Most people picture the familiar bright red liquid flowing through the veins of mammals when considering circulation systems. This redness is due to the iron-rich protein called hemoglobin, which transports oxygen throughout the body. However, the internal fluids of many invertebrates, including the American lobster, operate on a completely different biochemical principle. The color of a lobster’s circulating fluid is not red. This difference in color stems from a unique respiratory pigment and an ancient circulatory design that is perfectly suited for life in the deep ocean.
The Science Behind the Blue: Hemocyanin
The fluid circulating inside a lobster is called hemolymph, not blood. In its deoxygenated state within the animal, it is clear or colorless. The color change occurs because the lobster’s respiratory pigment is hemocyanin, a large protein that floats freely within the hemolymph. Hemocyanin binds and transports oxygen from the gills to the tissues, serving the same function as hemoglobin in vertebrates. The fundamental difference is the metal ion at their core: hemoglobin uses iron, while hemocyanin uses copper. These large, multi-subunit proteins are suspended directly in the circulating fluid, dictating the final color of the oxygenated fluid.
How Oxygenation Changes the Hue
The distinct blue color appears when oxygen binds to the copper ions embedded in the hemocyanin protein. When not carrying oxygen, the copper atoms exist in a colorless, reduced state, known as cuprous ion (Cu(I)). Binding an oxygen molecule to a pair of these copper centers causes a chemical reaction that oxidizes the copper to the cupric ion state (Cu(II)). This change in the copper’s oxidation state causes the visible shift in color from clear to a vivid blue, a result of the light absorption properties of the oxidized copper complex. When the hemolymph reaches tissues needing oxygen, the oxygen molecule is released, and the copper reverts to its colorless Cu(I) state, allowing the hemolymph to cycle between colorless and blue.
The Lobster’s Open Circulatory System
The hemolymph that carries the hemocyanin moves through a circulatory system fundamentally different from that of humans. Lobsters possess an open circulatory system, meaning their fluid is not confined to a continuous network of vessels like arteries and veins. Instead, the hemolymph is pumped by the heart into a few short arteries that empty directly into the hemocoel, the main body cavity. This fluid then flows freely around the organs and tissues, bathing them directly in the oxygen-rich hemolymph. Nutrients, oxygen, and hormones are distributed through this open space before the fluid is collected and returned to the heart for re-oxygenation. Although this system is less pressurized and generally less efficient than a closed circulatory system, it is sufficient for the metabolic needs of the lobster. The hemolymph also contains immune cells and factors that aid in the lobster’s innate immune response.