Cardiovascular adaptations to changing environments such as climate change may include short term phenotypic responses, longer term effects such as developmental programming, or evolutionary adaptations to associated physiological challenges. Within this context, and given the degree of similarity among biological processes across the vertebrates, those species with unusual anatomical or physiological traits help us explore limits to function in ways not feasible with experimental interventions using model species. Fish have been at the forefront of many novel biomedical research opportunities, including discovery of cardiac stem cells in zebrafish that permit ventricular regeneration. In general, fish have a low metabolic rate and corresponding low heart rate, which in the absence of compensatory factors regulating sinus rhythm would be expected to increase the risk of arrhythmogenesis. Orthologs of mammalian K+ channels regulate action potential duration at low temperatures. The two-chambered heart powers a single-pass circulation, meaning that ventricular myocytes derive their oxygen uptake from venous return in those species lacking a coronary circulation. By comparing species that have an additional limitation, i.e. genetic mutations that have resulted in a lack of facilitated oxygen transport due to absence of myoglobin and/or haemoglobin (Chaenocephalus aceratus), with sympatric red-blooded species (Notothenia coriiceps) allows unique insight into extreme cardiovascular physiology. These icefishes live in the frigid waters around Antarctica (<0oC all year round), and the associated increase in oxygen solubility together with wholesale remodelling of the cardiovascular system (including cardiomegaly and large diameter blood vessels, which in other contexts are associated with various pathologies) and autonomic control (having a low catecholamine synthetic capacity but high vagal tone), support healthy populations across the Southern Ocean. Their large hearts display a normal ECG waveform, and generate an impressive stroke volume/cardiac output, but the lack of respiratory pigments is associated with a poor tolerance of afterload when measured using an in situ perfused heart preparation. Extant organisms are the product of selection pressure, and we need to understand mechanisms underlying both the drivers of change and resultant physiology, in order to assess how well the genetic plasticity and phenotype capacity may limit response to future environmental change. We reasoned that these animals would be more sensitive than others to current rises in seawater temperature, so our latest expedition explored their thermal sensitivity. Instrumented animals showed an impressive resilience to an acute temperature ramp, with loss of sinus rhythm only seen >13oC due to failure of atrioventricular conductance. N. coriiceps (Hb+Mb+) had a higher routine and maximal heart rate than C. aceratus (Hb-Mb-), but similar critical thermal maximum (14-16oC). Comparison of in situ function with a species having an intermediate phenotype, Chionodraco rastrospinosus (Hb-Mb+), suggest that loss of Hb conveys poor pressure generating ability, but additional loss of cardiac Mb reduces intrinsic heart rate and maximum cardiac output, which may limit resilience to near-future ocean warming.