Internal Clocks and Behavior Patterns in Littorina Littorea
Aka Snail Trails
By Maisie Kaiser & Tess McCabe
Chapter 8, 2012, pg 1
This project is an exploration of the behaviour of Littorina littorea. Originally, the
goal was to determine if Littorina littorea’s movements reflected an internal sense
of the tides. Time lapse photography was used to document the movement of
twenty marked snails. In addition, several on-beach tank experiments and a
circumference survey were conducted. It was discovered that for the most part,
Littorina littorea remain stationary during the low tides, favoring movement while
submerged. Other interesting behavioural patterns were found, including size and
location correlations, group vs solitary migration, and a strong tendency to favor
counterclockwise movement.
Introduction:
The goal of this project is to investigate whether or not periwinkles have an
“internal clock” mechanism that allows them to predict high and low tides. The creature
chosen to study is an invasive species of snail found on Racing Beach. The Littorina
littorea, commonly known as the periwinkle, is a member of the mesogastropod family
Litorinidae, and lives in rocky shores, mango groves and marshes (Barnes, 1968, 353). It
is an ideal creature to study because its behavior changes in relation to the tides. During
low tide it attaches the lip of its shell to the substratum, but during high tide it grazes on
algae. A study by Gendron in 1977 showed that periwinkles prefer tide lines when on a
vertical plane.4 What is more interesting is that when displaced, periwinkles will always
migrate back to their original placement. They’ve been shown to know intuitively which
direction is upshore and which is downshore, and where they are in relation to the tides.
This supports the hypothesis that periwinkles have a tide sensing “clock”. The only
question to be answered then, is the one raised by John D. Palmer in his 1996 article on
biological clocks: "Why would an organism have an internal clock instead of relying on
the environment to keep time? The adaptive significance of a biological clock lies in its
ability to alert the owner of periodic environmental changes in advance of those
changes."5 For this reason, the hope is to find evidence that periwinkles have internal
tidal clocks by finding that their movements anticipate the high and low tides.
Hovel, Bartholomew, and Lipcius hypothesized that snails move up and down
with the tides in order to escape predators.6 They also thought that light levels might be
an indicator for the snails as to when they should move. They found that lone snails and
snails in clumps moved roughly the same amount, suggesting that the movement was not
a socially cued behavior. Though they studied the species L. irrorata, their theories may
Chapter 8, 2012, pg 2
also hold true for L. littorea.
In 1995, a Cambridge School student conducted a study on snails who had been
taken out of their natural environment. He noticed that tides covered periwinkles with
water at regular intervals on the shore, and he wondered if the periwinkles would move in
and out of the water themselves at the same intervals if there were no tides to do it for
them.8
To find this out, he took three different types of periwinkles from the beach and
put them in a tank filled partially with water. He would then note the times of the tides
and check on the periwinkles several times a day to see whether they were above or
below water. He found no correlation between the snails’ movement and the tides. He
thought that the water quality was the bigger factor in the snails’ movement.
One crucial error this researcher made is that he didn’t factor in the species of
each snail. Their species could have had a huge impact on their movement, because snails
that usually live in the lower intertidal would probably want to spend more time in the
water and the opposite. He also set up his tank indoors, and the warmer inside
temperature could definitely have affected the snails’ movement. He rated the water
quality on a scale of 1-5, but didn’t say what the numbers referred to.
In 2006 Flint and Adams-Gregg tracked the movements of periwinkles in three
tide pools.9 In their study, they recommended using nail polish and numbers to track
individual snails and their movements. In this study, it might help to track only one
species of snail (perhaps the species that move the most), and set up the artificial
environment outdoors instead of indoors. This project is based on a study done in 2011
by Pyle where she both tracked the population of the periwinkles and their movements in
a lab setting.7 There was some doubt as to whether the movement of the snails in the lab
was due to the temperature inside the lab. Based on her data, temperature can be
identified as a factor or not.
According to an article by Susan Millus, it’s very possible that the periwinkles
will follow one another’s paths in order to save energy. Researchers Davies & Blackwell
studied Littorina littorea and discovered that it takes 35 times more energy for a
periwinkle to make a new mucus trail than it does for the periwinkle to crawl along a
mucus trail made by another individual.1 Therefore, it makes sense that the periwinkles
Chapter 8, 2012, pg 3
would try to travel along pre-existing paths. The Ecological Society of America journal
suggested that periwinkles prefer to move in “highly directional” paths perpendicular to
the shoreline, no matter what the angle of the beach.3
If snails have internal clocks that relate to the tides, they will move at a faster
rate during the two hours before and after the tide comes in. If they start moving faster
during those times, that would suggest that they can sense that the tide is coming.
Photography was used to map not only the distance that the snails move but the
directional paths they take. The expectation is to find that periwinkles prefer traveling on
pre-existing trails to creating new trails. If there is time, another possible experiment is
continuing to track the population of snails, as others have done in previous years.
Materials & Methods:
Experiment #1 (Time Lapse):
A rock was located on the jetty in the lower intertidal zone which has a
relatively vertical face towards the ocean. 20 snails on that face were chosen to track. 2
snails each were marked with one color of nail polish. This was repeated until there were
20 marked snails, 2 with each color. Some pairs had to have color combinations because
there were not enough colors (some snails were half green/half pink, for example). The
monopod was secured with rocks in a place where the camera could capture the entire
face of the rock. The spot just underneath the camera was marked with nail polish so that
the camera could more precisely be placed in the same location each day. The camera
was set to take one picture of the rock face every 20 minutes, starting 1-2 hours before
low tide each day, and stopping 1-2 hours after low tide each day. The corners of the rock
face were marked with nail polish. The length and width of the rock face was recorded
(70x70 cm in this case). This allowed for going back later and figuring out the snails’
distance traveled using the pictures and Photoshop. Each day at the time of the first
photo, the temperature, weather conditions, and the number of snails still present in
several places were recorded. The places that were tracked were: original face, left face,
right face, water below original face, and all locations combined.
Chapter 8, 2012, pg 4
Experiment #2 (Tank/ Beaker Tracking):
8 snails were selected from the sides of rocks on the sandy beach area. 1 snail
was placed in each of 3 beakers filled with about an inch of water. The remaining 5 snails
were each marked with a different color of nail polish and all 5 placed in an aquarium
tank filled with about an inch of water. After starting the timer, every 2 minutes the
height (in cm) each snail has climbed was recorded (from the bottom of the beaker/tank),
with one researcher keeping track of the snails in the beakers and one watching the ones
in the tank. The directional movement of the snails in the tank was noted every 2 minutes
as well. Which way they were moving, if they appeared to be following one another, and
other movement behaviors were recorded.
Experiment #3 (Snail Circumference Measurements):
With the help of landmarks, the beach was divided into 6 zones: the sandy
middle intertidal, the sandy lower intertidal, the rocky middle, the rocky lower, the jetty
middle, and the jetty lower. In each zone, the closest 30 periwinkles from the sides of
rocks were measured with a measuring tape around the bottom of their shells (where the
snail’s foot came out) and their circumferences were recorded.
Chapter 8, 2012, pg 5
Results:
Average Snail Movement by Day
6
5
cm Traveled
4
3
2
1
0
May 14th
May 15th
Figure #1: There was no statistically significant difference between the two days.
Chapter 8, 2012, pg 6
Distribution of cm Traveled
30
Number of Snails
25
20
15
10
5
0
0-5cm
5-10cm
10-15cm
15-20cm
20-25cm
Distance Traveled
25-30cm
30-35cm
Figure #2: Two numbers were clear outliers.
Height of Snails Over Time
3
Height (cm)
2.5
2
Snail 1
1.5
Snail 2
1
Snail 3
0.5
0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40
Time (min)
Figure #3 three isolated snails in their own separate beakers and tracked the height they had climbed to
every two minutes. Snail 2 did not move very much at all, and it appeared to be the smallest snail, while
Snail 1, the biggest, moved the most. We found that two of the three snails went up to various heights and
down repeatedly, rather than just climbing up high and going back down once.
Chapter 8, 2012, pg 7
Height traveled by Beaker Snails
7
6
Hieght (cm)
5
Solo
4
White
3
Pink
Green
2
Terquiose
1
Blue
0
2
4
6
8
10
12
Time (Mins)
14
16
18
Figure #4: The solo snail traveled up the sides but most snails didn't travel up the sides at all.
Time Spent Moving Counterclockwise
13%
87%
Clockwise
counterclockwise
Figure #5: The snails favored traveling in the counterclockwise direction.
Chapter 8, 2012, pg 8
For Figure #6 see end page attached.
Snail Circumference
6
Average Circumference (cm)
5
4
3
2
1
0
Jetty-lower
Jetty-middle Rocky-lower Rocky-middle Beach-lower Beach-middle
Figure #7: The middle intertidal zone on the jetty had the highest average snail circumference,
and the middle intertidal on the sandy beach had the lowest. All of the t-tests we did were
significant except for the lower beach and the lower rocky areas, and the lower rocky and the
middle rocky areas. The significant p-values ranged from 0.0075 to 3.06E-13.
Conclusion:
Based on the photographs and the tank experiments, it was concluded that
snails tend to follow the waterline, so when they are above water, they hardly move at all.
This suggests that snails do not have a tide-sensitive biological clock and disproves our
hypothesis.
In Figure 1 you see the average movement of periwinkles above water. The
fact that there wasn’t a statistically significant difference in figure 1 suggests that the
movement observed doesn’t change drastically from day-to-day, or at least that the
movement wasn’t directly affected by the weather. Which leads to the question: Why
would snails move at all during low tide? If they aren't anticipating any tides, one would
argue that the snails would stay in a single spot to prevent loss of moisture, and wait until
the water returns to move. As shown in figure 4, that’s what most of the snails did. If
Chapter 8, 2012, pg 9
they moved at all they moved very little. What set the averages off were there few
sizeable numbers from 5-10 and the two in the 20-35 range.
Of the 37 snails looked at, seven of them moved more than 5 centimeters. This
could be for a number of reasons. The first is that the snails that moved might have been
of a different species. While the snails were being painted, the species of the snail wasn’t
taken into account as much as might have been wise, as it was assumed that periwinkles
were the dominant snail species. The second is that those seven snails were in search of a
safe crack in the rock or a nook, and moved because where they were in our initial picture
was unsafe. The third is that it was noticed that while painting them, the snails had a
tendency to fall off the rock when disturbed, so something could have disturbed the
snails, and they had been in the process of climbing back to their original location. This is
fairly likely, as it’s been demonstrated by Gendron that displaced snails will return to
their original location.4 The last possibility is human error. In the process of using
Photoshop, some of the dots used to mark measurements might have been mistaken for a
snail of the same color. However, that would only account for one or two of the
measurements so some of the bigger numbers are still worth investigating.
Figure 3 suggests that there might be a connection between the size of the
periwinkles and how much they move. The biggest snail moved the most, and the
smallest moved the least. This could be a function of the fact that bigger snails can hold
more water in their bodies, which they can then use to make more mucus in order to
propel them further.
In the second experiment, as shown in figure 4, the water level was changed in
the beaker and immediately caused a rise in the heights the snails went to. This suggests
that snails prefer to travel while submerged, but don’t have an internal sense of the
waterline as was originally thought. Still, three of the snails didn’t move vertically, and
the three that did move along the sides of the beaker did so intermittently. This could be
that snails use social context to get a bearing, so if one snail stays on the ground then
more are likely to follow. This theory is somewhat negated by the work of Hovel,
Bartholomew, and Lipcius6. Another theory is that the snails are looking for shelter, and
the ones on the ground are clumping, and the ones on the sides are looking for niches.
This would explain why the solitary snail traveled along the sides more, and why the
Chapter 8, 2012, pg 10
snails were so fond of the corners and edges of the tank- those would be the most
“sheltered” spots of the tank. In the future, if this study were to be repeated, it would
definitely be important to track all types of movement, not just vertical. This was done on
the map, but not on this graph.
Figure 5 shows the snails’ tendency to move counterclockwise. The data in the
graph is only the numbers from one experiment, but the phenomenon was observed in the
initial tank experiment as well. That’s only two experiments, and it’s been established
that the snails are likely to follow one another, so it might have been coincidental. The
first snail started moving counterclockwise randomly, and the rest tended to follow.
Secondly, it could have been that the displaced-snail-homing instinct that Gendron
found.4 The snails weren’t taken from the same spot, but they all might have been to the
left of the beakers which would explain why the snail’s first instinct is to go left and then
around the edge. Third, it might have been that the elevation of the beach had something
to do with it. The experiments were conducted at the same point, so if there was
something inherent to that point, it would explain why the same effect was observed both
times.
The circumference data from Figure 7 proved that many of our observations
translated into statistically significant differences. The biggest snails were in the middle
intertidal on the jetty, and the smallest were in the lower intertidal on the rocky beach,
which had been expected based on in-field observations. This could have to do with the
size of the rocks in each location. A possible theory is that snails in the rocky area were
smaller because they had to fit into smaller crevices on smaller rocks, and since jetty
rocks were bigger, the jetty snails could grow to be bigger. Another possible explanation
could have to do with the food supply, which for periwinkles means algae. There could
be more nutrients available on the jetty, since there is more exposure to the water there
and more rocks for algae to grow on.
The jetty was the only place where there was a significant difference between
snail sizes in the lower intertidal and the middle intertidal. There was not a significant
difference in sizes between the lower and middle zones in the rocky or sandy parts of the
beach. This could fit in with our rock size hypothesis because there is a larger range of
rock sizes on the jetty and so the larger range of snail sizes might have developed to
Chapter 8, 2012, pg 11
match the available rocks.
In the course of these experiments our hypothesis was disproved. In doing so, a
rich vein of snail characteristics and habits was discovered. This experiment provides a
good foundation for several new possible projects that could be continued. These could
include a further investigation of snail size versus location correlation, the relationship
between size and movement, and the tendency to move counterclockwise.
Works Cited:
1. Milius, Susan. (2007). Snail Highways: By following Trails, Periwinkles Save Slime.
Science News Vol. 171, No 9, pp. 132-133.
2. Invertebrate Zoology. Saunders. 353-55. Barnes, Robert D.
3. Petraitis, Peter S. (1983). Grazing Patterns of the Periwinkle and Their Effect On
Sessile Intertidal Organisms. Ecology
Vol. 64, No. 3, pp. 522-533
4. Gendron, Robert P. (1977). Habitat Selection and Migratory Behaviour of the Intertidal
Gastropod Littorina Littorea. Journal of Animal Ecology Vol. 46, No. 1, pp. 79-92.
5. Palmer, John D. (1996). Time, Tide and the Living Clocks of Marine Organisms.
American Scientist, Vol. 84, No. 6, pp. 570-578.
6. Hovel, K.A., Bartholomew, A., Lipcius, R.N. (2001). Rapidly Entrainable Tidal
Vertical Migrations in the Salt Marsh Snail Littorraria Irrorata. Estuaries, Vol. 24, No. 6,
pp. 808-816.
7. Pyle, Seraphina. (2011). Periwinkle Population and Behavioral Study. CSW Marine
Biology Study.
8. Smith, Ben. (1995). Snail Behavioral Study. CSW Marine Biology Study.
9. Flint, Adams-Gregg. (2006). Littorina Littorea Populations and Movement Patterns in
Three Intertidal Pools. CSW Marine Biology Study.
Chapter 8, 2012, pg 12