| Summary: | Freeze avoidance is vital for the survival of Antarctic notothenioid fishes inhabiting the ice-laden waters of the high-latitude Southern Ocean. Most notothenioids avoid freezing primarily by the action of their blood-borne antifreeze glycoproteins (AFGPs). AFGPs arrest the growth of environmental ice crystals that enter the fish, thereby preserving the body fluids in liquid state. AFGPs are ubiquitous in all adult Antarctic notothenioids examined, except for an early report that one species, Lepidonotothen squamifrons (from Balleny Islands waters near the Ross Sea) lacked measurable AFGP activity or thermal hysteresis (TH). How it survives in the ice-covered Southern Ocean is unclear. Analyses of the sera of L. squamifrons from the West Antarctic Peninsula (WAP) confirmed that TH was indeed minimal or absent. The sera osmolality was 429 ± 19 mOsm/Kg equivalent to a freezing point depression (f.d.p) of 0.8°C while the hysteresis (melting point-freezing point) was only 0.017°C. The combined f.p.d of 0.817°C was well above the freezing point of seawater (-1.9°C) which puts this species at risk of freezing should it encounter ice-laden freezing seawater. To determine whether L. squamifrons might be using an alternative freeze avoidance strategy, by inhabiting only the warm ice-free waters beneath the freezing surface layer, their thermal preference was tested by placing specimens with an attached temperature logger in a horizontal thermal gradient aquarium (-1°C to +1°C), along with AFGP-fortified species used as a control. L. squamifrons exhibited the warmest preferred median temperature at 0.78°C while AFGP fortified species preferred lower water temperatures. Expendable bathythermograph (XBT) casts and historical water column profiles at L. squamifrons catch sites indicated that their habitat temperatures were well above freezing, suggesting this species preferred non-freezing water temperatures in the wild. This is the first demonstration of a behavioral freeze avoidance in an Antarctic notothenioid fish as a freeze avoidance strategy in the frigid Southern Ocean.
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