Crabs have a unique biological design, differing significantly from humans in their internal transport systems. Unlike humans and other vertebrates with closed circulatory systems where blood is confined within veins and arteries, crabs utilize an open circulatory system. This means their internal fluids, known as hemolymph, are not continuously enclosed within vessels.
The Crab’s Unique Circulatory System
A crab’s circulatory system features a heart that pumps hemolymph into open spaces throughout its body. This fluid, which combines the functions of blood and interstitial fluid, fills a large body cavity called the hemocoel. Within this hemocoel, the hemolymph directly surrounds and bathes the internal organs and tissues, delivering nutrients and removing waste products. This direct contact facilitates efficient exchange without the need for an intricate network of fine capillaries.
From the heart, hemolymph initially travels through a limited number of arteries. These arteries are not part of a continuous closed loop; instead, they empty hemolymph into sinuses and spaces within the hemocoel. This contrasts with a closed system where blood remains within vessels, from the heart through arteries, capillaries, and veins, before returning. The internal movement of the crab itself also contributes to the circulation of this fluid within the hemocoel.
Hemolymph collects in specific areas before re-entering the heart. This return to the heart occurs through specialized openings in the heart wall called ostia. These ostia have valves that allow hemolymph to flow inward, preventing backflow and ensuring a unidirectional path back to the heart to restart circulation.
Oxygen Delivery
Crabs obtain oxygen through specialized respiratory organs called gills. These feather-like structures are located beneath the carapace. Water containing dissolved oxygen is drawn over the gills, which possess a very thin membrane that allows for the efficient diffusion of oxygen into the hemolymph and carbon dioxide out of it.
Oxygen entering the hemolymph is transported by a unique respiratory protein, hemocyanin. Unlike hemoglobin, the iron-based protein that gives human blood its red color, hemocyanin is copper-based. When oxygenated, this copper content causes the hemolymph to appear bluish.
Hemocyanin-rich hemolymph circulates through the hemocoel, delivering oxygen to tissues and organs for metabolic processes. After releasing its oxygen, the deoxygenated hemolymph then flows back towards the gills. There, it releases carbon dioxide into the water and reabsorbs fresh oxygen, completing the gas exchange cycle.