Effect of temperature and thermal acclimation on locomotor performance of Macrobiotus harmsworthi Murray (Tardigrada, Macrobiotidae)

We investigated the effects of acute and acclimation temperature on the locomotor performance and behavior of the tardigrade Macrobiotus harmsworthi collected from Qinling Mountains in central China. Tardigrades were acclimated to either 10 or 25 °C for 2 weeks. Then we recorded their walking speed, percentage of time moving, and the maximum distance covered by continuous locomotion at either 10 or 25 °C as the rate parameters of locomotor performance. The walking speeds of M. harmsworthi varied from 1.98 to 4.8 mm min^–1. The locomotor performance rates were significantly influenced by both acclimation temperature and performance temperature and by the interaction of the performance temperature and acclimation temperature. The data from our studies support the Beneficial Acclimation Hypothesis (BAH) which predicts that animals acclimated to a particular temperature have enhanced performance or fitness at that temperature in comparison with animals acclimated to other temperatures. The data, at least potentially, also support the Warmer is Better Hypothesis which predicts that organisms raised at high temperatures have higher relative fitness across all temperatures than do those raised at intermediate or cool temperatures. Some of the results from our studies testify the inference from the BAH that performance temperature that deviates from the acclimation temperature could cause the reduction of the locomotor performance rate.     

温度和热驯化对胡氏大生熊虫运动行为的影响

对外温脊椎动物体温和运动能力关系的研究表明,外温动物的运动行为对身体温度的变化高度敏感(Bennett,1990)。在许多动物种类中,体温的升高或降低显著影响动物的运动能力(Huey,1982;Miller,1982;Watkins,2000),而运动能力的下降则影响外温动物逃避捕食者(Christian andTracy,19     

EVOLUTIONARY ADAPTATION TO TEMPERATURE. VI. PHENOTYPIC ACCLIMATION AND ITS EVOLUTION IN ESCHERICHIA COLI

Acclimation refers to reversible, nongenetic changes in phenotype that are induced by specific environmental conditions. Acclimation is generally assumed to improve function in the environment that induces it (the beneficial acclimation hypothesis). In this study, we experimentally tested this assumption by measuring relative fitness of the bacterium Escherichia coli acclimated to different thermal environments. The beneficial acclimation hypothesis predicts that bacteria acclimated to the temperature of competition should have greater fitness than do bacteria acclimated to any other temperature. The benefit predicted by the hypothesis was found in only seven of 12 comparisons; in the other comparisons, either no statistically demonstrable benefit was observed or a detrimental effect of acclimation was demonstrated. For example, in a lineage evolutionarily adapted to 37°C, bacteria acclimated to 37°C have a higher fitness at 32°C than do bacteria acclimated to 32°C, a result exactly contrary to prediction; acclimation to 27°C or 40°C prior to competition at those temperatures confers no benefit over 37°C acclimated forms. Consequently, the beneficial acclimation hypothesis must be rejected as a general prediction of the inevitable result of phenotypic adjustments associated with new environments. However, the hypothesis is supported in many instances when the acclimation and competition temperatures coincide with the historical temperature at which the bacterial populations have evolved. For example, when the evolutionary temperature of the population was 37°C, bacteria acclimated to 37°C had superior fitness at 37°C to those acclimated to 32°C; similarly, bacteria evolutionarily adapted to 32°C had a higher fitness during competition at 32°C than they did when acclimated to 37°C. The more surprising results are that when the bacteria are acclimated to their historical evolutionary temperature, they are frequently competitively superior even at other temperatures. For example, bacteria that have evolved at either 20°C or 32°C and are acclimated to their respective evolutionary temperatures have a greater fitness at 37°C than when they are acclimated to 37°C. Thus, acclimation to evolutionary temperature may, as a correlated consequence, enhance performance not only in the evolutionary environment, but also in a variety of other thermal environments.     

Temperature acclimation alters cardiac performance in the lobster Homarus americanus

The American lobster is a poikilotherm that inhabits a marine environment where temperature varies over a 25°C range and depends on the winds, the tides and the seasons. To determine how cardiac performance depends on the water temperature to which the lobsters are acclimated we measured lobster heart rates in vivo. The upper limit for cardiac function in lobsters acclimated to 20°C is approximately 29°C, 5°C warmer than that measured in lobsters acclimated to 4°C. Warm acclimation also slows the lobster heart rate within the temperature range from 4 to 12°C. Both effects are apparent after relatively short periods of warm acclimation (3–14 days). However, warm acclimation impairs cardiac function at cold temperatures: following several hours exposure to frigid (<5°C) temperatures heart rates become slow and arrhythmic in warm acclimated, but not cold acclimated, lobsters. Thus, acclimation temperature determines the thermal limits for cardiac function at both extremes of the 25°C temperature range lobsters inhabit in the wild. These observations suggest that regulation of cardiac thermal tolerance by the prevailing environmental temperature protects against the possibility of cardiac failure due to thermal stress.     

温度对中华倒刺鲃不同生理生态性能的影响:驯化有益假说的验证

驯化有益假说(Beneficial acclimation hypothesis)认为生物表型的适应性变化会增强其在诱导这些变化产生的环境中的生理机能或适合度。然而,由于动物不同生理生态性能对环境驯化的响应可能不一致,那么,测试表型性状的选择对驯化有益假说的验证就尤为关键。为此,整合表征动物生存适合度的不同生理生态性能并探究其对环境驯化的响应模式就十分必要。以我国长江中上游广泛分布的中华倒刺鲃(Spinibarbus sinensis)为对象,考察了驯化温度(18℃、28℃)和测试温度(18℃、28℃)及其交互作用对该物种有氧运动能力和无氧运动能力的影响,为驯化有益假说等相关假说的验证提供参考。研究发现,中华倒刺鲃不同生理生态性能对温度驯化的响应存在差异:(1)驯化温度对表征中华倒刺鲃无氧运动能力的快速启动游泳无显著影响(除最大加速度外)(P>0.05),研究数据倾向于支持无益假说(No-advantage hypothesis);(2)驯化温度对表征中华倒刺鲃有氧运动能力的临界游泳速度(Critical swimming speed,Ucrit)和最大代谢率(Maximum metabolic rate, MMR)影响显著(P<0.05),18℃驯化-18℃测试下的Ucrit和MMR均优于28℃驯化-18℃测试下的Ucrit和MMR,结果部分支持驯化有益假说和冷有益假说(Cooler is better hypothesis);(3)驯化温度、测试温度、游泳速度对中华倒刺鲃的运动代谢率(Active metabolic rate,MO2)和单位距离能量消耗(The energetic cost of transport, COT)影响显著(P<0.05)。值得关注的是,当游泳速度小于30 cm/s时,驯化温度对MO2和COT无影响,结果支持无益假说;而当游泳速度大于30 cm/s时,在特定的流速下经过28℃驯化的中华倒刺鲃无论在28℃还是18℃的测试环境下MO2和COT均较低,结果倾向于支持热有益假说(Warmer is better hypothesis)。研究结果提示:驯化有益假说并不具有普遍性,热驯化相关假说的验证不仅受表型性状选择的影响,而且还与测试的环境选择压力有关。     

Thermal acclimation of locomotor performance in tadpoles and adults of the aquatic frog Xenopus laevis

Among amphibians, the ability to compensate for the effects of temperature on the locomotor system by thermal acclimation has only been reported in larvae of a single species of anuran. All other analyses have examined predominantly terrestrial adult life stages of amphibians and found no evidence of thermal acclimatory capacity. We examined the ability of both tadpoles and adults of the fully aquatic amphibian Xenopus laevis to acclimate their locomotor system to different temperatures. Tadpoles were acclimated to either 12 °C or 30 °C for 4 weeks and their burst swimming performance was assessed at four temperatures between 5 °C and 30 °C. Adult X. laevis were acclimated to either 10 °C or 25 °C for 6 weeks and their burst swimming performance and isolated muscle performance was determined at six temperatures between 5 °C and 30 °C. Maximum swimming performance of cold-acclimated X. laevis tadpoles was greater at cool temperatures and lower at the highest temperature in comparison with the warm-acclimated animals. At the test temperature of 12 °C, maximum swimming velocity of tadpoles acclimated to 12 °C was 38% higher than the 30 °C-acclimation group, while at 30 °C, maximum swimming velocity of the 30 °C-acclimation group was 41% faster than the 12 °C-acclimation group. Maximum swimming performance of adult X. laevis acclimated to 10 °C was also higher at the lower temperatures than the 25 °C acclimated animals, but there was no difference between the treatment groups at higher temperatures. When tested at 10 °C, maximum swimming velocity of the 10 °C-acclimation group was 67% faster than the 25 °C group. Isolated gastrocnemius muscle fibres from adult X. laevis acclimated to 10 °C produced higher relative tetanic tensions and decreased relaxation times at 10 °C in comparison with animals acclimated to 25 °C. This is only the second species of amphibian, and the first adult life stage, reported to have the capacity to thermally acclimate locomotor performance. Accepted: 28 October 1999     

General introduction to the lists of threatened biotopes,flora and fauna of the trilateral Wadden Sea Area (red data book)

The effect of the acclimation temperature on the temperature tolerance ofPorphyra leucosticta, and on the temperature requirements for growth and survival ofEnteromorpha linza was determined under laboratory conditions. Thalli ofP. leucosticta (blade or Conchocelis phases), acclimated to twenty-five degrees, survived up to 30°C, i.e. 2°C more than those acclimated to 15°C which survived up to 28°C. Lower temperature tolerance of bothPorphyra phases that were acclimated to 15°C was −1°C after an 8-week exposure time at the experimental temperatures. The upper temperature tolerance ofE. linza also increased by 2°C, i.e. from 31 to 33°C, when it was acclimated to 30°C instead of 15°C. The lower temperature tolerance increased from 1 to −1°C, when it was acclimated to 5°C instead of 15°C.E. linza thalli acclimated for 4 weeks to 5 or 10°C reached their maximum growth at 15°C, i.e. at a 5°C lower temperature than those acclimated to 15 or 30°C. These thalli achieved higher growth rates in percent of maximal growth at low temperatures than those acclimated to 15 or 30°C. Thalli acclimated for 1 week to 5°C reached their maximum growth rate at 20°C and achieved growth rates at low temperatures similar to those recorded for thalli acclimated to 15°C. Thalli ofE. linza acclimated for 4 weeks to 5°C lost this acclimation after being post-cultivated for the same period at 15°C. That was not the case with thalli acclimated for 8 weeks to 5°C and post-acclimated for 4 weeks to 15°C. These thalli displayed similar growth patterns at 10–25°C, while a decline of growth rate was observed at 5 or 30°C. The significance of the acclimation potential ofE. linza with regard to its seasonality in the Gulf of Thessaloniki, and its distribution in the N Atlantic, is also discussed.     

Supercooling Ability, Water Content and Hardiness of Rhododendron Flower Buds during Cold Acclimation and Deacclimation

The relationship between supercooling ability and water contentand killing temperature of flower buds during cold acclimationand deacclimation were studied using R. kiusianum and R. x akebono.The occurrence of multiple floret exotherms and their shiftto a narrow range at lower subzero temperatures, as well asthe marked decrease of florets water content, were observedas the symptoms of cold acclimation occuring in flower budsfrom fall to winter, and vice versa in spring buds during deacclimation.In R. kiusianum, the fully acclimated period was from Novemberto March and two months longer than that of R. x akebono. Thesupercooling ability of the former was about –25°Cand about –20°C in the latter. Although the watermigration within bud tissues during the freezing process wasdetermined in the acclimated and deacclimated buds for R. xakebono, no significant water changes could be observed, evenin the acclimated buds. Thus, it is conceivable that deep supercoolingin florets may result not necessarily from water migration fromflorets and bud axes to scales in response to freezing, butfrom low water content in situ of cold-acclimated or artificiallydehydrated flower buds. (Received July 29, 1981; Accepted October 12, 1981)     

Temperature requirements ofScytosiphon lomentaria (Scytosiphonales,Phaeophyta) from the Gulf of Thessaloniki,Greece, in relation to geographic distribution

Temperature requirements for growth, reproduction and formation of macrothalli of a day-neutral strain ofScytosiphon lomentaria from the Gulf of Thessaloniki were experimentally determined and correlated with the geographic distribution in the North Atlantic Ocean. The microthallus grew in a wider temperature interval and better at higher temperatures than did the macrothallus. Germlings acclimated to 5 or 15°C grew sufficiently (>20% of maximum rate) and developed into macrothalli at 5–25°C and 5–27°C. Macrothalli acclimated to 10 or 15°C grew sufficiently at 5–20°C. Macrothalli acclimated to 15°C survived at −1°C and reproduced at 5 to 23°C. Regardless of the acclimation temperature, germlings and macrothalli grew optimally (>80% of maximum rate) at 15–25°C and at 10–15°C. The experimental data explain only the southern distribution boundary ofScytosiphon in the North Atlantic. This boundary is composite in nature: on the European coasts it is a growth boundary, whereas on the American coasts it is a lethal one.     

Thermal acclimation of locomotor performance in tadpoles of the frog Limnodynastes peronii

Previous analyses of thermal acclimation of locomotor performance in amphibians have only examined the adult life history stage and indicate that the locomotor system is unable to undergo acclimatory changes to temperature. In this study, we examined the ability of tadpoles of the striped marsh frog (Limnodynastes peronii) to acclimate their locomotor system by exposing them to either 10 °C or 24 °C for 6 weeks and testing their burst swimming performance at 10, 24, and 34 °C. At the test temperature of 10 °C, maximum velocity (U_max) of the 10 °C-acclimated tadpoles was 47% greater and maximum acceleration (A_max) 53% greater than the 24 °C-acclimated animals. At 24 °C, U_max was 16% greater in the 10 °C-acclimation group, while there was no significant difference in A_max or the time taken to reach U_max (T-U_max). At 34 °C, there was no difference between the acclimation groups in either U_max or A_max, however T-U_max was 36% faster in the 24 °C-acclimation group. This is the first study to report an amphibian (larva or adult) possessing the capacity to compensate for cool temperatures by thermal acclimation of locomotor performance. To determine whether acclimation period affected the magnitude of the acclimatory response, we also acclimated tadpoles of L. peronii to 10 °C for 8 months and compared their swimming performance with tadpoles acclimated to 10 °C for 6 weeks. At the test temperatures of 24 °C and 34 °C, U_max and A_max were significantly slower in the tadpoles acclimated to 10 °C for 8 months. At 10 °C, T-U_max was 40% faster in the 8-month group, while there were no differences in either U_max or A_max. Although locomotor performance was enhanced at 10 °C by a longer acclimation period, this was at the expense of performance at higher temperatures. Accepted: 25 June 1999     

Critical thermal minima and maxima of three freshwater game-fish species acclimated to constant temperatures

A total of 120 critical thermal maxima (CT maxima) and 120 critical thermal minima (CT minima) were determined for channel catfish, largemouth bass and rainbow trout acclimated to three constant temperatures: 20, 25 and 30 °C in catfish and bass, and 10, 15 and 20 °C in trout. Highest mean CT maximum and lowest mean CT minimum measured over these acclimation temperatures were 40.3 and 2.7 °C (catfish), 38.5 and 3.2 °C (bass) and 29.8 and ∼ 0.0 °C (trout). Temperature tolerance data were precise with standard deviations generally less than 0.5 °C. Channel catfish had the largest thermal tolerance scope of the three species while rainbow trout had the lowest tolerance of high temperatures and the highest tolerance of low temperatures. In all species CT minima and CT maxima were highly significantly linearly related to acclimation temperature. Within each species, slopes relating CT maxima to acclimation temperature were approximately half as large as those relating CT minima to acclimation temperature, suggesting that acclimation temperature has a greater influence on tolerance to low rather than high temperatures. Slopes relating both CT minima and CT maxima to acclimation temperature for the two warm-water species were similar and approximately twice those for the rainbow trout. This revised version was published online in July 2006 with corrections to the Cover Date.     

Sub-lethal temperature thresholds indicate acclimation and physiological limits in brook trout Salvelinus fontinalis

The upper thermal tolerance of brook trout Salvelinus fontinalis was estimated using critical thermal maxima (CT_max) experiments on fish acclimated to temperatures that span the species' thermal range (5–25°C). The CT_max increased with acclimation temperature but plateaued in fish acclimated to 20, 23 and 25°C. Plasma lactate was highest, and the hepato-somatic index (I_H) was lowest at 23 and 25°C, which suggests additional metabolic costs at those acclimation temperatures. The results suggest that there is a sub-lethal threshold between 20 and 23°C, beyond which the fish experience reduced physiological performance.     

Northern grass lizards (<Emphasis Type="Italic">Takydromus septentrionalis</Emphasis>) from different populations do not differ in thermal preference and thermal tolerance when acclimated under identical thermal conditions

We acclimated adults of Takydromus septentrionalis (northern grass lizard) from four localities (populations) under identical thermal conditions to examine whether local thermal conditions have a fixed influence on thermal preference and thermal tolerance in the species. Selected body temperature (Tsel), critical thermal minimum (CTMin), and critical thermal maximum (CTMax) did not differ between sexes and among localities in lizards kept under identical laboratory conditions for ∼5 months, and the interaction effects between sex and locality on these measures were not significant. Lizards acclimated to the three constant temperatures (20, 25, and 35°C) differed in Tsel, CTMin, and CTMax. Tsel, CTMin, and CTMax all shifted upward as acclimation temperature increased, with Tsel shifting from 32.0 to 34.1°C, CTMin from 4.9 to 8.0°C, and CTMax from 42.0 to 44.5°C at the change-over of acclimation temperature from 20 to 35°C. Lizards acclimated to the three constant temperatures also differed in the range of viable body temperatures; the range was widest in the 25°C treatment (38.1°C) and narrowest in the 35°C treatment (36.5°C), with the 20°C treatment in between (37.2°C). The results of this study show that local thermal conditions do not have a fixed influence on thermal preference and thermal tolerance in T. septentrionalis.     

Dormancy in Rice Seed: IV. VARIETAL RESPONSES TO STORAGE AND GERMINATION TEMPERATURES

Serial germination tests were carried out on dormant seeds ofsix rice varieties (four varieties of Oryza sativa L. and twovarieties of O. glaberrima Steud.) stored at several differentconstant temperatures within the range 27° C to 57°C. Probit analyses of the results were carried out to determmethe mean dormancy period for each variety at each temperature.Regression lines fitted to these data showed that there is adirect negative relationship between storage temperature andlog mean dormancy period over the range 27° C to 47°C, thus confirming a previous result obtained on a single variety.At 7° C there were indications of a slight departure fromthis relationship in that the mean dormancy periods at thistemperature were slightly longer than would have been predictedby extrapolation of the regressions calculated from the resultsobtained at lower temperatures. In all cases where the resultswere unambiguous (i.e. in all the sativa varieties and one ofthe glaberrima varieties) a constant Q₁₀ of 3.13 was shown forthe rate of loss of dormancy over the range of storage temperaturesfrom 27° C to 47° C. In the remaining glaberrima variety,where the results were less reliable, a Q₁₀ of 2.54 was found. Germination tests on all varieties were carried out at 32°C, but in the case of one sativa variety germination tests forall storage treatments were also duplicated at 27° C. Thisinvestigation showed that, in contrast to the effect of storagetemperature, the higher temperature during the germination testconsistently resulted in a lower percentage germination. Inaddition the results demonstrated that there is no interactionbetween storage temperature and germination temperature: consequentlythe storage-temperature coefficient has the same value irrespectiveof germination temperature. Some theoretical implications ofthe results are discussed.     

Control of the Acclimation of Photosynthesis to Light and Temperature in Relation to Partitioning of Photosynthate in Developing Soybean Leaves

Photosynthetic acclimation was examined by exposing third trifoliolateleaves of soybeans to air temperatures of 20 to 30°C andphotosynthetic photon flux densities (PPFD) of 150 to 950µmolphotons m^–2 s^–1 for the last 3 d before they reachedmaximum area. In some cases the environment of the third leafwas controlled separately from that of the rest of the plant.Photosynthesis, respiration and dry mass accumulation were determinedunder the treatment conditions, and photosynthetic capacity,and dry mass and protein content were determined at full expansion.Photosynthetic capacity, the light-saturated rate of net carbondioxide exchange at 25°C and 34 Pa external partial pressureof carbon dioxide, could be modified between 21 and 35 µmolCO₂ m^–2 s^–1 by environmental changes after leaveshad become exporters of photosynthate. Protein per unit leafmass did not differ between treatments, and photosynthetic capacityincreased with leaf mass per unit area. Photosynthetic capacityof third leaves was affected by the PPFD incident on those leaves,but not by the PPFD on other leaves on the plant. Photosyntheticcapacity of third leaves was affected by the temperature ofthe rest of the plant, but not by the temperature of the thirdleaves. Photosynthetic capacity was linearly related to carbondioxide exchange rate in the growth regimes, but not to daytimePPFD. At high PPFD, and at 25 and 30°C, mass accumulationwas about 28% of the mass of photosynthate produced. At lowerPPFD, and at 20°C, larger percentages of the photosynthateproduced accumulated as dry mass. The results suggest that photosynthatesupply is an important factor controlling leaf structural growthand, consequently, photosynthetic acclimation to light and temperature. Key words: Glycine max (L.) Merr., photosynthesis, temperature acclimation, light acclimation, photosynthate partitioning     

Energy reserves during food deprivation and compensatory growth in juvenile roach: the importance of season and temperature

The effect of 21 days of starvation, followed by a period of compensatory growth during refeeding, was studied in juvenile roach Rutilus rutilus during winter and summer, at 4, 20 and 27° C acclimation temperature and at a constant photoperiod (12L : 12D). Although light conditions were the same during summer and winter experiments and fish were acclimated to the same temperatures, there were significant differences in a range of variables between summer and winter. Generally winter fish were better prepared to face starvation than summer fish, especially when acclimated at a realistic cold season water temperature of 4° C. In winter, the cold acclimated fish had a two to three‐fold larger relative liver size with an approximately double fractional lipid content, in comparison to summer animals at the same temperature. Their white muscle protein and glycogen concentration, but not their lipid content, were significantly higher. Season, independent of photoperiod or reproductive cycle, was therefore an important factor that determined the physiological status of the animal, and should generally be taken into account when fish are acclimated to different temperature regimes. There were no significant differences between seasons with respect to growth. Juvenile roach showed compensatory growth at all three acclimation temperatures with maximal rates of compensatory growth at 27° C. The replenishment of body energy stores, which were utilized during the starvation period, was responsible for the observed mass gain at 4° C. The contribution of the different energy resources (protein, glycogen and lipid) was dependent on acclimation temperature. In 20 and 27° C acclimated roach, the energetic needs during food deprivation were met by metabolizing white muscle energy stores. While the concentration of white muscle glycogen had decreased after the fasting period, the concentrations of white muscle lipid and protein remained more or less constant. The mobilization of protein and fat was revealed by the reduced size of the muscle after fasting, which was reflected in a decrease in condition factor. At 20° C, liver lipids and glycogen were mobilized, which caused a decrease both in the relative liver size and in the concentration of these substrates. Liver size was also decreased after fasting in the 4° C acclimated fish, but the substrate concentrations remained stable. This experimental group additionally utilized white muscle glycogen during food deprivation. Almost all measured variables were back at the control level within 7 days of refeeding.     

Long-term temperature acclimation in Daphnia magna: effects on filtering rates

Populations of a laboratoiy clone of Daphnia magna were acclimatedat 5, 10, 15 and 24C, and a varying temperature regin for aminimum of 70 days. The effect of temperature (5–25C)on filtering rates was measured for animals acclimated to theconstant temperatures and for a wild population collected froma reservoir. Acclimation temperature strongly influenced ffltcrmgrates at test temperatures. Animals with acclimation temperaturesclosest to the test temperature tended to have highest filteringrates at that temperature.     

Temperature and photoperiodic control of developmental responses in climatic races of Mimulus

Two strongly differentiated climatic races of the Mimulus cardinalis-lewisiicomplex were grown at a variety of temperatures (3–27°C)and photoperiods (8 and 16 hr) under controlled environmentalconditions. M. cardinalis (the lowland race, 400 m) and M. lewisii(the sub-alpine race, 3200 m) were found to differ in theirphysiological responses to the varied environments in severalsignificant ways: 1) At 27°C (16 hrphotoperiod), M. lewisiisustained 100% mortality in contrast to the substantial growthand flowering of M. cardinalis under these conditions; 2) In8 hr photoperiods at all temperatures, there was little growthand no flowering in M. lewisii whereas there was considerablegrowth at all temperatures, and flowering at 23 and 27°Cin M. cardinalis; 3) At low temperatures (7–15°C),16 hr photoperiods, flowering occurred a week or two earlierin M. lewisii than in M. cardinalis. The lowland race has asignificantly wider temperature and photoperiodic tolerancethan has the sub-alpine race. Applications of gibberellic acidto rosette Mimulus plants under non-inductive conditions (15°C,8 hr photoperiod) promoted vigorous stem elongation withoutflowering. The application of steroids, other hormones and metaboliteshad no observable effects. ¹Present address: Faculty of Botany, Ohio State University,Columbus, Ohio, U.S.A. (Received March 16, 1970; )     

Thermal tolerance and acclimation response of larvae of the sub-Antarctic beetle Hydromedion sparsutum (Coleoptera: Perimylopidae)

Hydromedion sparsutum is a locally abundant herbivorous beetle on the sub-Antarctic island of South Georgia, often living in close association with the tussock grass Parodiochloa flabellata. Over a 4-day period in mid-summer when the air temperature varied from 0 to 20°C, the temperature in the leaf litter 5–10 cm deep at the base of tussock plants (the microhabitat of H. sparsutum) was consistently within the range of 5–7.5°C. Experiments were carried out to assess the ability of H. sparsutum larvae collected from this thermally stable environment to acclimate when maintained at lower (0°C) and higher (15°C) temperatures. The mean supercooling points (freezing temperature) of larvae collected in January and acclimated at 0°C for 3 and 6 weeks and 15°C for 3 weeks were all within the range of −2.6 to −4.6°C. Larvae in all treatment groups were freeze tolerant. Acclimation at 0°C significantly increased survival in a 15-min exposure at −8°C (from 27 to 96%) and −10°C (from 0 to 63%) compared with the field-fresh and 15°C-treated larvae. Similarly, survival of 0°C-acclimated larvae in a 72-h exposure at −6°C increased from 20 to 83%. Extending the acclimation period at 0°C to 6 weeks did not produce any further increase in cold tolerance. The concentrations of glucose and trehalose in larval body fluids increased significantly with low temperature acclimation. Larvae maintained at 15°C for 3 weeks (none survived for 6 weeks) were less able to survive 1-h exposures between 30 and 35°C than the 0°C-treated samples. Whilst vegetation and snow cover are an effective buffer against low winter temperatures in many polar insects, the inability of H. sparsutum larvae to acclimate or survive at 15°C suggests that protection against high summer temperatures is equally important for this species. Accepted: 2 August 1999