All animals in the phylum Echinodermata possess tube feet. Echinoderms, including sea stars, sea urchins, and sea cucumbers, use these specialized structures for nearly all life functions. Tube feet are external, fluid-filled extensions that are part of a unique internal network. This complex system enables slow movement, secure attachment, and feeding.
The Defining Feature: What Tube Feet Are
Tube feet, technically known as podia, are small, flexible, and muscular projections that extend from the echinoderm’s body, typically grouped in five rows along the ambulacral grooves. They represent the external, functional endpoint of the animal’s internal water vascular system. Each individual tube foot is a cylindrical, thin-walled structure that can be lengthened or shortened by internal fluid pressure.
The structure is composed of two main parts: the internal sac called the ampulla and the external projection called the podium. In many species, the tip of the podium ends in a terminal disc or suction cup, which allows for strong adhesion to a substrate. This adherence is achieved through a combination of the suction created by the disc and the secretion of temporary adhesive mucus.
Tube feet lack rigid skeletal support, relying instead on hydrostatic pressure for movement and function. While the podium contains longitudinal muscles for retraction, the internal ampulla is encased in circular muscles that control fluid flow. These structures must pass through small openings in the calcareous skeletal plates, or ossicles, to reach the outside environment.
Powering the Movement: The Water Vascular System
The water vascular system (WVS) is a network of fluid-filled canals that operates the tube feet. This system begins with the madreporite, a sieve-like plate usually located on the animal’s upper surface, which filters and regulates seawater intake. Water flows from the madreporite down the stone canal, which connects to the main ring canal encircling the central disc or mouth.
Five radial canals then branch off the ring canal, extending outward along the length of each arm or ambulacral area. These radial canals give rise to numerous lateral canals, each leading to a pair of the internal ampullae and their corresponding external tube feet. The movement of the tube foot is a direct result of muscular contraction in the ampulla, which acts like a bulb syringe.
When the muscular ampulla contracts, it squeezes the internal fluid, forcing it into the attached podium and causing the tube foot to extend outward. Conversely, when the longitudinal muscles in the podium contract, they push the fluid back into the relaxing ampulla, which shortens and retracts the tube foot. This continuous, coordinated sequence of extension and retraction across hundreds of tube feet allows the echinoderm to achieve slow but steady locomotion, managed by pressure changes within the hydraulic network.
Diverse Functions Across Echinoderm Classes
While the underlying mechanism is shared, the specific roles of tube feet vary significantly depending on the echinoderm class. In sea stars (Asteroidea), tube feet are primarily used for locomotion and powerful adhesion, enabling them to cling to rocks and slowly crawl across the seafloor. The strong suction created by the terminal discs is also employed in feeding, allowing sea stars to pull apart the tightly closed shells of prey like clams and oysters.
For sea urchins (Echinoidea), the podia function mainly as sensory organs and for anchoring. Since many tube feet in sea urchins are thin-walled, they also play a significant part in respiration, absorbing oxygen from the seawater. Sea cucumbers (Holothuroidea) often modify the tube feet around their mouth into specialized, branching tentacles used for gathering food particles from the water or substrate.
In brittle stars (Ophiuroidea), the tube feet typically lack the strong terminal suction cups found in other classes. They are less involved in crawling, focusing instead on sensory detection and passing food along to the mouth. The thin walls of the tube feet across all classes allow them to function secondarily in gas exchange and waste removal. This versatility has allowed echinoderms to colonize diverse marine habitats.