This starfish walks on four legs, using its fifth leg as a "head"

There are a lot of things you take for granted when you have a head. One of those is the direction that you walk in. But what if you had no head? Surprisingly, no scientist had ever thought to investigate that. Until now.

A simple experiment, done by grabbing some brittle starfish and plopping them down on a sandy spot in an inflatable pool, helped Brown University evolutionary biologist Henry Astley understand something fundamental about how these starfish move around, and how close that comes to the way humans move.

All humans, and most creatures everywhere, have what's called bilateral symmetry. There's only one place where you can drop down a dividing line and have both halves be symmetrical; right between the eyes. This comes from having a head that guides the limbs on either side, walking forward and walking backward. Starfish, on the other hand, don't have bilateral symmetry. They have five limbs, all the same angle from each other, giving them a radial symmetry and no head.

So researchers were surprised to see the brittle stars simply designate a limb to act as head, stretching it forward and using its other four arms symmetrically to walk themselves forward. Often to turn, it will choose another limb to act as head, and walk its suddenly bilateral limbs behind that. Remember, starfish don't have actual brains, and they have crude eyes on each of their limbs, so there is nothing coordinating this effort. It might be assumed that, since starfish eyes are at the tips of their limbs, it is only designating something to go in front so it can feel and see where it's going. However, a brittle star will sometimes actually walk backwards, blindly, keeping its designated head in the same direction as it did when moving forward.

This indicates that there has to be either some kind of residual bilateral instinct that the sea stars fall back on, or a definite evolutionary advantage toward bilateral symmetry when moving. Is it easier to coordinate two symmetrical sides rather than five separate limbs? Does there need to be some single focus one one direction, whether the animal is walking to or away from that direction? What is it about one line and two sides? The only way to find out is to play with more starfish.

Image: Henry Astley/Brown University

[Journal of Experimental Biology]