Scientists at Hokkaido University and Hiroshima University have found that green brittle stars with five arms show a different "pumping" movement pattern than those with six arms.
Using a mathematical model, they have shown that such movements can be coordinated by the flow of internal body fluid alone, rather than neuronal activity.
Animals constantly make rhythmic movements such as breathing, feeding and walking.
Physiological studies and robotics have shown that neuronal activity and physical structure, respectively, are involved in coordinating those movements.
However, how physical structure affects such movement in animals is unknown.
A team consisting of Hitoshi Aonuma, Daiki Wakita and Yumino Hayase studied the green brittle star Ophiarachna incrassata, a starfish-like aquatic animal found in tropic and sub-tropic oceans of the Indo-Pacific region with typically five and occasionally six arms.