Sea stars are marine invertebrates known for their unique body forms and fascinating methods of movement. Unlike many marine creatures that rely on fins or muscular contractions, sea stars employ a distinct system for locomotion. This process allows them to navigate diverse underwater environments, from rocky shores to sandy seabeds.
The Water Vascular System Explained
The water vascular system is a specialized hydraulic network that enables a sea star to move, transport food, and respire. This system begins with the madreporite, a sieve-like plate on the sea star’s upper surface, which acts as an entry point for seawater. From the madreporite, water flows into a calcified tube, the stone canal, which then connects to a circular ring canal around the sea star’s mouth.
Extending from this central ring canal are radial canals, which run lengthwise into each arm. Along these radial canals, numerous lateral canals branch off, each connecting to a bulb-like muscular sac called an ampulla. Each ampulla is linked to a tube foot, an external projection on the underside of the sea star’s arms. The entire system is filled with a fluid that is essentially seawater but contains higher concentrations of potassium salts and some proteins, creating the necessary internal pressure for operation.
How Tube Feet Propel Movement
Sea star movement is driven by the coordinated action of its numerous tube feet. To extend a tube foot, muscles surrounding the ampulla contract, forcing fluid into the podium, the external, tube-like part of the tube foot. This influx of fluid causes the podium to elongate and push outwards.
At the tip of most tube feet is a small, disc-like structure that secretes an adhesive substance, allowing the tube foot to grip onto various surfaces. This adhesion is not solely based on suction, but involves a specialized duo-gland system that releases both adhesive and de-adhesive secretions. Once attached, muscles within the podium contract, pushing the fluid back into the ampulla. This retraction of the tube foot pulls the sea star’s body forward, initiating movement.
The Dynamics of Sea Star Locomotion
Sea star locomotion involves the continuous, wave-like coordination of thousands of individual tube feet. Despite lacking a centralized brain, their nervous system, composed of a nerve ring and radial nerves extending into each arm, effectively coordinates these movements. While each tube foot can respond autonomously to local stimuli, a global directional command from the sea star guides the overall movement.
Sea stars are generally slow movers, with an average speed of about 6 inches per minute. However, speeds can vary significantly among species; for instance, the sunflower sea star (Pycnopodia helianthoides) is notably faster, capable of moving up to 1 meter per minute.
Their ability to adjust the attachment and retraction of their tube feet allows them to navigate diverse terrains, including climbing vertical surfaces and burrowing into soft sediments. This versatile hydraulic system enables sea stars to explore their marine habitats.