Lobsters, like all crustaceans, do not possess the red, iron-based blood found in mammals and other vertebrates. Instead, the fluid that circulates within their bodies is a functional equivalent that performs the necessary tasks of gas exchange and internal transport. This circulatory substance, along with its unique color and movement throughout the body, sets the lobster apart from the more familiar circulatory systems.
What Lobsters Use Instead of Blood
The circulatory fluid in a lobster is scientifically termed hemolymph, a substance that combines the roles of blood and the interstitial fluid found in vertebrate tissues. This specialized liquid is composed primarily of water, various inorganic salts, and organic compounds like carbohydrates and lipids. The dissolved salts help maintain the necessary osmotic balance within the animal. Hemolymph also contains specialized circulating cells called hemocytes, which are responsible for internal defense and clotting. These cells protect the lobster from foreign invaders and quickly form a clot to seal wounds.
Why Lobster Hemolymph is Blue
The blue color of oxygenated hemolymph is due to the presence of a respiratory protein called hemocyanin. Unlike the red hemoglobin in human blood, which uses iron atoms to bind oxygen, hemocyanin relies on copper atoms for the same purpose. When hemocyanin is deoxygenated, the copper is in a reduced state, and the fluid appears clear or colorless. As oxygen binds to the copper atoms, the fluid takes on a distinct blue hue. This copper-based protein is dissolved directly into the hemolymph plasma, allowing for efficient oxygen transport throughout the body.
How Circulation Works in Lobsters
Lobsters employ an open circulatory system, meaning their hemolymph is not fully contained within a network of closed vessels like arteries and veins. A single, simple heart pumps the hemolymph out through a series of arteries that branch throughout the body. These arteries do not connect directly to veins, but instead end abruptly, releasing the hemolymph into large, open cavities called sinuses, or the hemocoel. In these sinuses, the hemolymph directly bathes the organs and tissues, allowing for the direct exchange of oxygen, nutrients, and waste products. After the exchange occurs, the hemolymph collects and returns to the heart through small openings in the heart muscle called ostia.