Dr. Gregory Sloop is a Montana physician who knows a thing or two about the cardiovascular system. He has a new article in the journal BIO-Complexity highlighting the sleek design of the Antarctic icefish that allows it to live in super-cold waters without freezing to death.

Icefish, aka the family Channichthydiae, survive at 0^oC in the Southern Ocean by maintaining blood viscosity at the set point of 3.27 centipoise — a level nearly identical to human blood. They do this without hemoglobin, which is the primary determinant of human and other red-blooded animals’ blood viscosity.

Since icefish don’t have hemoglobin, how do they maintain blood viscosity? Also, how do they breathe? Turns out they maintain viscosity at freezing temperatures primarily by using a special type of glycoprotein: the antifreeze protein. And because oxygen has a higher solubility at lower temperatures, icefish don’t need an oxygen transport molecule like hemoglobin. 

Why Jack Froze

In case you were wondering, blood viscosity is the technical reason why Jack (Rose’s buddy aboard the Titanic) froze in less than 23 minutes, but icefish can survive for 15 years in water of a freezing temperature. Viscosity increases at lower temperatures, and at 0^oC human blood reaches a viscosity that is not compatible with life. This is because hemoglobin as a protein is not able to keep viscosity low enough at freezing temperatures. But the antifreeze protein can (think: custom design).

While a viscosity too high is incompatible with life, a low viscosity is also unsuitable for sustaining life. This is because a properly functioning cardiovascular system must have optimized laminar flow and low vascular resistance, which can be achieved only through coordinated control of blood viscosity and specification of vascular geometry.

Stop Criticizing Icefish

Dr. Sloop says icefish have been criticized for expending nearly 22 percent of their basal metabolic rate pumping their hemoglobinless blood compared to at most 5 percent in temperate fish. But he reminds his readers that’s just the cost of doing business in the chilly — well technically, freezing — waters of the Southern Ocean.

Sloop also emphasizes that everything about the icefish is like a custom-fitted suit — appropriate for niche needs. Features included for dealing with the extreme cold are a high-output, low resistance vasculature where the diameter of muscle capillaries is 2-3 times larger than those of other fish.

These fish also have a heavy heart which delivers a larger stroke and therefore higher volume. Together these features enable a high-output, high-velocity, low-pressure, and low-resistance circulation.

Truly, every part of these incredible creatures is optimized for cold. Could all these custom changes be the result of random mutation? Dr. Sloop thinks that is very unlikely. What do you say?