How Brainless Brittle Stars Move Their Limbs

The difference between five- and six-legged green brittle stars has helped scientists understand how the sea creatures coordinate movement without neural activity.

AsianScientist (Aug. 8, 2019) – Scientists at Hokkaido University and Hiroshima University have shown that movement in green brittle stars can be coordinated by the flow of internal body fluid alone without the need for neuronal activity. These findings have been published in Scientific Reports.

Animals constantly make rhythmic movements such as breathing, feeding and walking. Physiological studies and robotics have shown that neuronal activity and physical structures are both involved in coordinating those movements. However, how physical structure affects such movement is unknown.

A team led by Associate Professor Hitoshi Aonuma of Hokkaido University, Japan, 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-neighbour part and not the immediate-neighbour part. To explain this behavior, the team built a mathematical model and found that such 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-neighbour parts shrank and expanded in unison followed by the same synchronous movements in the other three. They then 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,” said Aonuma. “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.”

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


The article can be found at: Wakita et al. (2019) Different Synchrony in Rhythmic Movement Caused by Morphological Difference between Five- and Six-armed Brittle Stars.

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Source: Hokkaido University.
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