"Starfishlike brittle stars have five thin arms and no central brain, but even so, they move in a carefully coordinated fashion similar to four-limbed animals (including humans).
Symmetry is at the heart of the mystery of brittle star movement.
Symmetry influences how an animal moves about. Animals with bilateral symmetry, like humans, have bodies specialized to move in one direction — forward. Many animals with radial symmetry don't move or do so slowly. When they do travel, most of these animals do so in a direction determined by their body's central axis, defined by the location of their mouths. Think of a jellyfish moving up and down in the water column.
This is why brittle stars are strange. Despite their five-way symmetry, the stars don't move according to their central axis. Instead, they move perpendicular to it using their five multijointed limbs to propel them along the seafloor.
Henry Astley, at Brown University, filmed 13 blunt-spined brittle stars crawling in an inflatable pool and digitized their movements to better analyze them.
He found that, about 75 percent of the time, brittle stars oriented their movement around a central limb, which pointed the way for the rest of the body. The left and right forelimbs made large, coordinated movements. To turn, the brittle star simply picked a new lead limb.
When not "rowing" forward, the brittle stars reversed, with a central limb trailing and the other four making large movements.
While these patterns of movement resemble that of a bilaterally symmetrical animal, the brittle stars do not alternate limbs as many four-limbed animals do. (When walking, for example, you alternate between your left and right foot; the brittle stars moved both of their forelimbs at the same time.)
The study is detailed in the Journal of Experimental Biology."
Live Science
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