Adaptation of Antarctic Icefish Vision to Extreme Environments.

Extreme environments, such as Antarctic habitats, present major challenges for many biological processes. Antarctic icefishes (Crynotothenioidea) represent a compelling system to investigate the molecular basis of adaptation to cold temperatures. Here we explore how the sub-zero habitats of Antarctic icefishes have impacted rhodopsin (RH1) function, the temperature-sensitive dim-light visual pigment found in rod photoreceptors. Using likelihood-models and ancestral reconstruction, we find that accelerated evolutionary rates in icefish RH1 underlie unique amino acid mutations absent from other deep-dwelling fishes, introduced before (S160A) and during (V259M) the onset of modern polar conditions. Functional assays reveal that these mutations red-shift rhodopsin spectral absorbance, consistent with spectral irradiance under sea-ice. These mutations also lower the activation energy associated with retinal release of the light-activated RH1, and accelerate its return to the dark-state, likely compensating for a cold-induced decrease in kinetic rates. These are adaptations in key properties of rhodopsin that mediate rod sensitivity and visual performance in the cold dark seas of the Antarctic.