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Temperature has a profound impact on ectotherms. Warming increases the metabolic oxygen demand of ectotherms, which could result in a mismatch between their oxygen demand and their ability to extract and deliver sufficient oxygen to meet demand. This hypothesis has been mainly tested using short-term exposure to intense thermal stress. However, the thermal responses of organisms can be different on longer timescales, where physiological acclimation becomes increasingly important. Such thermal acclimation effects may reduce the vulnerability of ectotherms to warming on the long term. Thus, responses to intense, short-term thermal stress may be different from responses to moderate, prolonged thermal stress. Here, we examine the effect of thermal acclimation on heat tolerance and metabolism in the aquatic ectotherm Gammarus fossarum (Koch, 1836). Amphipods were acclimated to either 11.1 ± 0.1 °C or 19.8 ± 0.1 °C and after thermal acclimation we measured both their metabolism and their survival time at different temperatures. Our results show that metabolism strongly increased with increasing temperatures in the cold-acclimated group, but less so in the warm-acclimated group. Cold-acclimated amphipods were also more sensitive to thermal stress, especially during prolonged exposure. Thus, the differences between both thermal acclimation groups support the idea of oxygen-limited heat tolerance: cold-acclimated amphipods showed increased oxygen consumption and decreased thermal tolerance. However, across individuals, those that sharply increased oxygen consumption with increasing temperature did not differ in heat tolerance from individuals whose metabolism was much less sensitive to temperature. Thus, acclimation to different temperatures appeared to be beneficial, but a role for oxygen limitation could not be demonstrated unambiguously. Beneficial effect of acclimation were much larger during prolonged exposure, with the acclimation response ratio (ARR) ranging from 0.03 to over 0.5 depending on the time scale (minutes to months). Thus, the acclimatory capacity may have been underestimated by short-term experimental studies.

Thermal history can plastically alter the response of ectotherms to temperature, and thermal performance curves (TPCs) are powerful tools for exploring how organismal-level performance varies with temperature. Plasticity in TPCs may be favoured in thermally variable habitats, where it can result in fitness benefits. However, thermal physiology remains insufficiently studied for freshwater insects despite freshwater biodiversity being at great risk under global change. Here, we assess how acclimation at either summer or winter average temperatures changes TPCs for locomotion activity and metabolism in Enochrus jesusarribasi (Hydrophilidae), a water beetle endemic to shallow saline streams in SE Spain. This beetle is a bimodal gas exchanger and so we also assessed how aerial and aquatic gas exchange varied across temperatures for both acclimation treatments. Responses of locomotory TPCs to thermal acclimation were relatively weak, but high temperature acclimated beetles tended to exhibit higher maximum locomotor activity and reduced TPC breadth than those acclimated at lower temperature. High temperature acclimation increased the thermal sensitivity of metabolic rates, contrary to the response generally found in aquatic organisms. Higher metabolic rates upon high temperature acclimation were achieved by increasing aerial, rather than aquatic oxygen uptake. Such plastic respiratory behaviour likely contributed to enhanced locomotor performance at temperatures around the optimum and thermal plasticity could thus be an important component in the response of aquatic insects to climate change. However, high temperature acclimation appeared to be detrimental for locomotion in subsequent exposure at upper sublethal temperatures, suggesting that this narrow range endemic may be vulnerable to future climate warming. This study demonstrates that TPCs are context-specific, differing with performance metric as well as thermal history. Such context dependency must be considered when using TPCs to predict organismal responses to climate change.

Temperature affects the physiology and life-history of ectothermic animals, often increasing metabolic rate and decreasing body size. Oxygen limitation has been put forward as a mechanism to explain thermal responses of body size and the ability to survive stress. However the time-scales involved in growth performance and heat tolerance differ radically. In order to increase our understanding of oxygen and temperature effects on body size and heat tolerance and the time scale involved, we reared Lymnaea stagnalis under six combinations of temperature and oxygen tension from hatching up to an age of 300 days and recorded shell length during this whole period. At the end of this period, we determined scope for growth by measuring food intake rate, assimilation efficiency, respiration rate and ammonium excretion rate at two different temperatures. We also measured the snails’ ability to survive heat stress (CTmax), both at normoxia and hypoxia. We found that scope for growth and long term growth performance were much more affected by interactions of chronic oxygen and temperature conditions during rearing than by acute conditions during testing. Furthermore, our study shows that individual variation in growth performance can be traced back to individual differences in rates of food and oxygen consumption. Developmental acclimation also gave rise to differences in CTmax, but these were relatively small and were only expressed when CTmax was tested under hypoxia. The large effects of rearing oxygen conditions on growth and other physiological rates compared to modest effects of test oxygen conditions on CTmax suggest that small effects of hypoxia on the short term (e.g. heat tolerance) may nevertheless have large repercussions on the long term (e.g. growth and reproduction), even in a pulmonate snail that can compensate for hypoxia to some extent by aerial respiration.

Temperature is an important environmental factor that influences key traits like body size, growth rate and maturity. Ectotherms reared under high temperatures usually show faster growth, but reach a smaller final size, a phenomenon known as the temperature-size rule (TSR). Oxygen may become a limiting resource at high temperatures, when demand for oxygen is high, especially in water as oxygen uptake is far more challenging under water than in air. Therefore, in aquatic ectotherms, the TSR might very well be mediated by temperature effects on oxygen availability and oxygen demand. To distinguish between the direct effects of temperature and oxygen mediated effects, growth rate and final size were measured in the aquatic ectotherm Asellus aquaticus (Linnaeus, 1758) reared under different temperature and oxygen conditions in a factorial design. Growth could be best described by a modified Von Bertalanffy growth function. Both temperature and oxygen affected age at maturity and growth. Growth responses to temperature were dependent on oxygen conditions (interactive effect of temperature and oxygen). Only under hypoxic conditions, when oxygen was most limiting, did we find a classic TSR. Moreover, when comparing treatments differing in temperature, but where the balance between oxygen demand and supply was similar, high temperature increased both growth rate and final size. Thus effects of oxygen may resolve the life-history puzzle of the TSR in aquatic ectotherms.