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

We tested the hypothesis that Blackfin icefish (Chaenocephalus aceratus), 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 (f_H) measurements were made during a thermal ramp until cardiac failure in C. aceratus and compared with the closely related red-blooded Black rockcod (Notothenia coriiceps). Then, integrative cardiovascular adjustments were more extensively studied using flow probes and intravascular catheters in C. aceratus during acute warming (from 0 to 8°C) at rest and after imposed activity. C. aceratus had a lower routine f_H than N. coriiceps (9 min⁻¹ vs. 14 min⁻¹) and a lower peak f_H during acute warming (38 min⁻¹ vs. 55 min⁻¹) with a similar cardiac breakpoint temperature (13 and 14°C, respectively). Routine cardiac output (Q̇) for C. aceratus at ∼0°C was much lower (26.6 ml min⁻¹ kg⁻¹) than previously reported, likely because fish in the present study had a low f_H (12 min⁻¹) indicative of a high routine vagal tone and low stress. C. aceratus increased oxygen consumption during acute warming and with activity. Correspondingly, Q̇ increased considerably (maximally 86.3 ml min⁻¹ kg⁻¹), as did vascular conductance (five-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.