Abstract
Previous reports on the behavior of Macclintockia scabra have implied that the
intertidal limpet moves every time it is wetted by the incoming tide. In a three-week
study on the potential effects of location, splash and light level on limpet movement,
frequency of movement (number of instances individual limpet moved divided by the
number of instances limpet was wet - averaged over all observed limpets) was calculated
to be 12%. This is based on 2660 field observations on seventy-nine limpets, dispersed
among three sites, differing in degree of wave exposure. I also made 600 observations on
twenty-five limpets that had established home scars on rocks transported to the
laboratory. Frequency of movement in the lab was 15%. A two-factor analysis of
variance (ANOVA) was carried out on frequency of movement, with degree of splash
and field site as the two factors. Data from during the day, and at night had to be
analyzed separately because all possible combinations of "splash", "site" and "light
level" did not occur during my field study. Frequency of movement did not differ
significantly among sites. No movement was observed at low tide in either the lab or
field. An increased frequency of movement was found to accompany a corresponding
increase in splash level, both at night and during the day. However, lab results showed a
decreased frequency of movement when limpets were submerged rather than sprinkled by
a spray bar. A significantly greater amount of movement (25%) was seen at night than
during the day (7%) - a finding that could be due to the higher nighttime tidal heights.
Lab results showed no significant difference in day and night movement frequency. In a
separate experiment, limpets were exposed to an artificial high tide, during a normally
low tidal period. The limpets responded by producing a significantly lower-than-normal
frequency of movement (8%). 10/15 tidal simulations resulted in a movement frequency
of zero. My findings suggest that Macclintockia scabra may use a combination of splash
level, light, and other as yet identified "tidal-timing" cues to induce movement. Location
appears to play a less significant role.
Introduction
The movement behavior and foraging patterns of Macclintockia scabra have been
observed and studied for over one hundred years. All of the classic reports present a
similar story regarding the limpets' movement:
The movements of the limpets are largely controlled by the tides. When
the tide is out, they remain practically motionless on the rocks and present
no visible sign of life. With the first dash of spray from the incoming tide
they begin to move and are apparently active until the water recedes once
more" (Wells, 1917).
Macclintockia scabra find a very specific location on a rock and grow their shell to fit
that location. They establish the site as their home scar and return to rest on the scar
every time the tide is low. As described above, during the incoming high tide, the
limpets lift up their shells and begin to forage around for food - once again returning to
their home scars when the tide recedes. This movement pattern, closely correlated with
the natural tidal cycle, has been found to be invariable and all-inclusive. In an
observational study on the movement of the limpets, Willis Hewatt (1940) notes:
"After the animals had been submerged several times by the oscillating
water, they lifted their shells and started moving very slowly in various
directions. The movements began at 8:45 a.m., and by 9:00 a.m. all of the
marked limpets had moved away from their spots."
In making preliminary observations of Macclintockia scabra, living in the high-
intertidal region of the Hopkins Marine Life Refuge, it was quickly apparent that rather
than foraging actively during the high tide, the majority of limpets remained motionless
on their home scars. This low-level and inconsistency of movement was unexpected.
After several daytime high tide observations of-0% movement frequency, I observed the
limpets' behavior during a nighttime high tide, and was surprised to see a substantially
greater number of limpets moving. This was another unexpected discovery, as previous
studies have found no significant difference in the daytime and nighttime movement
frequencies of Macclintockia scabra (Sutherland, 1969).
Tembarked on a three-week observational study, with the purpose of determining
an overall movement frequency for the limpets, as well as exploring what cues the
limpets were using to induce them to move. Did the limpets move significantly less
during the day than at night? If so, was this due to a difference in day/night tidal
conditions or were the limpets responding to the amount of light exposure they were
receiving? Did the movement frequency of the limpets vary significantly in different
locations?
Materials and Methods
1 conducted a three-week observational study of Macclintockia scabra at three
field sites, each characterized by a different degree of wave exposure. I used a variety of
brightly colored nail polish to identify limpets and their home scar location at each field
site (Table 1). I made observations of every limpet during daytime and nighttime high
and low tides. For each low tide observation, I located each limpet and checked to see
whether it had returned to its home scar or had established a new home scar location. For
each high tide observation, I categorized the degree of splash (Table 2) each limpet was
receiving and noted which limpets had left their home scars to forage for food. Limpets
were considered wetted at splash 1 category or above. I calculated a movement
frequency (E'times moved
times wetted) for each limpet and used these to tabulate overall
frequencies for each combination of splash level, site and degree of light exposure (day
or night). These were compared using analysis of variance (ANOVA). No degree 3
splash occurred at either of the protected sites during the day throughout the
observational period, making it impossible for me to conduct a three factor ANOVA of
the data. Instead, 1 conducted a two factor ANÖVA using splash and site as the factors
for day and night data separately, and a single factor ANOVA with degree of light
exposure (day or night) as the single factor.
In addition to my field observations, I brought granite rocks, along with twenty-
five limpets from the intertidal zone into a 250 gallon, outdoor tank. I suspended a spray
bar with running seawater over the rocks. The tank could be filled to submerge the rocks.
By controlling the extent and duration of water sprayed onto the rocks, along with the
degree to which the rocks were submerged, I could simulate artificial tides, consisting of
either degree 2 (spray, but no submergence) or degree 3 (spray combined with
submergence) splash conditions. Äfter giving the limpets a week to adjust to their
surroundings and establish home scars, day and night observations were made on the
individual limpets in a similar fashion to those in the field. For each observation, the
splash level was recorded, along with which limpets were moving. I calculated
movement frequencies for each limpet, and used these to calculate an average movement
frequency for each degree of splash, both during the day and at night. I conducted a two-
factor ANOVA on the lab data, using degree of splash and degree of light exposure (day
or night) as the two factors.
To test whether the timing of the natural tidal cycle was a cue the limpets were
using to move, 1 exposed field limpets to an artificial high tide at an unnatural time, by
spraying them with salt water during low tide. This technique produced a level of
wetness equivalent to a degree 2 splash. To determine a movement frequency for each
spraying session, 1 simply counted the limpets that were moving after a given period of
spray and divided that number by the number I was spraying. A total of fifteen spraying
sessions occurred, divided amongst daytime and nighttime hours. The movement
frequencies of each session were averaged, and this number was compared to the
calculated field movement frequency.
Results
Frequency of movement ( times limpet movec
times limpet was "wet"), pooled over all 2660
day and night field observations was 12%. Frequency of movement, pooled over all 600
day and night lab observations, was 15%.
Frequencies of movement did not differ notably from site to site (Fig. 1). It
appears that overall differences in wave exposure do not affect limpets' tendency to
move.
For day and nighttime observations, frequency of movement in the field increased
as the degree of splash increased (Fig. 2 and 3). No degree 3 splash occurred at either
protected site during the day, so each site, splash combination was analyzed as a separate
level of a single factor ANÖVA (Table 3). Although the overall ANOVA was highly
significant (PS.001), no unambiguous pattern of significant differences emerged from the
post hoc SNK test (Table 3). However, there was a consistent trend of more frequent
movement at higher degrees of splash during the day (Fig. 2, Table 3). Maximum
daytime movement was 29% at the exposed site with splash level 3 (Fig. 2).
At night, movement was significantly greater at each increasing level of splash
within each site (with one exception - see Table 4 and Fig. 3). The maximum frequency
of movement 1 observed in the entire study was 41% at protected site 2, with splash level
3, at night (Fig. 3).
In the lab, frequency of movement showed the opposite trend from the field data,
with decreasing movement as the degree of splash increased for both day and nighttime
observations (Fig. 4 and 5).
The frequency of movement of field limpets was 8% when exposed to an
artificial degree 2 splash. Of fifteen spraying sessions, ten resulted in no movement at all
(Fig. 6).
Frequency of movement in the field was significantly greater at night than during
the day, pooled over all levels of splash and sites (Table 6). Although no statistical
comparisons could be made, frequency of movement for each degree of splash was
consistently higher for the nighttime field observations than the day (Fig. 7). Frequency
of movement was also greater at each field site during the night than during the day (Fig,
8). In the lab, limpets also moved more frequently at night than during the day, but these
differences were not significant (Table 5).
Although frequency of movement did not differ among sites (wave exposures), a
combination of splash and time of day appear to be the primary stimuli that induce high¬
intertidal limpets to move.
Discussion
Based on previous studies, I expected to see -100% movement frequency of
Macclintockia scabra at high tide, rather than the observed 12%. Very little literature
actually addresses the movement and feeding habits of Macclintockia scabra, but rather
focuses on the species' characteristic return to a specified home scar at low tide; perhaps
my expectation of 100% movement frequency was unreasonable. However, it is unclear
why I saw such a low movement frequency. Enright's (1977) study of vertical migration
by zooplankton suggested that under certain conditions of food availability, it might be
metabolically advantageous for herbivores to feed intermittently rather than continuously.
Little (1989) proposed a similar mechanism for benthic grazers in the rocky intertidal. If
algal films are sufficiently scarce in the high-intertidal zone, perhaps it is metabolically
beneficial for Macclintockia scabra to forage only when necessary, rather than every
tidal cycle. Foraging 12% of the time could be sufficient to meet their energetic
requirements. It would be insightful to compare the movement frequencies of the high
and mid-intertidal Macclintockia scabra and combine this information with whether there
is a significant difference in the amount of micro-algae on the rocks in both regions.
The consistency of movement at each field site suggests that location is not one of
the major factors affecting the limpets' observed movement frequency.
In comparing the movement frequencies for each degree of splash, I found
significant differences in the majority of the comparisons, regardless of site (Tables 3 and
4). Though not all differences were significant, the general trend was to see more
movement as the splash level increased.
The difference in lab results is perplexing (Fig. 4 and 5), as both daytime and
nighttime movement frequencies were lower for a splash 3 than a splash 2 high tide.
Perhaps lab splash simulations did not mimic sufficiently the field splash 2 and 3
conditions. For example, the lab splash 3 was a gentle, gradual submergence of the
limpets and rocks, while a field splash 3 consisted of more wavy, variable conditions. If I
could replicate and categorize the lab and field degrees of splash more closely, perhaps I
would obtain more consistent results.
It would not be valid to conclude that the limpets possess an internal tidal timing
factor (endogenous clock?), based on the spraying data alone. A series of more
convincing isolation studies would need to occur in order to legitimately approach this
subject. However, because the limpets responded to the spraying with an appreciably
lower frequency of movement than that seen under their natural conditions, it appears as
if they are not simply responding to the degree of splash they are being exposed to, but
the timing of the splash as well. This topic bears further investigation.
Because the field movement frequency, observed during the day, was
significantly less than the nighttime field movement frequency (Table 6), it appears as if
limpets use the amount of light they are exposed to as a movement cue. This difference
was significant when pooled over all splash level and site combinations. Once again, it is
perplexing why there was no significant difference between day and night movement in
the lab (Table 5). Perhaps the artificial conditions caused the lab limpets to behave in an
unnatural manner.
Because no nighttime low high tide and daytime high high tide occurred during
my observational period, it is difficult to infer whether the limpets are responding to a
reduced amount of light, or an increased amount of splash by moving more at night. IfI
were to continue this study, and given the chance to compare the movement frequencies
at a nighttime low high tide and a daytime high high tide, I could more readily separate
the day v. night and degree of splash effects on the limpets. However, because the
limpets did respond with a greater frequency of movement at night than during the day
over the given observational period, the possibility remains that degree of light is indeed
a movement cue utilized by the limpets.
Acknowledgements
Thank you to Freya Sommer for assisting me with my lab setup and Chris Patton
for helping me with my computer "woes
A heart-felt thank you to my advisor, Professor Jim Watanabe, for the time,
wisdom and patience he gave to me throughout this project, and for the dedication he
devotes to undergraduate education on a daily basis. I am privileged to have had this
chance to work with him.
Literature Cited
Enright, J.T. 1977. Diurnal vertical migration: adaptive significance and timing.
Part 1. Selective advantage: a metabolic model. Limnology and Oceanography.
22: 856-872.
Hewatt, Willis G. 1940. Observations on the homing limpet, Acmaea scabra
Gould. American Midland Naturalist. 24.1: 205-208.
Little, Colin. 1989. Factors governing patterns of foraging activity in littoral
marine herbivorous molluscs. Journal of Molluscan Studies. 55: 273-284.
Sutherland, John P. 1969. Dynamics of high and low populations of the limpet,
Acmaea Scabra (Gould). Ecological Monographs. 40.2: 169-188.
Wells, Morris M. 1917. The behavior of limpets with particular reference to the
homing instinct. Journal of Animal Behavior. 7.6: 387-395.
Tables
Table 1. The number of limpets observed at each field and lab site.
Limpets
Site
Marked
Exposed
Protected
Protected
Lab
Table 2. Definition of Splash Categories
Degree
Wetness of
Lab
of Splash
Splash
Condition:
Rock
none
dry
inconsistent
damp
consistent,
moderate
wet
times of
excessive
submergence
—
Table 3. Analysis of Variance of Daytime Field Observations
Due to missing cells, combinations of site and splash were analyzed as separate levels of
a single factor ANOVA.
Factors: Site, Splash
Transformation: Log (percentage + 1)
Weighted Average Sample Size: 27.3
Sum-of-Squares df
Source
Mean-Square F-ratio
Treatment
103.094
17.182
9.310
—0.000
Within
354.338 192
1.846
SNK Test
KEY
Ex: Exposed Site
P1: Protected 1 Site
P2: Protected 2 Site
1: Degree 1 Splash
: Degree 2 Splash
3: Degree 3 Splash
Ex-2 Ex-3 P2-2 P1-2 P2-1 Ex-1 P1-1
Table 4. Analysis of Variance of Nighttime Field Observations
Factors: Site, Splash
Transformation: Log (percentage + 1)
Weighted Average Sample Size: 24.5
Source
Sum-of-Squares df
Mean-Square F-rat
14.635
7.318
2.691
LASH
130.696
24.034
65.348
TE'SPLASH
24.584
6.146
2.260
Withii
2.719
595.465
219
SNK Test
(See Table 3 Key)
EXPOSED:
PROTECTEDI:
PROTECTED2
SPLASH 3:
P2
SPLASH 2:
SPLASH 1:
PI Ex
0.070
0.000
0.064
Table 5. Analysis of Variance of Lab Observations
Factors: Degree of Light Exposure (Day or Night), Splash
Sum-of-Squares
Source
Mean-Square
F-Ratio
DayNight
0.012
0.279
0.012
0.595
Splash
0.595
13.706
DayNight“
0.134
0.134
3.085
Splash
Within
90
3.910
0.043
Table 6. Analysis of Variance of Field Observations
Factor: Day or Night
Source
Sum-of-Squares df
Mean-Square F-ratio
36.840 12.212 0.001
LIGHTDARK
36.840
Withir
3.017
1282.085 425
0.599
0.000
0.082
Figure Legend
Fig. 1. Movement frequencies calculated at each field and lab site. Error bars are
standard errors of the mean.
Fig. 2. Daytime movement frequencies, calculated at each field site for each degree of
splash. Error bars are standard errors of the mean.
Fig. 3. Nighttime movement frequencies, calculated at each field site for each degree of
splash. Error bars are standard errors of the mean.
Fig. 4. Movement frequencies, based on daytime lab observations, calculated for each
degree of splash. Error bars are standard errors of the mean.
Fig. 5. Movement frequencies, based on nighttime lab observations, calculated for each
degree of splash. Error bars are standard errors of the mean.
Fig. 6. Number of instances a given movement frequency was observed, when limpets
were exposed to an artificial degree 2 splash high tide, at an unnatural time.
Fig. 7. A comparison of daytime and nighttime movement frequencies, calculated at each
degree of splash pooled over all field sites. Error bars are standard errors of the mean.
Fig. 8. A comparison of daytime and nighttime movement frequencies, calculated at each
site, pooled over all levels of splash. Error bars are standard errors of the mean.
Figure 1.
Figure 2.
0.6
E
0.0
0O T
Splash



SITE
exposed
protected1
protected2
Figure 3.
Figure 4.
08
0.4
50.2

Splash
0.7
0.4
0.1
O.O
Splash
SITE
exposed
protected1
protected2
Figure 5.
Figure 6.

86—0
—3
Splash
04 0.5
Movement Frequency
Figure 7.
Figure 8.
0.2
2 0.1
OO

Splash
20.
0.0
exposed lab protectedlprotected2
Site
DAYNIGHT
day
night
DAYNIGHT
night