Icefish

The Ocellated Icefish (Chionodraco rastrospinosus) is truly unique, boasting an arsenal of fascinating physiological and biochemical adaptations that allow it to survive in the freezing, ice-laden waters of the Southern Ocean that encircle Antarctica.

Sixteen species of Antarctic icefishes comprise the family Channichthyidae, which falls under the larger suborder Notothenioidei. Together, the Notothenioids and icefishes dominate the frigid waters they call home, making up approximately 90 percent of the fish biomass in the Southern Ocean and thus making them the dominant fish taxa in the cold continental shelf waters surrounding Antarctica

The salt dissolved in these waters lowers the freezing point and allows temperatures to regularly drop to –2°C without solidifying into ice. Yet not only do icefishes thrive in some of the coldest aquatic conditions on earth, but it is completely unique amongst vertebrates in that it has no red blood cells and almost no haemoglobin, meaning that its blood is entirely clear.

Haemoglobin is the iron-rich protein in blood that’s responsible for binding to oxygen and helping to ferry it around the body where it’s needed, and it also incidentally makes blood appear red. But whilst the blood of almost all vertebrates the world over has a haemoglobin concentration of about 45%, the Icefish manages to survive and thrive with a concentration of only 1%.

Scientists are as yet uncertain how it achieves this as the fish are extremely difficult to observe in the wild, normally living at depths of over 3,200 feet (1 kilometre). However, it’s thought that the hardy icefish is apparently able to pull off this nifty trick in part because the cold polar waters of the Antarctic Ocean are richer in oxygen content than warmer waters, and partly because it has no scales to get in the way, allowing it to absorb oxygen directly through its ultra-thin skin and into its blood plasma where it is circulated around the body by the fish’s abnormally large heart.

Indeed the icefish has to expend about twice the amount of cardiac energy per second than other fish with normal amounts of haemoglobin in their blood, while the blood itself is extremely thin and the blood vessels are wider than normal. Similarly, it boasts an “antifreeze” glycoprotein in its body tissue and fluids that prevent ice crystals from forming in its blood and making the hardy survivor’s name all too literal.

Only a single mating pair exists in captivity at the Tokyo sea life park in Japan where scientists are hoping that the male and female will help them unlock the secrets of their unique physiology. However the evolutionary mechanism that led to this remarkable biological quirk is still a mystery that may take scientists years to solve.