Thermal Tolerance Polygon Research Articles

Effective practices for thermal tolerance polygon experiments using mottled catfish Corydoras paleatus

Temperature is an important environmental factor that affects how organisms allocate metabolic resources to physiological processes. Laboratory experiments that determine absolute thermal limits for representative species are important for understanding how fishes are affected by climate change. Critical Thermal Methodology (CTM) and Chronic Lethal Methodology (CLM) experiments were utilized to construct a complete thermal tolerance polygon for the South American fish species, Mottled catfish (Corydoras paleatus). Mottled catfish showed Chronic Lethal Maxima (CLMax) of 34.9 ± 0.52 °C and Chronic Lethal Minima (CLMin) of 3.8 ± 0.08 °C. Fish were chronically acclimated (∼2 weeks) to 6 temperatures ranging from 7.2 ± 0.05 °C →32.2 ± 0.16 °C (7 °C, 12 °C, 17 °C, 22 °C, 27 °C, and 32 °C), and CTM used to estimate upper and lower acute temperature tolerance. Linear regressions of Critical Thermal Maxima (CTMax) and Minima (CTMin) data with each acclimation temperature were used along with CLMax and CLMin to create a complete thermal tolerance polygon. The highest CTMax was 38.4 ± 0.60 °C for fish acclimated to 32.2 ± 0.16 °C, and the lowest CTMin was 3.36 ± 1.84 °C for fish acclimated to 7.2 ± 0.05 °C. Mottled catfish have a polygon measuring 785.7°C2, and the slope of the linear regressions showed the species gained 0.55 °C and 0.32 °C of upper and lower tolerance per degree of acclimation temperature, respectively. We compared slopes of CTMax or CTMin regression lines to each other using a set of comparisons between 3, 4, 5, or 6 acclimation temperatures. Our data demonstrated that 3 acclimation temperatures were as sufficient as 4 → 6 to pair with estimates of chronic upper and lower thermal limits for accurately determining a complete thermal tolerance polygon. Construction of this species' complete thermal tolerance polygon provides a template for other researchers. The following is sufficient to generate a complete thermal tolerance polygon: Three chronic acclimation temperatures that are spread somewhat evenly across a species’ thermal range, include an estimation of CLMax and CLMin, and are followed by CTMax and CTMin measurements.

Thermal tolerance of the male freshwater prawn Cryphiops caementarius exposed to different acclimation temperatures

The variation in water temperature influences metabolic and biochemical processes in ectothermic organisms, affecting development, behavior, and thermal responses. We conducted laboratory experiments based on different acclimation temperatures to determine the thermal tolerance in male specimens of the freshwater prawn Cryphiops caementarius. During 30 days, male prawns were exposed to acclimation temperatures of 19 °C (control), 24 °C, and 28 °C treatments. The Critical Thermal Maxima (CTMax) values at these acclimation temperatures were 33.42 °C, 34.92 °C, and 36.80 °C; whereas values for the Critical Thermal Minimum (CTMin) were 9.38 °C, 10.57 °C, and 13.88 °C. All acclimation temperature treatments had a positive effect (P < 0.05) on CTMax and CTMin, with high and significant correlations (CTMax: r = 0.992, P < 0.01; CTMin: r = 0.946, P < 0.01). The area of the thermal tolerance polygon over the three acclimation temperatures was 211.32 °C2 and the acclimation response rate values were high (CTMax from 0.30 to 0.47; CTMin from 0.24 to 0.83) but similar to those from other tropical crustacean species. These results demonstrate that adult males of the freshwater prawn C. caementarius can tolerate extreme water temperatures through a thermal plasticity response, which could be advantageous during a global warming scenario.

Extreme cold or warm events can potentially exacerbate chemical toxicity to the marine medaka fish Oryzias melastigma.

Marine ecosystems are currently subjected to dual stresses of chemical pollution and climate change. Through a series of laboratory experiments, this study investigated the impact of exposure to chemical contaminant such as DDT or copper (Cu), in combination with cold or warm temperature extremes on the marine medaka fish Oryzias melastigma. The results showed that extreme seawater temperatures (i.e., 15 and 32°C in sub-tropical Hong Kong) exacerbated adverse chemical impacts on the growth performance of O. melastigma, in particular at the high thermal extreme. This was likely associated with an interruption of oxygen consumption and aerobic scope. Most importantly, the results of acclimation experiments, as reflected by thermal tolerance polygons, showed that chemical exposure substantially narrowed the thermal tolerance of the medaka, making them more vulnerable to temperature changes and extreme thermal events. Under dual stresses of thermal extremes and chemical exposure, the medaka switched their metabolic pathway to anaerobic respiration that might deplete their energy reserve for chemical detoxification. Although stress proteins such as heat shock proteins (HSP90) were up-regulated for cellular protection in the fish, such a defensive mechanism was repressed with intensifying dual stresses at high temperature and high chemical concentration. Bioconcentration of DDT or Cu generally increased with increasing temperature and its exposure concentration. Overall, these complex chemical-temperature interactions concomitantly exerted a concerted adverse impact to O. melastigma. The temperature-dependent toxicity of DDT or Cu shown in this study clearly demonstrated the potential challenge brought by the risk of chemical pollution under the impact of global climate change.

Thermal tolerance of Acartia tonsa: In relation to acclimation temperature and life stage.

The Acartia tonsa, a calanoid copepod species, has high survival and thermal acclimation capacity in aquatic environments characterized by temperature variations. Dynamic and static thermal polygon areas of this species are 495 °C2 and 267 °C2 for nauplii stage, while adult stage has 747 °C2 and 411 °C2 dynamic and static thermal polygon area, respectively. In addition, Acartia tonsa is a copepod species which is more resistant to both high and low lethal temperatures, with its resistance zone of 105 °C2 and 131 °C2 for nauplii and adults, respectively. Acartia tonsa nauplii acclimated to 18°C, 23°C and 28°C have lover and upper thermal limit (CTMin-CTMax) of 6.82-26.15°C, 8.65-29.49°C, and 11.70-34.10°C, respectively. This species in the adult stage has a CTMin-CTMax of 4.47-30.30°C, 6.35-33.94°C, and 9.92-35.90°C at acclimation temperatures mentioned above. Its broad dynamic and static thermal tolerance polygon areas and, accordingly, its significant thermal limits allow Acartia tonsa to survive at warm or cold extremes in their natural environment.

Farklı alıştırma sıcaklıklarında kılıçkuyruk (Xiphophorus helleri) ve plati balıklarının (X. maculatus) termal tolerans parametrelerinin belirlenmesi

Thermal tolerance parameters of swordtail (Xiphophorus maculatus) and platy (X. helleri) at three acclimation temperatures were determined in the study. The CTMin values at 20, 24 and 28°C acclimation temperature were 9.41,10.42 and 11.95°C respectively for platy and 9.38,11.5 and 13.23°C for swordtail, while CTMax values were 37.41, 39.19 and 40.52°C for platy and 36.94, 38.89 and 40.07°C for swordtail. Accordingly, acclimation temperature affected the lower and upper temperature tolerances of fish by 3-4 °C. The CTMin ARR values varied between 0.42-0.42 in swordtail and 0.20-0.34 in platy, while CTMax ARR ranged between 0.29-0.48 in swordtail and 0.33-0.44 in platy depending on acclimation temperature. Thermal tolerance polygon area of platy (232°C2) was slightly higher than that of swordtail (217.3°C). The fact that both fish species have lower temperature tolerances limits their geographic distribution and aquaculture in subtropical climates where the water temperature drops to 10°C in winter.

Thermal responses of two sub-Antarctic notothenioid fishes, the black southern cod Patagonotothen tessellata (Richardson, 1845) and the Magellan plunderfish Harpagifer bispinis (Forster, 1801), from southern South America

The Notothenioidei are a typical example of stenothermal fishes since most species have evolved and lived in Antarctic waters, where the water temperature is low and stable. This fact enabled them to evolve physiological characteristics related to cold. Nevertheless, some species came out of Antarctic waters a few million years ago and coped with more variable thermal regimes. This work aims to determine the thermal tolerance and preference of two sub-Antarctic notothenioid species found in Southern South America, Patagonotothen tessellata and Harpagifer bispinis, adding valuable information about thermal adaptation mechanisms. Experiments were conducted after exposing their juveniles for three weeks at 4, 7, 10 and 12 °C. Their thermal tolerance limits were established using the Critical Thermal Methodology and their acute thermal preferenda, employing a horizontal thermal gradient tank. Fishes acclimated to different exposure temperatures had small to intermediate thermal tolerance polygons (P. tessellata: 593.85°C2, H. bispinis: 475.40 °C2) and positive relationships between preferred and acclimation temperatures. The Final Temperature Preferenda were estimated to be 14.25 °C for P. tessellata and 13.05 °C for H. bispinis, allowing to characterize them as cold eurythermal species, with P. tessellata more tolerant to heat and H. bispinis more tolerant to cold. Their different thermal sensitivities are in agreement with their different thermal histories and distributions. In a climate change context, the increase of sea surface temperatures is likely to reduce the northern boundaries of their distributions. Conversely, it can potentially enhance both species’ performances at their southernmost distribution limits since those environments are cooler than their maximum thermal tolerances.

Combined effect of pCO2 and temperature levels on the thermal niche in the early benthic ontogeny of a keystone species

We evaluated the effects of projected, near future ocean acidification (OA) and extreme events of temperature (warming or cooling) on the thermal tolerance of Concholepas concholepas, a coastal benthic keystone species. Three separate trials of an experiment were conducted by exposing juvenile C. concholepas for 1 month to one of two contrasting pCO2 levels (~500 and ~1200 μatm). In addition, each pCO2 level was combined with one of four temperature treatments. The control was 15 °C, whilst the other temperatures were 10 °C (Trial 1), 20 °C (Trial 2) and 25 °C (Trial 3). At the end of each trial, we assessed Critical Thermal maximum (CTmax) and minimum (CTmin) via self-righting success, calculated partial thermal tolerance polygons, measured somatic growth, determined transcription of Heat Shock Proteins 70 (HSP70) and measured oxygen consumption rates. Regardless of pCO2 level, HSP70 transcript levels were significantly higher in juveniles after exposure to extreme temperatures (10 °C and 25 °C) indicating physiological stress. Oxygen consumption rates increased with increasing temperature from 10 °C to 20 °C though showed a decrease at 25 °C. This rate was not affected by pCO2 or the interaction between temperature and pCO2. Juveniles exposed to present-day and near future pCO2 levels at 20 °C showed similar thermal tolerance polygonal areas; whilst changes in both CTmin and CTmax at 25 °C and 10 °C caused narrower and broader areas, respectively. Temperature affected growth, oxygen consumption and HSP70 transcription in small juvenile C. concholepas. Exposure to elevated pCO2 did not affect thermal tolerance, growth or oxygen consumption at temperatures within the thermal range normally experienced by this species in northern Chile (15-20 °C). At elevated pCO2 conditions, however, exposure to warmer (25 °C) or colder (10 °C) temperatures reduced or increased the thermal area, respectively. This study demonstrates the importance of examining the thermal-tolerance edges to better understand how OA and temperature will combine to physiologically challenge inter-tidal organisms.

Ocean warming and acidification pose synergistic limits to the thermal niche of an economically important echinoderm.

To make robust projectios of the impacts of climate change, it is critical to understand how abiotic factors may interact to constrain the distribution and productivity of marine flora and fauna. We evaluated the effects of projected end of the century ocean acidification (OA) and warming (OW) on the thermal tolerance of an important living marine resource, the sea urchin Loxechinus albus, a benthic shallow water coastal herbivore inhabiting part of the Pacific coast of South America. After exposing young juveniles for a 1-month period to contrasting pCO2 (~500 and 1400 μatm) and temperature (~15 °C and 20 °C) levels, critical thermal maximum (CTmax) and minimum (CTmin) as well as thermal tolerance polygons were assessed based on self-righting success as an end point. Transcription of heat shock protein 70 (HSP70), a chaperone protecting cellular proteins from environmental stress, was also measured. Exposure to elevated pCO2 significantly reduced thermal tolerance by increasing CTmin at both experimental temperatures and decreasing CTmax at 20 °C. There was also a strong synergistic effect of OA × OW on HSP70 transcription levels which were 75 times higher than in control conditions. If this species is unable to adapt to elevated pCO2 in the future, the reduction in thermal tolerance and HSP response suggests that near-future warming and OA will disrupt their performance and reduce their distribution with ecological and economic consequences. Given the wider latitudinal range (6 to 56°S) and environmental tolerance of L. albus compared to other members of this region's benthic invertebrate community, OW and OA may cause substantial changes to the coastal fauna along this geographical range.

Thermal tolerance of thirteen popular ornamental fish Species

Thermal tolerance parameters of thirteen ornamental fish species (Carassius auratus, Cyprinus carpio: koi, Brachydanio rerio, Puntius tetrazona, Garra rufa, Poecilia latipinna, P. reticulata, Pterophyllum scalare, Labidochromis caeruleus, Cyrtocara moorii, Carydoras paleatus, Trichogaster trichopterus and Hypostomus plecostomus) at three acclimation temperatures (20, 24 and 28 °C) were determined in this study. The CTMin values of the species ranged from 2.55–6.54 °C (C. auratus) to 11.66–13.94 °C (P. tetrazona), while CTMax values ranged from 33.91–37.51 °C (P. cory) to 39.71–41.80 °C (P. reticulata). The acclimation response ratio (ARR) values between 20 and 28 °C acclimation temperature (AT) ranged from 0.18 (H. plecostomus) to 0.63 (B. rerio) for CTMin, while these values ranged between 0.17 (G. rufa) and 0.67 (P. tetrazona) for CTMax values. Thermal tolerance polygon was widest in C. auratus (281 °C2) and narrowest in P. tetrazona (188 °C2). Despite being in the same family (Cyprinidae), it is interesting and unusual that C. auratus is a typical eurythermal and P. tetrazona is a typical stenothermal species. C. auratus and C. carpio followed by G. rufa, P. latipinna and B. rerio are the species which are advantageous in terms of aquaculture due to their wide temperature tolerance range and high ARR values.

Regional differences in thermal adaptation of a cold-water fish Rhynchocypris oxycephalus revealed by thermal tolerance and transcriptomic responses

Understanding how populations adapt to different thermal environments is an important issue for biodiversity conservation in the context of recent global warming. To test the hypothesis that populations from southern region are more sensitive to climate change than northern region in cold-water species, we determined the thermal tolerance of two geographical populations of a cold-water fish, Rhynchocypris oxycephalus: the Hangzhou population from southern region and the Gaizhou population from northern region, then compared their transcriptomic responses between a control and a high temperature treatment. The results showed that the thermal tolerance range and thermal tolerance polygon area of Hangzhou population were narrower than the Gaizhou population, indicating populations from southern region were possibly more vulnerable. Further transcriptomic analysis revealed that the Gaizhou population expressed more temperature responding genes than the Hangzhou population (583 VS. 484), corresponding with their higher thermal tolerance, while some of these genes (e.g. heat shock protein) showed higher expression in the Hangzhou population under control condition, suggesting individuals from southern region possibly have already responded to the present higher environmental temperature pressure. Therefore, these results confirm the prediction that populations from southern region are more sensitive to global warming, and will be important for their future conservation.

Thermal tolerance of paralarvae of Patagonian red octopusEnteroctopus megalocyathus

Patagonian red octopus, Enteroctopus megalocyathus, is a merobenthic octopus whose paralarvae have been successfully cultured up to juvenile octopuses. At present, high mortality during the paralarval period prevents the scaling from experimental rearing to commercial aquaculture. The aim of the study was to determine upper (CTMax) and lower (CTMin) thermal tolerance, acclimation response ratio (ARR) and thermal tolerance polygon of paralarvae from different culture conditions and subjected to seven acclimation temperatures (6, 8, 10, 12, 14, 16, 18°C) during the first 5 days of paralarval life. Culture conditions were two types of egg incubation (maternal care and artificial incubators) and two feeding regimes (fed or starved). Fed paralarvae showed thermal preferendum, while unfed paralarvae preferred much higher temperatures than those of acclimation. CTMin and CTMax increased along with the acclimation temperature. Lower values of ARR were obtained in paralarvae from artificial incubation, with this type of paralarva showing the least adaptability to thermal changes. Starved paralarvae showed the lowest values for thermal tolerance range (TTR) and smaller areas of thermal polygon than fed paralarvae. Rearing temperatures above 16°C may be considered suboptimal to paralarvae and affected by the conditions during the embryonic incubation. Rearing temperatures below 8°C may be considered suboptimal for all hatched paralarvae. Therefore, the other temperatures within this range could be used in the context of improving the culture management of paralarvae.

The effects of acclimation temperature, salinity, and behavior on the thermal tolerance of Mozambique tilapia (Oreochromis mossambicus).

Mozambique tilapia have been shown to be incredibly stress tolerant with respect to environmental salinity, hypoxia, and ammonia concentrations. Temperature challenges to this species have shown that they have difficulty with cold acclimation. The purpose of this study was to measure the effects of acclimation temperature and salinity on the thermal tolerance of Mozambique tilapia as assessed by critical thermal maxima (CTMax) and critical thermal minima (CTMin). We also monitored fish behavior and quantified ventilation rate. To our knowledge, this study was the first to investigate upper and lower thermal tolerances, and the effect of environmental salinity in this physiologically impressive species. Using predictive regression analyses of the thermal limits, thermal tolerance polygons were constructed and total areas were calculated 678.9°C2 for freshwater (FW)-acclimated tilapia, and 739.4°C2 seawater (SW)-acclimated tilapia. During the thermal challenges, we observed two novel behaviors in response to thermal challenge, ventilation cessation behavior (VCB) and aquatic surface respiration (ASR), and we conclude that the use of these behaviors extended the thermal limits of these fish in both FW and two-thirds SW by limiting the exposure of the gill epithelium to the changing environment.

Thermal ecology of Galaxias platei (Pisces, Galaxiidae) in South Patagonia: perspectives under a climate change scenario

The native freshwater fish Galaxias platei shows a wide latitudinal distribution in Patagonia, being found on both sides of the Andes. Currently, climate change poses one of the main threats to native fish, and its effects are appearing faster in high southern latitudes. The aim of this work was to analyse the possible effects of climate change in G. platei through its thermal responses. We hypothesized that juveniles of this species would be affected by indirect rather than by direct consequences of climate change. We determined the thermal tolerance polygon using Critical Thermal Methodology and preferred temperatures using a thermal gradient. Additionally, we evaluated routine metabolic rate using stop-flow respirometry. Results showed an intermediate to large polygon, with a non-negligible portion acquired through acclimation. Preferred temperatures and routine metabolic rates were positively related to acclimation temperature. Results suggest that G. platei thermal tolerance is dependent on its prior thermal history, have a eurythermal nature but maintain high levels of cold tolerance. Moreover, G. platei would be better suited at maintaining homeostasis at highest temperatures where more energy could be available for growth. This is the first time that thermal ecology data are registered for this species at its southernmost distribution.

Thermal tolerance and phenotypic plasticity in juvenile Hippocampus erectus Perry, 1810: Effect of acute and chronic exposure to contrasting temperatures

The seahorse Hippocampus erectus inhabits estuaries and coastal areas from southern Canada to northern Brazil, including the Yucatan Peninsula (YP), Mexico. Fish species living in thermally heterogeneous environments have developed physiological and behavioural adjustments that provide individuals with the phenotypic plasticity to respond to environmental changes within certain limits. When thermal changes exceed the ability of fish to adjust, energy supply is predominantly directed to fuel essential maintenance costs at the expense of growth and reproduction. In such circumstances, not only temperature, but also time of exposure, are elements that determine the adaptive value of phenotypic plasticity. The present study was designed to examine the consequences of acclimation of juvenile H. erectus to 18 and 30°C on the phenotypic plasticity associated with exposure to acute thermal events (measured through the thermal limits) and chronic exposure during longer periods (measured through growth). Similar survival and growth was found in all seahorses that experienced acute exposure (~1min) to temperatures ranging from 7 to 33°C and 12 to 36°C in both acclimation treatments, respectively. Chronic exposure to 30°C compared to 18°C, however, significantly reduced seahorse growth. A thermal tolerance polygon calculated as 302°C2 suggests that H. erectus has a wide tolerance to the extreme conditions commonly present in estuarine environments. Given that the amplitude in the thermal tolerance polygon was mainly due to an increase in the lower limits of the interval and that no minimum temperature threshold could be determined, thus suggesting that the populations from YP are near their upper thermal limit.

Metabolic rate and thermal tolerance in two congeneric Amazon fishes: Paracheirodon axelrodi Schultz, 1956 and Paracheirodon simulans Géry, 1963 (Characidae)

Temperature is the main factor affecting the distribution of the sympatric Amazon fishes Paracheirodon axelrodi and Paracheirodon simulans. Both species are associated with flooded areas of the Negro river basin; P. axelrodi inhabits waters that do not exceed 30°C, and P. simulans lives at temperatures that can surpass 35°C. The present work aimed to describe the biochemical and physiological adjustments to temperature in those species. We determined the thermal tolerance polygon of species acclimated to four temperatures using critical thermal methodology. We also determined the chronic temperature effects by acclimating the two species at 20, 25, 30, and 35°C and measured the critical oxygen tension (PO2crit) for both species. Additionally, we evaluated the metabolic rate and the enzymes of energy metabolic pathways (CS, MDH, and LDH). Our results showed a larger thermal tolerance polygon, a higher energetic metabolic rate, and higher enzyme levels for P. simulans acclimated to 20 and 35°C compared to P. axelrodi. Paracheirodon simulans also presented a higher hypoxia tolerance, indirectly determined as the PO2cri. Thus, we conclude that the higher metabolic capacity of P. simulans gives this species a better chance to survive at acutely higher temperatures in nature, although it is more vulnerable to chronic exposure.

Seasonal variation in thermal tolerance, oxygen consumption, antioxidative enzymes and non-specific immune indices of Indian hill trout, Barilius bendelisis (Hamilton, 1807) from central Himalaya, India

We studied the season dependent thermal tolerance, oxygen consumption, respiratory burst response and antioxidative enzyme activities in juveniles of Barilius bendelisis. The critical thermal maximum (CTmax), lethal thermal maximum (LTmax), critical thermal minimum (CTmin) and lethal thermal minimum (LTmin) were significantly different at five different seasons viz. winter (10.64°C), spring (16.25°C), summer (22.11°C), rainy (20.87°C) and autumn (17.77°C). The highest CTmax was registered in summer (36.02°C), and lowest CTmin was recorded during winter (2.77°C). Water temperature, dissolved oxygen and pH were strongly related to CTmax, LTmax, CTmin and LTmin suggesting seasonal acclimatization of B. bendelisis. The thermal tolerance polygon area of the B. bendelisis juveniles within the range of seasonal temperature (10.64–22.11°C) was calculated as 470.92°C2. Oxygen consumption rate was significantly different (p<0.05) between seasons with maximum value during summer (57.66mgO2/kg/h) and lowest in winter (32.60mgO2/kg/h). Total white blood cell count including neutrophil and monocytes also showed significant difference (p<0.05) between seasons with maximum value during summer and minimum number in winter and were found correlated to temperature, dissolved oxygen, pH and respiratory burst activity. Respiratory burst activity of blood phagocytes significantly differed (p<0.05) among seasons with higher value during summer (0.163 OD540nm) and minimum in winter season (0.054 OD540nm). The activity of superoxide dismutase, catalase and glutathione-s-transferase both in liver and gill, also varied significantly (p<0.05) during different seasons. Overall results of this study suggest that multiple environmental factors play a role in seasonal acclimation in B. bendelisis, which modulate the thermal tolerance, oxygen consumption, respiratory burst activity and status of anti-oxidative potential in wild environment.

Thermopreference, tolerance and metabolic rate of early stages juvenile Octopus maya acclimated to different temperatures

Thermopreference, tolerance and oxygen consumption rates of early juveniles Octopus maya (O. maya; weight range 0.38–0.78g) were determined after acclimating the octopuses to temperatures (18, 22, 26, and 30°C) for 20 days. The results indicated a direct relationship between preferred temperature (PT) and acclimated temperature, the PT was 23.4°C. Critical Thermal Maxima, (CTMax; 31.8±1.2, 32.7±0.9, 34.8±1.4 and 36.5±1.0) and Critical Thermal Minima, (CTMin; 11.6±0.2, 12.8±0.6, 13.7±1.0, 19.00±0.9) increased significantly (P<0.05) with increasing acclimation temperatures. The endpoint for CTMax was ink release and for CTMin was tentacles curled, respectively. A thermal tolerance polygon over the range of 18–30°C resulted in a calculated area of 210.0°C2. The oxygen consumption rate increased significantly α=0.05 with increasing acclimation temperatures between 18 and 30°C. Maximum and minimum temperature quotients (Q10) were observed between 26–30°C and 22–26°C as 3.03 and 1.71, respectively. These results suggest that O. maya has an increased capability for adapting to moderate temperatures, and suggest increased culture potential in subtropical regions southeast of México.

Thermal tolerance of the invasive Belonesox belizanus, pike killifish, throughout ontogeny.

The goal of this study was to characterize the variability of thermal tolerances between life-history stages of the invasive Belonesox belizanus and attempt to describe the most likely stage of dispersal across south Florida. In the laboratory, individuals were acclimated to three temperatures (20, 25, or 30°C). Upper and lower lethal thermal limits and temperatures at which feeding ceased were measured for neonates, juveniles, and adults. Thermal tolerance polygons were developed to represent the thermal tolerance range of each life-history stage. Results indicated that across acclimation temperatures upper lethal thermal limits were similar for all three stages (38°C). However, minimum lethal thermal limits were significantly different at the 30°C acclimation temperature, where juveniles (9°C) had an approximately 2.0°C and 4.0°C lower minimum lethal thermal limit compared with adults and neonates, respectively. According to thermal tolerance polygons, juveniles had an average tolerance polygonal area almost 20°C(2) larger than adults, indicating the greatest thermal tolerance of the three life-history stages. Variation in cessation of feeding temperatures indicated no significant difference between juveniles and adults. Overall, results of this study imply that juvenile B. belizanus may be equipped with the physiological flexibility to exercise habitat choice and reduce potential intraspecific competition with adults for limited food resources. Given its continued dispersal, the minimum thermal limit of juveniles may aid in continued dispersal of this species, especially during average winter temperatures throughout Florida where juveniles could act to preserve remnant populations until seasonal temperatures increase.

Thermal tolerance of European Sea Bass ( Dicentrarchus labrax) juveniles acclimated to three temperature levels

This study was carried out to determine upper (CTMax) and lower (CTMin) thermal tolerance, acclimation response ratio (ARR) and thermal tolerance polygon of the European sea bass inhabiting the Iskenderun Bay, the most southeasterly part of the Mediterranean Sea, at three acclimation temperatures (15, 20, 25 °C). Acclimation temperature significantly affected the CTMin and CTMax values of the fish. At 0.3 °C min −1 cooling or heating rate, CTMin ranged from 4.10 to 6.77 °C and CTMax ranged from 33.23 to 35.95 °C in three acclimation temperatures from 15 to 25 °C. Thermal tolerance polygon for the juveniles at the tested acclimation temperatures was calculated to be 296.14 °C 2. In general, the current data show that our sea bass population possesses acclimation response ratio (ARR) values (0.25–0.27) similar to some tropical species. The cold tolerance values attained for this species ranged from 4.10 to 6.77 °C, suggesting that cold winter temperatures may not pose danger during the culture of European sea bass in deep ponds or high water exchange rate systems. Upper thermal tolerance is more of a problem in the southern part of the Mediterranean as maximum water temperature in ponds may sometimes exceed 33–34 °C, during which underground cool-water should be used to lower ambient water temperature in the mid-summer. For successful culture of sea bass in ponds, temperature should be maintained around 25 °C throughout the year and this can be managed under greenhousing systems using underground well-waters, commonly available in the region.

Thermal tolerance of Litopenaeus vannamei (Crustacea: Penaeidae) acclimated to four temperatures

Critical thermal minima (CTMin) and maxima (CTMax) values were determined for the Pacific white shrimp Litopenaeus vannamei post-larvae and juveniles at four different acclimation temperatures (15, 20, 25, and 30 °C). The CTMin of shrimp at these acclimation temperatures were 7.82, 8.95, 9.80, and 10.96 °C for post-larvae and 7.50, 8.20, 10.20, and 10.80 °C for juveniles, respectively, at 1 °C h −1 cooling rate. The CTMax values were 35.65, 38.13, 39.91, and 42.00 °C for post-larvae and 35.94, 38.65, 40.30, and 42.20 °C for juveniles at the respective acclimation temperatures. Both acclimation temperature and size of the shrimp affected CTMin values of L. vannamei ( P<0.01). Overall, juveniles displayed significantly lower CTMin values than the post-larvae ( P<0.0001). However, the CTMax response by post-larvae and juveniles were not significantly different from each other and no interaction was determined between the acclimation temperature and development stage ( P>0.01). The area of the thermal tolerance polygon over four acclimation temperatures (15, 20, 25, and 30 °C) for the post-larvae of L. vannamei was calculated to be 434.94 °C 2. The acclimation response ratio (ARR) values were high ranging from 0.35 to 0.44 for both post-larvae and juveniles. L. vannamei appears to be more sensitive to low temperatures than other penaeid species and its cold tolerance zone ranged from 7.5 to 11 °C. In successful aquaculture temperature must never fall below 12 °C to prevent mortalities. Upper thermal tolerance is less of a problem as in most subtropical regions maximum water temperature rarely exceeds 34 °C, but care should be given if shallow ponds with low water renewal rate are being used.