The Journal of Experimental Biology 2348 © 2014. Published by The Company of Biologists Ltd | The Journal of Experimental Biology (2014) 217, 2348-2357 doi:10.1242/jeb.098798 ABSTRACT For many aquatic species, the upper thermal limit (T max ) and the heart failure temperature (T HF ) are only a few degrees away from the species’ current environmental temperatures. While the mechanisms mediating temperature-induced heart failure (HF) remain unresolved, energy flow and/or oxygen supply disruptions to cardiac mitochondria may be impacted by heat stress. Recent work using a New Zealand wrasse (Notolabrus celidotus) found that ATP synthesis capacity of cardiac mitochondria collapses prior to T HF . However, whether this effect is limited to one species from one thermal habitat remains unknown. The present study confirmed that cardiac mitochondrial dysfunction contributes to heat stress-induced HF in two additional wrasses that occupy cold temperate (Notolabrus fucicola) and tropical (Thalassoma lunare) habitats. With exposure to heat stress, T. lunare had the least scope to maintain heart function with increasing temperature. Heat-exposed fish of all species showed elevated plasma succinate, and the heart mitochondria from the cold temperate N. fucicola showed decreased phosphorylation efficiencies (depressed respiratory control ratio, RCR), cytochrome c oxidase (CCO) flux and electron transport system (ETS) flux. In situ assays conducted across a range of temperatures using naive tissues showed depressed complex II (CII) and CCO capacity, limited ETS reserve capacities and lowered efficiencies of pyruvate uptake in T. lunare and N. celidotus. Notably, alterations of mitochondrial function were detectable at saturating oxygen levels, indicating that cardiac mitochondrial insufficiency can occur prior to HF without oxygen limitation. Our data support the view that species distribution may be related to the thermal limits of mitochondrial stability and function, which will be important as oceans continue to warm. KEY WORDS: Fish, Thermal limits, Cardiac mitochondria, Heart failure, Oxidative phosphorylation INTRODUCTION Critical environmental stressors can affect the dynamics and distribution of fish populations (Booth et al., 2011; Sunday et al., 2012). Currently, ocean temperatures are influenced by climate change and drive changes in the distribution of fish populations and species (Sunday et al., 2011). As ocean temperatures rise, the thermal limitation of fishes is indicated by a decreased capacity in aerobic performance (Pörtner and Knust, 2007). An understanding of the thermal tolerance windows for fishes has been called for so that the effects of ocean warming on temperature-related geographic RESEARCH ARTICLE 1 Applied Surgery and Metabolism Group, School of Biological Sciences, University of Auckland, Auckland 1142, New Zealand. 2 International Centre for Sturgeon Studies, Vancouver Island University, Nanaimo, BC, Canada, V9R 5S5. 3 School of Physiotherapy and Exercise Science, Griffith University, Gold Coast, QLD 9726, Australia. *Author for correspondence (a.hickey@auckland.ac.nz) Received 20 October 2013; Accepted 26 March 2014 distributions can be predicted (Pörtner, 2002). A comparative approach provides a powerful means to test the physiological mechanisms responsible for differences in thermal tolerances among related taxa with different thermal niches (Somero, 2010; Somero, 2011). The thermal tolerances of fishes and other ectotherms appear to be reflected by the temperatures at which their hearts fail (T HF ), and therefore the heart is contended to be the most temperature-sensitive organ in fish (Pörtner and Farrell, 2008; Pörtner and Knust, 2007). The scope of thermal tolerance has also been proposed to result from limitations imposed by mitochondrial function and density (Pörtner, 2002). Alterations in mitochondrial function with temperature can contribute to trade-offs in energy budgets that in turn affect fish growth and fertility, and can ultimately influence population dynamics (Pörtner, 2002). However, some studies have indicated that the temperature at which liver and skeletal muscle mitochondrial respiration fail (T mt ) occurs above critical habitat temperatures (Pörtner et al., 2000; Pörtner, 2002; Somero et al., 1998; Weinstein and Somero, 1998). These studies, however, focused on maximal respiration capacities in terms of flux, with single electron inputs into respiratory chains, and did not use heart muscle or explore respirational efficiencies. Recent work has demonstrated that T mt can occur below the T HF in heart muscle exposed to increasing environmental temperatures (Iftikar and Hickey, 2013). This work on the wrasse Notolabrus celidotus showed that, while increasing temperature elevated oxygen flux through respiring mitochondria, a greater fraction of the flux at high temperatures was to meet elevated inner mitochondrial membrane proton leak (proton leak represents non-phosphorylating respiration). This study was the first to directly measure ATP production simultaneously with respiration, and this declined at temperatures well below T HF , indicating a loss of oxidative phosphorylation (OXP) system efficiency prior to T HF (Iftikar and Hickey, 2013). The study also showed that mitochondrial coupling – traditionally measured as the respiratory control ratio (RCR), provides a reasonable measure of the coupling between the electron transport system (ETS) and OXP system. Mitochondria from fish acclimated to 17.5°C showed significant depressions of ATP synthesis at 25°C. Additionally, the non-OXP respiration fluxes with respiratory complex I substrates (Leak-I) had increased by 60% relative to Leak-I at 17.5°C, while the RCRs decreased to less than 4, such that Leak-I accounted for 25% of OXP (Iftikar and Hickey, 2013). Therefore, cardiac mitochondria had lost considerable efficiency prior to T HF . The aim of the present study was: (i) to determine whether heart T mt occurs below T HF in other wrasse species from habitats with different temperatures; and (ii) to explore differences in thermal responses among species. The family Labridae is a large group of wrasse species occupying reef habitats in both temperate and tropical waters (Cowman et al., 2009). Two species were investigated: the cold temperate Notolabrus fucicola (banded/purple Could thermal sensitivity of mitochondria determine species distribution in a changing climate? Fathima I. Iftikar 1 , Julia R. MacDonald 1 , Daniel W. Baker 2 , Gillian M. C. Renshaw 3 and Anthony J. R. Hickey 1, *