Zebra mussels, like many invertebrates, have an open circulatory system, i.e., blood (hemolymph) is not completely contained within blood vessels. Tissues are bathed in hemolymph for gas and nutrient exchange before the hemolymph returns to the primary blood vessel, the heart. The heart is three-chambered (one ventricle and two auricles) and is located just beneath the mussel’s ligament. Blood flows around the body of the mussel, is oxygenated in the gills and mantle, and returns to the heart via the nephridium to the auricles, where oxygenated blood mixes with deoxygenated blood. The ventricle, which surrounds the rectum, pumps oxygenated hemolymph away from the heart via the kidney through a network of blood vessels to the body organs and foot. The hemolymph continues to hemocoels where nutrients, gases, and wastes are exchanged. The hemolymph is then again reoxygenated in the gills and mantle before returning to the heart.
Although most of the circulation of the hemolymph is performed by the heart, circulation may also be influenced by local movements from other parts of the body, such as the foot (Jones 1983).
A pediveliger uses its foot to maneuver.
The hemolymph has no specialized oxygen-transporting pigments. Therefore, its oxygen concentration is essentially that of the water taken into the mantle cavity. The low metabolic rate and large surface area available for gas exchange across the gills in the mussel allow for an aerobic metabolism.
Jones (1983) and Coleman (1974) describe the circulatory system of bivalves in detail. The types of zebra mussel hemocytes are described by Giamberini et al. (1996).