Sea urchins are fascinating marine invertebrates, often recognized by their spherical, spiny appearance. While they are aquatic creatures, a common misconception is that they can actively swim. Sea urchins do not swim in the conventional sense, unlike fish or other free-moving marine animals. Their movement is unique and specifically adapted for life on the seafloor.
How Sea Urchins Move
Sea urchins primarily move across the seafloor using hundreds to thousands of tiny, flexible appendages called tube feet, also known as podia. These tube feet operate through a water vascular system, which allows the urchin to extend and retract them by changing internal fluid pressure. Each tube foot has an adhesive tip that secretes a sticky substance, enabling it to grip surfaces like rocks or coral. The urchin extends a tube foot, adheres it to the substrate, contracts the foot to pull its body forward, and then detaches it in a coordinated, wave-like motion.
Their movable spines also assist in locomotion, helping them push off surfaces, maneuver over uneven terrain, or right themselves if inverted. While these spines can contribute to movement, the tube feet are the main drivers of their slow, deliberate pace. A sea urchin typically moves at a very slow speed, around 3 inches (8 cm) per minute, although some can move up to 6 inches (15 cm) per minute when escaping predators. This method allows them to adhere firmly to surfaces, even in turbulent conditions.
Why People Might Think Sea Urchins Swim
Any perceived “swimming” by adult sea urchins is typically passive displacement caused by water currents or tidal movements, rather than self-propelled action. They do not have fins or other structures designed for active swimming through the water column. Their anatomy is built for bottom-dwelling, requiring them to anchor themselves to the substrate to avoid being swept away.
While adult sea urchins are benthic (bottom-dwelling) and cannot swim, their larval stage, known as a pluteus larva, is planktonic and can actively swim. These microscopic larvae are part of the zooplankton, drifting with currents. This early life stage allows for dispersal before they settle on the seafloor and transform into their familiar adult form.
Life on the Seafloor
Sea urchins are found in marine environments across the globe, from shallow coastal waters to deep ocean floors, inhabiting diverse habitats such as rocky shores, coral reefs, and kelp forests. They prefer areas with hard surfaces where their tube feet can effectively attach. Their slow, crawling movement is perfectly suited for their lifestyle as grazers.
As herbivores, sea urchins play an important ecological role by consuming algae and detritus from rocks and other surfaces. They use a specialized chewing apparatus called Aristotle’s lantern, equipped with five teeth, to scrape food. By controlling algal growth, they help maintain the balance of marine ecosystems, preventing algal overgrowth that could otherwise smother coral reefs and disrupt biodiversity. Their bottom-dwelling existence and unique locomotion strategy are well-adapted to their specialized niche within the marine food web.