> They are the only known vertebrates to lack hemoglobin in their blood as adults. https://en.wikipedia.org/wiki/Channichthyidae
> In February 2021, scientists discovered and documented a breeding colony of Neopagetopsis ionah icefish estimated to have 60 million active nests across an area of approximately 92 square miles at the bottom of the Weddell Sea in Antarctica. The majority of nests were occupied by one adult fish guarding an average of 1,735 eggs in each nest. https://www.sciencenews.org/article/ice-fish-nest-antarctica...
>To compensate for the absence of hemoglobin, icefish have adapted:
>Larger blood vessels (including capillaries) and low-viscosity (RBC-free) blood to enable very high flow rates at low pressures.
>Greater blood volumes (four times those of other fish)
>Larger hearts with greater cardiac outputs (five times greater) compared to other fish.
>Hearts lacking coronary arteries, and the ventricle muscles are very spongy, which enables them to absorb oxygen directly from the blood they pump.
>Larger cardiac mitochondria and increased mitochondrial biogenesis to facilitate enhanced oxygen delivery by increasing mitochondrial surface area, and reducing distance between the extracellular area and the mitochondria in comparison to red-blooded notothenioids.