| Summary: | Extrinsic neural and humoral influences on heart rate ('f'H) and ventilation frequency ('f'V) were examined following varying periods of post-surgical recovery in eight related Antarctic fish species inhabiting an array of inshore niches. Resting 'f'H after recovery from handling was lower than previous reports, and the novel measurement of routine 'f'H in free-swimming 'Dissostichus mawsoni' (6·14 beats min⁻¹, bpm) is the lowest recorded for any fish. The extent of cardio-depressive cholinergic (vagal) tonus explained the large range of 'f'H among species and varied with behavioural repertoire, being lower in the more active species, apart from 'Notothenia coriiceps'. Adrenergic tonus was low compared with cholinergic tonus, with the exception of 'Trematomus newnesi'. Hence, high cardiac cholinergic tonus may be a genotypic trait of the notothenioids that diverged with ecotype. Power spectral analysis showed that the vagal influence produced comparable spectra among species of similar morphology and ecotype. Removal of autonomic tonus resulted in a remarkably similar intrinsic 'f'H between species. Simultaneous measurements of cardio-respiratory variables and oxygen consumption (inline image) were made in the benthic 'Trematomus bernacchii' and cryopelagic 'Pagothenia borchgrevinki'. The slopes of the relationship between 'f'H and inline image were similar. 'Trematomus bernacchii', however, had a higher inline image for a given 'f'H than 'P. borchgrevinki', and 'P. borchgrevinki' required a two-fold larger range in fH to reach a similar maximum inline image, suggesting that there is a difference in cardiovascular fitness between the two species. Overall, the data suggest that cardio-respiratory control in Antarctic nototheniids is largely determined by activity levels associated with a given ecotype.
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