Crayfish are recognized by their hard shells and the surprising color of their internal fluids. The question of whether these crustaceans possess “blood” and why that fluid is often blue has long interested scientists. Examining the crayfish reveals a circulatory system that functions differently from that of vertebrates. This system uses a specialized circulatory fluid and a unique molecular pigment to transport oxygen throughout the body.
Hemolymph: The Circulatory Fluid of Crayfish
The fluid circulating within a crayfish is not technically considered blood. It is called hemolymph, a term reflecting the fundamental difference in how the fluid is contained and circulated. Crayfish utilize an open circulatory system, meaning the internal fluid does not remain sealed within a network of vessels like arteries and veins.
In this open system, the heart pumps hemolymph through short vessels, which then empty directly into large internal cavities known as the hemocoel. The hemolymph mixes freely with the interstitial fluid surrounding the organs and tissues. The organs are bathed in this circulating fluid, allowing for the direct exchange of nutrients, waste, and gases.
The hemolymph returns to the heart through small openings in the cardiac muscle called ostia. These openings allow the fluid to re-enter the heart after circulating through the body cavity. This contrasts with the closed circulatory system of mammals, where true blood remains confined to vessels and is separate from the interstitial fluid.
Hemocyanin: The Copper-Based Pigment
The blue color of crayfish hemolymph is caused by the presence of the respiratory protein called hemocyanin. This oxygen-carrying molecule is dissolved directly in the hemolymph, unlike the red blood cells found in humans. Hemocyanin is second only to hemoglobin in its frequency of use as an oxygen transport molecule across the animal kingdom.
The molecular basis for the blue hue is the element copper, which sits at the core of the hemocyanin structure. Each functional subunit contains two copper atoms that reversibly bind a single oxygen molecule. In contrast, the hemoglobin found in human blood uses an iron atom to bind oxygen, which results in its red color.
When hemocyanin binds to oxygen, the copper atoms change their oxidation state, altering how they absorb and reflect light. The copper shifts from a colorless state to a blue state upon oxygenation. When oxygen is released to the tissues, the copper reverts to its original state, causing the hemolymph to become colorless or pale gray.
The Role of Blue Blood in Oxygen Transport
The function of hemocyanin is to bind, transport, and release oxygen to meet the metabolic demands of the crayfish’s body. The process begins after the hemolymph is pumped across the gills, where oxygen is absorbed from the surrounding water. Once oxygenated, the hemocyanin-rich fluid turns blue and circulates throughout the hemocoel.
The hemocyanin unloads the oxygen in the tissues, facilitating cellular respiration. This oxygen release is influenced by internal factors, such as the acidity level (pH) and the concentration of certain ions within the hemolymph. For example, the presence of L-lactate, a byproduct of anaerobic metabolism, can increase the hemocyanin’s oxygen affinity in some crustaceans.
This copper-based system provides advantages for animals living in aquatic environments, especially those that are cold or low in oxygen. Under these conditions, the effectiveness of hemocyanin can surpass that of iron-based hemoglobin. Although generally less efficient at transporting oxygen per volume compared to hemoglobin, the hemocyanin system is well-suited for the lifestyle and metabolic activity of many crustaceans.