A Model of Sea Star Locomotion Using Tube Feet

Sea star inspired crawling and bouncing

by: Sina Heydari, Amy Johnson, Olaf Ellers, Matthew J. McHenry, and Eva Kanso

Summarized by Max Botwin, a geology major at the University of South Florida. He is currently a senior and will be graduating in Summer 2021. He is planning to move to Texas in May and looks forward to learning more about the world of geology as he works in the field. In Max’s free time, he enjoys playing with his dog Nilla and exploring the local trails around USF’s campus.

What data were used? The model was created by the researchers using parameters that were observed and measured from Asterias rubens sea stars, such as tube foot length during extension or contraction, density of water, buoyancy, and others.

Methods: To effectively model the locomotion of sea stars, the researchers adhered to what they called “hierarchical control laws”, which was the idea that the sea star controls the direction of movement for all of its tube feet, but the power and the recovery from a contraction is determined by each tube foot at an individual level. The researchers observed sea star movement and created a mathematical model to encompass as many parameters involved with tube foot locomotion as possible, including buoyancy of the feet, density of water, extension of tube feet, etc. Researchers were also able to quantify the pull of tube feet from extending and push from contracting using length of tube feet. Researchers were able to use their model and the data they collected to run many simulations with varying numbers of tube feet over different terrain to test the speed, stability, coordination, and overall effectiveness of tube feet locomotion. Examples of the simulations run by the researchers can be found in the link here.

Results: From this study, researchers have found that sea stars give a general command to the tube feet to move in a certain direction, but the power and the recovery from a contraction of the tube feet is controlled by the individual tube feet. This allows for a crawling movement from the sea star which allows it to travel across most terrain underwater and even swim. Tube feet are only effective underwater, since they work through jet propulsion where the tube foot will extend and fill a vacuole with water, then contract and squeeze out that water to propel themselves. The tube feet on sea stars are only related to one another by their attachment to the sea star and nothing else; there is no communication between the tube feet. Despite this, tube feet seem to fall into coordinated movements, where some tube feet are angled backwards and push forwards and some are angled forwards to “pull” the sea star forward. This pseudo coordination of movement is not limited to tube feet that are adjacent to one another, which means that tube feet can fall into the same locomotion “group”, even though they are not located near one another. When this occurs, the sea star goes from crawling to a bouncing gait which is much faster, but also requires more energy and was therefore found to only be useful under certain circumstances. Researchers saw that sea stars preferred to “pull” when moving vertically and pushing when moving horizontally.

Figure 2 from Heydari et al. (2020). (a) Common sea star used in the study called Asterias rubens. (b) Image of tube feet on Asterias rubens. (c) Image showing the bouncing gait of Asterias rubens. (d) Cross section of Asterias rubens to show the parts of its nervous system. (e) The anatomy of tube feet for mature sea stars. (f) The different actions that tube feet can undertake to perform locomotion. (g) This is a schematic of the mechanical rendition of a sea star including tube feet to help model sea star movement. (h) A flow chart to describe the hierarchical motor control used to command the tube feet in the mechanical sea star.

Why is this study important? This study is important because of the versatility that sea stars have in their movement across most surfaces underwater. If this locomotion can be accurately modeled, it can be recreated for robots, vehicles, or other such applications. The ability to traverse underwater terrain can have impactful applications in many industries that need to do work on the bottom of a body of water. Imagine a vehicle that is able to remain submerged and move along the ground while underwater to repair a bridge or to clean the bottom of a lake- the possibilities are endless!

The Big Picture: This study has shed light on unknown factors involved in how tube feet are controlled by the sea star and researchers were able to study and simulate sea star locomotion but were not able to match the complexity of real sea star locomotion. Answers beget more questions and that rings true here as well; the researchers were able to answer some of the “how” for sea star locomotion but were unable to explain the “why” behind it. This study can be used as a base for future models of the same type and can go further in detail using new parameters and improve their models to better depict the movement of tube feet on sea stars.

Citation: Heydari S, Johnson A, Ellers O, McHenry MJ, Kanso E. Sea star inspired crawling and bouncing. J R Soc Interface. 2020;17(162):20190700. doi:10.1098/rsif.2019.0700