Abstract
Cryonotothenioids constitute a subgroup of notothenioid fishes endemic to the Southern Ocean that are specialized to exist in a narrow range of near-freezing temperatures. Due to the challenges of reliably collecting and maintaining larval cryonotothenioids in good condition, most thermal tolerance studies have been limited to adult and juvenile stages. With increasing environmental pressures from climate change in Antarctic ecosystems, it is important to better understand the impacts of a warming environment on larval stages as well. In this study, we determine the critical thermal maxima (CTmax) of cryonotothenioid larvae collected in pelagic net tows during three research cruises near the western Antarctic Peninsula. We sampled larvae of seven species representing three cryonotothenioid families—Nototheniidae, Channichthyidae, and Artedidraconidae. For channichthyid and nototheniid species, CTmax values ranged from 8.6 to 14.9 °C and were positively correlated with body length, suggesting that younger, less motile larvae may be especially susceptible to rapid warming events such as marine heatwaves. To our knowledge, this is the first published test of acute thermal tolerance for any artedidraconid, with CTmax ranging from 13.2 to 17.8 °C, which did not correlate with body length. Of the two artedidraconid species we collected, Neodraco skottsbergi showed remarkable tolerance to warming and was the only species to resume normal swimming following trials. We offer two hypotheses as to why N. skottsbergi has such an elevated thermal tolerance: (1) their unique green coloration serves as camouflage within near-surface phytoplankton blooms, suggesting they occupy an especially warm near-surface niche, and (2) recent insights into their evolutionary history suggest that they are derived from taxa that may have occupied warm tide-pool habitats. Collectively, these results establish N. skottsbergi and larval channichthyids as groups of interest for future physiological studies to gain further insights into the vulnerability of cryonotothenioids to a warming ocean.