INTRODUCTION
Tigriopus californicus, a harpacticoid copepod which inhabits high
splashpools along the Northwest Pacific coast, is capable of withstanding
extreme physical stresses. In the present study, the effect of high
temperatures on the torpor response of Tigriopus californicus was examined.
Different geographic populations were used to test whether there is a
difference in temperature tolerance due to varying environmental conditions
associated with latitudinally separated habitats. The test populations were
acclimated at 20°C to equilibrate them to similar recent temperature regimes.
To mimic natural conditions the temperature was raised 2°0 every 15 to 30
minutes. The torpor response of males and egg-carrying females at the same
temperature was studied for any sex differences. These animals were also
followed individually for several consecutive trials to ascertain individual
responses and the effects of repeated temperature stress.
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High-Temperature Induced Torpor in Tigriopus californicus
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MATERIALS AND METHODS
Five populations of Tigriopus californicus were collected from four
different geographic locations along the California coast: a North pop-
ulation from Santa Cruz at Natural Bridges State Beach (37° latitude);
a Hopkins population from 17-Mile Drive, Pacific Grove, just south of Point Joe
(36.6° latitude); a South population from Los Padres National Park at
Willow Creek (35.9° latitude); and a L.A. population from Flat Rock, Palos
Verdes (33.75° latitude). (Figure 1) In Los Angeles, two populations were
collected from two separate tidepools located approximately 3 meters apart.
These two tidepools were very similar in composition and there was no
significant vertical difference, however, one of the L.A. populations (LA,)
is noticeably smaller than the other L.A. population (LA2) as well as the
other three populations.
The collecting pools were all closely matched according to size having
a surface area of approximately 300 to 400 cm. and ranging in depth from
to 10 cm. The tidepools all had total sunlight, were in the high splash
zone with little, if any, flora and fauna, and contained a very dense
Tigriopus californicus population. The substrate of the pools was also
controlled for, however, it varied slightly due to geological factors of
the location - the tidepools at Santa Cruz and Flat Rock are in sandstone
while the other areas are primarily granite. The test populations were
stored in 400 ml. beakers containing 300 ml. of fresh sea water which had
been filtered using a vacuum millipore technique with whatman GF/C glass
fibre paper on Hawp HA 0.45 millipore filters. The animals were fed
TetraMin fish food and the water was changed every four days. All five
test populations were acclimated for a minimum of 20 days at 200c 1.50
using a Forma-temp constant temperature water bath. Kontogiannis (1973)
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High-Temperature Induced Torpor in Tigriopus californicus
found that Tigriopus californicus completely acclimated to 200c in 10 days
he found no further gain or loss of heat resistance with prolonged acclimation
at this temperature.
A sixth population of Tigriopus californicus was collected at China Point,
Pacific Grove for use in the experiment on sex differences in the high temperature
torpor response. This population was acclimated at 20°C for 48 hours prior to
testing.
In the general test for geographic differences in the high temperature
torpor response, 20 individuals, randomly selected from the same population,
were placed in a depression well containing 2 ml. of fresh filtered sea
water. A total of 200 individuals were tested for each of the five test
populations. The tray of depression wells was placed in a Forma-temp water
bath and the temperature was raised 2°0 every 15 minutes until 33°0. At
33°C the temperature was raised 2°C every 1/2 hour. For all the experiments,
torpor was defined as no body movement when the animal was prodded with the
end of a pipette. Readings were taken every 15 minutes to determine whether
any animals were in torpor and the temperatures in the test wells was
maintained using a Tele-thermometer. When an animal went into torpor it
was removed from the test conditions using a pipette and placed in a petri
dish of filtered sea water at room temperature to recover.
The torpor response to a steady high temperature was tested using a
select group of animals from the North, Hopkins, and L.A., populations. Only
egg-carrying females, selected from the last active 10 of each geographic
population (908 in torpor) when run under the previous test conditions, were
used. 10 individuals from each of the 3 populations were tested. Each
female was placed individually in a depression well containing 2 ml. of
filtered sea water. The tray of depression wells was allowed to equilibrate
in a Forma-temp water bath which was held at 370C for 60 minutes, then at
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38°C for 60 minutes, and then raised 1°0 every 15 minutes until all the animals
were in torpor. The readings were taken in the same method as was previously
described. In this experiment and the one following, the animals were always
kept separated in depression wells in order to keep track of the individual
animals.
To test sex differences and the consistency of individual Tigriopus
californicus on consecutive high temperature runs, 100 animals, 50 males and
50 egg-carrying females, from China Point, Pacific Grove, were tested
individually. in depression wells. The test animals were studied on 3
consecutive runs, 24 hours apart. The temperature was raised in the same
method as in the first experiment: 2°0 per 15 minutes to 33°C and 2°C per
1/2 hour from 33°C on up.
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High Temperature Induced Torpor in Tigriopus californicus
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RESULTS
In Figure 2 it can be seen that while there is some variation in the
temperatures at which Tigriopus californicus enters a torpor state, 3800
is the temperature which consistently puts over 908 of the animals into
torpor. The North population had the lowest temperature tolerance while
the animals from L.A. were able to survive the highest temperatures. Nost of
the populations went into torpor at 3800, however, there was a statistically
significant difference in the amount of time which the various populations
could withstand the high temperatures. (RC Contigency Test, G-306: giving
a very high level of significance) (Figure 3). Throughout all the experiments
there was a consistent ranking of the geographic populations in increasing
temperature tolerance: North, Nopkins, South, L.A.., L.A.,. Figure 4 displays
the time until 508 of the population was in a torpor state graphed according to
its geographic location along the California coast. The line shown is a
least squares regression fit to the points.
Figure 5 confirms the differences between the populations. The mean
time to torpor and the mean temperature to torpor with standard error bars
are plotted for egg-carrying females from North, Hopkins, and L.A.
populations. The high-temperature tolerance differences between the groups
are all statistically significant. (Student-T-test, Po .0004 level).
The results plotted in Figure 6 show a statistically significant difference
between males and egg-carrying females. (Mann-Whitney U Test, P£ .0001).
The individual Tigriopus californicus proved to be inconsistent in their
torpor response to repeated high temperatures. As a whole population, both
the males and the females significantly increased in temperature tolerance
from Run 1 to Run 2 and the males decreased significantly from Run 2 to Run 3.
(Chi-Square Test, P« .005). The females, however, did not significantly decrease
overall from Run 2 to Run 3, although there were significantly more decreases
than increases. (Chi-
.01).
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High Temperature Induced Torpor in Tigriopus californicus
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DISCUSSION
One of the most important factors in an organism's response to temperature
is its thermal history. Generally, an animal from a warmer climate can with-
stand higher temperatures than animals from colder situations just as animals
from colder environments can tolerate lower temperatures than those from
warmer situations. These may be acclimation differences or reflect different
genetic races. The present study supports this general principle as high-
temperature induced torpor in populations of Tigriopus californicus was
positively correlated with their latitudinally separated habitats. The
colder environment North population was least tolerant of high-temperatures,
followed by the Hopkins group, then the South population, and finally the
most tolerant L.A. populations from the warmest climate. These studies
were carried out on individuals acclimated for at least 20 days at 2000.
Thus it is suggested these may reflect genetic differences in a latitudinal
cline, or may be phenotypic expressions resulting from different thermal
histories. In order to conclusively separate these two factors, further
testing with cross-breeding experiments must be done.
The upper temperature limit for the majority of Tigriopus californicus
is 3800 as only individuals from the L.A. populations remained active above
this temperature. During some preliminary tests, a few individuals from
both the L.A.j and the L.A., populations survived temperatures up to 400c.
Time to enter torpor after reaching the critical temperature appears as a
major adaptive variable with mean differences of 1-1/2 hours between Santa
Cruz and Los Angeles populations. The L.A., population survived consistently
longer in the high temperatures than did the L.A., population. This difference
is statistically significant, suggesting that there are differences among
microhabitat populations as well as major habitat differences. Also, the
L.A. animals appeared to recover from the torpor much more rapidly than the
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High Temperature Induced Torpor in Tigriopus californicus
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other populations when placed in normal conditions.
The varying high temperature tolerance is further emphasized by the
tests with the select egg-carrying females from the North, Hopkins, and
L.A., populations. In this experiment, very discrete temperature and time
ranges were found for the populations. Again, in the experiment involving
sex differences, the populations as a whole maintained their high-temperature
tolerance order. The egg-carrying females did significantly better in high
temperatures than did the males. These results agree with field studies on
Tigriopus californicus (Egloff, 1966) which showed significantly different
sex ratios (8 males) associated with tidepool temperatures; the sex ratios
were 263 and 588 in the warm and cool pool, respectively. In Lebistes
reticulatas, the females also tends to be more tolerant of extreme water
temperatures than males (Tsukuda, 1958 as cited in Vernberg 6p; Vernberg,
1970). This hardiness of Tigriopus californicus gravid females has been
documented under other physical stresses, such as salinity (Nimkin), and
has obvious adaptive significance for survival of the species.
Individuals of Tigriopus californicus in consegutive runs showed much
variation in their repeated high-temperature torpor response. The increased
time to torpor between Run 1 and Run 2 indicates a temperature tolerance
step-up with a possible leveling off effect from Run 2 to Run 3. Under
natural conditions, this phenomena should be helpful for short-term as well
as longer seasonal climate changes.
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SUMMARY (ABSTRACT)
There appears to be both microhabitat and major habitat differences
in the high-temperature torpor response of Tigriopus californicus corresponding
to their geographic origins. 38°0 is a critical temperature for all the
populations. The determining factor in high-temperature related torpor is
time, as the tolerance periods vary significantly between the populations.
Egg-carrying females can withstand high temperatures significantly better
than the males. There is a large variation within an individual's torpor
response to repeated high temperature stresses. However, the populations
as a whole do maintain a consistent order of the least tolerant to the
most resistant of temperature: North, Hopkins, South, L.A.,, L.A..
These findings suggest a genetic basis for geographic differences in
temperature tolerance as the 20-day minimum acclimation at209C would
presumably eliminate any different short-term adaptations due to different
environments.
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High Temperature Induced Torpor in Tigriopus californicus Page
ACKNOWLEDGEMENTS
I would like to express my sincere thanks to the faculty and staff of
Hopkins Marine Station, especially to my advisor Charles H. Baxter for his
endless help and encouragement, to Dr. Robin Burnett for his advise and his
help with statistics, and to Lynn Hodgson for her assistance and support
throughout the quarter.
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High Temperature Induced Torpor in Tigriopus californicus
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FIGURE CAPTIONS
Figure 1:
The four different geographic locations along the California coast
from which the five Tigriopus californicus populations were collected:
the North population from Santa Cruz; the Hopkins population from
1/ Mile Drive; the South population from Willow Creek; and the two
L.A. populations, L.A. and L.A.,, from Palos Verdes.
Figure 2:
The difference in temperatures, in degrees Centigrade, at which
latitudinally separated populations of Tigriopus californicus
enter a torpor state.
Key for latitudinally separated populations in Figures 2 & 3.
— —
-North
— — — — — — — - - Hopkins
- South
.... L.A.
— L.A.,
Figure 3:
The difference in times, in minutes, at which latitudinally
separated populations of Tigriopus californicus enter a torpor
state.
Figure 4:
The time until 50% of the population was in a torpor state
graphed according to its geographic location along the California
coast. The line shown is a least squares regression fit to the
points. Key to the latitudinally separated populations: 1 - North;
2 - Hopkins; 3 - South; 4 - L.A.j; 5 - L.A.9.
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Figure 5:
The mean time to torpor, in minutes, and the mean temperature to
torpor, in degrees Centigrade, with standard error bars plotted
for egg-carrying female Tigriopus californicus from North, Hopkins,
and L.A. populations.
Figure 6: The difference in time to torpor, in minutes, for egg-carrying
females and males of the same Tigriopus californicus population
on three consecutive test runs.
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LITERATURE CITED
Egloff, D.A. 1966. Ecological Aspects of Sex Ratio and Reproduction in
Experimental and Field Populations of the Marine Copepod, Tigriopus
californicus. PhD. dissertation, Stanford University.
Kontogiannis, J.E. 1973. Aquisition and Loss of Heat Resistance in Adult
Tide-pool Copepod Tigriopus californicus. Physiol. Zoology 46:
50-54.
Nimkin, K. 1977. Differential Mortality to Salinity Stress and its Relation
to Sex Ratios in the Marine Copepod, T.c. unpublished manuscript
on file at Hopkins Marine Station, Pacific Grove, Ca.
Vernberg, F.J. and Vernberg, W.B. 1970. The Animal and the Environment,
Holt, Rinehart and Winston, Inc., New York.
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High Temperature Induced Torpor in Tigriopus californicus
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