By Amanda Biederman
Chief Science Correspondent
Millions of years ago, long before Norwegian explorer Roald Amundsen became the first person to reach the South Pole, a group of fishes achieved a remarkable feat: they surrendered their red blood cells.
This family of fishes, known officially as Channichthyidae but more casually as the icefishes, are the only known vertebrates on Earth that lack hemoglobin, the oxygen-binding molecule responsible for the red color of blood. Hemoglobin carries oxygen to an organism's tissues, allowing individual cells to generate ATP and harness energy necessary for the organism's survival.
There are 16 known species of icefishes. My research team is currently collecting and studying three species of icefishes: C. aceratus (the blackfin icefish), P. georgianus (the South Georgia icefish), and C. rastrospinosus (the ocellated icefish). We are also studying N. coriiceps (the black rock cod), a red-blooded species (i.e. "normal") from the same suborder (Notothenioidei).
Scientists believe that the icefishes have been able to withstand this mutation due to their extremely cold, stable environment (generally below 0 degrees Celsius). Cold water dissolves oxygen more readily than warm water, so these fishes are constantly in an oxygen-saturated environment. Additionally, the cold waters lower these fishes' metabolisms, resulting in a low oxygen demand.
The icefish have evolved a suite of adaptations to compensate for the loss of hemoglobin. These fishes have a large, slow-beating heart and large vessels to transport a large volume of blood throughout the body under a low-pressure system.
Icefish are very fragile. Unlike other fish, they lack scales, and they are transparent under the right light conditions. They are largely colorless; their gills, intestines and muscle tissue are white. C. aceratus has lost the expression of a similarly structured molecule, myoglobin, which offloads oxygen from the blood. As a result, C. aceratus have hearts that are very light in color.
A fish's thermal tolerance can be limited by a number of factors. Our research team is looking at these factors from a systems (cardiovascular and neurological) to a sub-cellular (energy production, membrane integrity and molecular (hyopoxia-inducible factor expression) level. Essentially, we are analyzing the points at which these organisms fail at high temperatures, and what factors have contributed to these failures.
So why should you care about the fate of the icefish? Until very recently, we knew very little of Antarctic fish biology. Much still remains unknown of these species and their unique adaptations. However, the loss of hemoglobin provides a particularly novel opportunity to study how the presence and function of hemoglobin shapes an organism's normal function. As Kristin O'Brien (the PI on our project) explains here, there are many advantages to using natural knockouts over artificial knockouts. Some research on icefish biology has already been applied to cases of anemia in humans, as well as delayed bone development patterns.
I'm comparing differences in membrane properties between red-blooded and white-blooded fishes. In a future post, I will describe the process of membrane isolation, as well as some of the background behind membrane biology in general.