Circulatory system
(Courtesy Drs. Reiber and McGaw © 2009).
Decapod crustaceans have traditionally been classified as having open circulatory systems, meaning that the organism's hemolymph is contained not within a closed circuit of vessels but rather freely leaves the vessels to mingle with interstitial fluid in the body cavity and thereby perfuse the organs.
In this model, a single-chambered heart pumps hemolymph throughout the body through seven arteries. As in mammals, these arteries branch into smaller arterioles, which divide into still finer vessels resembling capillaries. Some of the capillaries terminate in dead ends, while others form capillary beds resembling anastomoses. The capillaries subsequently drain into sinuses within the body cavity. Unlike the circulatory systems of vertebrates, however, the arteries of decapods do not possess smooth muscle or endothelial linings. Consequently, blood flow is controlled not by muscular action but by valves within the arteries (Reiber and McGaw 2009).
(Courtesy Drs. Reiber and McGaw © 2009).
(Courtesy Drs. Reiber and McGaw © 2009).
Research has emerged, however, that indicates decapods possess a much more complex circulatory system than was formerly believed. It turns out that the capillaries in the brain and antennal gland do in fact form a closed circuit. Furthermore, images obtained from scanning electron microscopy have revealed that these sinuses are not simple open spaces but rather possess a more organized structure and are essentially isolated from the body cavity at large. This finding has lead researchers to conclude that the decapod circulatory system is not open in the classical sense but is in fact a hybrid that should be classified as "incompletely closed" (Reiber and McGaw 2009).