How Do Sea Urchins Move? (Explained)

Sea urchins’ movement, like other Echinoderms, is always an interesting topic because, at first glance, they don’t have obvious limbs to do that. I remember when I was wondering myself, how they move forward, or how they stay on the bottom during strong currents. In this post, we’ll talk all about that. However, let’s start with a quick answer:

Sea urchins move horizontally or vertically using their sticky tube feet located on the bottom of their bodies. They can also use their long spines controlled by their muscles or even on their teeth. Regular sea urchins can move equally within any direction.

However, this certainly doesn’t tell the whole story and in this post, I’ll explain more about sea urchins’ movement, their movement patterns, and how they are able to stay on the bottom. Furthermore, I’ll explain why sea urchins move, if they can swim, or see. Read on!

Sea urchins movement

Sea urchins are living animals that are part of the phylum Echinodermata, which means “spiny skin” in Greek. The movement of these marine animals is accompanied by structures including arms, tube feet, spines, and the body wall. However, how these marine creatures move may differ and depend on the class.

Most sea urchins (regular sea urchins) use their spines located on the bottom of their bodies (called tube feet) to move around the seafloor. They use them in a rowing motion to move through the sediment front-end forward. They are able to move equally within any direction because of their radial symmetry.

The tube feet are generally the primary organs used for walking, but they can also use their long spines controlled by their muscles or even on their teeth. These sticky tube feet are essential in keeping sea urchins in place, especially in a highly turbulent environment. Thanks to them, the animal can climb rocks even vertically or algal stipes.

Irregular sea urchins such as sand dollars, sea biscuits, or heart urchins move by using their spines rather than tube feet. They use them in a rowing motion and move only in a linear path. Unlike regular sea urchins, they adapted to move around sandy bottoms and buried themselves in the sand to escape predators.

Sea urchins movement patterns

Scientists have studied sea urchins’ movement patterns to better understand ecological processes regulating predator and grazing community structures. We can divide their movement patterns into Brownian or ballistic motion.

The Brownian motion is random and slow sea urchins’ movements without direction. This movement occurs when they can’t smell any predators. They move around with an average speed of 3 inches (8 cm) per minute.

The situation changes when they start smelling predators. When in the ocean, they usually stay still in their shelters, like holes or cracks in the rocks. However, scientists studied what happens when they don’t have a place to hide. What happened is that when sea urchins smelled a predator, they responded with ballistic motion. This motion is typical for escaping – fast, straight and directional movements. Sea urchins can move with an average speed of 6 inches (15 cm) per minute during this motion.

How do sea urchins are able to stay on the bottom?

Sea urchins’ tube feet are essential in keeping sea urchins in place. They are sticky and allow sea urchins to stay on the bottom during strong currents or climb the rocks even vertically. But how exactly do they work?

Tube feet are the extension of a water vascular system – a complex set of canals characteristic of all Echinoderms. Their original function as simple respiratory – suspension feeding structures evolved into a broad range of functions like feeding, respiration, locomotion, excretion, and sensory reception.

As mentioned above, regular sea urchins use their tube feet primarily to walk on the seafloor and stay on the bottom without being washed away by the currents. Sea urchins’ each tube foot extends when they pump fluid into it, allowing the foot to reach out. Next, the suction cup (called podia) sticks to an object (for example, a rock) at the end of each foot. As the feet contract, sea urchins are pulled towards the rocks.

Sea urchins produce a sticky liquid at the end of each foot and help them to glue themselves to their objects. This adhesion is fully reversible, and they can detach their feet from the object without damaging them. Sea urchins can stay attached to their substrate for a few seconds up to several days.

Why do sea urchins move?

Like all animals on the planet, including humans, sea urchins move to feed themselves. They mostly feed on algae, plankton, kelp, periwinkles, and sometimes barnacles and mussels. They eat by scraping food off rocks and grinding up using their sharp teeth, called Aristotle’s lantern.

Sea urchins can also move in order to escape predators. As I mentioned above, they may use their ballistic motion to escape if there are no places to hide. However, they usually hide in their shelters like cracks in the rocks because of their slow movements.

In order to protect themselves from predators they move their spines but not necessary to walk. They move them because they want to directly point them at predators. When shadows pass over, they react and point at the potential danger with a rate of about 1-2 inches (2.5-5 cm ) per minute.

Can sea urchins see?

So, I mentioned that sea urchins move to escape predators or move their spines towards predators when the shadow passes over. But how do they see it without eyes? Based on the research, sea urchins are currently the only animals discovered to see without having actual eyes. They can see thanks to their light-sensitive cells in their tube feet.

As you see, tube feet are incredibly developed organs and play a significant role in sea urchins’ lives.

Can sea urchins swim?

Sea urchins, like their other relatives (sea stars, sand dollars, sea cucumbers), cannot swim. They’re well adapted to living on the seafloor and protect themselves with their hard skeleton and spiky spines.

Sources

• John D. Kirwan, Michael J. Bok, Jochen Smolka, James J. Foster, José Carlos Hernández, Dan-Eric Nilsson; The sea urchin Diadema africanum uses low resolution vision to find shelter and deter enemies. J Exp Biol 15 July 2018; 221 (14): jeb176271. doi: https://doi.org/10.1242/jeb.176271
• In Byrne, M., In O’Hara, T. D., CSIRO (Australia),, & Australian Biological Resources Study,. (2017). Australian echinoderms: Biology, ecology and evolution.