Exploring nature's natural knockouts: cardiorespiratory performance of Antarctic fishes during acute warming.

We tested the hypothesis that blackfin icefish (), one of the six species in the family Channichthyidae (the icefishes) that do not express haemoglobin and myoglobin, lack regulatory cardiovascular flexibility during acute warming and activity. The experimental protocols were designed to optimize the surgical protocol and minimize stress. First, minimally invasive heart rate () measurements were made during a thermal ramp until cardiac failure in and compared with those from the closely related red-blooded black rockcod (). Then, integrative cardiovascular adjustments were more extensively studied using flow probes and intravascular catheters in during acute warming (from 0 to 8°C) at rest and after imposed activity. had a lower routine than (9 beats min versus 14 beats min) and a lower peak during acute warming (38 beats min versus 55 beats min) with a similar cardiac breakpoint temperature (13 and 14°C, respectively). Routine cardiac output () for at ∼0°C was much lower (26.6 ml min kg) than previously reported, probably because fish in the present study had a low (12 beats min) indicative of a high routine vagal tone and low stress. increased oxygen consumption during acute warming and with activity. Correspondingly, increased considerably (maximally 86.3 ml min kg), as did vascular conductance (5-fold). Thus, unlike earlier suggestions, these data provide convincing evidence that icefish can mount a well-developed cardiovascular regulation of heart rate, cardiac output and vascular conductance, and this regulatory capacity provides flexibility during acute warming.