Lower Critical Thermal Maximum Research Articles

Flies on the rise: acclimation effect on mitochondrial oxidation capacity at normal and high temperatures in Drosophila melanogaster.

Increased average temperatures and extreme thermal events (such as heatwaves) brought forth by climate change impose important constraints on aerobic metabolism. Notably, mitochondrial metabolism, which is affected by both long- and short-term temperature changes, has been put forward as an important determinant for thermal tolerance of organisms. This study examined the influence of phenotypic plasticity on metabolic and physiological parameters in Drosophila melanogaster and the link between mitochondrial function and their upper thermal limits. We showed that D. melanogaster acclimated to 15°C have a 0.65°C lower critical thermal maximum (CTmax) compared with those acclimated to 24°C. Drosophila melanogaster acclimated to 15°C exhibited a higher proportion of shorter saturated and monounsaturated fatty acids, concomitant with lower proportions of polyunsaturated fatty acids. No mitochondrial quantitative changes (fractional area and number) were detected between acclimation groups, but changes of mitochondrial oxidation capacities were observed. Specifically, in both 15°C- and 24°C-acclimated flies, complex I-induced respiration was increased when measured between 15 and 24°C, but drastically declined when measured at 40°C. When succinate and glycerol-3-phosphate were added, this decrease was however compensated for in flies acclimated to 24°C, suggesting an important impact of acclimation on mitochondrial function related to thermal tolerance. Our study reveals that the use of oxidative substrates at high temperatures is influenced by acclimation temperature and strongly related to upper thermal tolerance as a difference of 0.65°C in CTmax translates into significant mitochondrial changes.

Critical thermal maxima in aquatic ectotherms

Critical Thermal Maxima (CTMax) is one of the main used techniques to infer thermal tolerance of a certain organism. Aquatic animals, which are mainly ectotherms, are believed to present growing CTMax from poles towards the equator while their thermal tolerance, in relation to their environmental temperature would present the opposite trend. In this work such pattern is analysed in a global scale by performing metanalysis with previous work data. Also, differences between organisms living in fresh and saltwater ecosystems and between different taxa were assessed. Finally, the effect of other stressors over the thermal tolerance is compiled and discussed. It was possible to prove that, as it has been proposed, CTMax values have an inverse relation with thermal tolerance being the tropical organisms closer to their limit than in other climates. Also, temperate organisms present, in general, the highest thermal tolerance. Marine animals from the temperate region presented lower CTMax and Thermal Safety Margin (TSM) than freshwater organisms from the same region. Such pattern was not verified on the tropical region. The two mollusc classes: Gastropoda and Bivalvia, presented the highest CTMax and TSM values.Several stress factors other than temperature analysed in previous works presented significant effect. An analysis to the possible nefarious effects of such stressors over thermal tolerance was performed.

A weight‐of‐evidence approach for defining thermal sensitivity in a federally endangered species

Abstract Managing for threatened and endangered species under changing environmental conditions is a challenge faced by resource managers worldwide. Lack of basic knowledge of the biology and habitat requirements of these species can contribute to this difficulty but is exacerbated by the limitations of working with rare species (i.e. few individuals) or unrefined and non‐lethal methods for evaluating stress. A weight‐of‐evidence approach was used to evaluate the thermal biology of the federally endangered dwarf wedgemussel (Alasmidonta heterodon), using cumulative results from multiple experimental assessments, co‐occurring species, and their host fish to begin defining the thermal limits and optimal conditions for the species. Results suggest that dwarf wedgemussel and its host fish are thermally sensitive species compared with other Atlantic‐slope mussels, with a lower critical thermal maximum (CTM) and the selection of reduced temperatures during choice experiments. Physiological studies resulted in a lack of statistical significance, primarily owing to low power, which was a function of sample size – an unavoidable problem when studying rare species. Given these limitations, thermal choice and CTM may be more useful end points than physiological processes such as clearance and respiration rates when dealing with sample‐size limitations. These results suggest that management strategies that avoid exposing dwarf wedgemussel and its thermally sensitive host fish to extreme temperatures could be important for species conservation.

The effect of salinity and photoperiod on thermal tolerance of Atlantic and coho salmon reared from smolt to adult in recirculating aquaculture systems

Land-based, closed containment salmon aquaculture involves rearing salmon from smolt to adult in recirculating aquaculture systems (RAS). Unlike in open-net pen aquaculture, rearing conditions can be specified in RAS in order to optimize growth and physiological stress tolerance. The environmental conditions that yield optimal stress tolerance in salmon are, however, unknown. To address this knowledge gap, we reared Atlantic (Salmo salar) and coho (Oncorhynchus kisutch) salmon in 7 separate RASs for 400 days post-smoltification under 2 photoperiods (24:0 or 12:12, light:dark) and 4 salinities (2.5, 5, 10 or 30 ppt.) and assessed the effects of these conditions on thermal tolerance. We found that over the first 120 days post-smoltification, rearing coho under a 24:0 photoperiod resulted in a ~2 °C lower critical thermal maxima (CTmax) than in coho reared under a 12:12 photoperiod. This photoperiod effect did not persist at 200 and 400 days, which was coincident with an overall decrease in CTmax in coho. Finally, Atlantic salmon had a higher CTmax (~28 °C) compared to coho (~26 °C) at 400 days post-smoltification. Overall, these findings are important for the future implications of RAS and for the aquaculture industry to help identify physiologically sensitive time stages.

Assessing thermal tolerance of vulnerable alpine stream insects as part of a long-term monitoring project in the Teton Range, Wyoming

Alpine streams are predicted to decline as air temperatures warm and their water sources dry. Stream temperatures are expected to increase as glaciers and permanent snowfields decrease in size. For aquatic insects that are cold-adapted and restricted to small, high elevation streams fed by glaciers or snowfields, warmer water temperatures could be lethal. Conversely, less water in streams may increase the likelihood of insects freezing during winter months. We measured the critical thermal maximum (CTMAX) – the highest non-lethal temperature an insect can survive, and supercooling temperature – the temperature at which an insect freezes, of three alpine stoneflies, Zapada sp., Lednia tetonica and Lednia tumana, collected in Grant Teton and Glacier National Parks. CTMAX and supercooling point varied among species and with stream source (glacier-fed, snowmelt-fed and icy seep) and population (seven populations). Supercooling temperature was lowest in an alpine tarn and highest in glacier- and snowmelt-fed streams. Zapada sp. had the lowest CTMAX of the three species. Stoneflies from icy seeps had lower CTMAX than individuals from glacier- or snowmelt-fed streams. Individuals that likely experience the coldest winter temperatures had the lowest supercooling temperature. Similarly, stoneflies that experienced warmer water temperatures also had higher CTMAX values. Investigating the thermal tolerances of alpine stoneflies allows us to predict how these insects may respond to future climate change scenarios.
 
 Featured photo by Nicole Y-C on Unsplash. https://unsplash.com/photos/9XixVlnUCbk

Hotter nests produce hatchling lizards with lower thermal tolerance.

In many regions, the frequency and duration of summer heatwaves is predicted to increase in future. Hotter summers could result in higher temperatures inside lizard nests, potentially exposing embryos to thermally stressful conditions during development. Potentially, developmentally plastic shifts in thermal tolerance could allow lizards to adapt to climate warming. To determine how higher nest temperatures affect the thermal tolerance of hatchling geckos, we incubated eggs of the rock-dwelling velvet gecko, Amalosia lesueurii, at two fluctuating temperature regimes to mimic current nest temperatures (mean 23.2°C, range 10-33°C, 'cold') and future nest temperatures (mean 27.0°C, range 14-37°C, 'hot'). Hatchlings from the hot incubation group hatched 27 days earlier and had a lower critical thermal maximum (CTmax 38.7°C) and a higher critical thermal minimum (CTmin 6.2°C) than hatchlings from cold incubation group (40.2 and 5.7°C, respectively). In the field, hatchlings typically settle under rocks near communal nests. During the hatching period, rock temperatures ranged from 13 to 59°C, and regularly exceeded the CTmax of both hot- and cold-incubated hatchlings. Because rock temperatures were so high, the heat tolerance of lizards had little effect on their ability to exploit rocks as retreat sites. Instead, the timing of hatching dictated whether lizards could exploit rocks as retreat sites; that is, cold-incubated lizards that hatched later encountered less thermally stressful environments than earlier hatching hot-incubated lizards. In conclusion, we found no evidence that CTmax can shift upwards in response to higher incubation temperatures, suggesting that hotter summers may increase the vulnerability of lizards to climate warming.

Hyperoxia Does Not Extend Critical Thermal Maxima (CTmax) in White- or Red-Blooded Antarctic Notothenioid Fishes.

Understanding what limits the capacity of organisms to tolerate increasing temperatures is a critical objective in comparative biology. Using an experimental system of Antarctic notothenioid fishes, we sought to determine whether a mismatch between oxygen demand and oxygen supply was responsible for setting thermal tolerance limits. Previous studies have shown that Antarctic icefishes (family Channichthyidae), which lack hemoglobin, have lower critical thermal maxima (CTmax) than red-blooded notothenioids collected from the same region of the Antarctic (Western Antarctic Peninsula). In addition, within the notothenioid fishes there exists a positive correlation between CTmax and hematocrit. We tested the hypothesis that the lower CTmax of icefishes is associated with reduced oxygen supply. We employed an experimental heat ramp (4°C h(-1)) to determine CTmax under both normoxic and hyperoxic conditions and quantified correlates of oxygen limitation (lactate levels and expression of hypoxia-inducible factor-1α) in white-blooded Chaenocephalus aceratus and red-blooded Notothenia coriiceps. Hyperoxia, corresponding to a three- to fourfold increase in seawater Po2, did not extend CTmax in either species despite an overall mitigation in the rise of plasma and muscle lactate compared with the normoxic treatment. Our results also indicate that cardiac HIF-1α mRNA levels were insensitive to changes in both temperature and oxygen treatments. The absence of a change in CTmax with hyperoxia is likely to represent the contribution of factors beyond oxygen supply to physiological failure at elevated temperatures.

Effect of warming rate on the critical thermal maxima of crabs, shrimp and fish

The threat of global warming has prompted numerous recent studies on the thermal tolerance of marine species. A widely used method to determine the upper thermal limit has been the Critical Thermal Maximum (CTMax), a dynamic method, meaning that temperature is increased gradually until a critical point is reached. This method presents several advantages over static methods, however, there is one main issue that hinders interpretation and comparison of CTMax results: the rate at which the temperature is increased. This rate varies widely among published protocols. The aim of the present work was to determine the effect of warming rate on CTMax values, using different animal groups. The influence of the thermal niche occupied by each species (intertidal vs subtidal) and habitat (intertidal vs subtidal) was also investigated. CTMax were estimated at three different rates: 1°Cmin−1, 1°C30min−1 and 1°Ch−1, in two species of crab, Eurypanopeus abbreviatus and Menippe nodifrons, shrimp Palaemon northropi and Hippolyte obliquimanus and fish Bathygobius soporator and Parablennius marmoreus. While there were significant differences in the effect of warming rates for some species, for other species warming rate produced no significant differences (H. obliquimanus and B. soporator). While in some species slower warming rates lead to lower CTMax values (P. northropi and P. marmoreus) in other species the opposite occurred (E. abbreviatus and M. nodifrons). Biological group has a significant effect with crabs' CTMax increasing at slower warming rates, which did not happen for shrimp and fish. Subtidal species presented lower CTMax, at all warming rates tested. This study highlights the importance of estimating CTMax values at realistic rates that species encounter in their environment and thus have an ecological value.

The effects of dissolved oxygen and triploidy on critical thermal maximum in brook charr (Salvelinus fontinalis)

This study examined the effect of dissolved oxygen (DO) level on critical thermal maximum (CTMax) in diploid and triploid brook charr (Salvelinus fontinalis) exposed to a temperature increase of 3°C/h. Because gas solubility is inversely proportional to temperature, DO declines during standard CTMax tests. With this treatment as a baseline, oxygen or nitrogen injection was used to provide three other DO conditions during CTMax tests: two hyperoxic (maintenance at initial 10mg/L and increase from 10mg/L at 2mg/L/h) and one hypoxic (decrease from 10mg/L at 2mg/L/h). Hyperoxia had no effect on temperature at CTMax or time taken to reach CTMax. Hypoxia, on the other hand, resulted in a significantly lower CTMax and shorter time to CTMax than under standard or hyperoxic conditions, with both indices affected by triploidy but not in a consistent fashion: in one experiment triploids had a lower CTMax and shorter time to CTMax than diploids and in a second experiment they had a higher CTMax and longer time to CTMax than diploids. Indices of the secondary stress response (plasma glucose and ions) during CTMax tests under hypoxia responded as would be predicted for an acute stress, with no difference between triploids and diploids.

Foraging by forest ants under experimental climatic warming: a test at two sites

Climatic warming is altering the behavior of individuals and the composition of communities. However, recent studies have shown that the impact of warming on ectotherms varies geographically: species at warmer sites where environmental temperatures are closer to their upper critical thermal limits are more likely to be negatively impacted by warming than are species inhabiting relatively cooler sites. We used a large-scale experimental temperature manipulation to warm intact forest ant assemblages in the field and examine the impacts of chronic warming on foraging at a southern (North Carolina) and northern (Massachusetts) site in eastern North America. We examined the influence of temperature on the abundance and recruitment of foragers as well as the number of different species observed foraging. Finally, we examined the relationship between the mean temperature at which a species was found foraging and the critical thermal maximum temperature of that species, relating functional traits to behavior. We found that forager abundance and richness were related to the experimental increase in temperature at the southern site, but not the northern site. Additionally, individual species responded differently to temperature: some species foraged more under warmer conditions, whereas others foraged less. Importantly, these species-specific responses were related to functional traits of species (at least at the Duke Forest site). Species with higher critical thermal maxima had greater forager densities at higher temperatures than did species with lower critical thermal maxima. Our results indicate that while climatic warming may alter patterns of foraging activity in predictable ways, these shifts vary among species and between sites. More southerly sites and species with lower critical thermal maxima are likely to be at greater risk to ongoing climatic warming.

Thermal tolerance and potential impacts of climate change on coastal and estuarine organisms

The study of thermal tolerance is the first step to understanding species vulnerability to climate warming. This work aimed to determine the upper thermal limits of various fish and crustaceans in a temperate estuarine ecosystem and an adjacent coastal area. Species were ranked in terms of thermal tolerance and intraspecific variability was evaluated. The method used was the Critical Thermal Maximum (CTMax). The CTMax was found to be higher for species typically found in thermally unstable environments, e.g. intertidal, supratidal, southern distributed species and species that make reproduction migrations because they are exposed to extreme temperatures. Subtidal, demersal and northern distributed species showed lower CTMax values because they live in colder environments. Species from different taxa living in similar habitats have similar CTMax values which suggests that they have evolved similar stress response mechanisms. This study showed that the most vulnerable organisms to sea warming were those that occur in thermally unstable environments because despite their high CTMax values, they live closer to their thermal limits and have limited acclimation plasticity. Among the demersal species studied, two sea-breams (Diplodus bellottii and Diplodus vulgaris) are potentially threatened by sea warming because their CTMax values are not far from the mean water temperature and they are already under thermal stress during current heat waves.

The Critical Thermal Maximum of Juvenile Red Drum Reared for Out‐of‐Season Stocking in Texas

AbstractHatchery‐propagated red drum Sciaenops ocellatus are released by the Texas Parks and Wildlife Department (TPWD) during the spring, summer, and fall to supplement natural stocks. Reported success of red drum stocked out of season in summer has been low or nonexistent; several potential causes, including mortality due to high temperatures, have been suggested. The present study was devised to determine the critical thermal maximum (CTM) of pond‐raised juvenile red drum that had previously been exposed to diurnal temperatures of the summer season. Laboratory studies designed to mimic the rate of temperature increase (0.25°C per hour) typical of TPWD rearing ponds were used to examine rates of mortality in juvenile red drum that were collected from three separate ponds exposed to the ambient fluctuating temperatures of the summer season. The CTM (temperature that was lethal to 50% of the test fish [LT50]) was significantly lower (P < 0.0001) for two of the fish trials (LT50 = 37.1°C and 37.0°C) than for a third trial (LT50 = 38.7°C). This difference correlated with the size of fish in each trial; the two trials with smaller‐sized fish (range = 18–32 mm total length) had the lower CTMs, whereas the trial with larger‐sized fish (range = 31–50 mm total length) had the higher CTM, suggesting positive size dependence in the CTM. The CTMs obtained here are higher than the maximum temperatures encountered in grow‐out ponds during summer, indicating that high temperature exposure may not be the sole cause for the low success of red drum stocked out of season.

Thermal Tolerance and Growth Differences in Orangethroat Darter (Etheostoma spectabile) from Thermally Contrasting Adjoining Streams

Orangethroat darters Etheostoma spectabile occupying thermally contrasting streams within the same watershed were tested for differences in thermal responses by comparing their critical thermal maxima (CTM) and short-term growth. Individuals from a spring branch population had a lower CTM, better growth at low temperatures and more inhibited growth at higher temperatures compared to individuals collected from a thermally variable adjoining stream reach. The findings support the contention that intraspecific differences in thermal tolerance and growth may exist at relatively localized scales within a small watershed.

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.

Thermal tolerance in the tree lizard (Urosaurus ornatus) from a desert population and a low montane population

We studied the thermal tolerance of two populations of the tree lizard Urosaurus ornatus from a desert environment in southwestern New Mexico and a low montane environment in southeastern Arizona. In general, there were no differences in thermal tolerance between the two populations. Desert individuals tended to have higher panting thresholds in the summer than low montane individuals, whereas the reverse held for late summer and fall. Males had lower critical thermal maxima than females in the low montane population, but there was no difference in the desert population. Thermal tolerance varied from month to month, tolerances being highest during the summer. Thermal tolerance was related to body size (measured as both snout–vent length and body mass), explaining up to 39% of the variance.

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).

The effects of dissolved oxygen and aquatic surface respiration on the critical thermal maxima of three intermittent-stream fishes

Critical thermal maxima (CTM) were determined for three species of fish in hypoxic (1.2 mg l-1), normoxic (7 mg l-1) and hyperoxic (12 mg l-1) oxygen tensions. All fish were acclimated to 30°C under a regulated photoperiod of 12:12. During each CTM determination, one-half of the test fish were allowed free access to the surface (to permit aquatic surface respiration) while the remaining one-half were denied access to the surface. Under hypoxic conditions, CTMs of Fundulus notatus, Notropis lutrensis and Pimephales vigilax given access to the surface were significantly higher than those of conspecifics without access to the surface. With access to the surface, all species had significantly lower CTMs under hypoxic conditions than under normoxic and hyperoxic conditions. CTMs measured under normoxic and hyperoxic conditions were not different for any of the test species. Under all comparable oxygen tensions with surface access, F. notatus had the highest CTM, N. lutrensis intermediate and P. vigilax had the lowest CTM. The critical oxygen concentration for N. lutrensis during CTM determinations occurred between 1.2 and 2 mg l-1.

Thermal Relations of Some Tropical Frogs Along an Altitudinal Gradient

Temperate-zone amphibians generally show an acclimation response of tolerance to high temperatures (CTMax), and several wideranging species have been found to have a negative correlation between elevation and CTMax. Fewer tropical species have been examined, but Heatwole et al. (1965) reported that the Puerto Rican frog Eleutherodactylusportoricensis failed to show an acclimation response, but did show a negative correlation between elevation and CTMax. Subsequently, E. portoricensis, as originally understood, was found to consist of a widespread species (Eleutherodactylus coqui) and a species restricted to high elevations (E. portoricensis) (Thomas 1965). CTMax was determined for these two species, and we found that differences previously believed to be attributable to elevation are, in fact, interspecific differences. There were no differences in CTMax of E. coqui from different elevations, but E. portoricensis had significantly lower CTMax. E. coqui failed to show an acclimation response. The few species of tropical amphibians that have been investigated suggest that, unlike most temperate-zone species, some tropical species fail to show an acclimation response, and some fail to show a negative correlation between CTMax and elevation. ANIMALS CAN RESPOND to environmental change by behavioral adjustments (fast response), physiological adjustments such as acclimatization (slower response), or over a longer time, by evolutionary adjustments such as structural and enzymatic changes (Hertz 1981). The relative degree to which animals respond to environmental change at any of these three levels can be qualitatively described by the term plasticity. In this paper we will discuss responses to thermal environments in terms of both physiological plasticity, the ability to acclimate, and evolutionary plasticity, the ability of populations to adjust to local conditions via selection. These two levels of response are not independent because the ability to make physiological adjustments is undoubtedly genetically controlled and is therefore subject to evolutionary change. Given the differences in the thermal regimes of temperate and tropical regions, a better understanding of the physiological plasticity and evolutionary plasticity of organisms is important to our understanding of the evolution of tropical and temperatezone species. Correlation between critical thermal maxima (CTMax) and environmental temperatures suggest that tolerances to high temperatures reflect adaptations of populations to their local thermal environments (Snyder & Weathers 1975, Miller & Packard 1977, Howard et al. 1983). Thus, tropical species tend to have higher CTMax than temperate species (Brattstrom 1968, 1970; Snyder & Weathers 1975). Brattstrom (1968, 1970) found, in general, no difference between temperate and tropical species in their ability to adjust their tolerances to high temperatures (CTMax) by acclimation. However, acclimation temperature does not alter the standard metabolic rates of most tropical amphibians, whereas all temperate-zone amphibians that have been tested show significant acclimation of metabolism (Feder 1978, 1982). For amphibians in temperate regions, individuals from cool areas at high elevation have lower CTMax than conspecifics from lower, warmer sites (Brattstrom 1968 provides an example and an apparent exception; Cupp & Brodie 1972, Miller & Packard 1977, Hoppe 1978, Howard et al. 1983, Delson & Whitford 1973 demonstrated this trend, but only in neotenic individuals). Fewer tropical species have been examined along an elevational transect, but Heatwole et al. (1965) reported that Eleutherodactylus portoricensis from Puerto Rico showed a trend in temperature tolerance similar to that of the temperate species mentioned above, with individuals from high elevations having lower CTMax than individuals from low elevation. However, the tadpoles of another Puerto Rican species (Leptodactylus albilabris) did not show a relationship between elevation and CTMax (Heatwole et al. 1968). Subsequent to the study of Heatwole et al. (1965), E. portoricensis, as then understood, was found to consist of two species that differ in subtle morphological and vocal characters: Eleutherodactylus coqui, widespread throughout Puerto Rico, and E. portoricensis, restricted to high elevations (Thomas 1965). This raised the possibility that the sample of frogs collected by Heatwole et al. (1965) from the high elevation site may have consisted of either E. portoricensis, E. coqui, or a mixture of the two. We reexamined the CTMax of these Puerto Rican frogs to look for interspecific differences and to compare E. coqui from high and low elevations. In addition, we measured the critical thermal minima (CTMin) of the three groups I Received 19 November 1985, revision received 29 November 1986, accepted 15 December 1986. 2 Present address: University College of the Northern Territory, GPO Box 1341, Darwin, NT 0801, Australia. 236 BIOTROPICA 20(3): 236-239 1988 This content downloaded from 157.55.39.120 on Mon, 05 Sep 2016 06:09:18 UTC All use subject to http://about.jstor.org/terms of frogs, and E. coqui were acclimated at 20 and 27?C to determine if the CTMax of these frogs were affected by acclimation. To confirm the results of the experiment (performed in April 1984), the CTMax experiment was repeated in late October 1984 on frogs that were tested within 4 hr of capture.

Concordant variation in thermal tolerance and allozymes of the red shiner, Notropis lutrensis, inhabiting tailwater sections of the Brazos River, Texas

Critical thermal maxima (CTM) and genetic variation were compared for red shiners, Notropis lutrensis, from regulated and unregulated sites on the Brazos River in northcentral Texas. Tailwater fish acclimated to 25°C had significantly lower CTM's than those from a site upstream from the dam and unregulated downstream sites. Significantly different intrasite variances were observed, with two- and four-fold larger CTM variances in fish from within 1 km and 30 km of the dam. Genetic variation was determined from electrophoretic comparisons at 21 structural gene loci. Mean heterozygosity was greatest at regulated sites. Tests for locus heterogeneity at five variable loci indicated that regulated and unregulated populations are not homogeneous. Fish under regulation were genetically more similar to each other than they were to those not affected by regulation. The proportions of the gene variance attributable to habitat alteration were partitioned, and fully one-third of the gene variation was attributed to stream regulation. Patterns of variation in thermal tolerance and metabolic enzymes in the red shiner correlated closely with temperature regimes associated with hypolimnion release from the dam. These adaptive responses have occurred in less than 40 years.

Ecology, Maturation and Reproduction of Thamnophis Sauritus Proximus

Salamanders of several species were heated in experiments designed to reveal differences between species with respect to the critical thermal maximum. Also the effect of acclimation on the upper thermal level tolerated was tested. It was found that salamanders of the species Desmognathus fuscus and D. quadramacula-tus have significantly lower critical thermal maxima after being kept for several days at five degrees Centigrade than when kept at fifteen degrees before being used in experiments. At the five-degree level of acclimation, the widely distributed fuscus has a slightly but statistically significantly higher critical thermal maximum than quadramnaculatus, a species restricted to montane streams. Desmognathus ochrophaeus carolinensis has possibly the lowest CTM among the species tested, while Diemiictylus viridescens proved to have greater resistance to high temperatures than the other species. This greater tolerance of high temperature enables viridescens to utilize warm standingwater habitats unfavorable to other species, and possibly plays an important part in the wide geographic distribution of the species.