Thermal preference and activity thresholds in populations of Argia vivida (Odonata: Coenagrionidae)...

Hydrobiologia 140: 85-92, (1986) 0 Dr W. Junk Publishers. Dordrecht - Printed in the Netherlands

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Thermal preference and.activity thresholds in populations of Argia vivida (Odonata: Coenagrionidae) from habitats with different thermal regimes

Mark Leggott 8z Gordon Pritchard’ Department of Biology University of Calgary, 2500 University Drive N. IX, Calgary, Alberta, Canada T2N IN4 ‘Author for correspondence.

Keywords: Odonata, Zygoptera, Argia vivida, thermal preferendum, activity thresholds

Abstract

The hypothesis was tested that isolated populations of Argia vivida Hagen living in habitats with different thermal regimes would show similar larval temperature preferenda, similar distributions in a temperature gradient, similar larval upper temperature acitivity thresholds, and similar adult minimum temperature flight thresholds. The hypothesis was supported in all cases, except for distribution within the gradient, where there were significantly fewer observations below 22 “C in a population from a habitat with a fluctuating die1 and annual temperature regime than in a population from a more thermally stable habitat. Larval modal temperature preferendum was 28 “C; escape temperature (EST) was 35.4- 36.4 “C, critical thermal maximum (CTM) was 39.1-41.O”C, and upper lethal temperature (ULT) was 44.4-46.O”C. While technical difficulties affected the estimates of flight thresholds, there was no difference between the field estimates from different sites. Minimum body temperature for flight appears to be about 25 “C, apparently higher than for several other zygopterans, while larval activity thresholds and 96 hr LDSO of 36.8 “C are similar to those recorded for other odonates.

Introduction Methods

The genus Argia is limited to the New World. Pritchard (1982) argued for a Central American origin for the genus and discussed possible mechanisms whereby successful colonization of North America might have occurred. This led to the hypothesis that range extension in the most northerly ranging species, Argia vivida Hagen, had been accomplished by the exploitation of suitable larval habitats (geothermally heated streams) and by seasonal regulation of the life cycle, rather than by adaptations to the prevailing temperatures. The present paper examines certain responses to temperature in populations of this species living in habitats with different thermal regimes.

Field Sites

Three sites were used: Cave & Basin Hot Springs, Banff, Alberta (51” lO’N, 115” 34’W); Albert Can- yon, British Columbia (50’ 50’N, 117” 55 ‘W); and Deep Creek, Idaho (42” 24’N, 112’ 44’W). All sites are in geothermal areas but the influence of geothermal activity on the three streams differs considerably. The water at the Cave & Basin site is maintained at a fairly constant 26°C year-round, whereas Albert Canyon has a smoothly changing annual cycle that varies between 5 “C in winter and 20 “C in summer, with very small daily fluctuations (about 2 “C in summer), and Deep Creek varies from 0 “C to 33 “C, with ice cover for 3 months and daily fluctuations during the summer of 15 -20 “C. The annual degree-day accumulations above 0°C for the 3 sites are 9 500, 3 800, and 2 560 respective-

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ly. However, air temperatures are considerably higher at Deep Creek than at the other 2 sites. The Cave & Basin site became unavailable during this work and thus most of the experiments reported here were performed on insects from the other 2 sites.

Temperature preference in larvae

Studies of thermal preference usually consider either the acute or final preferenda, and there is some disagreement as to whether they represent the same parameter. Acute preferenda are determined about 2 hours after placing an organism in a thermal gradient, and are thus usually influenced by the acclimation temperature. Final preferenda are determined only after the organism has been in the gradient for a longer time (12-96 hr), during which reacclimation would occur and the final

I 1

preferendum would therefore be unaffected by pri- or acclimation temperature (Reynolds & Casterlin, 1979). Thus the final preferendum is considered to be a species-specific trait (Fry, 1947), although differences between populations showing a positive correlation with ambient temperature have been reported (e.g. Hall et al., 1978). Final preferenda were determined in the present study.

Larvae from Albert Canyon and from Deep Creek were collected on 28 July 1983 and 5 August 1983 respectively; mean temperature over the previ- ous 2 months had increased from 15” to 20°C at Albert Canyon and from 15” to 23” at Deep Creek. Larvae were transported in ice-cooled containers to the laboratory where they were placed in individual petri-dishes with dechlorinated water and an abundance of enchytraeid worms at 10°C and a photoperiod of L:D 1410. After 12 hr larvae were transferred to the experimental apparatus (Fig. I),

I I c; Refrigeration Units 3

9

I Gravel

-

oz 0 Cooling Coil

Pump

Fig. 1. Temperature preference apparatus.

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which consisted of 10 galvanized metal troughs (8 x 8 x 150 cm), the ends of which were sealed into waterproof wooden boxes (35 x 35 x250 cm); in one of these the water was heated to 45 “C and in the other it was cooled to 6°C. The metal troughs were lined with white aquarium gravel and filled with dechlorinated water 2 cm above the gravel. Two additional metal troughs were prepared in the same manner but were not sealed into the wooden boxes and served as controls.

The entire apparatus was placed in a walk-in in- cubator at 22 “C with continuous illumination. Af- ter 24 hr a linear temperature gradient of 9-41 “C was established in each trough and 10 larvae were distributed evenly in each trough. Approximately equal numbers of final (F), penultimate (F-l), F-2, and F-3 instars were used in each trial. Be- ginning 12 hr after introduction, the position of each larva in the gradient was recorded at 15 min intervals for 24 hr. To simplify the recording of position, each trough was divided into 33 equal zones, representing 1 “C differences in temperature, and larval position was recorded by zone number.

In another experiment, the water in 5 experimental troughs and 1 control trough was aerated via perforated plastic tubing affixed to the bottom of each trough along the entire length. This did not affect the thermal gradient and served to test whether oxygenation of the water affected larval distribution.

The null hypotheses were that the modal temperature preferences and the distributions of larvae in the gradient were the same; the latter was tested between sites by comparison of the frequency distributions by Chi-square in a 2 x 33 contingency table.

Activity thresholds of larvae

Larvae were collected at all three sites, placed in a cooler with ice, and returned to the laboratory, where they were placed in individual petri-dishes with dechlorinated water and an abundance of enchytraeid worms. Larvae from each of 4 head-width classes (cl, l-2, 2-3, >3 mm) from Cave & Ba- sin were placed at constant temperature regimes of 10, 15, 25, and 3O”C, at a photoperiod of L:D 14:10, and 10 larvae were acclimated at each temperature for each of the periods 24, 36, 48, 60 and 200 hr. Five larvae from each size-class from Deep Creek and from Albert Canyon were acclimated at

10°C and L:D 14:lO for 200 hr. Temperatures were maintained within + 1 “C.

After the acclimation period larvae were transferred individually to 1000 ml beakers containing dechlorinated water at the acclimation temperature and after 30 min the beakers were placed in a water bath also at the acclimation temperature. The temperature of the water in the beakers was then raised at 0.5 “C/min and recorded to the nearest 0.1 “C when larvae showed certain behaviour patterns. Three activity thresholds were used: Escape temperature (EST) was that temperature at which larvae exhibited sustained, vigorous swimming; critical thermal maximum (CTM) was the temperature at which larvae became disoriented (Cowles & Bogert, 1944), frequently turning over on their backs; upper lethal temperature (ULT) was that temperature at which larvae ceased all movement, specifically tarsal twitching. At the end of each trial larvae were placed at 20°C to confirm that death had occurred. Thresholds were determined for 1 larva of each size-class from each site after each acclimation period at each temperature.

The null hypothesis, that activity thresholds in the 3 populations are the same, was tested by a comparison of the effects of acclimation temperature, acclimation period, and site in an analysis of variance (ANOVA). Where treatment effects were significant, Student-Newman-Keuls (SNK) multi- ple range tests were performed.

The temperature at which 50% of the larvae were dead after 96 hr exposure (the 96 hr LDSO) was determined for larvae from Cave & Basin. Larvae were acclimated at 20°C for 10 days, during which time they were fed enchytraeid worms. Then, 10 late-stage larvae were placed without food at each of the constant temperatures 34, 36, 38, and 40°C and checked for mortality at 12 hr intervals up to 96 hr.

Adult flight thresholds

Minimum thermal thresholds for flight were determined by the procedures of May (1976). Adults were caught at Deep Creek and Albert Canyon and placed in individual vials on ice for about 2 hr. Then they were placed on a grey substrate, fully ex- posed to the sun, and prodded until they took flight. They were recaptured and a hypodermic thermister probe coupled to a digital thermometer

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was inserted into the thorax between the wing bases. If the time between flight initiation and determination of thoracic temperature exceeded 15 s, the result was excluded. The experiment was also performed in the laboratory under fluorescent lighting at 22 “C on adults from Albert Canyon.

The null hypothesis was that the threshold temperatures for flight at the 2 sites were the same.

Results

Larval temperature preference

There was no difference in the distributions of individuals in the temperature gradient whether the water was aerated or not, nor were there any differences between the 4 instars, and so the results of all treatments are pooled in Fig. 2. There was higher

200

160

120

80

160

80

r

AC

DC

Temperature (“C ) Fig. 2. Frequency of observations of larvae at each temperature in the gradient. Solid columns experimental values and open columns experimental values. AC, larvae from Albert Canyon; DC, larvae from Deep Creek.

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mortality among Deep Creek larvae and so fewer observations were made than on Albert Canyon larvae. The controls showed a distinct end-effect at both ends of the trough, but otherwise the control distributions were random and differed significantly from each of the experimental distributions (P < 0.001).

Larvae from Deep Creek showed a distinct preference for the range 23-32 “C, with a modal preference of 28 “C. There were few observations outside this range. The results for Albert Canyon were identical for preferred temperature range, modal preference, and number of observations above 32 “C. However, the proportion of observations below 22°C (36.8%) for Albert Canyon larvae was much higher than for Deep Creek larvae (19.5%), and the 2 distributions were significantly different from each other (P < 0.001).

In order to shed light on this difference, the read- iness to move in cold (temperature < 16 “C) and warm (temperature >19”C) zones was compared between larvae from the two sites. The mean numbers of 10 min intervals spent in a zone before moving to another zone are shown in Table 1. Mann- Whitney U-tests showed that the activity of Deep Creek larvae was significantly higher (P < 0.001) than activity of Albert Canyon larvae in cold zones, but not in warm zones. There was no significant difference between movement of Albert Can- yon larvae in warm and cold zones, but Deep Creek larvae moved significantly more frequently (P < 0.001) in cold zones.

Larval activity thresholds

Neither acclimation time nor temperature had a significant effect on ULT, but there were signifi-

Table 1. Mean number of consecutive 10 min intervals spent in cold and warm temperature zones by Argia vivida larvae from Deep Creek (DC) and Albert Canyon (AC) before moving to another zone.

Site N x S.E.

COLD ZONE DC 16 1.7424 0.1856 AC 15 3.5966 0.5499

WARM ZONE DC 21 3.0106 0.3704 AC 20 3.8926 0.5206

0

ULT

CTM

EST

32 ! 10 15 20 25 30

Acclimation Temperature

(“C )

Fig. 3. Relationship between acclimation temperature and activity thresholds (means +l SD). EST, escape temperature; CTM, critical thermal maximum; ULT, upper lethal temperature.

cant effects of acclimation time and temperature on EST (P < O.OOl), and of acclimation temperature on CTM (P < 0.05). There was a gradual rise in EST and CTM values with increase in acclimation temperature (Fig. 3), but the SNK test showed that the EST value at 30 “C alone accounted for the significant difference detected by ANOVA, while the CTM value at 30°C was significantly higher than those at 10” and 15 “C. The significant effect of acclimation time on EST was due to the low ESTs at the intermediate acclimation times of 48 and 60 hr. The mean threshold values from Deep Creek, Al- bert Canyon and Cave & Basin (Table 2) were not deemed to be significantly different by ANOVA.

The 96 hr LDSO, estimated by plotting percent

Table 2. Mean Activity thresholds (X + 1SE) for larvae from the three sites. Values in parentheses represent sample size.

Site EST CTM ULT

Cave & Basin’ (48) 36.4kO.4 41.OkO.2 44.4 -I 0.4 Deep Creek2 (12) 35.9kO.3 39.1 * 0.2 45.6kO.3 Albert Canyon2 (12) 35.4 * 0.3 39.7kO.3 46.OkO.2

I Larvae acclimated at lo”, 15”, 20” and 25 “C for 24, 36, and 200 hr.

2 Larvae acclimated at 10°C for 200 hr only.

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Table 3. Mean thoracic temperatures at which flight occurred. Values in parentheses represent sample size.

Site Ambient Flight Temperature (“C) Temperature (“C)

(x+ 1SE)

Deep Creek (12) 33 32.OkO.3 Albert Canyon’ ( 6) 27 31.OkO.3 Albert Canyon2 ( 6) 22 26.5 kO.8

I Field determination. * Laboratory determination.

survival after 96 hr on a probability scale against temperature, was 36.8”C. Fifty percent of the larvae were dead after 12 hr at 40°C and after 38 hr at 38°C.


The mean values recorded in the field (Table 3) were not significantly different between the two sites (P >0.05), but the value determined in the laboratory was significantly lower (P < 0.01) than those recorded in the field.

different populations to respond to a thermal gradient. The fact that Deep Creek larvae are found less frequently at low temperatures in there thermal gradient than are Albert Canyon larvae, suggests that the former either have a better ability to select optimal temperatures and do not enter the low temperature zone as frequently, or they move more rapidly once in it. The fact that development rates at low temperatures in Deep Creek eggs and larvae are faster than in individuals from Albert Canyon (Leggott & Pritchard, 1985) lends support to the second hypothesis, which is confirmed by the results of the comparison between activity in warm and cold zones by larvae from the two sites. Deep Creek larvae did, indeed, move more readily than Alberta Canyon larvae in cold zones but not in warm zones.

Thus we postulate that the fluctuating thermal regime at Deep Creek might have presented a selection pressure for the establishment and main- tenance of a precise temperature preference response, and in the absence of this selection pressure at Albert Canyon, the physiological and behavioural mechanisms allowing the detection of temperature changes might have been lost or modified.

Larval activity thresholds Larval temperature preferendum

Display of a temperature preference is an important thermoregulatory mechanism in ectothermic animals, whose body temperatures are influenced largely by ambient temperatures (Reynolds & Casterlin, 1979). Behavioural thermoregulation may allow individuals to take better advantage of the thermal regime, possibly maximizing growth rates and other fitness components (Magnuson et al., 1979). Thus, temperature preferenda have been examined extensively, especially in fish (see Cou- tant, 1977), and to a lesser extent in terrestrial insects (e.g. Fraenkel & Gunn, 1940; Deal, 1941). Few aquatic insects have been studied (Ivanova, 1940; Omardeen, 1957; Linley & Evans, 1971).

Fry (1947) suggested that the final preferendum was a species-specific trait. The fact that both populations of A. vivida examined here had identical modal preferenda lends support to this state- ment. However, our results also indicate that there are differences in the abilities of individuals from

The results of this study compare well with other studies. The increase in threshold temperatures with increase in acclimation temperature observed here was also recorded for several Anisoptera by Martin & Gentry (1974) and by Garten & Gentry (1976), and the CTM and ULT values reported by these authors were in the same range as those observed for A. vivida. Also, the 96 hr LD50 value of 36.8 “C from the Cave & Basin is similar to that determined for A. vivida from a desert stream by Gaufin et al. (1972).

Although the mechanisms allowing ectotherms to acclimate to changes in environmental temperatures are probably numerous, one of the more important may be the ability to effect changes in enzyme systems (Hochachka & Somero, 1973). Schott & Brusven (1980) reported changes in enzyme systems of A. vivida larvae, with both the amount of particular enzymes and appearance of different iso- zymes increasing with an increase in acclimation temperature. Such changes may underly the accli-

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mation response in EST and CTM, while the lack of an acclimation response in ULT may be due to the fact that change in this threshold would un- doubtedly involve truly major changes in enzyme systems which cannot be accomplished simply. The lack of a statistically significant elevation in EST and CTM between 10” and 25 “C is perhaps not un- expected, since even 25 “C is well below lethal temperature. However, we are unable to explain the significant effect of intermediate acclimation times on EST.


While species or seasonal differences in minimum flight thresholds have been observed (May, 1979), the various factors that can affect measure- ments of this parameter make the detection of differences between populations extremely diffi- cult. May (1977) omitted data for small species of Micrathyria (Odonata, Libellulidae) because of un- avoidable warming during measurement. Our laboratory value of 26.5 “C clearly suffers from similar problems because, given the assumption that damselflies are not endothermic, the maximum temperature that could have been achieved at take-off was 22°C. Furthermore, we determined minimum flight temperatures after initiation of flight by prodding rather than by tossing the insects into the air (May, 1976, 1977) and this probably led to further inflation of the true value. Given that adult A. vivida did fly in the laboratory at 22”C, adults should be active in the field at this temperature, even on overcast days.

Comparisons with field observations

Adult Argia vivida were never seen flying at temperatures below 2O”C, even in sunny conditions when they could raise their body temperature above ambient by sun-basking. At higher temperatures on overcast days activity was sporadic. This contrasts markedly with the behaviour of other zygopterans such as Enallagma boreale Selys and Amphiagrion abbreviatum Selys which were often seen flying at the study sites at lower temperatures and light in- tensities. A similar difference was observed between Argia difficilis Selys and Heteragrion erythrogas- trum Selys in Panama by Shelley (1982); the Argia perched in the sun and maintained body tempera-

tures well above ambient, whereas H. erythrogas- trum perched and foraged only in shaded areas and maintained a body temperature within 1 “C of ambient. Clearly, the high flight threshold for A. vivida will limit the amount of time spent in reproductive activity at high latitudes and has possibly limited the species’ northern range extension.

We have never found larvae in the field in water above 35 “C and only rarely above 32 “C. The aver- age EST of 35 “C and the fact that few larvae were observed above 35 “C in the thermal gradient, indicate that there is behavioural avoidance of high temperatures. These observations also correlate with the fact that larvae in the laboratory did not grow past the third instar at 30°C (Leggott, 1984) and that females have not been observed oviposit- ing in water above 30°C at geothermal sites in Al- berta, British Columbia, Idaho and Montana, even though adults were active along such streams and suitable oviposition substrates were present. The high modal temperature preference of 28 “C would serve to keep larvae at the highest sub-lethal temperatures available and thereby maximize growth rates.

Thermal equilibrium theory

An important assumption of the thermal equilibrium model (Sweeney & Vannote, 1978; Van- note & Sweeney, 1980) is that aquatic insects have little or no ability to acclimatize to changed environmental conditions. Although this generaliza- tion has been stated on several occasions (e.g. Pat- tee, 1955; Keister & Buck, 1974; Sweeney, 1978), the bulk of evidence indicates that insects are able to effect physiological and behavioural adjustments to temperature (Wigglesworth, 1972; Precht et al., 1973; Prosser, 1973). This ‘phenotypic plasticity’ is probably inherent in all organisms (Precht et al., 1973) and is a continuous process fitting insects to prevailing conditions (Chapman, 1982). However, our results suggest that no evolutionary adaptation has occurred in these A. vivida populations in the particular traits that we have studied, except perhaps for the behavioural response to low temperatures hypothesized in the section on larval temperature preference.

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Acknowledgements

Financial support from the Natural Sciences & Engineering Research Council of Canada is grate- fully acknowledged. Kelvin Conrad, Michael Ben- ton, and an anonymous reviewer made comments which improved the manuscript.

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