Crustaceans, such as crabs, lobsters, and shrimp, do not possess the red fluid that vertebrates call “blood.” Instead, their circulatory system uses a fluid known as hemolymph. This fluid performs the same functions as blood in transporting substances throughout the body, but it operates differently and has a distinct chemical makeup.
Defining Hemolymph
Hemolymph is the general term for the circulatory fluid found in most arthropods, including all crustaceans, where it acts as a combination of vertebrate blood and lymph. This fluid is responsible for circulating oxygen, nutrients, hormones, and waste products throughout the animal’s body.
The fluid combines the functions of oxygen transport, the primary role of blood, with the function of interstitial fluid (lymph), which bathes the tissues directly. It contains circulating immune cells and various dissolved chemicals, including plasma proteins, analogous to components found in vertebrate blood plasma.
The hemolymph fills the entire internal body cavity of the crustacean, an open space called the hemocoel. Unlike blood in a closed system, this fluid is not continuously confined to arteries, veins, and capillaries. It moves through the hemocoel, making direct contact with internal organs and tissues to facilitate substance exchange.
The Open Circulatory System
The mechanism by which hemolymph circulates is described as an open circulatory system. This system contrasts significantly with the closed system of humans and other vertebrates, where blood remains enclosed within vessels at all times. In a crustacean, a simple, muscular heart pumps the hemolymph through a short series of arteries and vessels.
These vessels, however, do not return the fluid directly to the heart through veins. Instead, the hemolymph is discharged from the vessels into the open spaces of the hemocoel, where it directly bathes the organs. The crustacean circulatory system is often considered an “incompletely closed” system because higher crustaceans, like crabs and lobsters, have a complex arterial network that includes fine vessels similar to capillaries in some tissues.
After circulating through the hemocoel, the hemolymph collects in open spaces, or sinuses, before returning to the heart. The heart possesses small openings called ostia, which are fitted with valves, allowing the hemolymph to be drawn back into the heart chamber. The heart then contracts to propel the fluid once again through the arterial system, completing the circuit of fluid distribution.
The Unique Chemistry of Crustacean Fluid
The hemolymph’s unique chemistry is most apparent in the substance it uses for oxygen transport. Unlike the iron-based hemoglobin that gives vertebrate blood its red color, crustaceans generally use a large protein called hemocyanin. This specialized metalloprotein contains two copper atoms that reversibly bind to a single oxygen molecule.
The presence of copper in the oxygen-carrying pigment is what gives the hemolymph its unusual appearance. When oxygenated, the copper atoms are oxidized, causing the hemolymph to appear blue or blue-green. Conversely, when the hemocyanin is deoxygenated, the copper reverts to a reduced state, and the fluid becomes colorless or pale yellow.
The hemolymph also contains cellular components called hemocytes, which perform functions similar to the white blood cells in vertebrates. These cells are suspended directly in the fluid and are the primary mediators of the crustacean’s innate immune system. Crustaceans lack the complex adaptive immune system found in vertebrates, relying entirely on these cellular and humoral responses for defense.
These cells are involved in crucial defense reactions like phagocytosis, the encapsulation of foreign bodies, and the clotting of the hemolymph for wound healing. Furthermore, the hemolymph transports essential nutrients, such as glucose, and carries metabolic waste products to the excretory organs for removal.