These Arms Were Made For Walking

Unlike their Echinoderm brethren, brittle stars do not move along on tube feet that can propel them in any direction.  Instead, brittle stars ‘walk’.  This mode of movement by brittle stars is even more astonishing when it is considered that brittle stars, like all echinoderms, are not bilaterally symmetrical, i.e mirror imaged halves.  All echinoderms are radially symmetrical, like a less tasty pie or pizza. Indeed, they exhibit pentamerism with five equal parts.

It’s easier to have a gait when you are bilateral.

At the beginning of the video above you can see that brittle stars move by using only 4 of their five limbs.  A central limb appears to point forward and do much of nothing else. New work with the fantastic title “Getting around when  you’re round” by by a graduate student at Brown University, Henry Astley, provides the first quantitative analysis of this behavior.

Brittle stars pull this synchronized movement, i.e. rowing (A Below), off without a brain.  Yet, in brittle stars the forlimbs are tightly coupled. “The forelimbs appear to function as a rotating strut during rowing, propelling the body disk via the sweeping motions of long limbs.” But even more amazing, as shown by Astley, is that brittle stars have found a mechanism to choose any of its five limbs to direct its movement.  Any of the arms are capable of being the front, i.e. a lazy leg gets to ‘lead’ and apparently gets a free ride (Left).  When the brittle start wants to change direction, it picks a new leg to be the front and starts moving in that direction.  This is unlike how we move.  We first rotate our bodies and then move in the new direction.

Astley also describes another locomotory behavior, reverse rowing (C below). At 0:22 in the video above you can see a brittle star moving by reverse rowing.  In this behavior there is no lead leg, but rather a straggler.  During reverse rowing, the forelimbs both rotate and pull the body disk forward by shortening via a series of bends.

Why do brittle stars row? Astley suggest that the preference for symmetrical arm propulsion may be due to mechanical benefits. The body rests on the seafloor and thus continued support of limbs, like in us, is not needed. Thus synchronous movements of the arms can be free to work in unison and generate symmetrical forces and displacement. Interestingly this also in other animals that scrap their tummies on the ground–sea turtles and mudskippers.

I guess there is only one thing to do now.  Listen to some Nancy Sinatra.

Dr. M (1730 Posts)

Craig McClain is the Assistant Director of Science for the National Evolutionary Synthesis Center, a National Science Foundation supported initiative. He has conducted deep-sea research for 20 years and published over 50 papers in the area. He has participated in and led dozens of oceanographic expeditions taken him to the Antarctic and the most remote regions of the Pacific and Atlantic. Craig’s research focuses on how energy drives the biology of marine invertebrates from individuals to ecosystems, specifically, seeking to uncover how organisms are adapted to different levels of carbon availability, i.e. food, and how this determines the kinds and number of species in different parts of the oceans. Craig’s research has been featured on National Public Radio, Discovery Channel, Fox News, National Geographic and ABC News. In addition to his scientific research, Craig also advocates the need for scientists to connect with the public and is the founder and chief editor of the acclaimed Deep-Sea News (, a popular ocean-themed blog that has won numerous awards. His writing has been featured in Cosmos, Science Illustrated, American Scientist, Wired, Mental Floss, and the Open Lab: The Best Science Writing on the Web. His forthcoming book, Science of the South (, connects cultural icons of South such as pecan pie with the science behind them.

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2 comments on “These Arms Were Made For Walking
  1. you accidentally tagged this under “brittle starT”, not “brittle star”. Just a heads-up, since it’s a bitch finding mistagged stuff later :)

  2. Pingback: Brittle Star info

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