Seasonal acclimation in resting metabolism of the skink, Mabuya brevicollis (Reptilia: Scincidae) from southwestern Saudi Arabia

The resting metabolic rate (RMR) of seasonally-acclimated Mabuya brevicollis of various body masses was determined at 20, 25, 30, 35 and 40 °C, using open-flow respirometry. RMR (ml g⁻¹ h⁻¹) decreased with increasing mass at each temperature. RMRs increaProd. Type: FTPsed as temperature increased. The highest and lowest Q₁₀ values were obtained for the temperature ranges 20–25 °C and 30–35 °C for the summer-acclimated lizards. The exponent of mass “b” in the metabolism-body mass relation ranged from 0.41 to 0.61. b values were lower in the autumn and winter-acclimated lizards than in spring and summer-acclimated lizards. Seasonal acclimation effects were evident at all temperatures (20–40 °C) for M. brevicollis. Winter-acclimated skinks had the lowest metabolic rates at different temperatures. The pattern of acclimation exhibited by M. brevicollis may represent a useful adaptation for lizards inhabiting subtropical deserts to promote activity during their active seasons.     

Lack of metabolic temperature compensation in the intertidal gastropods,Littorina saxatilis (Olivi) and L. obtusata (L.)

Two intertidal snails, Littorina saxatilis (Olivi, 1972) (upper eulittoral fringe/maritime zone) and Littorina obtusata (Linnaeus, 1758) (lower eulittoral) were collected from a boulder shore on Nobska Point, Cape Cod, Massachusetts, in July and acclimated for 15–20 days at 4 ° or 21 °C. Oxygen consumption rate (Vo₂) was determined for 11–15 subsamples of individuals at 4 °, 11 ° and 21 °C with silver/platinum oxygen electrodes. Multiple factor analysis of variance (MFANOVA) of lo₁₀ transformed values of whole animal Vo₂ with log₁₀ dry tissue weight (DTW) as a covariant revealed that increased test temperature induced a significant increase in Vo₂ in both species (P<0.00001). In contrast, MFANOVA revealed that temperature acclimation did not affect Vo₂ in either L. saxatilis (P= 0.35) or L. obtusata (P= 0.095). Thus, neither species displayed a capacity for the typical metabolic temperature compensation marked by an increase in Vo₂ at any one test temperature in individuals acclimated to a lower temperature that is characteristic of most ectothermic animals. Lack of capacity for metabolic temperature acclimation has also been reported in other littorinid snail species, and may be characteristic of the group as a whole. Lack of capacity for respiratory temperature acclimation in these two species and other littorinids may reflect the extensive semi-diurnal temperature variation that they are exposed to in their eulittoral and eulittoral fringe/maritime zone habitats. In these habitats, any metabolic benefits derived from longer-term temperature compensation of metabolic rates are negated by extreme daily temperature fluctuations. Instead, littorinid species appear to have evolved mechanisms for immediate metabolic regulation which, in L. saxatilis and L. obtusata and other littorinids, appear to centre on a unique ability for near instantaneous suppression of metabolic rate and entrance into short-term metabolic diapause at temperatures above 20–35 °C, making typical seasonal respiratory compensation mechanisms characteristic of most ectotherms of little adaptive value to littorinid species.     

Respiratory responses to temperature and hypoxia in the nonindigenous Brown Mussel, Perna perna (Bivalvia: Mytilidae), from the Gulf of Mexico

The respiratory responses to increasing temperature and progressive hypoxia were examined relative to temperature acclimation in the nonindigenous, brown mussel, Perna perna (Mytilidae) from the Gulf of Mexico. When oxygen uptake rate (V?_O2) was recorded at near full air O₂ saturation, rate-temperature curves for Texas specimens of P. perna were sigmoidal, V?_O2 generally increasing with increasing temperature but becoming suppressed as temperatures approached 10 and 30 °C, corresponding closely to this species' incipient thermal limits. At each tested temperature, V?_O2 did not differ among individuals acclimated to 15, 20, or 25 °C. Lack of thermal acclimation was also reflected in acclimatory Q₁₀ values>1.0 (range=1.34-2.14) recorded across acclimation groups at test temperatures equivalent to acclimation temperature. Low acute respiratory Q₁₀ values in all acclimation groups across 15-20 °C indicated a limited capacity for thermal regulation of V?_O2 within this temperature range. The ability of P. perna to regulate O₂ uptake with progressive hypoxia was temperature-dependent, increasing from poor O₂ regulation at 10 °C to good regulation at 30 °C. The O₂ regulatory ability of P. perna and other open-water mytilids in declining O₂ concentrations does not greatly differ from that of estuarine heterodont bivalves, suggesting that it is not a major factor preventing open-water species, such as P. perna, from invading estuarine environments. However, P. perna's inability to regulate O₂ uptake at temperatures>25 °C combined with its relatively low upper thermal limit of 30 °C will likely prevent it from establishing permanent estuarine populations on Gulf of Mexico shores.     

The use of a temperature-profiled position transducer for the study of low-temperature growth in Gramineae

A device is described for measuring linear extension of grass leaves during controlled cooling and heating of the growing region. The instrument was employed to investigate the sensitivity to temperature of the expanding third and fourth leaves of Lolium temulentum L. seedlings. Using a stepped temperature profile it was established that there was no lag in the response of growth rate to rapid changes in temperature below 16°C. If cooling was continued to the point where growth ceased (1°C) but no further, then rates of growth on rewarming were enhanced over the chilling range and reverted to the original rate at 20°C. Cooling to successively lower subzero temperatures before rewarming abolished the hysteretic enhancement, progressively raised the temperature at which growth resumed and decreased the rate of extension until, at-5.3°C, no recovery occurred. The temperature sensitivity of growth, measured as Q₁₀, was essentially constant when cooling from 20°C to 5°C, with 5°C-grown leaf tissue exhibiting a higher mean Q₁₀ than tissue developed at 20°C. The possible physiological significance of these data is discussed.Abbreviations LVDT linear variable displacement transformer - Pe, Fx temperatures at which growth ceases during cooling and resumes during rewarming     

Enhancement of frost tolerance in olive shoots in vitro by cold acclimation and sucrose increase in the culture medium

The evaluation of frost tolerance in olive shoots in vitro has been successfully accomplished. The behavior of in vitro shoots at freezing temperatures was comparable to that of intact plants. Cold acclimation was found to increase frost tolerance in cv. Moraiolo and the LT₅₀ was about 4 °C lower compared to nonacclimated shoots. Damage in acclimated shoots occurred at –15 °C, whereas control shoots were damaged at –10 °C. Olive shoots were unable to withstand freezing temperatures of –20 °C, even when acclimated. The effects of sucrose were also determined. 6% (w/v) sucrose in the medium conferred the highest frost tolerance in both acclimated and nonacclimated plants.     

Effect of constant temperatures on germination, radial growth and virulence of Metarhizium anisopliae to three species of African tephritid fruit flies

The effect of temperatureon conidial germination, mycelial growth, andsusceptibility of adults of three tephritidfruit flies, Ceratitis capitata(Wiedemann), C. fasciventris (Bezzi) andC. cosyra (Walker) to six isolatesof Metarhizium anisopliae were studied inthe laboratory. There were significantdifferences among the isolates in the effect oftemperature on both germination and growth.Over 80% of conidia germinated at 20, 25 and30°C, while between 26 and 67% conidiagerminated at 35°C and less than 10% at15°C within 24 hours. Radial growth was slowat 15°C and 35°C with all of theisolates. The optimum temperature forgermination and mycelial growth was 25°C. Mortality caused by the six fungal isolatesagainst the three fruit fly species varied withtemperature, isolate, and fruit fly species.Fungal isolates were more effective at 25, 30and 35°C than at 20°C. The LT₉₀values decreased with increasing temperature upto the optimum temperature of 30°C. Therewere significant differences in susceptibilitybetween fly species to fungal infection at allthe temperatures tested.     

Effect of acute pH depression on the survival of the freshwater amphipod Hyalella azteca at variable temperatures: field and laboratory studies

Field observations on temperature and pH of a small pond showed that a amphipod population of Hyalella azteca was exposed to variable seasonal pH between 5.10–5.85, and water temperatures between 2–21 °C. Laboratory experiments were designed to simulate seasonal temperatures and field pHs of a small pond habitat. Laboratory bioassay experiments were conducted to determine the survival of Hyalella azteca at pHs 4, 5, 6 and 7, and varying temperatures of 5°, 10°, 15°, 20° and 25 °C.The LT₁₀₀ at pH 4 and 25 °C was 5.7 ± 0.47 days, compared to 47.3 ± 2.49 days at 5 °C. An Analysis of Variance (ANOVA) showed temperature was a significant (p > 0.0001) source of variation in the acute lethality of pH to H. azteca. A Duncans Multiple Range Test (DMRT) further showed that in laboratory experiments at pH 4, there was a significant difference ( = 0.01) between the LT_100s at 5°, 10°, 15° and 20 °C, but not between temperatures 20° and 25 °C.     

Influence of temperature and ambient oxygen on the swimming energetics of cyprinid larvae and juveniles

Synopsis The relationship between respiration and swimming speed of larvae and juveniles (2–100 mg fresh mass) of Danube bleak, Chalcalburnus chalcoides (Cyprinidae), was measured at 15° and 20° C under hypoxic (50% air saturation), normoxic, and hyperoxic (140% air saturation) conditions. In a flow-tunnel equipped with a flow-through respirometer the animals swam at speeds of up to 8 lengths · s^-1; speeds were sustained for at least two minutes. The mass specific standard, routine, and active respiration rates declined with increasing body mass at both temperatures. Metabolic intensity increased with temperature, but also the critical swimming speed (at which oxygen uptake reached its maximum) was higher at 20° than at 15° C by about 30%. Nevertheless, the oxygen debt incurred by the fish at the highest speeds was about 40%, and the net cost of swimming about 32%, lower at 20° than at 15°C. The standard metabolic rate was more strongly dependent on temperature (Q₁₀ around 2.5) than the maximum active rate (Q₁₀ below 2). Whereas standard and routine respiration rates were well regulated over the pO₂-range investigated (8.5–25.8 kPa), the active rates showed a conformer-like pattern, resulting in factorial scopes for activity between 2 and 4. Under hypoxia, the critical swimming speed was lower than under normoxia by about 1.51 · s^-1, but the net cost of swimming was also lower by about 30%. On the other hand, hyperoxia neither increased the swimming performance nor did it lead to a further increase of the metabolic cost of swimming. The hypoxia experiments suggest that in response to lowered tensions of ambient oxygen maintenance functions of metabolism not directly related to swimming may be temporarily reduced, leading to increased apparent swimming efficiency under these conditions. The responses of the larvae of Danube bleak to low temperature and low ambient oxygen are discussed in terms of the metabolic strategies by which energy-limited animals meet the challenge of environmental deterioration.     

Reduction of metabolic rate and thermoregulation during daily torpor

Physiological mechanisms causing reduction of metabolic rate during torpor in heterothermic endotherms are controversial. The original view that metabolic rate is reduced below the basal metabolic rate because the lowered body temperature reduces tissue metabolism has been challenged by a recent hypothesis which claims that metabolic rate during torpor is actively downregulated and is a function of the differential between body temperature and ambient temperature, rather than body temperature per se. In the present study, both the steady-state metabolic rate and body temperature of torpid stripe-faced dunnarts, Sminthopsis macroura (Dasyuridae: Marsupialia), showed two clearly different phases in response to change of air temperature. At air temperatures between 14 and 30°C, metabolic rate and body temperature decreased with air temperature, and metabolic rate showed an exponential relationship with body temperature (r ²=0.74). The Q ₁₀ for metabolic rate was between 2 and 3 over the body temperature range of 16 to 32°C. The difference between body temperature and air temperature over this temperature range did not change significantly, and the metabolic rate was not related to the difference between body temperature and air temperature (P=0.35). However, the apparent conductance decreased with air temperature. At air temperatures below 14°C, metabolic rate increased linearly with the decrease of air temperature (r ²=0.58) and body temperature was maintained above 16°C, largely independent of air temperature. Over this air temperature range, metabolic rate was positively correlated with the difference between body temperature and air temperature (r ²=0.61). Nevertheless, the Q ₁₀ for metabolic rate between normothermic and torpid thermoregulating animals at the same air temperature was also in the range of 2–3. These results suggest that over the air temperature range in which body temperature of S. macroura was not metabolically defended, metabolic rate during daily torpor was largely a function of body temperature. At air temperatures below 14°C, at which the torpid animals showed an increase of metabolic rate to regulate body temperature, the negative relationship between metabolic rate and air temperature was a function of the differential between body temperature and air temperature as during normothermia. However, even in thermoregulating animals, the reduction of metabolic rate from normothermia to torpor at a given air temperature can also be explained by temperature effects.Abbreviations BM body mass - BMR basal metabolic rate - C apparent conductance - MR metabolic rate - RMR resting metabolic rate - RQ respiratory quotient - T _a air temperature - T _b body temperature - T _lc lower critical temperature - T _tc critical air temperature during torpor - TMR metabolic rate during torpor - TNZ thermoneutral zone - T difference between body temperature and air temperature - VO₂ rate of oxygen consumption     

Effects of acclimation and acute temperature change on specific dynamic action and gastric processing in the green shore crab, Carcinus maenas

The effects of temperature acclimation and acute temperature change were investigated in postprandial green shore crabs, Carcinus maenas. Oxygen uptake, gut contractions and transit rates and digestive efficiencies were measured for crabs acclimated to either 10 °C or 20 °C and subsequently exposed to treatment temperatures of 5, 15, or 25 °C. Temperature acclimation resulted in a partial metabolic compensation in unfed crabs, with higher oxygen uptake rates measured for the 10 °C acclimated group exposed to acute test temperatures. The Q₁₀ values were higher than normal, probably because the acute temperature change prevented crabs from fully adjusting to the new temperature. Both the acclimation and treatment temperature altered the characteristics of the specific dynamic action (SDA). The duration of the response was longer for 20 °C acclimated crabs and was inversely related to the treatment temperature. The scope (peak oxygen consumption) was also higher for 20 °C acclimated crabs with a trend towards an inverse relationship with treatment temperature. Since the overall SDA (energy expenditure) is a function of both duration and scope, it was also higher for 20 °C acclimated crabs, with the highest value measured at the treatment temperature of 15 °C. The decline in total SDA after acute exposure to 5 and 25 °C suggests that both cold stress and limitations to oxygen supply at the temperature extremes could be affecting the SDA response. The contractions of the pyloric sac of the foregut region function to propel digesta through the gut, and contraction rates increased with increasing treatment temperature. This translated into faster transit rates with increasing treatment temperatures. Although pyloric sac contractions were higher for 20 °C acclimated crabs, temperature acclimation had no effect on transit rates. This suggests that a threshold level in pyloric sac contraction rates needs to be reached before it manifests itself on transit rates. Although there was a correlation between faster transit times and the shorter duration of the SDA response with increasing treatment temperature, transit rates do not make a good proxy for calculating the SDA characteristics. The digestive efficiency showed a trend towards a decreasing efficiency with increasing treatment temperature; the slower transit rates at the lower treatment temperatures allowing for more efficient nutrient absorption. Even though metabolic rates of 10 °C acclimated crabs were higher, there was no effect of acclimation temperature on digestive efficiency. This probably occurred because intracellular enzymes and digestive enzymes are modulated through different control pathways. These results give an insight into the metabolic and digestive physiology of Carcinus maenas as it makes feeding excursions between the subtidal and intertidal zones.     

Ventilation and respiratory metabolism in the thermophilic desert ant,Cataglyphis bicolor (Hymenoptera,Formicidae)

The discontinuous ventilation cycle of the Saharan desert ant Cataglyphis bicolor was studied over the range 15–40°C, corresponding to a >2-fold increase in the rate of CO₂ output and hence metabolic rate (Q ₁₀=2.1). Over this range, metabolic rate modulated only ventilation frequency; the volume of CO₂ emitted per ventilation remained constant. The closed-spiracle phase accounted for a small, constant proportion (ca. 14%) of total CO₂ output. In the flutter phase, the rate of CO₂ output increased at a greater than exponential rate from 29% of total CO₂ output at 15°C to 52% at 40°C. CO₂ output rate in the ventilation phase increased, and its duration decreased, exponentially with temperature. Relative to total duration of discontinuous ventilation cycle, the length of each phase was constant over the entire range of metabolic rates measured. These data are the first thorough characterization of the effect of changing metabolic rate on all phases of the discontinuous ventilation cycle of an adult insect. Clearly, C. bicolor maximizes ventilation-phase emission volumes and enhances the contribution of the flutter phase to total CO₂ release relative to other ants for which comparable data are available, and does so in ways that may reduce respiratory water loss rates.Abbreviations BM body mass - C-phase closed-spiracle phase - cVCO₂ rate of carbon dioxyde leakage during the C phase - DVC discontinuous ventilation cycle - F-phase fluttering-spiracle phase - MR metabolic rate - Q ₁₀ factorial increase in MR with 10°C increase in temperature - RQ respiratory quotient - SMR standard metabolic rate - T body temperature (°C) - VCO₂ rate of carbon dioxide output - V-phase ventilation phase - vVCO₂ rate of CO₂ emission during the V-phase - fVCO₂ rate of CO₂ emission during the F-phase - VO₂ rate of oxygen consumption     

Effects of temperature on acaricidal and insecticidal activities of the benzoylureas flucycloxuron and diflubenzuron

Effects of temperature on the activity of flucycloxuron on larval stages of Panonychus ulmi (Koch), based on LC₅₀ values, were highly significant (P < 0.001) with temperature coefficients of-1.7 in both the ranges of 15° to 25°C and 20° to 30°C. The slopes of probit regression lines at 15° and 20°C were significantly steeper than those at 25° and 30°C. As a consequence the temperature coefficients based on LC₉₀ values were-4.4 and-2.2, for the 2 temperature ranges. The ovicidal activity of flucycloxuron on P. ulmi was low and was only statistically detectable at 20°C (LC₉₀ of 84 mg a.i./l). In studies with larvae of Aedes aegypti (Linnaeus), Leptinotarsa decemlineata (Say), Plutella xylostella (Linnaeus), Spodeptera exigua (Hübner) and Spodoptera littoralis (Boisduval) probit regression lines were parallel over temperature. The activity of flucycloxuron on these five insect species was not affected by temperature. Based on LC₅₀ values, diflubenzuron showed positive temperature coefficients on P. xylostella of + 2.1 at 15° to 25°C and + 2.5 at 20° to 30°C. For S. littoralis the temperature coefficient was positive (+ 2.4) at 15° to 25°C but negative (-1.9) at the 20° to 30°C range. Temperature coefficients of diflubenzuron were neutral for A. aegypti, L. decemlineata and S. exigua. In the design and analysis of these studies special allowance was made for date effects and variation in natural mortality over temperature.     

Effects of light and temperature on sporangiophore initiation in Pilobolus crystallinus (Wiggers) Tode

Sporangiophore initiation in Pilobolus crystallinus grown in white light was induced by either a dark or a low-temperature treatment. The period of darkness necessary to induce sporangiophore initiation was shortened by lowering the temperature. Arrhenius plots for the sporangiophore-suppressing reaction in both light and darkness consisted of two straight lines with a Q₁₀ of about 2 at lower temperatures and 8–11 at higher temperatures. The temperature at which the Q₁₀ changed was the lower, the higher the fluence rate: 14° C at 8 W/m², 19.5° C at 0.24 W/m² and 24.5° C in darkness. Possible interpretations of these results are briefly presented.Abbreviations %SP percentage of trophocysts initiating sporangiophores - D50% duration of treatment required to 50% sporangiophore initiation     

Effect of temperature on the oxygen consumption in the larvae of dineutes indicus aube (gyrinidae,coleoptera)

Oxygen uptake of the larvae of Dineutes indicus Aube has been measured at different temperatures. It has been observed that the oxygen uptake increases with the rise in the temperature but falls sharply after 34°C.The data obtained corresponds to a straight line when plotted on an Arrhenius graph.Q₁₀ values decrease with the increase in weight and it has been observed that the smallest individual has the highest Q₁₀ value. Nevertheless, the calculated energies of activation reported here apply only to the limited temperature range studied.     

Metabolic responses of sand fiddler crabs, Uca pugilator, in northwest Florida to seasonal temperature change

Metabolic responses of sand fiddler crab, Uca pugilator, populations in northwest Florida are greatly influenced by seasonal temperature fluctuations. Crabs acclimated at 20 °C and immediately transferred to either 14 or 26 °C produced an acute metabolic response with respective temperature quotient (Q₁₀) values of 3.46 and 3.91. Crabs acclimated at 10 and 20 °C exhibited a Q₁₀ of 2.62 indicating a partial compensation response. A brumation (reverse) response (Q₁₀ value of 20.11) was observed for acclimated crabs between 5 and 10 °C. Brumation is advantageous during winter when food supplies are scarce and crabs must survive extensive periods of inactivity.     

Photosynthetic responses of cassava cultivars (Manihot esculenta Crantz) from different habitats to temperature

Maximum photosynthetic CO₂ exchange rates (P_n) of single attached leaves were determined for several cassava cultivars selected from different habitats and grown in pots outdoors at CIAT, Colombia, S.A. P_n rates were in a narrow range of 22 to 26 mol CO₂ m^–2s^–1 for all cultivars tested when measured at high photon flux density, normal air, optimum temperature and with low leaf-air vapor pressure differences. For all tested cultivars (9 cvs.), there was a broad optimum temperature for P_n between 25 to 35°C. At temperatures below and above this range P_n declined in all cultivars with P_n rates reaching 80% of maximum at 20 and 40°C. P_n temperature coefficient (Q₁₀) from 15–25°C was 1.6±0.2 across cultivars. No consistent relation existed between P_n, optimum temperature, and the original habitat.     

Influence of root temperature on growth of Pinus sylvestris,Fagus sylvatica,Tilia cordata and Quercus robur

One-year-old tree seedlings were incubated in a greenhouse from April to July, under natural daylight conditions, with their root systems at constant temperatures of 5, 10, 15, 20, 25, 30 and 35 °C and with the above ground parts kept at a constant air temperature of 18–20 °C. The course of height growth, total mass increment, root, shoot and leaf weight as well as leaf areas were measured. The results indicate that clear differences exist in the optimal root zone temperatures for various growth parameters in different tree species. Pinus sylvestris had a maximal height increment at about 5–10 °C and maximal total mass increment at 15 °C root temperature. In contrast, the optimum for Quercus robur was at 25 °C. Tilia cordata and Fagus sylvatica had their optima for most growth parameters at 20 °C. The root temperature apparently indirectly influenced photosynthesis (dry weight accumulation) and respiration loss. From the observed symptoms and indications in the literature it seems probable that a change in hormone levels is involved as the main factor in the described effects. Variation of root temperature had only an insignificant effect on bud burst and the time at which the shoots sprouted. Apparently species of northern origin seem to have lower root temperature optima than those of more southern origin. This is to be verified by investigation of other tree species.     

Interactive effects of temperature and photoperiod on the daily activity and energy metabolism of pouched mice (Saccostomus campestris: Cricetidae) from southern Africa

The daily activity and energy metabolism of pouched mice (Saccostomus campestris) from two localities in southern Africa was examined following warm (25 °C) and cold (10 °C) acclimation under long (LD 14:10) and short (LD 10:14) photoperiol. There was no differential effect of photoperiod on the daily activity or metabolism of pouched mice from the two localities examined, which suggests that reported differences in photoresponsivity between these two populations were not the result of differences in daily organisation. Neverthe-less, there was a significant increase in metabolism at 10 °C, irrespective of photoperiod, even though seven cold-acclimated animals displayed bouts of spontaneous torpor and saved 16.4–36.2% of their daily energy expenditure. All but one of these bouts occurred under short photoperiod, which suggests that short photoperiod facilitated the expression of torpor and influenced the daily energy metabolism of these individuals. As expected for a noctureal species, the amount of time spent active increased following acclimation to short photoperiod at 25 °C. However, there was a reduction in mean activity levels under short photoperiod at 10 °C, possibly because the stimulation of activity by short photoperiod was masked by a reduction in activity during bouts of spontaneous torpor. Cold temperature clearly had an overriding effect on the daily activity and metabolism of this species by necessitating an increase in metabolic heat production and eliciting spontaneous torpor which overrode the effect of short photoperiod on activity at an ambient temperature of 10 °C.Abbreviations 3-ANOVA three-way analysis of variance - %ACT percentage of time spent active - ADMR average daily metabolic rate - M _b body mass - MR metabolic rate - MR_dark metabolic rate recorded during the dark phase - MR_light metabolic rate recorded during the light phase - NST non-shivering thermogenesis - RQ respiratory quotient - STPD standard temperature and pressure, dry - T _a ambient temperature - T _b body temperature - VO₂ oxygen consumption     

Heat tolerance, behavioural temperature selection and temperature-dependent respiration in larval Octopus huttoni

This study reports temperature effects on paralarvae from a benthic octopus species, Octopus huttoni, found throughout New Zealand and temperate Australia. We quantified the thermal tolerance, thermal preference and temperature-dependent respiration rates in 1-5 days old paralarvae. Thermal stress (1 °C increase h⁻¹) and thermal selection (∼10-24 °C vertical gradient) experiments were conducted with paralarvae reared for 4 days at 16 °C. In addition, measurement of oxygen consumption at 10, 15, 20 and 25 °C was made for paralarvae aged 1, 4 and 5 days using microrespirometry. Onset of spasms, rigour (CT_max) and mortality (upper lethal limit) occurred for 50% of experimental animals at, respectively, 26.0±0.2 °C, 27.8±0.2 °C and 31.4±0.1 °C. The upper, 23.1±0.2 °C, and lower, 15.0±1.7 °C, temperatures actively avoided by paralarvae correspond with the temperature range over which normal behaviours were observed in the thermal stress experiments. Over the temperature range of 10 °C-25 °C, respiration rates, standardized for an individual larva, increased with age, from 54.0 to 165.2 nmol larvae⁻¹ h⁻¹ in one-day old larvae to 40.1-99.4 nmol h⁻¹ at five days. Older larvae showed a lesser response to increased temperature: the effect of increasing temperature from 20 to 25 °C (Q₁₀) on 5 days old larvae (Q₁₀=1.35) was lower when compared with the 1 day old larvae (Q₁₀=1.68). The lower Q₁₀ in older larvae may reflect age-related changes in metabolic processes or a greater scope of older larvae to respond to thermal stress such as by reducing activity. Collectively, our data indicate that temperatures >25 °C may be a critical temperature. Further studies on the population-level variation in thermal tolerance in this species are warranted to predict how continued increases in ocean temperature will limit O. huttoni at early larval stages across the range of this species.     

Multigenerational analysis of temperature and salinity variability affects on metabolic rate,generation time,and acute thermal and salinity tolerance in Daphnia pulex

Climate change, sea level rise, and human freshwater demands are predicted to result in elevated temperature and salinity variability in upper estuarine ecosystems. Increasing levels of environmental stresses are known to induce the cellular stress response (CSR). Energy for the CSR may be provided by an elevated overall metabolic rate. However, if metabolic rate is constant or lower under elevated stress, energy for the CSR is taken from other physiological processes, such as growth or reproduction. This study investigated the examined energetic responses to the combination of temperature and salinity variability during a multigenerational exposure of partheogenetically reproducing Daphnia pulex. We raised D. pulex in an orthogonal combination of daily fluctuations in temperature (15, 15–25, 15–30 °C) and salinity (0, 0–2, 0–5). Initially metabolic rates were lower under all variable temperature and variable salinity treatments. By the 6th generation there was little metabolic variation among low and intermediate temperature and salinity treatments, but metabolic suppression persisted at the most extreme salinity. When grown in the control condition for the 6th generation, metabolic suppression was only observed in D. pulex from the most extreme condition (15–30 °C, 0–5 salinity). Generation time was influenced by acclimation temperature but not salinity and was quickest in specimens reared at 15–25 °C, likely due to Q₁₀ effects at temperatures closer to the optima for D. pulex, and slowest in specimens reared at 15–30 °C, which may have reflected elevated CSR. Acute tolerance to temperature (LT₅₀) and salinity (LC₅₀) were both highest in D. pulex acclimated to 15–30 °C and salinity 0. LT₅₀ and LC₅₀ increased with increasing salinity in specimens raised at 15 °C and 15–25 °C, but decreased with increasing salinity in specimens raised at 15–30 °C. Thus, increasing temperature confers cross-tolerance to salinity stress, but the directionality of synergistic effects of temperature and salinity depend on the degree of environmental variability. Overall, the results of our study suggest that temperature is a stronger determinant of metabolism, growth, and tolerance thresholds, and assessment of the ecological impacts of environmental change requires explicit information regarding the degree of environmental variability.