Sea urchins are marine invertebrates belonging to the phylum Echinodermata, which also includes sea stars and sea cucumbers. These globe-shaped creatures are covered in a hard shell called a test and sharp movable spines. Sea urchins definitively have tube feet, as this feature is a defining characteristic of all echinoderms. These small, flexible appendages serve numerous functions essential to the urchin’s survival and are coordinated through the Water Vascular System.
Defining the Water Vascular System and Tube Feet
The tube feet, technically called podia, are small, active tubular projections that are part of the sea urchin’s unique hydraulic network, the Water Vascular System (WVS). This system is filled with a watery fluid and operates entirely on hydraulic pressure. The WVS begins with a sieve-like plate on the aboral (top) surface called the madreporite, which allows filtered seawater to enter the internal network of canals.
From the madreporite, the water flows through the stone canal and into the ring canal, which encircles the urchin’s esophagus. Five radial canals branch out from this ring, extending along the inside of the hard skeletal test and supplying fluid to hundreds of tube feet. Each tube foot is connected internally to a muscular bulb-like reservoir called an ampulla.
The mechanism for extending and retracting the feet relies on hydraulic pressure. When the muscles of the ampulla contract, they squeeze the internal fluid, forcing it down into the attached podium and causing the tube foot to extend outward. Conversely, muscles within the tube foot itself contract to push the fluid back into the ampulla, which causes the foot to retract. This coordinated contraction and relaxation allows the sea urchin to execute precise movements and maintain a firm grip on the substrate.
Locomotion and Adhesion: The Primary Use
The primary function of the tube feet is to facilitate movement and secure the animal to a surface, which is important in high-energy coastal habitats. Sea urchins use a coordinated stepping cycle to crawl slowly across the seabed or up vertical rock faces. During locomotion, a foot extends toward the desired direction, adheres to the substrate, and then its muscles contract to pull the body forward.
The terminal disc at the tip of the tube foot is specialized for temporary adhesion. Although it appears to function like a suction cup, the primary mechanism of attachment is the secretion of a dual-gland adhesive system. This system secretes a chemical adhesive to bond the foot to the rock and a de-adhesive to release it cleanly, allowing for reversible attachment.
This robust adhesion anchors the sea urchin against strong currents or wave action. The tube feet work in tandem with the animal’s movable spines, which provide leverage, push the body, and lift the test off the ocean floor. This combined effort allows the urchin to maneuver over complicated terrain and right itself if flipped upside down.
Respiration, Feeding, and Sensory Functions
Beyond movement, the tube feet perform several other biological functions. They serve as the primary sites for gas exchange, a respiratory role facilitated by their thin, permeable walls. Oxygen from the surrounding seawater diffuses across the tube foot wall into the internal coelomic fluid, while carbon dioxide moves out into the water. This function is especially pronounced in irregular sea urchins, such as sand dollars and heart urchins, which often lack specialized gills.
The podia also assist with food manipulation for the urchin’s herbivorous diet. Tube feet located near the mouth opening on the underside of the body grasp and handle food particles, such as scraped algae. They pass these small pieces of food toward the central mouth, where the chewing apparatus, known as Aristotle’s lantern, processes the material.
Finally, the tube feet are an important component of the urchin’s sensory apparatus. They are equipped with sensitive cells that aid in chemoreception, allowing the animal to detect dissolved chemicals and gradients in the water. This chemical sensing helps the sea urchin locate food sources and navigate its environment. Some species use their tube feet for photoreception, detecting light and shadows to help them orient and find shelter.