The answer to what color shark blood is simple: it is red, just like the blood of mammals, birds, and most other vertebrates. This familiar hue stems from the presence of a specific protein within the shark’s circulatory system that is responsible for transporting oxygen throughout the body.
The Chemistry That Makes Blood Red
The red coloration of shark blood is caused by the respiratory pigment known as hemoglobin, a complex protein contained within the red blood cells. Hemoglobin binds to oxygen molecules in the gills and releases them into the body’s tissues where they are needed for cellular respiration. The structure of this protein includes four subunits, each containing a heme group.
At the center of each heme group lies a single iron atom, which serves as the site for oxygen attachment. When the iron atom binds to oxygen, it undergoes oxidation, changing how the molecule absorbs and reflects light. The oxygenated pigment strongly reflects red wavelengths, giving the blood its characteristic bright color. If the blood loses its oxygen, the chemical structure shifts, causing the color to deepen to a darker, more maroon shade.
Biological Specifics of Shark Blood
Although the color is the same as in many other animals, the chemical composition of shark blood is adapted for its marine environment. Sharks and other cartilaginous fish employ a physiological strategy called osmoregulation to maintain their water balance in saltwater. This involves retaining high concentrations of organic compounds in their bloodstream.
The primary compounds responsible for this adaptation are urea, a nitrogenous waste product, and trimethylamine oxide (TMAO). By keeping these substances in the blood, the shark’s internal fluid concentration becomes nearly isotonic, or equal in salinity, to the surrounding seawater. The high urea concentration, which can reach up to 2.5% of the blood plasma, would be toxic to the enzymes of most other vertebrates, but TMAO acts as a counteracting stabilizer.
Furthermore, the red blood cells of sharks are nucleated, retaining their cell nucleus and genetic material throughout their lifespan. Specialized organs, such as the spleen and Leydig’s organ, are responsible for the production of these red blood cells, rather than the bone marrow found in bony vertebrates.
Contrasting Red Blood with Other Colors in Nature
The reliance on iron-based hemoglobin for oxygen transport is widespread, but red blood is not universal across the animal kingdom. The color of an animal’s blood is determined by the metal ion at the center of its respiratory pigment.
A notable contrast is seen in invertebrates such as octopuses, squids, and horseshoe crabs, which possess blue blood. Their oxygen-carrying protein is hemocyanin, which uses copper atoms instead of iron to bind to oxygen. When copper is oxidized, it reflects blue light, giving the blood a distinct azure tint.
Some species of marine worms, including certain segmented worms, use a pigment called chlorocruorin, which gives their blood a green color. Chlorocruorin is also iron-based, but a structural difference in the molecule’s heme group changes its light reflection properties. This pigment appears bright green when it is diluted in the plasma, although it can look faintly red when highly concentrated.