Blood, or a similar circulating fluid, transports oxygen, nutrients, and waste products throughout an organism’s body. While many animals have red blood, nature presents surprising variations in how this vital process occurs, leading to a spectrum of blood colors.
Why Most Blood is Red
The prevalence of red blood across many animal species, including humans, stems from hemoglobin. Hemoglobin is located within red blood cells and is rich in iron. When oxygen binds to the iron atoms within hemoglobin, it undergoes a structural change that gives blood its characteristic bright red color.
This oxygenated state makes arterial blood, carrying oxygen from the lungs or gills to the tissues, appear vibrant red. As oxygen is delivered to the body’s cells, the hemoglobin releases it, causing the blood to become a darker, purplish-red before returning to the oxygen-rich organs. The iron in hemoglobin is central to its efficient oxygen binding and release.
The Spectrum of Blood Colors
Beyond hemoglobin, the animal kingdom employs diverse molecules for oxygen transport, resulting in a variety of blood colors. These alternative respiratory pigments utilize different metal ions to bind oxygen, leading to distinct hues. The color of these oxygen carriers often changes depending on whether they are bound to oxygen or not.
One prominent alternative is hemocyanin, a copper-based protein found dissolved directly in the circulating fluid, or hemolymph, of many invertebrates. When hemocyanin binds to oxygen, the copper within its structure oxidizes, causing the hemolymph to turn a striking blue. In its deoxygenated state, hemocyanin is colorless or pale yellow.
Another oxygen-carrying molecule is hemerythrin, which, like hemoglobin, is iron-based but lacks the heme group. Hemerythrin is usually found within cells and appears violet-pink when oxygenated, becoming colorless when deoxygenated. Chlorocruorin, an iron-based pigment structurally similar to hemoglobin but with a modified heme group, is another unique example. This pigment appears green when diluted and oxygenated, though it can appear light red in concentrated solutions.
Creatures with Non-Red Blood
Animals with blue blood, due to hemocyanin, include a wide range of marine invertebrates. Octopuses, squids, and horseshoe crabs are well-known examples, as are various crustaceans like lobsters, crabs, and shrimp. Some spiders and snails also possess hemocyanin, giving their hemolymph a blue tint.
Violet-pink blood, characterized by hemerythrin, is less common but found in specific marine organisms. Brachiopods and certain marine worms, such as sipunculids (peanut worms), utilize hemerythrin for oxygen transport.
Green blood, stemming from chlorocruorin, is observed in some segmented marine worms. Polychaetes, a diverse group of marine annelids, particularly tube worms, are known to have chlorocruorin in their blood.
Adaptations of Blood Diversity
Different blood systems reflect diverse adaptations to specific environmental conditions. Hemocyanin, with its copper-based oxygen binding, is effective in cold, low-oxygen marine environments. Its efficiency allows animals like deep-sea cephalopods and horseshoe crabs to thrive where oxygen might be scarce.
Hemerythrin and chlorocruorin are often found in sessile or burrowing marine invertebrates. These pigments represent specialized solutions for oxygen transport in habitats that may experience fluctuating oxygen levels or unique physiological demands. Varied oxygen carriers highlight successful strategies for sustaining life, each suited to an organism’s niche and lifestyle.