C
HOMING BEHAVIOR
OF THE LIMPET COLLISELLA SCABRA
by Freya Sommer
Hopkins Marine Station
Stanford University
Pacific Grove, CA.
Direct all proofs and correspondence to:
Freya Sommer
Hopkins Marine Station
Pacific Grove, CA. 93950
Sommer
PART I: Homing behavior of small individuals of the limpet
Collisella scabra.
Introduction
A distinctive characteristic of the limpet Collisella scabra is
its ability to return to a very specific "home" site after foraging
on the rock while awash or submerged. This phenomenon was recently
reviewed by BRANCH (1981). Although homing in C. scabra has been
studied quite extensively, the exact mechanism is still unknown.
Homing behavior in relation to size of the limpet was studied
by JESSEE (1968). His results indicated that there was no significant
difference between the homing behavior of large C. scabra, greater
than 15mm in length, and that of small limpets between 6 and 10mm
long. He suggests, however, that individuals smaller than 6mm long
do not usually home. HEWATT (1940) had previously suggested that
C. scabra less than 14mm in length do not home.
The present study was undertaken to determine whether small C.
scabra have the ability to home, and whether the homing is a constant
occurence. My results show that C. scabra less than Amm long can
indeed home, and that there is less rigidity in homing, i.e. a
higher rate of home site changing, than shown in a previous study of
larger limpets (BRANT, 1950).
Materials and methods
The C. scabra studied were located on gently sloping granite.
between 2 and 3 feet above MLLW, on a protected shore of Mussel Point.
Pacific Grove, California. 26 individuals were chosen and measured
with calipers. They were grouped into 3 size classes: less than 2mm
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in length (8), 2-3mm (7), and 3-4mm (11). The limpets were marked
with a dot of red nail polish on the apex of the shell, and a dot was
painted on the granite imm anterior to the shell. Relative positions
of the dot and the marked limpet were noted. A number or letter
corresponding to each animal was painted on the rock in the vicinity
of the "home" site.
During 5 consecutive daytime high tides the limpets were observed
every hour for 7 hours, and movements away from home were noted. In
preliminary observations, no limpet was seen to start and complete
a foraging run in under an hour, so it is assumed that all forays
made during each high tide were recorded. No observations were made
during high tides at night. If a limpet failed to return to the site
where it was originally marked, its new location was marked at low
tide.
Results
Of 26 C. scabra, 6 were never seen to forage (Figure 1). These
may have moved during one or more night high tides, or they may not
have foraged at all during the study. Of the 20 animals which were
seen to move, not every one foraged every day. Only the 20 foraging
limpets were included in this analysis of homing behavior. The
average total distance travelled by an animal in one foray was about
5cm.
Homing was defined as returning to the same site that was
occupied before the onset of foraging at high tide, and taking up
the precise previous orientation. This was determined by alignment
of the painted dots and good fit to the substrate, as far as
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discernible by eye.
As can be seen from Figure 1, 85% of the limpets which were
seen to forage homed back to their original home sites. This analysis
does not explain the fate of the 15% which changed home sites, and
actually underestimates homing. The 3 limpets changing sites in
Figure 1 did actually show homing behavior as indicated in Figure 2,
which analyzes homing according to the total number of forays made
during each high tide. Levels of homing are diagrammed in Figure 2a,
" 1° homing" is homing to the site wherethe limpet was originally
marked. "Site changing" is failing to return after foraging to a
site to which homing previously occurred. "2° homing" is homing to
the site where the limpet was found after failing to return to the
original site. "3° homing" is homing to the site where the limpet
was found after failing to return to the 2° site. Figure 2b shows
that 84.1% of forays resulted in 1° homing, and a total of 10.1%
resulted in 2° or 3° homing. No foray ending in site changing was
ever followed by another non-homing foray, i.e. there was always
at least one instance of homing to the new site.
Discussion
The results indicate that small individuals of C. scabra can
indeed home, although there is a tendency to change home sites. It
is possible that the proportion of 1° homers to home site changers
is greater than that suggested by this study, since some C. scabra
may have moved and homed once or more during the night high tides.
For instance, of all 6 non-movers had foraged at night, then the
maximum fraction of 1° homers would be 88.5%, rather than the 84.1%
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reported. This is still less than the 97.8% homing reported by
BRANT (1950) over a 5 day sampling period for larger C. scabra on
low rocks. It would be interesting to study growth rates of small
C. scabra in relation to home site-changing frequency, to determine
whether a limpet retains its home site longer as it gets larger.
It may be adaptive for the young limpet to change sites every so
often, in the chance of finding the "perfect" home.
Other possible reasons for site changing include interference
from other organisms. In two cases, a larger, "intruding" C. scabra
wasffound on the home site of the small marked limpet which had settled
in a spot away from its home site. Even after the intruder had
moved away, 2-4 high tides later, the small limpet did not reclaim
its site. Home site changing could possibly be the result of environ¬
mental trauma, which might have smaller effect on the limpet as it
grows larger.
Sommer
PART II: Studies on possible mechanisms of homing in the limpet
Collisella scabra.
Introduction
As mentioned in Part I, the exact mechanism of homing in the
limpet Collisella scabra is still unknown. JESSEE (1968), and COOK
et al. (1969) attempted to prevent homing behavior in C. scabra by
scrubbing the rock surface around the home site, and by applying
NaoH or HCl, to destroy any chemical trail that may have been laid
down by the foraging animal. Homing was reduced, but not eliminated.
JESSEE (1968) believes that this signifies some appreciation of the
rock topography by the limpet. COOK ( ?) suggests that a reason
for the lack of homing prevention could be inefficiency of methods
used in attempting to remove a chemical trail. The purpose of
this study was to investigate further the possible mechanisms of
homing in C. scabra.
Materials and methods
There were four stages of experiments. First, home site burning
was attempted in the field and in the lab. Second, limpets were taken
from the field and put on glass in the lab in order to see whether
they 1) conformed to their new substrate, and 2) set up home sites
on a flat surface totally devoid of topographical information. Third,
a similar test was done, using rocks as substrate instead of glass.
Finally, a series of home site transplantation experiments was done
in the lab, in which quartz home sites were chiselled off rocks and
transplanted, together with the resident limpet, to glass or new rock
substrates.
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For all lab experiments, artificial high and low tides were
created in an outdoor sea-table (Figure 3). "Low tide" was
brought about by allowing only a small amount of sea water to run
into the table, and by placing a short, hollow plug in the drain, so
that the water level remained at about 3", and all the experimental
limpets were left high and dry. "High tide" was caused by replacing
the short plug with a taller one, and turning on "splash" from
perforated tubing connected between water nozzles above the sea-table.
All test animals were gradually submerged and able to forage. In this
manner, the limpets were subjected to two high tides per day, one at
about 8 a.m., and the other at about 7 p.m., each of 2-3 hours duration.
Results
Home site burning experiments.
Home site burnings were first carried out in the field. 20
limpets, 1-1.8cm in length, were selected on one rock face. All were
marked as in the small limpet experiments described in Part I. 10 of
these were removed from the rock at low tide. A circular area of radius
6-8cm, centered on the limpet's home site, was intensively heated with
a propane torch. After the rock had cooled, the limpets were
replaced on the rock 8cm from their home sites, aimed tangentially
to the treated area, and slightly wetted with sea water to aid in
sticking. 10 controls were similarly removed and replaced, on non¬
torched areas. Results are shown in Table 1.
By the second high tide after treatment of the limpets, there
did not appear to be any difference in the behavior of the two groups
of animals. The controls fared as poorly as the burned C. scabra,
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possibly resulting from traumatizing the animals when removing them
from the rock. A set of lab experiments was therefore done, where
disturbance to the limpet was minimized.
8 C. scabra, maintained in the sea-table and observed over 5-11
"high tides" to forage and to home consistently during this period,
were used as subjects. Controls were 30 other limpets which had been
similarly observed in the sea-table. When an experimental C. scabra
was foraging at least 5cm away from its home site, its rock was tilted
so that the limpet was barely submerged. Then an area around the
home site about 4-5cm in radius was heated intensively with a
propane torch. The rock was then re-submerged, and the sea-table
was allowed to drain slowly in simulation of a falling tide.
5 out of 8 limpets returned to their burned-out home sites with
no apparent hesitation in comparison to the controls. 2 Of 3 non-homers
returned to the area of the home site, but while traversing the
heated region, encountered patches of a crumbly brown substance,
possibly an encrusting alga burned by the heating process. The animals
did not traverse this substance, but turned away from it, or attempted
to reach the home site from another approach. This was not possible
beeause these patches completely surrounded the home sites. The
remaining non-homer did not even turn around to head toward home upon
the onset of "low tide," as the others had done. This limpet may
have been disturbed by the heating, or it may have been in the
process of changing its home site.
Displacement experiments.
All of the previous experiments involved limpets homing on their
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own rocks, whether in field or in the lab. In the following
experiments, limpets were taken from the field and placed on glass or
rock in the sea-table, and subjected to two high tides per day for
over 3 weeks.
Animals which were placed on glass ranged from.4-2cm in length.
Some limpets crawled off the glass at the beginning of the study.
Those which remained grew their shells to conform to the flat surface.
No homing was observed. At each high tide the animals would either
reorient themselves slightly, or move to a new place altogether.
Within 2 weeks, the larger limpets, 1.5-2.0cm in length, had grown
new shell around the bottom edge, discernible as a 1-2mm wide band
of lighter, thinner shell, conforming exactly to the glass.
Those limpets which were placed on new rocks were 1.0-2.Ocm in
length. During the first few "high tides," they wandered on the
rock, not returning to any previously occupied site (i.e., no homing).
After 3-4 "high tides," the limpets merely reoriented themselves by
180° on spots which, in one direction, seemed to fit their shells
fairly well. They continued this behavior regularly with the changes
of the tides until, in 1-2 weeks, they began homing behavior, leaving
these sites at "high tide" and returning to them at the onset of "low
tide." By this time their shells conformed very well to thiser new
sites.
Home site transplantation experiments.
A final series of experiments involved transplanting an animal
and its home site to new substrate. C. scabra home sites often occur
on superficial crystals in the granite. A sharp tap with a chisel
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10
to the crystal/granite interface is usually sufficient to dislodge
the whole crystal, complete with the limpet on its home site. A
number of crystal home sites were thus removed from the field and
transplanted to glass and new rocks in the sea-table. They were glued
to the new substrate with silicone sealant (Figure 4).
Once the limpets had crawled off the crystals onto the glass
substrate, they wandered with each high tide, never returning to
the region of their crystal home sites. Within 5 "high tides." 6 out
of 9 limpets had left the glass altogether, the remaining 3 never
having left their crystals through 20 days of observation. Varied
results were obtained from observations of limpets on crystals
transplanted to new rocks. After 2 weeks, 4 of 12 animals had never
left the crystals, 5 had left and set up new home sites on the rock,
never having moyed back in the direction of their crystals, 2 were
missing, and 2 limpets had left their crystals, foraged on the rock.
and returned via the outgoing path to their original home sites.
Homing on these transplanted sited was thus very poor. In an
attempt to reduce the possible adverse effect of steep cliffs (crystal
edges) on homing ability or desire to return home, similar transplant-
ation was performed, but this time the crystals were embedded in pits
chiselled into the new granite so that the edges were flush with the
surface of the rock. Gaps were filled in with a mixture of ground
granite and glue. 2 crystals were embedded in this manner. One
limpet never left its crystal in 10 days of observation. The other
left and foraged on the rock, homing back to its home site at the
onset of "low tide," on 3 separate occasions. The rest of the time,
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it did not leave its crystal. It would seem that the last method
of home site transplantation would be most useful in additional
future studies of this sort.
Discussion
Based on the results of sea-table home site-burning experiments.
it appears that a complex, information-bearing chemical trail, if laid
down by these limpets at all, is not necessary for homing to occur.
It seems that some kind of recognition of local topography could
allow the limpet to return home. It is possible, however, that a
combination of trail following and topography appreciation is involved
in homing behavior, not necessarily at the same time, but in sequence.
as the animal develops. For example, very small, newly settled
C. scabra might initially lay chemical trails on the not-yet-familiar
substrate, in order to return to their home sites, "learning" the
topography as they become familiar with it in foraging. Once this
occurred, chemical information might no longer be necessary.
The admittedly casual observations on home site transplantation
and limpet displacement show that limpet homing can be studied in the
lab. The fact that C. scabra were not seen to home on glass could
be indicative of a need for some kind of variance in the surface of
the substrate to facilitate homing via "learning" of the topography;
or it may simply suggest that in cases where C. scabra conform to
the surface everywhere, they do not need to home to a specific site.
The limpets which were displaced were, unfortunately, not rigorously
observed throughout every high tide after being taken from the field.
so it is not known whether they consistently followed their outgoing
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paths back to their homes after their initial foraging run. On
occasion they were seen to return home by the same path, and toward
the end of the study some animals were seen to take a circular route.
Further study is warranted on this topic. Perhaps, as hypothesized
above for developing limpets, the displaced animals follow a chemical
trail at first, until they become familiar with the surroundings.
Burning experiments could be performed with these animals as soon as
they begin to forage, to test this idea.
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Summary
1. C. scabra less than Amm long home. The rate at which they change
home sites may be greater than that of adults.
2. Homing in adults persists after burning the home site and surrounds.
3. Chemical trail following may play a role in homing of juveniles,
limpets which have voluntarily changed sites, and limplets
removed from the field and placed on new substrates,
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14
Acknowledgments
I would like to thank my advisor, Dr. W. Gilly, for encouragement
and advice provided throughout this study, and also for the
inspirational trips to the Seaside Dump. The assistance of the
many unfortunate C. scabra who were forced to give up their homes in
the name of science is also gratefully acknowledged.
Sommer
15
Literature cited
BRANCH, G. M.
1981.
The biology of limpets: physical factors, energy flow,
and ecological interactions. Oceanogr. Mar. Biol. Ann. Rev.
19: 235-380
BRANT, DANIEL H.
1950. A quantitative study of the homing behavior of the
limpet Acmaea scabra.
Unpubl. Spec. Prob. Reprt. Dept.
Zoology, Univ. Calif. Berkeley
COOK, ANTHONY
Homing in the Gastropoda.
Unpubl. manuscript, New Univ.
Ulster, coleraine, N. Ireland
HEWATT, WILLIS G.
1940.
Observations on the homing limpet, Acmaea scabra Gould.
Amer. Midland Naturalist 24(1): 205-208
JESSEE, WILLIAM F.
1968. Studies of homing behavior in the limpet Acmaea scabra.
The Veliger 11: 52-55 (supplement)
O
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16
CAPTIONS TO FICURES AND TABLES
Homing behavior of 26 C. scabra in 3 size classes over
Figure 1.
5 daytime high tides.
Figure 2.
a. Schematic representation of levels of homing.
b. Results of foraging runs of 26 C. scabra over 5
daytime high tides.
Figure 3. Sea-table arrangement for homing studies.
Figure 4. Crystal home site transplantation onto new substrates.
Table 1. Results of field home site burning experiment.
5OMHER
P.7
day
total forays
homing
change of site
2° homing
3° homing

19 homing
N 3-4 mm
2 — 3 mm
—
2 mm

changed no foraging
homing site
observed
12
13

change 0
2° homing
5 total
tota
16
% forays
84.1
5.8
7.2
1 2
2.9
S
3 homing
HER 9.18
E6.3
F16.4
SEA
WATER21
EXPERIHENTAL
6
C SCABRA
DN GRANITE
HOULDERS

7
eue
A
GLASS
„PERFORATED TUBING



:.
:.
Lrus

6
L
SEA-TABLE
S
CRYSTAL
HOME
4
SITE


GRANITE
GRANITE

g
en

N=9

BURNED
33.3 %
44.4 %
22.2%
n = 10
CONTROLS
30 %
40%
20 %
POSTTON AT Znd HIGH
TIDE AFTER BURNING
AT HOME SITE
AWAY FROM SITE
NISSING FROR ROK
TAGLE