Mean Critical Thermal Maximum Research Articles

Exercise training does not affect heat tolerance in Chinook salmon (Oncorhynchus tshawytscha)

The progression of climate warming will expose ectotherms to transient heatwave events and temperatures above their tolerance range at increased frequencies. It is therefore pivotal that we understand species' physiological limits and the capacity for various controls to plastically alter these thresholds. Exercise training could have beneficial impacts on organismal heat tolerance through improvements in cardio-respiratory capacity, but this remains unexplored. Using juvenile Chinook salmon (Oncorhynchus tshawytscha), we tested the hypothesis that exercise training improves heat tolerance through enhancements in oxygen-carrying capacity. Fish were trained once daily at 60% of their maximum sustainable swim speed, UCRIT, for 60 min. Tolerance to acute warming was assessed following three weeks of exercise training, measured as the critical thermal maximum (CTMAX). CTMAX measurements were coupled with examinations of the oxygen carrying capacity (haematocrit, haemoglobin concentration, relative ventricle size, and relative splenic mass) as critical components of the oxygen transport cascade in fish. Contrary to our hypothesis, we found that exercise training did not raise the CTMAX of juvenile Chinook salmon with a mean CTMAX increase of just 0.35 °C compared to unexercised control fish. Training also failed to improve the oxygen carrying capacity of fish. Exercise training remains a novel strategy against acute warming that requires substantial fine-tuning before it can be applied to the management of commercial and wild fishes.

Intraspecific variation in thermotolerance and morphology of the invasive European green crab, Carcinus maenas, on the west coast of North America

For marine ectotherms, environmental temperature plays an important role in helping to determine biogeographical distribu tions. Understanding the impacts that temperature can have on organismal function can lead to insights into the nature of adaptive variation in thermotolerance. This approach has been widely used to understand the spatial distribution of related species within the context of evolutionary time scales. Here, we measured the upper lethal thermal threshold of the invasive European green crab, Carcinus maenas (Linnaeus, 1758), sampled from two populations from the southern and northern limits of its recipient range on the west coast of North America: Sea Drift Lagoon, Stinson Beach, California (CA), and Pipestem Inlet, Vancouver Island, British Columbia (BC). These two populations differ in their natural thermal regimes; sea surface temperatures near the southern, CA population range from ~ 23 °C to ~ 11 °C while temperatures near the northern population in BC range from ~ 14 °C to ~ 6 °C during summer and winter, respectively. We demonstrate that after laboratory acclimation to 6 °C or 23 °C, the CA group had a significantly higher mean critical thermal maximum (CTmax) than its northern counterpart, in both acclimation groups. After acclimation to 6 °C, the CA group displayed a mean CTmax of 34.7 °C whereas the mean CTmax of the BC group was 31.7 °C. In the 23 °C acclimation groups, mean CTmax was 36.2 °C in the CA population and 35.5 °C in the BC one. To establish whether these differences in whole-organism thermotolerance were manifested at the molecular scale, levels of the inducible stress protein, heat shock protein 70 (Hsp70), were quantified in the chelae of the crabs used in the CTmax assay, at each individual's time-of-death. In the 6 °C acclimation comparison, the CA population had significantly greater Hsp70 levels than the BC population. No difference in Hsp70 levels was observed between the 23 °C acclimation groups. In a separate experiment, collection site-specific differences were found in the thermal threshold for induction of Hsp70 in isolated chelae tissue. The two populations also differed morphologically. Significant differences in carapace widths (CW) were measured for adult male and female green crabs from the two populations, sampled during 2008–2009. Observable divergences in both morphology and in physiological responses to heat stress have occurred within the relatively recent time span (~ 20 years) of the invasion of the European green crab on the west coast of North America and it is possible that a large, northern cold-water phenotype may have already arisen.

High-Temperature Tolerances of Fluvial Arctic Grayling and Comparisons with Summer River Temperatures of the Big Hole River, Montana

Critical thermal maximum (CTM) and resistance time to high temperature were determined for juvenile Arctic grayling Thymallus arcticus from the fluvial population of the Big Hole River, Montana. Grayling were tested after acclimation to 8.4, 16.0, and 20.0°C. Thermal tolerances increased with acclimation temperatures; mean CTM was 26.4°C for the 8.4°C acclimation group, 28.5°C for the 16.0°C group, and 29.3°C for the 20.0°C group; median resistance time at given test temperature also increased with acclimation. The upper incipient lethal temperature (UILT) was 23.0°C for fish acclimated to 8.4°C and 16.0°C, and was 25.0°C for those acclimated to 20.0°C, temperatures that were similar to the median tolerance limits of Arctic grayling in Alaska. Comparisons of mean CTM and UILT for juvenile Arctic grayling with levels and durations of maximum river temperatures recorded during summers 1992–1994 indicated that resident fish may occasionally be subjected to potentially lethal temperatures in the warmest reaches of the Big Hole River.

Critical thermal maxima of coho salmon (Oncorhynchus kisutch) fry under field and laboratory acclimation regimes

Juvenile coho salmon (Oncorhynchus kisutch) from three populations in Washington State were captured in the field and tested for critical thermal maximum (CTM). Tolerances varied among the populations (mean CTMs were 28.21, 29.13, and 29.23 °C) and exceeded published data from some laboratory tests. The population from a relatively cool stream had a lower CTM than the two populations from warmer streams. However, after the salmon had been in the laboratory for 3 months under constant, common temperature regimes, the CTMs no longer differed, indicating that the population-specific differences resulted from different acclimation regimes rather than from genetic adaptation.

Upper Temperature Tolerance of Four Species of Caddisflies (Insecta: Trichoptera)

ABSTRACT Critical Thermal Maximum (CTM) were determined for fifth instar larvae of Hydropsyche simulans Ross and Chimarra obscura (Walker) acclimated to 12, 19 and 26°C, and Ceratopsyche morosa (Hagen) and Chimarra aterrima Hagen acclimated to 19°C. The selected CTM endpoint yielded consistent data as the coefficients of variation for all eight species—temperature acclimation combinations were less than 10%. Mean CTM's ranged from 31.4°C to 38.5°C. At the common acclimation temperature of 19°C, a SNK multiple range test separated the mean CTM's of the four species into two distinct groups: C. obscura = H. simulans > C. aterrima = C. morosa. CTM was directly related to acclimation temperature in both Hydropsyche simulans and Chimarra obscura. The two species representing populations from more northern latitudes (Missouri) had lower CTMs than their congeners or closely related species from more southern latitudes (Texas).

Laboratory Spawning and Rearing of the Endangered Fountain Darter

Abstract Survival of the fountain darter (Etheostoma fonticola), a U.S. federally listed endangered species, may depend on captive propagation. Studies were conducted to determine the effect of temperature on spawning and to develop methods for culture. The fountain darter spawned and produced viable offspring in aquaria at 27, 24, 21, 18, 15, 12, 9, and 6°C. The fish also spawned at 3 and 30°C but did not produce viable eggs. Daily egg production of individual fish held at 27, 21, 15, and 9°C was variable. The mean critical thermal maximum for the fountain darter was 34.8°C. Early life stages, 4–14 mm long, were offered a variety of live protozoans, rotifers, and microcrustaceans. Food selection varied with fish size and food size. Fountain darters reached sexual maturity in about 180 d when maintained at 21°C. Three-year-old darters produced viable offspring, and several lived longer than 4 years. Tricaine methanesulfonate was an effective anesthetic at 60 mg/L but was fatal to subadults at 100 mg/L.

Geographic variation in thermal tolerance of a widespread minnow Notropis lutrensis of the North American mid‐west

Eighteen populations of Notropis lutrensis, representing all of the major river systems occupied by this species across an 1100km north‐south span of its range, showed no significant differences in mean critical thermal maximum (CTM), although general north‐south differences in climatic temperatures exist. There were no clinal trends in CTMs, and CTMs were unrelated to stream size or any other discernible feature of the local habitats. Previous research has shown that N. lutrensis can adapt rapidly to an artificially altered thermal regime. The present results suggest that even though a species may have potentially ‘labile’ thermal physiology, it may nonetheless be ‘static’ or evolutionarily conservative in such characters when natural populations are examined empirically.

Intraspecific differences in thermal tolerance of Etheostoma spectabile (Agassiz) in constant versus fluctuating environments

The critical thermal maximum (CTM) of Etheostoma spectabile differed significantly among four populations whose habitats ranged from thermally constant to highly fluctuating conditions. Mean CTM of the population at the most fluctuating location was highest, populations in intermediate thermal environments were intermediate in thermal tolerance, and a population in a constant temperature spring run had thelowest CTM. The results support the view that thermal physiology can be evolutionarily labile within a species, and that changes respond to directional selection.

Thermal Tolerance of Dragonfly Nymphs. II. Comparison of Nymphs from Control and Thermally Altered Environments

Dragonfly nymphs were collected from ponds and streams thermally altered by reactor cooling water and from control ponds and streams. The critical thermal maxima of seven species of dragonfly nymphs were estimated at acclimation temperatures of 12, 16, 20, 24, 28, and 32 C. Within each environment, species with greater mean body length exhibited a higher thermal tolerance than smaller species. Species differences in critical thermal maximum (CTM) were statistically significant within each environment examined, but these differences were trivial relative to the much greater influence of acclimation temperature. Mean CTM was significantly regressed against acclimation temperature in each species. Nymphs from a thermally elevated pond exhibited a greater increase in CTM per degree rise in acclimation temperature than nymphs from other habitats. Mean thermal tolerances of dragonfly nymphs from the four environments increased in the following order: control stream, control pond, thermal stream, and thermal pond.

Physiological Responses to Temperature in the Horned Lizards, Phrynosoma cornutum and Phrynosoma douglassii

In the laboratory, Phrynosoma cornutum and Phrynosoma douglassii from central New Mexico had mean preferred body temperatures of 38.5? C and 35? C respectively. The mean critical thermal minimum established in the laboratory for P. douglassii was 2.75? C and 9.46? C in P. cornutum. The mean critical thermal maximum in P. douglassii was 43.5? C and 47.9? C in P. cornutum. The range of thermal tolerance in P. douglassii was 2.30 greater than that of P. cornutum. Oxygen consumption and heart rate increased with temperature. There were no significant differences between the heart rates of P. douglassii and P. cornutum at various temperatures. Oxygen consumption in P. douglassii at 5? C and 15? C was almost double that in P. cornutum at those temperatures, but their rates of oxygen consumption were not significantly different at 25, 35, and 40? C. Differences in the oxygen pulse values of these species at high and low temperature extremes indicate differences in the cardiac output and efficiency. It is suggested that P. douglassii may have a more efficient transport of oxygen by the respiratory pigments at low temperatures than P. cornutum.

Temperature and Energy Characteristics of the Sagebrush Lizard (Sceloporus graciosus) in Yellowstone National Park

The mean cloacal temperature for 41 Sceloporus graciosus in the field was 30.9*C. Both the mean air temperature one inch above the ground (23.7*C) and the mean substratum temperature (25.7*C) were significantly lower than the mean cloacal temperature. The mean cloacal temperatures of discrete spring, summer, and fall samples of lizards collected in 1966 measured in a laboratory temperature gradient (N = 58, 70 and 60 respectively), differed significantly from each other, with the spring sample having the lowest mean (28.9*C) and the summer sample the highest (32.3*C). The mean critical thermal maximum of a sample of 14 lizards collected August 1965, was significantly higher than the mean critical thermal maximum of a sample of 10 lizards collected in June 1966 (43.6 vs 42.6*C). While the mean critical thermal maxima of three samples collected in 1966 were not significantly different from each other, they, as well as the mean temperatures obtained from the samples in the thermal gradient, followed the general seasonal trend in temperatures. Oxygen consumption measured at 30*C with a Warburg apparatus was approximately 30% of the value indicated for most species of lizards. Mean fat storage on 7 September 1966, was approximately 23% of the total dry body weight for both juvenile and older lizards. Using estimates from oxygen consumption and fat loss, an adult lizard produces 2000-2800 cal during a hibernation period of 220 days.

Physiological Responses to Temperature and Desiccation in the Endemic New Mexico Plethodontids, Plethodon neomexicanus and Aneides hardii

Measurements of the critical thermal maximum (CTM), oxygen consumption, and heart rate of Plethodon neomexicanus and Aneides hardii were made for comparison with similar data for other members of the family Plethodontidae. At an acclimation temperature of 20? C, the mean CTM for A. hardii was 33.3? C, and for P. neomexicanus, 33.5? C. Mean oxygen consumption increased from 30.9 /jl/g/hr at 50 C to 93.9 /l/g/hr at 25? C in A. hardii and from 35.4 Jl/g/hr at 50 C to 60.9 ul/g/hr at 25? C in P. neomexicanus. The metabolismtemperature (M-T) curve for P. neomexicanus was similar to that of Desmognathus quadramaculatus, an eastern plethodontid, but the M-T curve of A. hardii was more comparable to that of a lunged salamander. Heart rate increased directly with temperature in both species. The mean heart rate increased from 22/min at 10' C to 64/min at 25? C in A. hardii and from 24/min at 10' C to 73/min at 25? C in P. neomexicanus. The low CTM recorded for these species may be related to the low temperatures characteristic of their microhabitat. The difference in M-T curves in these species may reflect differences in surface area-mass relationships resulting in differences in cutaneous gas exchange.