| Summary: | Fishes inhabiting ice-laden waters of McMurdo Sound, Antarctica, have ice present on all external tissues. This includes the integument, gills, and intestinal tract. With one exception, all internal tissues and fluids including the heart, liver, red muscle, white muscle, blood, bile, urine and ocular fluids are ice-free. Ice is consistently identified in the spleens from three shallow water species. This, in vivo, presence of ice is presupposed by the in vitro mechanism of antifreeze activity. The presence of internal ice explains why all nototheniids in McMurdo Sound produce high systemic concentrations of antifreezes. Furthermore, its localization to the spleen suggests that one function of this organ is to remove ice crystals from the systemic circulation. The presence of ice in these fishes correlates with the depths where seasonal ice grows in McMurdo Sound. Using high resolution temperature and conductivity measurements, we can identify the depths where ice formation occurs. The uppermost 30 m of the Sound are below the seawater freezing point for most of the austral spring and early summer. These waters are sites of ice growth. In the western Sound, waters below the in situ freezing point extend to 123 m. This unusually cold, deep water originates from beneath the Ross Ice Shelf. Water temperatures begin to rise in mid-December and by late-January they are above the equilibrium melting point (${-}1.1\sp\circ$C) of ice crystals sequestered in fish spleens. This warming trend affects the uppermost 200 m; a range that overlaps the depths where we capture the three species of fish with splenic ice. Therefore, in addition to a putative biological mechanism, there is a seasonal mechanism for the elimination of in vivo ice in shallow water fishes. Below 200 m, water temperatures are nearly isothermal and temporally stable. The probability of ice progressively decreases with depth because of the effect of hydrostatic pressure on the freezing point. It would appear that benthic fish assemblages are unlikely to ...
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