[2] Icefish populations are known to reside in the Atlantic and Indian sectors of the Southern Ocean, as well as the continental shelf waters surrounding Antarctica.

[5] 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.

[10] Icefish are typically ambush predators; thus, they can survive long periods between feeding, and often consume fish up to 50% of their own body length.

[2] Their hearts lack coronary arteries, and the ventricle muscles are very spongy, which enables them to absorb oxygen directly from the blood they pump.

Additionally, icefish have larger cardiac mitochondria and increased mitochondrial biogenesis in comparison to red-blooded notothenioids.

[3] The cold, well-mixed, oxygen-rich waters of the Southern Ocean provided an environment where a fish with a low metabolic rate could survive even without hemoglobin, albeit less efficiently.

This current formed 25–22 million years ago, and thermally isolated the Southern Ocean by separating it from the warm subtropical gyres to the north.

During the mid-Tertiary period, a species crash in the Southern Ocean opened up wide range of empty niches to colonize.

Their unusual cardiovascular physiology, including large heart, high blood volume, increased mitochondrial density, and extensive microvasculature, suggests that icefish have had to evolve ways of coping with the impairment of their oxygen binding and transport systems.

However, when they repeated the test with each organism at a temperature that accurately reflected its native environment, the myoglobin performance was roughly equivalent between icefish and mammals.

To begin with, the Southern Ocean has been characterized by extremely cold but stable temperatures for the past 10–14 million years.

This minimization could have helped the icefish to survive 8.5 million years ago when Arctic diversity plummeted dramatically.

While emphasis is often placed and understandably so on the importance of hemoglobin and myoglobin in oxygen delivery and use, recent studies have found that both proteins are actually also involved in the process of breaking down nitric oxide.

[2] Nitric oxide plays a role in regulating various cardiovascular processes in icefish, such as the dilation of branchial vasculature, cardiac stroke volume, and power output.

[30] The presence of nitric oxide also can increase angiogenesis, mitochondrial biogenesis, and cause muscle hypertrophy; all of these traits are characteristics of icefish.

[2] In addition, the heightened levels of nitric oxide that followed as an inevitable consequence of the loss of hemoglobin and myoglobin may have actually provided an automatic compensation, allowing for the fish to make up for the hit to their oxygen transport system and thereby providing a grace period of the fixation of these less than desirable traits.