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Sci Rep
2019 Jun 05;91:8298. doi: 10.1038/s41598-019-44808-w.
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Different Synchrony in Rhythmic Movement Caused by Morphological Difference between Five- and Six-armed Brittle Stars.
Wakita D
,
Hayase Y
,
Aonuma H
.
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Physiological experiments and mathematical models have supported that neuronal activity is crucial for coordinating rhythmic movements in animals. On the other hand, robotics studies have suggested the importance of physical properties made by body structure, i.e. morphology. However, it remains unclear how morphology affects movement coordination in animals, independent of neuronal activity. To begin to understand this issue, our study reports a rhythmic movement in the green brittle star Ophiarachna incrassata. We found this animal moved five radially symmetric parts in a well-ordered unsynchronized pattern. We built a phenomenological model where internal fluid flows between the five body parts to explain the coordinated pattern without considering neuronal activity. Changing the number of the body parts from five to six, we simulated a synchronized pattern, which was demonstrated also by an individual with six symmetric parts. Our model suggests a different number in morphology makes a different fluid flow, leading to a different synchronization pattern in the animal.
16KT0099 MEXT | Japan Society for the Promotion of Science (JSPS), JPMJCR14D5 MEXT | JST | Core Research for Evolutional Science and Technology (CREST)
Figure 1. Rhythmic movement, “pumping”, in the five-armed individuals of the green brittle star Ophiarachna incrassata. (a) Temporal frequency of pumping phases. Each point represents a pumping phase, which comprises a series of movements shown in (b). The animals exhibit the first pumping phase 36 ± 17 min (N = 4) after feeding. Then, they periodically initiate pumping for more than 10 hrs. The interval between pumping phases is not consistent (20 ± 9 min) among individuals. Asterisks indicate no record from the arrows. (b) Temporal change in the radius of the five interradii in a pumping phase. Inset shows the aboral side of an individual at the moment indicated by the arrowhead in the graph. Radius is measured from the center of the disk to the midpoint of the edge of each interradius. The radii numbered anticlockwise are colored as in the inset, which corresponds to the graph in color. Colored horizontal bars under the graph represent shrinking periods of each interradius.
Figure 2. Simulation of rhythmic movement, “pumping”, in the green brittle star Ophiarachna incrassata, based on a phenomenological model. (a) Temporal change of the volume of five interradii. Cycles are unsynchronized as in the experiments of five-armed individuals (see Fig. 1b). (b) Temporal change of the volume of six interradii. Three distant interradii and the other three each make synchronized groups, anticipating the coordinated pattern of six-armed individuals. u in the y-axis represents the volume of internal fluid in the interradii. Each axis is given in an arbitrary unit. The color of the interradii in each inset corresponds to that of each graph. The black one in each panel represents the 1st interradius, which initially has a larger volume than the others.
Figure 3. Rhythmic movement, “pumping”, in the six-armed individual of the green brittle star Ophiarachna incrassata. (a) Temporal frequency of pumping phases. The animal frequently shows pumping phases for more than 10 hrs after feeding, as occurs in five-armed animals. (b) Temporal change in the radius of the six interradii in a pumping phase. Cycles are synchronized with two groups separated, which demonstrates the model simulation of six interradii (see Fig. 2b). Figures are shown as in Fig. 1.
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