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HARDIN, DANE D. (Hopkins Marine Station of Stanford University,
Pacific Grove, Calif.). A Comparative Study of Lethal Temperatures
in the Limpets Acmaea scabra and Acmaea digitalis (Mollusca:
Gastropoda: Prosobranchia).
The lethal temperatures of two specics of limpets which occupy the samc intertidal
levels were studied. It was found that A. scabra (with a critical range of 40-44 c)
is better able to withstand high tomperatures than is A. digitalis(with a critical
range of 39-42'C). Concurrent field studics showed that A. scabra experiences higher
microhabitat and body temperatures than docs A. digitalis at similar air temperatures.
It was found that the limpets' body temperatures were 2-6 higher than surrounding ex-
ternal temperatures. Intraspecific differences in temperature tolerance were also-
found, with the highest intertidal members of eachespecics:surviving better thancthe
lowest members. Possible causes of intraspecific differences arc acclimation or
size of the animal.

PLEASE DO NOT TVPE BELON THIS LINE
5.
0
C
A Comparative Study of Lethal Temperatures in the
Limpets Acmaea scabra and Acmaea digitalis
(Mollusca: Gastropoda: Prosobranchia)
by
Dane D. Hardin
Hopkins Marine Station of Stanford University,
Pacific Grove, California
Footnote 1
5
A Comparative Study of Lethal Temperatures
in the Limpets Acmaea scabra and Acmaca digitalis
Dane D. Hardin
INTRODUCTION
The importance of temperature as a limiting factor for the gco-
graphical distribution of intertidal animals is generally recog-
nized (Moorc, 1958). Many studies have been conducted in an effort
to correlate the distribution of some intertidal organisms with
among them
either air or sea temperature,y those of Hutchins (1947).
Southward (1950), and Southward and Crisp (1954). Segal (1956, 1961.
1962) has indicated that there is a difference between the highest and lowes
intertidal members of Acmaea limatula Carpenter, 1864,and tha lem-
att with respect to such body functions as heart rate and
oxygen consumption. Intraspecific differences in these processes
have also been detected in animals from different latitudes. Segal
has further demonstrated that these differences are a possible effect
of temperature and that acciimation of the processes to different
tidal levels takes place.
Studies have also been conducted to determinc the lethal temp-
(navee; ownl
eratures of many intertidal animals teroekhugeenodetowertreh
o Hayes,1126; BPovvsev,11).
and Hayeo, ayr However these investigations have
not dealt satisfactorily with intraspecific variation or niche
difference. For instance, there may be a difference between lethal
temperatures of members of a single species from the extreme boundaries
ofits vertical intertidal distribution at a single latitude.
18
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Dane D. Hardin
There may also be differences between different species which are
found at the same intertidal levels.
This study was undertaken in an attempt to answer threc main
questions: 1) Is there a difference in lethal temperatures of
and hodr stae
animals of the same species,taken from different tidal levels?
2) Is there a difference between lethal temperatures of two species
occupying the same intertidal levels? 3) How does an organism's
lethal temperature relate to the temperature of its microhabitat?
The organisms used in this study, the limpets Acmaea digitalis
Eschscholtz, 1833, and Acmaea scabra (Gould, 1846), are ideal for
rer wa
answering these question, since they occupy the same range in the
intertidal zone (-2 to +10 feet) along the central California coast
(Test, 1945).
MATERIALS AND METHODS
The members of cach species were collected from areas visibly
dominated by one or the other (at Point Pinos and Mussel Point on
the Monterey Peninsula) in order that the results gained by lab-
oratory experiments would be as representative as possible of the
normal population.
For the purposes of testing intraspecific differencesiinilethal
temperatures with respect to differences in intertidal location.
animals were taken from the extreme upper (above +7.0 feet) and
lower (below +4.5 fect) limits of the species in that area, and
from a region midway between. Teineurethatthe animaie seliete
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Dane D. Hardin
reprecented thenermalpopulation foreach regien, quadrate were
markeuontherocke frem which-alltheanimereweretaken Col-
lected animals were placed in running sea water at approximately
15°C. in the laboratory and used within 24 hours.
Two types of laboratory experiments were conducted to determine
lethal temperatures. The first type was run with the animals sub-
merged. High, mid, and low members of each species were placed in
continuously aereated beakers of sea water in a water bath and
lo ostant tempentr.
allowed to  equilibrate, The temperature was then raised
at a rate of fC. per 5 minutes, to allow for complete equilibration
of theeinternal and external temperatures of the animals (Southward,
1958). Fifteen high, mid, and low members of each species were
removed at l intervals, at the indicated temperatures and replaced
in running sea water at 15°C. The animals were allowed to recover
for 6-12 hours and checked for survival by pricking the mantle fold
with a needle. If no response was elicited, the animal was considered
dead.
The second type of experiment tested the animals' abilities to
survive prolonged exposure to higher than normal temperatures in air,
Members of each specics from each of the three intertidal levels
were placed in degsicätors in the water bath. The bottom of each
dessigator was filled with a nearly saturated solution of ammonium
chloride and potassium nitrate, resulting in a relative humidity, as
determined with a Honeywell relative humidity readout instrument.
of 85-90%.for each trial. Again the temperature in the containers
16
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Dane D. Hardin
was raised at the previously mentioned rate until the desired temp-
erature was reached. The animals were held at this temperature
(#0.3), with 15 animals from each species and each intertidal level
being removed a 5, 10, and 15 hours. Trials were run at 29, 31.5.
and 34. After each time period the removed animals were replaced
in running sea water at 15°C., and tested as above after 6-12 hours.
Field temperatures encountered by the two species were taken
eween
on several days-at 12-4:0Opmat Mussel Point. Temperatures were
taken with a portable thermistor (model 43TD Yellow Springs Tele-
Thermometer). Five readings were recorded: 1) air temperature
2-3cm. above the animal, 2) rock temperature next to the limpet,
3) the temperature on the surface of the animal's, shelle4) the temper-
ature beneath the animal's foot, and 5) the temperature within the
limpet's mantle cavity. The air, rock, and shell temperatures were
taken with a banjo-type probe, model 409, which was kept shaded to
prevent heating by direct sunlight. The foot and mantle cavity
temperatures were taken with a 716 inch flexible probe, model 402.
The foot and mantle cavity temperatures were taken in situ. Bach
animal was lifted from the rock, the probe was placed under the
foot or in the mantle cavity, and the animal returned to the spot
from which it was taken. The limpet was then held in place with the
blade of a knife and the temperature read-out recorded.
RESULTS
Figure 1 shows the results of the submerged temperature trials.
sorwed ketrrr
Acmaea scabra consistently hadhighessuzinalpescentagee than did
8
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15
Dane D. Hardin
Acmaca digitalis, and the higher members of each species consistently
survived better than did the lower intertidal animals.
The results of the prolonged temperature trials are shown in
Figures 2 and 3. There is no figure for the 34 trial as no indi-
Hlrough
viduals of either species survived this temperature. At 20 and24.
Acmaea scabra continued to survive better than Acmaca digitalis+ 29 and 37.5
the intraspecific diffcrences seen in the submerged trials
are less evident.
To check any possible correlations between size and survival.
shell dimensions of all animals used in the submerged temperature
experiments were determined with vernier calipers readable to 0.Olcm.
Twenty individuals from each group were randomly chosen, (by random
selection of pieces of paper containing data on each individual).
and the respective size distributions are shown in Table 1. The
data show a marked tendency for the low intertidal group to be
smaller than the high intertidal group.
The mean temperatures for 10 randomly selected limpets are
represented in each graph of Figure 4. Acmaca scabra consistently
exhibited higher microhabitat temperatures than did Acmaea digitalis
at similar air temperatures. It can also be seen that in all cases
the actual temperature of the limpet (mantle cavity temperature) is
higher than the microhabitat temperatures. No significant temper-
aturc differences of any of the measured variables were found
between the upper and lower areas(the temperatures were taken from
+3 to 47 feet).
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Danc D. Hardin
DISCUSSION
In all experiments Acmaca scabra survived high temperatures,
in both air and water, better than did Acmaea digitalis. These
differences betwcen the two species correlates with the higher mean
microhabitat temperatures which were found. Haven (1964) has observed
that A. scabra is seen in greatest abundance on surfaces which are
more horizöntal than the areas of highest A. digitalis concentration.
This also correlates well with the observed lethal temperatures,
since A. scabra would therefore receiw more and stronger sunlight
than would A. digitalis.
The observed intraspecific differences in ability to survive
high temperatures possibly result from temperature acclimation.
Thus, even though the high and low members within each species
experience much the same temperatures, the higher members would
experience any high temperatures for longer periods of time than
their lower counterparts. An alternative explanation, not involving
temperature acclimation, is that the greater resistance of the higher
andsor a90.
forms is a consequence of their greater size, This is suggested by
the results in Table 1, showing a continuum in size, with higher.
animals having larger shells than lower animals, and the work
of Frank (1965), which showed that A. digitalis move higher
in the intertidal zone with age. However, these results do
not establish a causal relationship between size and ability
to withstand high temperatures, and further research ig
pLanned.
The intraspecific differences in ability to survive high temp-
eratures were much less clear in the prolonged temperaturc trials.
16
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Dane D. Hardin
and
This was especially evident with Acmaca digitalisThie can
possibly be traced to the fact that even though the A. digitalis
population presents a size continuum(with the smallest animals
lowest in the intertidal zone and the largest animals highest)
this species is probably more mobile than Acmaea scabra. This
results in part from the greater percentage of homing behavior
found in A. scabra than in A. digitalis (Haven, 1964; Jessee, 1966;
Miller, 1966). It can be expected thereforc, that the effects of
acclimation would be more clearly defined in a population of ani-
mals which remain in rather fixed positions. This agrees with the
greater intraspecifec differences in survival at high temperaturess
seen in A. scabra.
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Dane D. Hardin
SUMMARY
Lethal temperatures of the limpets Acmaca scabra and Acmaea
digitalis were studied. A. scabra was found to survive high temp-
eratures better than A. digitalis. These results correlate with
field studies which showed that A. scabra experiences higher micro-
habitat and body temperatures than does A. digitalis at similar temp-
eratures. It was also found that the internai temperatures of limpets
are consistently above the external surrounding temperatures.
Members of the species coming from the highest intertidal ranges
of the species were found to survive high temperatures better than
members from the lowest intertidal ranges of the species. These
intraspecific differences may be a result of acclimation to the
length of exposure to high environmental temperatures and there are
indications that the size and/or age of a limpet may have an effect
on its ability to survive high temperatures.
ACKNOWLEDGMENTS
This work was made possible by Grant G1806 from the Under-
graduate Research Participation Program of the National Science
Foundation. My sincerest thanks are given to the faculty and staff
of Hopkins Marine Station of Stanford University for allowing me the
opportunity to do this study, and especially to Dr. David Bpel who
advised and directed me in my research. I am also gratefully
indebted to Dr. A. Todd Newberry of Cowell College at the University
of California, Santa Cruz, without whose encouragement and advice
this work would not have been possible.
O
Dane D Hardin
FOOTNOTES
page 1 - Present address: Cowell College and Division of
Natural Sciences, University of California, Santa Cruz,
California.
64
C
Dane D. Hardin
LITERATURE CITED
Broekhuysen, C. J. 1940. A preliminary investigation of the
importance of dessiccation, temperature and salinity as
factors controlling the vertical distribution of certain
marine gastropods in False Bay, South Africa. Trans.
Roy. Soc. S. Afr. 28: 255-292.
Frank, Peter W. 1965. The biodemography of an intertidal
snail population. Ecology 46: 831-844.
Gowanlach, J. N. and F. R. Hayes. 1926. Contributions to the
study of marine gastropods. I. The physical factors,
behavior and intertidal life of Littorina. Contr.
Canad. Biol., N.S. 3:133-166.
Haven, Stoner B. 1964. Habitat differences and competition
in two intertidal gastropods in central California.
Bull. Ecol. Soc. Amer. 45: 52.
Hutchins, L. W. 1947. The bases for temperature zonation in
geographical distribution. Ecol. Monogr. 17: 325-335.
Jessee, William F. 1966. Ecological and mechanistic studies
of homing behavior in the limpet Acmaea scabra (Gould,
1846). The Veliger.
Mayer, A. G. 1918. Ecology of the Murray Island coral reef.
Papers Tortugas Lab. 9: 1-48.
Miller, Allen C. 1966. Orientation and Movement of the limpet
Acmaea digitalis on vertical rock surfaces. The Veliger.
168
2 -
Dane D. Hardin
LITERATURE CITED
Moore, Hilary B. 1958. Marine ecology. John Wiley & Sons:
vii + 493 pp.
Segal, Earl. 1956. Microgeographical variation as thermal
acclimation in an intertidal mollusc. Biol. Bull.
111: (1) 129-152.
1961. Acclimation in molluscs. Amer. Zool. 1: 235-244,
1962. Initial response of the heart-rate of a gastro-
pod, Acmaea limatula, to abrupt changes in temperature.
Nature 195: (4842) 674-675.
Southward, A. J. 1950. Occurrence of Chthamalus stellatus in
the Isle of Man. Nature 165: (4193) 408-409.
1958. Note on the temperature tolerances of some
intertidal animals in relation to environmental tem-
peratures and geographical distribution. J. Mar. Biol.
Assn. U.K. 37: 49-66.
and D. J. Crisp. 1954. The distribution and abundance
of certain intertidal animals around the Irish coast.
Proc. R. Irish Acad. 57 (B): (1) 29 pp.
Test, Avery R. 1945. Ecology of California Acmaea. Ecology
26: (4) 395-405.
166
DANE D. HARDIN
FIGURE LEGENDS
Figure 1. Submerged exposure to high temperatures. Oschighiinter-
tidal animals. • - mid-intertidal animals. X= low intertidal
animals.
Figure 2. Prolonged exposure to 29 C. air temperature. Oz high
intertidal animals.o z mid-intertidal animals. X = low inter-
tidal animals.
Figure 3. Prolonged exposure to 31.5'C. air temperature.
O= high intertidal animals. o s mid-intertidal animals.
X  low intertidal animals.
Figure 4. Microhabitat and body temperatures. The midline of
each bar indicates the mean of ten measurements, and the
portion of each bar above and below the midline indicates the
standard deviation. a - air, r s rock, s z shell, fa foot.
me  mantle cavity.
Dane D. Hardin
TABLE CAPTIONS
Table 1. The mean shell dimensions for 20 randomly selected high,
mid, and low intertidal members of each species. All measure-
ments are in centimeters, and the standard deviation is indi-
cated in parentheses.
6
100-
90r
80-
270
9604
550
B40
aot
20
10
A. scabra
A. digitalis
V
350 400 410 420 430
39° 40° 41° 42° 43
TEMPERATURE IN C.
Egee 1
100
90T
807
707
2607
250
8407
30+
8820
10-

A. scabra
15
10
HOURS OF EXPOSURE AT 29° C.
agie 2

A. digitalis
15
10
C

100-
90
80+
270
60
50 7
540
330
20 +
10

55


A. scabra
HOURS OF EXPOSURE AT 31.5° C.
Lgie 3
—X
A. digitalisO
10
15
27
261
34
2237
22
220 2
18
17-
116
me
r
a
A. digitalis
A. scabra
ON CLEAR DAY
ge
me
me
f
I
8
a
A. digitalis
A. scabra
ON OVERCAST DAY
me
SEE
MICROHABITAT AND DODVMRMDERADURRS OF LIMPETS
LLENGTE
WIDTH
HEIGHT
A. scabra
HIGH
1.35(0.26) 1.05(0.21)
0.47(0.12)
O.45(0.10)
MID
1.34(0.17) 1.03(0.15,

LOW
1.27(0.19) 0.98(0.16
O.36(0.05)
A. digitalis
HIGH
1.46(0.19) 1.11(0.15) O.51(0.10)
MID
1.03(0.18) O.45(0.10)
1.37(0.19
LOM
1.20(0.17) 0.95(0.15) O.40(0.05)
1
toble