 1: P=0.0007; site 2:
p=0.002) rather than towards the release site (Figure 3 & 4). In regard to the
movement height, the snails who’s origin was the high intertidal zone moved higher
on average (site 1: p=0.0018; site 2: p=0.0016) than the snails who’s origin was the
low intertidal zone (Figure 5 & 6). The average vertical distance and height of snails’
movement in site 2 was higher than that in site 1 comparing within the same origin
groups (Figure 3, 4, 5 & 6).
Discussion:
The average movements of L. obtusata in both sites were upper-shore snails
moved upward, towards land, as expected. Particularly, 83% of the upper-shore snails
in site 1 and 86% of the upper-shore snails in site 2 moved upward from the released
line. At the same time, the movements of snails who’s origin was the low intertidal
zone was upward, towards land as well. However, the average movement of lowershore snails was significantly smaller than the upper-shore snails. These results
exhibits that the upper-shore snails moved further shoreward, while the lower-shore
snail stayed relatively close to the release line.
The mean movement of L. obtusata, regardless of origin, in one week, ranged
from 0.3cm to 43cm in height; the mean vertical distance movement was ranged from
20 cm to 167cm. This movement was not a significant distance. The low mobility of
L. obtusata might help to hinder the gene flow between the high intertidal zone snails
and the low intertidal zone snails (Erlandsson et al., 1998). Indeed, there were
relatively long movements that did occur (over 4 meters migration distance), mainly
in the upper-shore snails. This may be due to the effects of transplantation,
disturbance and unfamiliar habitat (Erlandsson et al., 1998). Since both sites of our
studies were conducted in the lower intertidal zone, the upper-shore snails were
transplanted far away from the high intertidal zone. Therefore, the transplantation
may contribute to the longer distance of migration of upper-shore snails, while a
shorter distance of migration can be seen in lower-shore snails.
In comparing the movements between the two sites, generally the vertical
distance of snails’ movement in site 2 was much higher than the vertical movement in
site 1. These is due to the slope of release site 2 being much steeper than in site 1.
Furthermore, when the vertical distances of movements were compared with the
height of migration, the height of migration is greatly smaller than the vertical
distance of movement. Therefore, although the snails moved a relatively long vertical
distance, the difference in height is small.
The recovery rate of the snails from different habitats was almost the same,
except for the lower-shore snails in site 1, which has a recovered rate of 88%. This
suggested that the two colors of paint did not have detectable difference influences on
the behavior of snail or the recovered rate of the snail.
In this study, the L. obtusata showed a similar pattern of movement as other
Littorea species (Rochette and Dill, 2000). The vertical distribution and the moving
pattern of snails is affected by many factors, which include both physical and
biological conditions such as the wave action, light availability (Evans, 1965), food
availability, and predator presence (Rochette and Dill, 2000). In the rocky shore
habitat, the wave action decreased as height. The light availability was higher in the
high intertidal zone but the heat stress was great. In the east coastal zone of Canada,
the Littorea species have a higher mortality rate due to more predators such as green
crabs, fish, and seagulls at low intertidal zones compared to high intertidal zones
(Rochette and Dill, 2000). The animals that live in different microhabitats would tend
to gain the maximum advantage by adapting to the local environmental conditions
through natural selection (Slatkin, 1987). The selection that is caused by the biotic
and abiotic factors from the lower- to upper-shore levels (Rolan-Alvarez et al., 1997)
and the movement pattern that promote individuals to return to their native habitat
would obviously be adaptive.
The gene flow and dispersal can have variety of effects on evolution in either
promoting or constraining the local adaptation (Garant et al, 2007). The motile
intertidal animals such as the snails have adapted to the most suitable habitat. The
problem is the maintenance of their position on the shore (Gendron, 1997), which
means they must be able to orient up or down the shore if dislocated out of its
preferred habitat or the habitat preference was changed (Gendron, 1997). Many
phenotypes that are related to local adaptation are genetic based (Rochette and Dill,
2000). To maintain the local adaptation, the specific genes have to be maintained
within the population. Immigrants and migration will introduce genes adapted to other
conditions and dilute the benefic specific genes through hybrids (Slatkin, 1987). The
tendency of snails to move back to their original habitat can reduce the chance of
hybrids of two ecotypes. The microhabitat preferences and reduced chance of
hybridization are the mechanisms evolved by intertidal gastropods in order to
maintain the phenotypic differences between ecotypes (Rochette and Dill, 2000). In
this study, it was found that the L. obtusata from high intertidal zone had a greater
tendency to move toward the shore side, where it is originally from. These movement
patterns of the low and high intertidal L. obstustata ensured that the two ecotypes
were separated by different preferences of microhabitat and avoided the loss of
benefit genotypes through gene flow. On the other hand , when the dispersal of the
animals is high, the movement patterns will prevent local adaptation by increasing the
gene flow between populations. For example, the three-spine sticklebacks
(Gasterosteus aculeatus) in Parapatric Lake experiencing higher gene flow differed
less in morphological traits (Hendry and Taylor, 2004) due to the high dispersal level
of the species.
The reinforcement that occurs by avoiding hybrids will increase the isolation
level of the population (Grahame et al, 2006) and increase the preferences of the
habitat as well through a movementpattern. Reinforcement was found with hybrids
being unviable and sterile (Servedio and Noor, 2003), which means that random
mating is prevented. It suggested that the genotypes that have the tendency to move
out of the population have less chance to reproduce successfully or are too unable to
pass their genes to the next generation. Therefore the genotypes that stay in the
original habitat will be reinforced, and the percentage of the favorable alleles will
increase overtime. In other words,, the genotype that prefersnot to hybrid will be
selected. In Coyne and Orr’s study (1997), the sympatria taxa of Drosophila showed
great levels of prezygotic isolation with short genetic distance, which was concluded
as the effort of reinforcement. It is highly possible that the different movement pattern
of the L. obstusata that were originally from low and high intertidal zone were due to
the reinforcement. The individuals that did not have the tendency to move back to
their original habitat would have less chance of survival in an unsuitable habitat and
would therefore be unlikely to pass their genes to next generation. Therefore, the
certain movement patterns of L. obstusata were highly selected.
Although the result of our study supported the hypothesis, that snails tend to
move to their original habitat, there might still be some inaccuracy in our study. The
tide of the recapture day was 1.6 m, which was only 0.81 m lower than the release
line. The percentage of recovered below the release line in the two sites was relatively
low, 27% and 26% respectively. This might be because the release line had been set
too low so that some of the lower-shore snails could not be found (submerged into
water) after released. Therefore, the values for snails recaptured below the release line
might be underestimated. Another explanation is that the presence of predators in the
water that is below the release line caused the entire snail survey to move upward
towards land. According to Palmer (1979), fishes and crabs are the most important
predators on lower intertidal gastropods. Therefore, both of the lower-shore and
upper-shore snails move upward towards land and the low recovery rate below the
release line might be explained by the pressure of predators in that area. Another
problem is that the two release lines were close to each other. Snails were found in
between the two release sites. Since the color was reversed in the second release site,
the snails that migrated high could not be distinguished between the other release site.
In this study, the snails were disturbed and dislocated from their origin,
however previous studies have shown that the movement of snails would not be
affected by such factors (Chapman, 1986). The densities of conspecifics in the
release spots were higher than in their original site . However, the density has little
effect on the movements of Littorina (Chapman, 1986). Nail polish was assumed to
have no impact on the snail’s behavior. The disturbance associated with painting had
very little effect on their subsequent movement (Chapman, 1986). The marked snail
should have random movement rather than directional. Future study should confirm
that different colors of nail polish would not cause other change, such aslight
absorption variations that could influence the body temperature.
The gradient in the release site was assumed to be the same as the entire
intertidal zone. The horizontal difference in this area was neglected. The movement of
the snails was assumed to be determined solely by the height. However, the migratory
patterns are possibly influenced by other factors such as food availability, light
sources, predators and water movement (Erlandsson et al., 1998). In future studies,
multiple sites should be selected to separate those factors in each batch to determine
the direction and movement pattern. Sites can include upper shore, mid-shore and
lower shore to see if the horizontal gradients have any effect on the snail movement.
Snails are inactive when light intensity and temperature are low (Newell, 1958). This
experiment should be done in different time of year to see the seasonal variance and
further confirm the conclusion. It may be important to measure movements several
times to get a general idea of patterns of movement. Different patterns of movement
may occur in low-origin and high-origin snails (Chapman, 1986).
Human error may also contribute to the uncertainty of this experiment
including, missing snails, limited searching radius or search duration. The searching
radius limited the recapture. Some snails may have migrated further than our
searching range and would have been missed. Recovery rate may increase by
increasing the searching radius and searching duration. The angle was estimated with
the protractor. The direction was determined by estimation according to the eight
intervals on the protractor, which may raise systematic error. Protractors with higher
precision should be used in future studies. The measuring tape might not be a straight
line on the ground between two spots when measuring the minimum distance, which
may also contribute to sources of error. The slope was obtained by measuring the
highest point and the distance and calculated by the trigonometric function. The
height of each snail was then calculated by using the slope. Since the releasing spot
was not a perfect incline as assumed, there was uncertainty created in this method.
The height of each snail could be measured to achieve a higher accuracy.
In conclusion, our study supported the hypothesis that L. obtusata snails have
a tendency to move towards their intertidal height of origin. Local adaptation has been
shown to be essential to the survival of the snails.
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Tables and Figures:
200
150
vertical distance (cm)
100
-100
Low intertidel snials
50
High intertidal snails
0
-50
0
50
100
150
200
-50
average of high intertidal
snails movment
average of low intertidal
sanils movemnt
-100
-150
Horizontal distance (cm)
Figure 1. The mean vectors and distributions of snails recaptured after have been
released for one week in site 1, in Letite, NB
500
400
300
vertical distance (cm)
-400
High intertidal snails
200
Low intertidal snails
100
average low intertidal snails
movemnt
0
-300
-200
-100
0
100
200
300
average high intertidal snails
movement
-100
-200
-300
Horizontal distance (cm)
Figure 2. The mean vectors and distributions of snails recaptured after have been
released for one week in site 2, in Letite, NB
Table 1. The recapture number and rate of snails from two releases sits after released
in Letite, NB.
Samples 1
Recapture number
Recapture rate 2
HI in site 1
23
46%
LI in site 1
44
88%
HI in site 2
22
44%
LI in site 2
21
42%
“HI” indicated the snail samples that were collected from high intertidal zone. “LI” indicated the snail
samples that were collected from low intertidal zone.
2
The total release number was 50 for each experimental group.
1
Table 2.The summary of position of snails collected from each released site, showing
the angle and direction of the movement in Letite, NB.
Samples1
Release site 1
HI
LI
n
Percentage
Angle
Direction2
19
4
3
28
13
83%
17%
7%
64%
29%
<180 (>0)
>180
0
<180 (>0)
>180
Land
Sea
H
Land
Sea
Release site 2
HI
19
86%
<180 (>0)
Land
3
14%
>180
Sea
LI
13
62%
<180 (>0)
Land
8
38%
>180
Sea
1
“HI” indicated the snail samples that were collected from high intertidal zone. “LI” indicated the snail
samples that were collected from low intertidal zone.
2 “H” indicated the snails were moving parallel with the horizontal sea level.
vertical moving distance (cm)
140
120
100
80
60
40
20
0
Low intertidal snails
High intertidal snails
Figure 3. The average vertical distance of movement from release point (positive
number means moving towards land) of upper-shore snails and lower-shore snails
released at site 1 in Letite, NB.
350
Vertical moving distance (cm)
300
250
200
150
100
50
0
Low intertidal snails
high intertidal snails
Figure 4. The average vertical distance of movement from release point (positive
number means moving towards land) of upper-shore snails and lower-shore snails
released at site 2 in Letite, NB.
30
25
Height (cm)
20
15
10
5
0
Low intertidal snails
High intertidal snails
Origin
Figure 5. The average height of movement from release point (positive number
means moving towards land) of upper-shore snails and lower-shore snails released at
site 2 in Letite, NB.
90
80
70
Height (cm)
60
50
40
30
20
10
0
Low intertidal snails
High intertidal snails
Origin
Figure 6. The average height of movement from release point (positive number
means moving towards land) of upper-shore snails and lower-shore snails released at
site 2 in Letite, NB.