Adaptations for cryopelagic life in the antarctic notothenioid fish Pagothenia borchgrevinki

Summary The cryopelagic circumantarctic notothenioid fish Pagothenia borchgrevinki lives near the undersurface of the sea ice in McMurdo Sound, Antarctica. When compared to closely related benthic species (especially Trematomus bernacchii), Pagothenia exhibited substantial morphological differences in a variety of organ systems. The values of the fineness ratio and the indices of trunk shape and flatness suggested streamlining and drag reduction, adaptations to life in the water column. Pagothenia also lacked substrate contact adaptations in the pelvic and anal fins. Silvery reflective layers (strata argentea) beneath the skin and in the iris and choroid of the eye provided camouflage when Pagothenia were viewed against a background of platelet ice. The retina had many cones indicating the eye was adapted to both diurnal and nocturnal vision. During the austral spring Pagothenia fed exclusively on nektonic organisms near the ice-water interface. Dietary diversity was low; copepods and amphipods were the most frequently occurring and amphipods were the most frequently occurring taxa. In conclusion, Pagothenia appear specialized for life in the water column.