Starfish are marine invertebrates with star-shaped bodies, typically featuring five or more arms radiating from a central disc. They inhabit diverse marine environments, thriving in all the world’s oceans, from warm tropical zones to frigid polar regions and various ocean depths. They exhibit a unique radial symmetry, distinguishing them from many other animals.
The Starfish’s Unique Locomotion System
Starfish do not possess traditional “feet” for movement. Instead, they navigate using specialized structures called tube feet, also known as podia. These small, active, tubular projections line the underside of each arm, typically arranged within grooves. Many species have small suction cups at their tips, enabling them to grip surfaces.
The number of tube feet varies significantly among species, ranging from hundreds to thousands. For example, a single adult sunflower sea star can employ as many as 15,000 tube feet for locomotion.
How Tube Feet Work
The operation of tube feet relies on an internal hydraulic system known as the water vascular system, unique to echinoderms. Water enters this system through a porous plate called the madreporite, located on the starfish’s upper surface. From there, water circulates through a network of internal canals, including the stone canal, ring canal, and radial canals, extending into each arm.
Each tube foot consists of a muscular sac, called an ampulla, located inside the arm and connected to the external podium. When the ampulla contracts, it forces water into the podium, causing it to extend and stiffen. Conversely, muscle contractions within the podium push water back into the ampulla, leading to retraction. This coordinated extension and retraction, coupled with adhesive chemicals secreted by the tube feet, allows the starfish to attach to surfaces and slowly glide along the seafloor or cling to rocks.
Beyond Movement: Other Roles of Tube Feet
Beyond locomotion, tube feet perform several other functions. They are important in feeding, especially for predatory species that prey on bivalves like clams and mussels. The starfish uses its tube feet to firmly grip shells and exert a steady pull, gradually prying them open.
Once a small gap is created, some starfish can evert their stomach into the bivalve’s shell to digest prey externally. Tube feet also serve as sensory organs, capable of detecting touch, texture, and temperature changes. They can also sense chemical cues, which helps them locate food sources, and perceive subtle vibrations in the water.