Starfish-like animal can coordinate movement without brain's help: Study

A computer model shows that a starfish-like animal can coordinate rhythmic motion based on body structure without their brain instructing them to do so. This provides insights useful for physiology and robotics, claim researchers.

Scientists 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, according to the study published in the journal of Scientific Reports.

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.

First, the researchers looked at five-armed brittle stars and discovered a repeated movement in five fan-shaped parts between the arms that shrink and expand, which they named "pumping."

They found that the pumping occurred in a coordinated, asynchronous manner: movement of one part was followed by that of the second-neighbor part, not the immediate-neighbor part, in a pentagonal star.

Then, the team built a mathematical model and found that coordinated movement can be achieved by an internal fluid flow created by changing volume and pressure in each part. When the researchers altered the number of parts from five to six in the simulation, it showed changes in the pumping patterns: three second-neighbor parts shrank and expanded in unison followed by the same synchronous movements in the other three.

They observed a six-armed brittle star and confirmed that the simulation was accurate compared to the real animal. "This suggests the rhythmic movement can be coordinated without neuronal interactions between body parts. The insight could inspire future robot designs for generating coordinated movements without a complex control system. Further research should investigate how different body structures affect movement patterns and how neuronal and non-neuronal activities each play a role in moving processes," said, the lead author of the study Hitoshi Aonuma.

Since pumping occurs after feeding, the team considered it a digestive process and suspected that different patterns in pumping create different flows in the animal's intestine, possibly affecting its digestive function.