Behavioural thermoregulation in two freshwater fish species

Temperature-dependent exercise recovery is not associated with behavioral thermoregulation in a salmonid fish

The relationship between behavioral thermoregulation and physiological recovery following exhaustive exercise is not well understood. Behavioral thermoregulation could be beneficial for exercise recovery; for example, selection of cooler temperatures could reduce maintenance metabolic cost to preserve aerobic scope for recovery cost, or selection of warmer temperatures could accelerate recovery of exercise metabolites. While post-exercise behavioral thermoregulation has been observed in lizards and frogs, little is known about its importance in fish. We examined the influence of post-exercise recovery temperature on metabolic rate, thermal preference, and metabolite concentrations in juvenile brook char (Salvelinus fontinalis). Fish were acclimated to and exercised at 15 °C, then recovered at either 15 °C or 10 °C while their metabolic rate was measured via respirometry. Metabolite concentrations were measured in fish after exercise at 15 °C and recovery under one of three thermal treatments (to simulate various behavioral thermoregulation scenarios): (i) 6 h recovery at 15 °C, (ii) 6 h recovery at 10 °C, or (iii) 3 h recovery at 10 °C followed by 3 h recovery at 15 °C. Thermal preference was quantified using a static temperature preference system (15 °C vs. 10 °C). Metabolic rates returned to resting faster at 10 °C compared with 15 °C, although at 10 °C there was a tradeoff of delayed metabolite recovery. Specifically, post-exercise plasma osmolality, plasma lactate, and muscle lactate remained elevated for the entire period in fish recovering at 10 °C, whereas these parameters returned to resting levels by 6 h in fish from the other two recovery groups. Regardless, fish did not exhibit clear behavioral thermoregulation (i.e., fish overall did not consistently prefer one temperature) to prioritize either physiological recovery process. The advantage of metabolic rate recovery at cooler temperatures may balance against the advantage of metabolite recovery at warmer temperatures, lessening the usefulness of behavioral thermoregulation as a post-exercise recovery strategy in fish.

Species interactions alter the selection of thermal environment in a coral reef fish

Increasing ocean temperatures and the resulting poleward range shifts of species has highlighted the importance of a species preferred temperature and thermal range in shaping ecological communities. Understanding the temperatures preferred and avoided by individual species, and how these are influenced by species interactions is critical in predicting the future trajectories of populations, assemblages, and ecosystems. Using an automated shuttlebox system, we established the preferred temperature and upper and lower threshold temperatures (i.e., avoided temperatures) of a common coral reef fish, the black-axil chromis, Chromis atripectoralis. We then investigated how the presence of conspecifics, heterospecifics (Neopomacentrus bankieri), or a predator (Cephalopholis spiloparaea) influenced the selection of these temperatures. Control C. atripectoralis preferred 27.5 ± 1.0 °C, with individuals avoiding temperatures below 23.5 ± 0.9 °C and above 29.7 ± 0.7 °C. When associating with either conspecifics or heterospecifics, C. atripectoralis selected significantly lower temperatures (conspecifics: preferred = 21.2 ± 1.4 °C, lower threshold = 18.1 ± 0.8 °C; heterospecifics: preferred = 21.1 ± 1.1 °C, lower threshold = 19.2 ± 0.9 °C), but not higher temperatures (conspecifics: preferred = 28.9 ± 1.2 °C, upper threshold = 30.8 ± 0.9 °C; heterospecifics: preferred = 29.7 ± 1.1 °C, upper threshold = 31.4 ± 0.8 °C). The presence of the predator, however, had a significant effect on both lower and upper thresholds. Individual C. atripectoralis exposed themselves to temperatures ~ 5.5 °C cooler or warmer (lower threshold: 18.6 ± 0.5 °C, upper threshold: 35.2 ± 0.5 °C) than control fish before moving into the chamber containing the predator. These findings demonstrate how behavioural responses due to species interactions influence the thermal ecology of a tropical reef fish; however, there appears to be limited scope for individuals to tolerate higher temperatures unless faced with the risk of predation.

Temperature Effects During a Sublethal Chronic Metal Mixture Exposure on Common Carp (Cyprinus carpio)

The aquatic environment is the final sink of various pollutants including metals, which can pose a threat for aquatic organisms. Waterborne metal mixture toxicity might be influenced by environmental parameters such as the temperature. In the present study, common carp were exposed for 27 days to a ternary metal mixture of Cu, Zn, and Cd at two different temperatures, 10 and 20°C. The exposure concentrations represent 10% of the 96 h-LC₅₀ (concentration lethal for the 50% of the population in 96 h) for each metal (nominal metal concentrations of Cu: 0.08 μM; Cd: 0.02 μM and Zn: 3 μM). Metal bioaccumulation and toxicity as well as changes in the gene expression of enzymes responsible for ionoregulation and induction of defensive responses were investigated. Furthermore the hepatosomatic index and condition factor were measured as crude indication of overall health and energy reserves. The obtained results showed a rapid Cu and Cd increase in the gills at both temperatures. Cadmium accumulation was higher at 20°C compared to 10°C, whereas Cu and Zn accumulation was not, suggesting that at 20°C, fish had more efficient depuration processes for Cu and Zn. Electrolyte (Ca, Mg, Na, and K) levels were analyzed in different tissues (gills, liver, brain, muscle) and in the remaining carcasses. However, no major electrolyte losses were observed. The toxic effect of the trace metal ion mixture on major ion uptake mechanisms may have been compensated by ion uptake from the food. Finally, the metal exposure triggered the upregulation of the metallothionein gene in the gills as defensive response for the organism. These results, show the ability of common carp to cope with these metal levels, at least under the condition used in this experiment.

A framework for understanding gene expression plasticity and its influence on stress tolerance

Phenotypic plasticity can serve as a stepping stone towards adaptation. Recently, studies have shown that gene expression contributes to emergent stress responses such as thermal tolerance, with tolerant and susceptible populations showing distinct transcriptional profiles. However, given the dynamic nature of gene expression, interpreting transcriptomic results in a way that elucidates the functional connection between gene expression and the observed stress response is challenging. Here, we present a conceptual framework to guide interpretation of gene expression reaction norms in the context of stress tolerance. We consider the evolutionary and adaptive potential of gene expression reaction norms and discuss the influence of sampling timing, transcriptomic resilience, as well as complexities related to life history when interpreting gene expression dynamics and how these patterns relate to host tolerance. We highlight corals as a case study to demonstrate the value of this framework for non-model systems. As species face rapidly changing environmental conditions, modulating gene expression can serve as a mechanistic link from genetic and cellular processes to the physiological responses that allow organisms to thrive under novel conditions. Interpreting how or whether a species can employ gene expression plasticity to ensure short-term survival will be critical for understanding the global impacts of climate change across diverse taxa.

Different sensitivity to heatwaves across the life cycle of fish reflects phenotypic adaptation to environmental niche

Predicting responses of marine organisms to global change requires eco-physiological assessments across the complex life cycles of species. Here, we experimentally tested the vulnerability of a demersal temperate fish (Sparus aurata) to long-lasting heatwaves, on larval, juvenile and adult life-stages. Fish were exposed to simulated coastal (18 °C), estuarine (24 °C) summer temperatures, and heatwave conditions (30 °C) and their physiological responses were assessed based on cellular stress response biomarkers (heat shock protein 70 kDa, ubiquitin, antioxidant enzymes, lipid peroxidation) and phenotypic measures (histopathology, condition and mortality). Life-stage vulnerability can be ranked as larvae > adults > juveniles, based on mortality, tissue pathology and the capacity to employ cellular stress responses, reflecting the different environmental niches of each life stage. While larvae lacked acclimation capacity, which resulted in damage to tissues and elevated mortality, juveniles coped well with elevated temperature. The rapid induction of cytoprotective proteins maintained the integrity of vital organs in juveniles, suggesting adaptive phenotypic plasticity in coastal and estuarine waters. Adults displayed lower plasticity to heatwaves as they transition to deeper habitats for maturation, showing tissue damage in brain, liver and muscle. Life cycle closure of sea breams in coastal habitats will therefore be determined by larval and adult stages.

Physiological mechanisms linking cold acclimation and the poleward distribution limit of a range-extending marine fish

Extensions of species' geographical distributions, or range extensions, are among the primary ecological responses to climate change in the oceans. Considerable variation across the rates at which species' ranges change with temperature hinders our ability to forecast range extensions based on climate data alone. To better manage the consequences of ongoing and future range extensions for global marine biodiversity, more information is needed on the biological mechanisms that link temperatures to range limits. This is especially important at understudied, low relative temperatures relevant to poleward range extensions, which appear to outpace warm range edge contractions four times over. Here, we capitalized on the ongoing range extension of a teleost predator, the Australasian snapper Chrysophrys auratus, to examine multiple measures of ecologically relevant physiological performance at the population's poleward range extension front. Swim tunnel respirometry was used to determine how mid-range and poleward range edge winter acclimation temperatures affect metabolic rate, aerobic scope, swimming performance and efficiency and recovery from exercise. Relative to 'optimal' mid-range temperature acclimation, subsequent range edge minimum temperature acclimation resulted in absolute aerobic scope decreasing while factorial aerobic scope increased; efficiency of swimming increased while maximum sustainable swimming speed decreased; and recovery from exercise required a longer duration despite lower oxygen payback. Cold-acclimated swimming faster than 0.9 body lengths sec-1 required a greater proportion of aerobic scope despite decreased cost of transport. Reduced aerobic scope did not account for declines in recovery and lower maximum sustainable swimming speed. These results suggest that while performances decline at range edge minimum temperatures, cold-acclimated snapper are optimized for energy savings and range edge limitation may arise from suboptimal temperature exposure throughout the year rather than acute minimum temperature exposure. We propose incorporating performance data with in situ behaviour and environmental data in bioenergetic models to better understand how thermal tolerance determines range limits.

Temperature preference of juvenile lumpfish (Cyclopterus lumpus) originating from the southern and northern parts of Norway

Fish are ectothermic animals and have body temperatures close to that of the water they inhabit. They can still control their body temperatures by selecting habitats with temperatures that maximize their growth, feed conversion and wellbeing. Lumpfish, Cyclopterus lumpus, is widely distributed in the North Atlantic Ocean and therefore exposed to variable water temperatures. Lumpfish is extensively used as cleanerfish in salmon farming in Norway and exposed to a wide temperature range along the north-south axis of the Norwegian coastline. But, if these temperature ranges correspond to the preference temperatures of lumpfish is not known. If lumpfish has adapted to regional temperatures along the Norwegian coast, differences in preference temperature for fish from different regions should be evident. In a selective breeding perspective, different selection lines for preference temperature would then be useful for further development of lumpfish as a cleanerfish. We subjected lumpfish juveniles weighing 154-426g originated from northern (Group North - GN) and southern (Group South - GS) Norway to a temperature preference test, using an electronic shuttle box system. The system allowed the fish to control the water temperature by moving between two chambers, and thereby choosing its preferred temperature in the range from 5 to 16 °C. We started the temperature at 7.8 ± 1.37 °C for GN and 7.58 ± 1.34 °C for GS, but all the fish except four (two each from GN and GS) chose lower temperatures (5.03-7.6 °C) in the first 18 h and stayed closer to that temperature during the next 30 h. Based on the results, GN and GS lumpfish preferred 6.92 ± 1.8 and 6.2 ± 1.2, respectively, and there was no significant difference between the groups. Neither was there any significant difference in growth rates (SGR) between the two groups. Based on our results, we suggest that lumpfish from any geographical origin along the Norwegian coast can be used anywhere in Norway. It follows that lumpfish from a single selection line could be used at any salmon farm in Norway independent of its location.

No experimental evidence of stress-induced hyperthermia in zebrafish (Danio rerio)

Stress-induced hyperthermia (SIH) is characterised by a rise in body temperature in response to a stressor. In endotherms, SIH is mediated by the autonomic nervous system, whereas ectotherms must raise their body temperature via behavioural means by moving to warmer areas within their environment (behavioural thermoregulation). A recent study suggested that zebrafish (Danio rerio), an important model species, may move to warmer water in response to handling and confinement and thus exhibit SIH, which, if accepted, may have important practical and welfare implications. However, an alternative hypothesis proposed that the observed movements may be produced by avoidance behaviour rather than behavioural thermoregulation. Investigating the claims for SIH in zebrafish further, we conducted two experiments that extend the earlier study. The first experiment incorporated new conditions that considered fish behaviour in the absence of thermal variation, i.e. their null distribution, an important condition that was not performed in the original study. The second was a refined version of the experiment to reduce the numbers of fish and aid movement between areas for the fish. In contrast to the previous study, we saw no effect of handling or confinement on preference for warmer areas, and no evidence for SIH in either experiment. Instead, we observed a short-lived reduction in preference for warmer areas immediately post-stress. Our work suggests that zebrafish may not experience SIH, and claims regarding fish consciousness based on SIH may need to be revised.

Physical tidepool characteristics affect age- and size-class distributions and site fidelity in tidepool sculpin (Oligocottus maculosus)

Rapidly changing environments pose unique challenges to the resident organisms. Tidepools in coastal environments vary in biophysical characteristics spatially and temporally, and how they vary determines their short- and long-term suitability as habitats and therefore influence on the distributions of tidepool organisms. Biophysical effects on distribution could differ between age classes, depending on their intrinsic ontogenetic requirements and dominance relations. In this study, we investigate the influence of physical pool characteristics on the site fidelity and population distribution of tidepool sculpin (Oligocottus maculosus Girard, 1856). We assessed short-term recapture of marked individuals and size-class distribution among four pool sets. The proportion of adults varied between pools primarily in association with water temperature and pool volume. Smaller adult and larger juvenile fish occupied warmer, small-volume pools, whereas larger adults occupied larger, cooler pools. Between 24% and 56% of marked fish were recaptured, with a higher probability of recapture in pools with “smooth” basins than in those with more rugose basins. Few fish moved among study pools, but the proportion of adults declined with repeated sampling, suggesting greater pool fidelity of juveniles. These results illustrate that intrinsic habitat features influence age- and size-class distributions in a resident tidepool sculpin species, with corresponding consequences for site fidelity.

Altered thermoregulation as a driver of host behaviour in glochidia-parasitised fish

Parasites alter their host behaviour and vice versa as a result of mutual adaptations in the evolutionary arms race. One of these adaptations involves changes in host thermoregulation, which has the potential to harm the parasite and thereby act as a defence mechanism. We used a model of the brown trout Salmo trutta experimentally parasitised with ectoparasitic larvae called glochidia from the endangered freshwater pearl mussel Margaritifera margaritifera to reveal whether parasitation alters fish behavioural thermoregulation. A study based on radio telemetry temperature sensors was performed during almost one year M. margaritifera parasitic stage. Glochidia infested S. trutta altered its thermoregulation through active searching for habitats with different thermal regimes. General preference for lower temperature of infested fish varied, being sometimes above, sometimes below the temperature preferred by uninfested individuals. Infested fish also preferred different temperatures across localities, while uninfested fish maintained their thermal preference no matter which stream they inhabited. Glochidia further induced the expression of a behavioural syndrome among S. trutta personality traits, suggesting that it might increase the probability that the fish host would occur in the glochidia temperature optimum. Our findings present the first evidence that thermoregulation plays a fundamental role in the relationship of affiliated mussels and their fish hosts. Incorporating thermoregulation issue in the study of this relationship can help to interpret results from previous behavioural studies as well as to optimise management measures related to endangered mussels.

Getting into hot water: sick guppies frequent warmer thermal conditions

Ectotherms depend on the environmental temperature for thermoregulation and exploit thermal regimes that optimise physiological functioning. They may also frequent warmer conditions to up-regulate their immune response against parasite infection and/or impede parasite development. This adaptive response, known as 'behavioural fever', has been documented in various taxa including insects, reptiles and fish, but only in response to endoparasite infections. Here, a choice chamber experiment was used to investigate the thermal preferences of a tropical freshwater fish, the Trinidadian guppy (Poecilia reticulata), when infected with a common helminth ectoparasite Gyrodactylus turnbulli, in female-only and mixed-sex shoals. The temperature tolerance of G. turnbulli was also investigated by monitoring parasite population trajectories on guppies maintained at a continuous 18, 24 or 32 °C. Regardless of shoal composition, infected fish frequented the 32 °C choice chamber more often than when uninfected, significantly increasing their mean temperature preference. Parasites maintained continuously at 32 °C decreased to extinction within 3 days, whereas mean parasite abundance increased on hosts incubated at 18 and 24 °C. We show for the first time that gyrodactylid-infected fish have a preference for warmer waters and speculate that sick fish exploit the upper thermal tolerances of their parasites to self medicate.

Microhabitat selection by marine mesoconsumers in a thermally heterogeneous habitat: behavioral thermoregulation or avoiding predation risk?

Habitat selection decisions by consumers has the potential to shape ecosystems. Understanding the factors that influence habitat selection is therefore critical to understanding ecosystem function. This is especially true of mesoconsumers because they provide the link between upper and lower tropic levels. We examined the factors influencing microhabitat selection of marine mesoconsumers – juvenile giant shovelnose rays (Glaucostegus typus), reticulate whiprays (Himantura uarnak), and pink whiprays (H. fai) – in a coastal ecosystem with intact predator and prey populations and marked spatial and temporal thermal heterogeneity. Using a combination of belt transects and data on water temperature, tidal height, prey abundance, predator abundance and ray behavior, we found that giant shovelnose rays and reticulate whiprays were most often found resting in nearshore microhabitats, especially at low tidal heights during the warm season. Microhabitat selection did not match predictions derived from distributions of prey. Although at a course scale, ray distributions appeared to match predictions of behavioral thermoregulation theory, fine-scale examination revealed a mismatch. The selection of the shallow nearshore microhabitat at low tidal heights during periods of high predator abundance (warm season) suggests that this microhabitat may serve as a refuge, although it may come with metabolic costs due to higher temperatures. The results of this study highlight the importance of predators in the habitat selection decisions of mesoconsumers and that within thermal gradients, factors, such as predation risk, must be considered in addition to behavioral thermoregulation to explain habitat selection decisions. Furthermore, increasing water temperatures predicted by climate change may result in complex trade-offs that might have important implications for ecosystem dynamics.