Sea stars pump filtered seawater through their bodies instead of blood
Unlike vertebrates and most invertebrates, sea stars completely lack blood and a traditional circulatory system. Instead, they rely on a hydraulic network known as a water vascular system. They draw in ocean water through a specialized sieve plate called the madreporite, filter it, and pump it throughout their arms. This pressurized seawater transports nutrients, oxygen, and metabolic waste while mechanically powering thousands of tiny suction-cup tube feet.
An Open Hydraulic Alternative to Blood
Most animals depend on an internal fluid—blood or hemolymph—enclosed within vessels and propelled by a muscular heart to sustain their tissues. This internal medium carries dissolved gases, nutrients, and cellular waste across organ systems. Sea stars, along with other members of the phylum Echinodermata such as sea urchins and sea cucumbers, operate on an entirely different architectural principle. They possess neither a central heart, nor blood vessels, nor true blood. Instead, they run their internal physiology on an open, hydraulic network that draws in and circulates the surrounding ocean water.
This network is known as the water vascular system. Originating as a specialized compartment of the animal's body cavity, or coelom, during development, the water vascular system acts simultaneously as a circulatory apparatus, a respiratory surface, and an engine for physical movement. By taking ambient seawater directly into its body, filtering it, and pressurizing it throughout a labyrinth of internal canals, a sea star bypasses the need for specialized blood proteins like hemoglobin to move vital materials to its living tissues.
The Anatomy of the Seawater Network
The intake of ocean water begins at a distinct, light-colored structure on the upper surface of the sea star's central disc called the madreporite. Often resembling a tiny, porous button or sieve plate, the madreporite is perforated by micro-channels lined with beating cilia. These cilia generate a microscopic current that pulls seawater into the system while preventing large debris, sediment particles, and foreign organisms from clogging the internal plumbing.