Evolutionary legacy and thermal sensitivity: a comparative study of aerobic scope in two sub-Antarctic notothenioids.

Metabolic physiology provides insights into fish's capacity to tolerate thermal stress under climate change scenarios. Our work analyzed the effects of increasing temperature on the respirometric performances of the black southern cod Patagonotothen tessellata and the Magellan spiny plunderfish Harpagifer bispinis, two sympatric notothenioids with different life habits and evolutionary origins, from the Beagle Channel, their southernmost distribution limit. We hypothesized that, because temperature influences O2 uptake, the standard (SMR) and maximum (MMR) metabolic rates would increase with increasing environmental temperature in both species. Interspecific differences in aerobic scope (AS) would be expected, based on their evolutionary histories, distributions and habitat use. Fish were exposed for three weeks to four acclimation temperatures (4, 7, 10, and 12 °C). The AS of fish was calculated as the difference between MMR and SMR, determined by intermittent-flow respirometry in static chambers. Our results revealed significant interspecific differences in SMR at 7 °C and in MMR at 10 °C, whereas AS did not differ significantly between species. Factorial analyses confirmed that P. tessellata exhibited higher SMR, MMR, and AS values than H. bispinis, consistent with its habitat use. Maximal AS and, hence, its optimal temperature differed between species, with P. tessellata reaching higher values at 10 °C compared to H. bispinis at 7 °C, suggesting species-specific thermal optima. At the highest temperatures, AS in P. tessellata was more than twice that of H. bispinis, pointing to a greater capacity for aerobic activity. Our findings suggest a greater resilience of P. tessellata to warming, whereas H. bispinis may be more vulnerable.