The vast majority of fish possess red blood, a color shared with nearly all other vertebrates, including humans. This circulating fluid delivers necessary substances like oxygen and nutrients to tissues while removing waste products. The distinct crimson shade of fish blood is due to a specific respiratory protein responsible for binding and transporting oxygen from the gills throughout the fish’s body.
The Primary Color of Fish Blood
The red color in most fish blood comes from the protein hemoglobin, which is packaged inside red blood cells. Hemoglobin contains four iron-containing structures called heme groups, and it is the iron within these groups that gives the blood its characteristic hue. When oxygen binds to the iron atom in the heme group, it creates a bright red color.
This mechanism is virtually identical across almost all vertebrates, reflecting a shared evolutionary heritage. The concentration of red blood cells determines the intensity of the red color, but the fundamental chemical reason remains the same: iron binding with oxygen. Hemoglobin significantly increases the blood’s capacity to carry oxygen compared to dissolved oxygen in the plasma alone.
The Unique Case of Colorless Blood
The Antarctic Icefish, belonging to the family Channichthyidae, is a unique exception. These fish are the only known vertebrates that completely lack hemoglobin and mature red blood cells, resulting in colorless, or translucent, blood. The absence of the red oxygen-carrying pigment is a consequence of the deletion of the beta-globin subunit of the hemoglobin gene in their evolutionary history.
Icefish survive due to their specialized environment, the frigid Southern Ocean. The cold water near Antarctica holds a significantly higher amount of dissolved oxygen than warmer waters. This high environmental oxygen concentration allows the fish to absorb sufficient oxygen directly into their blood plasma through their skin and enlarged gills. To compensate for the low oxygen-carrying capacity of their plasma, icefish have evolved a much larger heart and wider blood vessels, which circulate a greater volume of blood faster throughout their bodies.
Specialized Oxygen Management in Fish
The red blood of most fish functions with specialized physiological adaptations. One such adaptation is the heightened sensitivity of their hemoglobin to changes in blood acidity, known as the Root effect. This effect is an extreme form of the Bohr effect, where an increase in carbon dioxide or protons (a decrease in pH) not only lowers hemoglobin’s affinity for oxygen but also significantly reduces its total oxygen-carrying capacity.
This mechanism is employed to perform specialized tasks, such as inflating the swim bladder, an organ that provides neutral buoyancy. In a dedicated network of capillaries near the swim bladder, fish tissues release lactic acid, creating an acidic environment. This localized drop in pH triggers the Root effect, causing the hemoglobin to “dump” a large volume of oxygen into the blood plasma. This generates the high gas pressures needed to fill the swim bladder.
The Root effect enhances oxygen delivery to specific tissues like the retina of the eye and the swim bladder. This specialized process allows teleost fishes, the largest group of vertebrates, to thrive in a wide range of aquatic conditions, utilizing their red blood efficiently.