Sea urchins have hundreds of tube feet that they use to explore their environment and adhere to the substrate. These tube feet are hydrostatic skeletons, i.e. hollow structures regulated through the interplay between a muscular envelope and the internal fluid pressure. Muscles in the tube wall allow the sea urchins to bend or retract their feet, while increased fluid pressure is used to elongate the feet.

Although the extremities of the feet look like suction naps, the actual mechanism used to hold on to the substrate is adhesion. Sea urchins basically produce a sticky liquid at the end of their feet and glue themselves to their substrate. This adhesion is reversible and the animals can detach their feet from the substrate without damaging them. Temporary adhesion in sea urchins can last for a few seconds up to several days.

For this project, we were interested in the forces produced by a single tube foot during locomotion. When walking, a sea urchin uses dozens of tube feet to pull itself forward. Tube feet go through a stepping cycle with different feet at different stages of the cycle at any given time. A stepping cycle goes something like:

1. tube foot is projected in the general direction of movement
2. tube foot adheres to the substrate
3. tube foot muscles contract to pull the body of the sea urchin forward
4. tube foot detaches from the substrate

And that is how I ended up spending many hours coaxing purple sea urchins (Paracentrotus lividus) into walking over a tiny force transducer...

So why were we so interested in the forces generated by a single sea urchin tube foot during walking? It turns out the adhesive secreted at the tip of the tube feet could be very useful for industrial and biomedical purposes, among others. We are looking at an adhesive that successfully holds together a soft tissue and a hard substrate, resists very high forces per unit area, is easy to detach, and does all of this in an aqueous environment. Dimitra Dodou, one of my advisers for this project, works on the development of devices that can be used during surgery or medical examinations to get a strong hold on wet and soft tissues while minimizing the risk of piercing or otherwise damaging organs. Using adhesives much like those found in echinoderm tube feet would make medical interventions safer and more comfortable, and recovery faster.

About this project

I worked on this research project from November 2010 to November 2011. It formed the basis of my master thesis titled "Locomotion and temporary adhesion in sea urchins: functional morphology of the tube feet and forces generated during walking in Paracentrotus lividus," which I completed at Wageningen University in the Netherlands.

I was advised by Dr. Johan L. van Leeuwen and Dr. Mees Muller from the Experimental Zoology Group at Wageningen University, and Dr. Dimitra Dodou from the Department of BioMechanical Engineering at the Delft University of Technology.