Final answer:
The icefish has evolved to lack red blood cells and hemoglobin, relying on dissolved oxygen in its plasma for respiration, which is a contrast to most vertebrates that use hemoglobin within red blood cells to transport oxygen.
Step-by-step explanation:
The icefish, a unique creature lacking red blood cells and hemoglobin, represents a fascinating evolutionary adaptation to the cold Antarctic waters it inhabits. In contrast to other fish and vertebrates which typically use hemoglobin in red blood cells to transport oxygen, the icefish relies on a different method. Hemoglobin is a complex protein comprising four subunits and a heme group which contains iron that can reversibly bind oxygen. However, icefish have lost the expression of functional hemoglobin genes, instead, their blood circulates freely dissolved oxygen in their plasma.
While most vertebrates, including other fish, use hemoglobin-contained within red blood cells, icefish blood lacks these cells entirely. This unique adaptation is thought to be a response to the high levels of dissolved oxygen in the cold Antarctic waters, in which these fish reside, allowing for efficient gas exchange without the need for hemoglobin. Other organisms may use different molecules like hemocyanin or hemerythrin, which contain copper or iron respectively, to transport oxygen. It's important to note that although both hemocyanin and hemerythrin have iron in their structure, they do not contain a heme group and are less efficient in oxygen-carrying compared to hemoglobin. The variation in oxygen transport across different species showcases the wide array of evolutionary solutions to the challenges of respiration.