Echinoderms, a phylum of marine animals including sea stars, sea urchins, and sea cucumbers, display secondary radial symmetry, often arranged in five parts. This unique body architecture supports a specialized internal system that manages movement, feeding, and respiration. This hydrostatic network extends outside the body to create numerous small, flexible appendages. This system allows them to anchor firmly to surfaces and navigate the ocean floor.
Defining the Tube Feet
Tube feet, or podia, are the external components of the echinoderm’s hydraulic system used for movement. Each tube foot is a soft, cylindrical extension protruding through skeletal pores. They are typically arranged in five radiating rows along the body, corresponding to the pentaradial symmetry.
Each tube foot consists of the internal ampulla and the external podium. The ampulla is a small, muscular, water-filled sac located inside the body cavity. The podium is the external, elongated part that contacts the substrate, often terminating in a sucker-like disc in sea stars and sea urchins.
Powering Locomotion The Water Vascular System
The tube feet are operated by the water vascular system (WVS), an internal network utilizing hydraulic pressure for movement. Seawater enters the system through the madreporite, a porous plate on the animal’s surface. The water flows through the stone canal, which connects to a circular ring canal surrounding the mouth region.
From the central ring canal, five radial canals extend outward along each arm or body section. Lateral canals branch off these radial canals, connecting to an ampulla and its corresponding podium. This system is filled with a watery fluid distinct from the surrounding seawater.
Locomotion is generated by the controlled movement of this internal fluid. When an ampulla’s muscles contract, they squeeze the fluid, forcing water into the attached podium, causing it to elongate. To retract the foot, longitudinal muscles in the podium contract, pushing the water back into the relaxing ampulla. The coordinated extension and retraction of thousands of feet allows the animal to slowly creep along a surface.
Beyond Movement Other Functions and Diversity
While locomotion is a primary role, tube feet are adapted for several other functions across different classes of echinoderms. Sea stars use the strong adhesion provided by the feet to pry open the shells of bivalve mollusks, such as clams and oysters. This adhesive capability uses both suction and the secretion of temporary chemical glues for secure attachment and easy release.
The thin walls of the podia also facilitate gas exchange and waste removal. Oxygen diffuses from the surrounding seawater directly into the fluid of the feet, and carbon dioxide moves out. In species like feather stars, the tube feet lack suckers but are covered in sticky mucus, which captures small planktonic particles and passes them toward the mouth.
Diversity in form reflects specialized function. Sea urchins have short, rigid tube feet that aid in anchoring the animal against strong currents on rocky substrates. Sea stars have long, flexible tube feet that allow for greater reach and manipulation. Sea cucumbers use their tube feet primarily for adhesion and slow crawling, with some species possessing modified, branched feet around the mouth that function as feeding tentacles.