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Typical brittle stars have five radially symmetrical arms that coordinate to move the body in a certain direction. However, some species have a variable number of arms, which is a unique trait since intact animals normally have a fixed number of limbs. How does a single species manage different numbers of appendages for adaptive locomotion? We herein describe locomotion in Ophiactis brachyaspis with four, five, six and seven arms to propose a common rule for the movement of brittle stars with different numbers of arms. For this, we mechanically stimulated one arm of individuals to analyse escape direction and arm movement. By gathering quantitative indices and employing Bayesian statistical modelling, we noted a pattern: regardless of the total number of arms, an anterior position emerges at one of the second neighbouring arms to a mechanically stimulated arm, while arms adjacent to the anterior one synchronously work as left and right rowers. We propose a model in which an afferent signal runs clockwise or anticlockwise along the nerve ring while linearly counting how many arms it passes through. With this model, the question on how ‘left and right’ emerges in a radially symmetrical body via a decentralized system is answered.

We report seven species of the genus Henricia Gray, 1840 that were found in Vostok Bay, and two species from adjacent area, known from museum collection or seen in underwater footage. while existing literature reported no confirmed species from this area. Most of these species: H. djakonovi, H. alexeyi, H. densispina, H. hayashii, H. granulifera, H. pacifica, H. asiatica, and H. oculata robusta were reported from the Sea of Japan previously. H. nipponica, known from Japan, is reported from Russian seas for the first time. All studied taxa are re-described here using a range of morphological characters and partial 16S rRNA nucleotide sequences, life colorations of several species are reported for the first time, and an identification key is provided. Lectotype designations are fixed for studied series of species described by AM Djakonov.

As a solution, we focused on individual difference in the number of radially symmetric arms and unique movement in brittle stars. We found that the green brittle star shrank and expanded interradii (parts of the disk partitioned by neighbor arms) with a rhythmic pattern. The movement among the interradii was unsynchronized in five-armed individuals but synchronized in a six-armed individual. To explain the relation between the pumping pattern and the body structure, we built a phenomenological model where internal fluid flows between the interradii. Based on the model, an interradius in five-armed ones makes an asymmetric flow into either neighbor, whereas that in six-armed ones makes symmetric flows into both neighbors. Main Text:All rights reserved. No reuse allowed without permission.(which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.The copyright holder for this preprint . http://dx.doi.org/10.1101/340471 doi: bioRxiv preprint first posted online Jun. 12, 2018; It is generally accepted that generating oscillatory patterns is crucial in virtually all animals to produce rhythmic movement. Animals utilize oscillatory neuronal activities within the central nervous systems so-called central pattern generators (CPGs) in locomotive movements (walking, swimming, flying etc.), mastication, ventilation and so on (1-4). However, there still remains gap between neuronal oscillation and rhythmic movement. In order to bridge the gap, many researchers built mathematical models. One of the successful models is to explain swimming pattern of lamprey, which was built based on the results of physiological experiments and simulated swimming movement (5). One the other hand, robotics researches suggested importance of physical communication in gait transition of animals (6). They demonstrated speed-dependent autonomous gait transition using a simple quadruped robot in which each limb had a CPG but there was no connection with electrical circuit between the CPGs. The brittle star-like robot designed by a similar concept was able to immediately change locomotion patterns by unexpected physical damage (7). Both robots coordinated limb movement by sensing local load without preprogramed patterns. However, it is still an issue how physical communication functions in animals.Brittle stars (Ophiuroidea, Echinodermata) are marine animals characterized by radial symmetry in the body plan. The central disk has typically five arms, which partition the disk into five symmetric fan-shaped parts, called as interradii. In this study, we found that the green brittle star, Ophiarachna incrassata (Lamarck, 1816), moved the interradii with a rhythmic pattern after feeding (S1). This rhythmic movement reminded us of pumps, so that we call it "pumping".Most individuals of the green brittle star have five arms with the same pumping pattern.Meanwhile, we paid attention to the fact that brittle stars sometimes have individual difference in the number of arms, namely, that of in...

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