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260
ADDITIONAL INFORMATION, IF ANY, CONCERNING AUTHORS, ADDRESS, TITLE, OR CITATION DATA
PLEASE TYPE ABSTRACT DOUBLE SPACED BELON
CarolMillard
Biologisal Ahstract
MILLARD, CAROL S. (Hopkins Marine Station of Stanford
University, Pacific Grove, Calif., USA.) The clustering
behavior of Acmaea digitalis. The Veliger
Acmaea digitalis popula-
tions often cluster on intertidal rocks at low tide.
Clustering occurs in areas which receive some shade and some
protection from direct wave action. Animals disperse on a
rising tide. Reclustering occurs during tidal ebb while
rocks are still wet, and takes place both day and night. A
cluster varying in size from 22 to 44 individuals occupied an
average of 15 sq. in. (range 8 - 20 sq. in.) each day over a
32 day period. Daily shifts in cluster position occurred;
total cumulative area occupied by the cluster in 32 days
was 77 sq. in. Only 17 members of the original cluster re¬
mained after 32 days, while a total of 22 new members entered.
PLEASE DO NOT IYPE BELOW THIS LINE
The Clustering Behavior of Acmaea digitalis
(Mollusca: Gastropoda: Prosobranchia)
by
Carol Spencer Millard
Hopkins Marine Station of Stanford University
Pacific Grove, California
(4 Text figures)
The limpet Acmaea digitalis Eschscholtz, 1833, is a
common inhabitant of the high intertidal rocky shores of the
North American Pacific coast. Populations of A. digitalis
are not randomly distributed but are often clustered into
groups in limited areas with comparatively few solitary
limpets on the surrounding regions. No specific studies of
clustering behavior have been made on A. digitalis, although
Frank (1966) mentions the occurrence of aggregations in de¬
pressions and other irregularities in the rock. Abe (1932)
studied the behavior of clusters of the limpet Acmaea
dorsuosa Gould in Japan. Abe's study, however, covered a
period of two years and his results showed mainly seasonal
variations.
*footnote 1.
26
Carol S. Millard
The present study, conducted at Hopkins Marine Station
on Mussel Point, Pacific Grove, California was carried out
to provide a better picture of the clustering behavior of
A. digitalis. On April 25, 1966, fourteen clusters were
placed under observation. Eleven of these persisted only
two to three days and only three clusters remained intact
with regard to the area occupied and members present for a
period of a month. This paper documents the activity of the
largest of the three remaining clusters.
For present purposes a cluster is defined as an aggre¬
gation of three or more animals, each within one centimeter
of its nearest neighbor. Animals farther than one centi¬
meter from the closest member of a cluster were not included
in the cluster. The clusters studied were located on the
east face of a large island of granite rock some eighty feet
offshore. Waves broke on the opposite side of the island.
rolled around and surged below the limpets in a narrow
channel, splashing them thoroughly at high water. The par-
ticular cluster studied was located six feet above 0.O water
level. No macroscopic algae were present in the area and the
microscopic algae seemed evenly distributed. The rock sur-
face was fairly smooth with a fold or crack rising up the
center. The upper end of this fold, where the cluster
studied was located, presented a shallow depression on the
2ed
Carol S. Millard
rock face which became shaded between 1:30 and 3:00 P.M.
The location offered some protection from the prevailing
winds.
To facilitate the plotting of cluster position on the
rock a grid of red dots was painted at 1½ inch intervals on
the rock surface. Within this grid the positions of indi¬
viduals could be designated with good accuracy. The limpets
present on April 25, 1966 were marked with India ink to
distinguish them from each other and from the original
members. The limpets were observed daily at low water for
a week and their positions mapped at each observation.
Thereafter, they were observed at intervals of one to
seven days. Several attempts were made to chart the move¬
ments of the limpets at rising and falling tides. For
these observations pencil notes were made on x-ray film
with the emulsion removed and surface roughened, facili¬
tating writing underwater. The water invariably became so
rough that plotting the movements of the cluster throughout
an entire tidal cycle was impossible.
General observation of the cluster showed that the
limpets move when they are being wetted by the splash of
incoming waves on a rising or falling tide. Within a half
hour after they have first been splashed, the majority of
26
Carol S. Millard
the limpets begin to move upward and laterally on the rock.
In this reaction, corroborated by Miller (1966), the lower
limpets begin to move before the higher ones; the low ani¬
mals are more frequently and heavily splashed. Once moye¬
ment has started the animals crawl upwards and laterally for
distances of two to six feet. Thus at high water the cluster
is dispersed. The animals remain more or less active while
the tide is in. As the water level recedes again, they tend
to regroup themselves into a cluster, in approximately the
position occupied by the previous cluster.
The first question posed was: does the cluster occupy
the same amount of space and the same area on the rock
consistently? To answer this question the daily positions
and areas occupied by the cluster at low water were compared
for successive days (see Figures 1 and 2). It can be seen
that the cluster shifted its position from day to day. The
area occupied averaged 15 sq. in. and varied from 8 to 20 sg.
in. The cumulative total area of rock surface occupied by
the cluster during the month of observation was 77 sg. in.
(see Figure 1,u). There was always some overlap between the
positions occupied by the cluster on successive days. This
overlap ranged from 37% to 75% of the total area occupied at
the time of observation (see Table 1). New area, not
276
Carol S. Millard
previously inhabited during the period of observation, was
added as the cluster changed daily in its shape and location
on the rock.
Another question to be answered concerns cluster member¬
ship (Figures 3 and 4). The membership of the cluster varied
from day to day as animals entered the cluster, remain asso¬
ciated with it for a period, and then either left the area
or remained nearby but not actually in the cluster. Later.
some of these limpets rejoined the cluster. Thus, it could
be observed that the turnover and variation in cluster mem¬
bership was something occurring in a relatively constant
group. Only a few unmarked limpets entered the cluster area
after the first week, during which eleven newcomers joined
the cluster. In the month that followed, fewer than ten un-
marked limpets joined. Two animals were observed to wander
one to four feet from the cluster area. The closer of the
two returned after five days; the other did not return.
Observations of Acmaea digitalis in several different
locations around the Monterey Peninsula indicate that, in
general, clusters are found in more protectéd areas. They
tend to occur in spots which receive more shade than the sur¬
rounding rock face due to a slight depression in the rock
barely noticeable when the full sun is on the whole rock face,
22
Carol S. Millard
On the rock island where most of this study was carried out,
between 1:30 and 3:00 P.M. the cluster areas would be out¬
lined by a shade pattern which covered them but left the
rest of the rock in full sunlight. Clustering also tends
to occur on surfaces at right angles to the sea which receive
some surge and splash from waves but do not bear the full
brunt of the breakers. In areas directly exposed to wave
action Acmaea digitalis have not been observed to cluster,
although their populations may be very dense. Here they
remain randomly dispersed, although small, temporary clusters
of three to six animals may aggregate in a crack or depres¬
sion in the rock.
The factors stimulating clustering are not fully
known. Animals cluster when the tide is out and disperse
when the tide is in, at night as well as during the day,
thus light does not seem to be a direct and immediate
causal factor. Tide level and wave action appear to play
an important role. The limpets recluster when they are
still being splashed on an ebbing tide and the area is still
totally wet, as though the frequency of splash and the amount
of water passing over them were being measured. The fact
that the animals recluster when the rock is wet and may do
so at night seems to indicate that clustering is not similar
29.
Carol S. Millard
to the phenomenon of aggregation in sow bugs which keep
moving until they end up in the dampest and shadiest spot
available (Allee 1926) It is also difficult to look at
clustering as a purely random occurrence because it is seen
that limpets return to a fairly stable cluster area.
Some observations made suggest that the height of the
sea at high water and the degree of wave action may influ¬
ence the height on the rocks where clustering occurs,
Toward the end of the month that the cluster shown in Figure
1 was observed, the weather grew progressively more stormy,
and wave action increased. During this period the cluster
rose in its position on the rock (Fig. 1), gradually occupy¬
ing a higher area. Also, the cluster underwent progressive
fragmentation with time. This behavior recalls the finding
of Abe (1932) that clustering is seasonal, and that the
clusters of A. dorsuosa break up shortly after the winter
storms begin.
The means by which animals are able to find their ways
back to the cluster area are not clear. There is a possibil¬
ity that mucous trails are involved. Another animal which
occurs slightly above A. digitalis in the intertidal region,
Littorina planaxis Philippi, has been proven to follow mucous
trails left on rocks by members of its own or other species
2.
Carol S. Millard
(Miyamoto, 1964; Peters, 1964). Whatever the mechanism, it
may be similar to that involved in "homing" in limpets.
During the month that the clusters were observed four of the
animals were seen to return to the same exact location and
orientation at successive low tides for period of two or
three days. They then changed resting spots and repeated
this behavior for several more days. Homing in some A.
digitalis was also noticed by Miller (1966).
SUMMARY
Acmaea digitalis populations often cluster on inter-
tidal rocks at low tide. Clustering occurs in areas which
receive some shade and some protection from direct wave
action. Animals disperse on a rising tide. Reclustering
occurs during tidal ebb while rocks are still wet, and takes
place both day and night.
A cluster varying in size from 22 to 44 individuals
occupied an average of 15 sq. in. (range 8 - 20 sq. in.)
each day over a 32 day period. Daily shifts in cluster
position occurred; total cumulative area occupied was 77 sq.
in.
Only 17 members of the original cluster remained after
32 days while a total of 22 new members entered.
2.
Carol S. Millard
ACKNOWLEDGMENTS
I would especially like to thank Dr. Donald P. Abbott of
Hopkins Marine Station for his assistance and guidance during
this study. I also wish to thank Galen Hilgard and Raymond
Markel who helped me obtain equipment, and Tudy Balesteri for
keeping an eye on me when field work was necessary at high
tide and access to my offshore rock was difficult. My sin¬
cere appreciation goes to Susan E. Fitzpatrick for the let¬
tering on the text figures. This work was made possible by
Grant GY 806 from the Undergraduate Research Participation
Program of the National Science Foundation.
29
10
Carol S. Millard
LITERATURE CITED
Abe, Noboru
1932. The colony of the limpet (Acmaea dorsuoso Gould).
Tohoku Imperial University Science Reports, 4th Ser.
(Biol.) 7:169-187.
Allee, Warder Clyde
1926. Studies in animal aggregations: causes and
effects of bunching in land isopods. Journ. Exp. Zool.
45:255-77.
Frank, Peter W.
1966. The biodemography of an intertidal snail popu¬
lation. Ecology 46(6):831-843.
Miller, Alan Charles
1966. Orientation and movement of the limpet Acmaea
digitalis (Gastropoda: Prosobranchia) on vertical rock
surfaces. Veliger ----
Miyamoto, Alan C.
1964. Clustering in Littorina planaxis. Unpublished
student research paper, Hopkins Marine Station of
Stanford University, 20 p.
Peters, Ronald S.
1964. Function of the cephalic tentacles in Littorina
planaxis Philippi. Veliger 7(2):143-148.
2e
e
11
Carol S. Millard
FIGURE CAPTIONS
Figure 1, A - U. Daily changes in size, shape, and number of
individuals in a cluster of Acmaea digitalis on a vertical
rock face, April 26 to May 27, 1966. Outlined clusters con¬
tain limpets one centimeter or less from nearest neighbors.
The number on the cluster shows the number of limpets in it,
Day and month are shown in arabic and roman numerals
respectively; hours of observation are given in the lower
right hand corner of each figure. Lines on the grid were
1.5 inches apart on the rock. The darkened square occupies
the same space on the rock in each figure. Figure 1,u shows
the cumulative total of all the area occupied by the cluster
during the month of the study,
Figure 2. Area occupied by the cluster on successive days,
based on the data in Figure 1. Line A represents the total
area covered by the cluster at any given observation time.
Line B represents the area occupied by the cluster during a
given observation which was not occupied at the previous
observation. Line C represents the overlap of the cluster at
any given reading with the area occupied at the previous
observation. Line D represents the area which has not been
occupied previously by the cluster at any time during the
present study.
27
0
Carol S. Millard
Figure 3. Turnover rate of limpets in cluster membership
between successive observations. Dates and times are the
same as those in Figure 1.
Figure 4. Composition of the cluster at each observation
period. Line A represents the total number of limpets pres¬
ent in the cluster during each observation. Line B represents
the number of limpets present which were members of the clus¬
ter when the study began; some individuals left the cluster
for a few days and later rejoined. Line C represents limpets
present which were not charter members but joined the cluster
after the study began.
27
1. Permanent address:
FOOTNOTES
Carer s. Mrlar
2
14
Carol S. Millard
TABLE CAPTIONS
Table I. Comparison of area occupied by the cluster with
number of animals in the cluster and the percentage of the
area occupied which overlaps that occupied the pre¬
vious observation.
DATE+ HOUR
26 - IV - 66: 1050
27 - IV - 66: 0930
28 - IV - 66: 0930
29 - IV - 66: 1030
2 - V - 66: 1345
3 -V - 66: 1645
5 -V. - 66: 0700
6 -V - 66: 1530
6 -V - 66: 1100
6 - V - 66: 1500
6 - V - 66: 1850
12 - V - 66: 1200
14 - V - 66: 1500
16 - V. - 66: 1530
17 -V - 66: 1100
17 - V - 66: 1700
19 - V - 66: 0100
19 -V - 66: 0300
26 - V - 66: 1300
27 - V - 66: 1300
-5-
TABLE I
AREA OCCUPIED
BY THE CLUSTER
IN SO. INCHES
14.75
16
18.75
20.25
16.75
13.5
19.5
18.7
16.75
16.25
17.25
14.75
15.25
10.25
11.5
8.25
13
15.25
11.25
Carol S. Millard
NUMBER
OF
ANIMALS
40
% OVERLAP
WITH
PREVIOUS DAY
—---
40 %
7%
57 %
64 %
46 %
75
56 %
43 %
41 %
40 %
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