Abstract
Tropical inland fishes are predicted to be especially vulnerable to thermal stress because they experience small temperature fluctuations that may select for narrow thermal windows. In this study, we measured resting metabolic rate (RMR), critical oxygen tension (P crit) and critical thermal maximum (CTMax) of the widespread African cichlid (Pseudocrenilabrus multicolor victoriae) in response to short-term acclimation to temperatures within and above their natural thermal range. Pseudocrenilabrus multicolor collected in Lake Kayanja, Uganda, a population living near the upper thermal range of the species, were acclimated to 23, 26, 29 and 32°C for 3 days directly after capture, and RMR and P crit were then quantified. In a second group of P. multicolor from the same population, CTMax and the thermal onset of agitation were determined for fish acclimated to 26, 29 and 32°C for 7 days. Both RMR and P crit were significantly higher in fish acclimated to 32°C, indicating decreased tolerance to hypoxia and increased metabolic requirements at temperatures only slightly (∼1°C) above their natural thermal range. The CTMax increased with acclimation temperature, indicating some degree of thermal compensation induced by short-term exposure to higher temperatures. However, agitation temperature (likely to represent an avoidance response to increased temperature during CTMax trials) showed no increase with acclimation temperature. Overall, the results of this study demonstrate that P. multicolor is able to maintain its RMR and P crit across the range of temperatures characteristic of its natural habitat, but incurs a higher cost of resting metabolism and reduced hypoxia tolerance at temperatures slightly above its present range.
Highlights
With the rise in global mean temperatures (Root et al, 2003; IPCC, 2013) species’ persistence may depend on their capacity to shift their distributions to more favourable environments, acclimate via phenotypic plasticity or adapt to changes within their current environment
The mean mass-adjusted rates with longer acclimation periods (RMR) of fish acclimated to 32°C was significantly higher than the RMR measured in all lower temperature treatments
While RMR is expected to exhibit a predictable increase in response to increasing temperature and did appear to rise over the tested range (Fig. 3), the Q10 value between the two highest treatments (5.41) in P. multicolor was dramatically higher than general predicted values for teleost fish (∼2.0–3.0, with tropical species falling onto the higher end of that range; McNab, 2002; Seifert and Chapman, 2006) and notably higher than those calculated between the lower temperature treatments
Summary
With the rise in global mean temperatures (Root et al, 2003; IPCC, 2013) species’ persistence may depend on their capacity to shift their distributions to more favourable environments, acclimate via phenotypic plasticity or adapt to changes within their current environment. Conservation Physiology Volume 3 2015 influenced by environmental temperature, and its ability to carry out vital functions over a range of temperatures is defined as its performance window (Huey and Stevenson, 1979) This window results from temperature-dependent trade-offs at all levels of functioning (Pörtner, 2010); beyond the upper and lower critical limits of their thermal windows (CTMax and CTMin, respectively), ectotherms will start to exploit their passive range of tolerance but can do so only for a limited time. While increasing temperature can have some positive effects on growth, this only occurs within an optimal range, beyond which growth and other important processes are compromised due to protein denaturation and enzyme inhibition, gradually reducing long-term fitness (Pörtner, 2001, 2010; Pörtner and Knust, 2007)
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