Generalized locomotion of brittle stars with a flexible number of arms

Abstract

18Typical brittle stars have five radially symmetrical arms, which coordinate to move their 19 body in a certain direction. However, some species of them show individual difference 20 in the number of arms: commonly five or six, rarely four or seven. We found this trait 21 unique since intact legged animals each own a fixed number of limbs in general. How 22 34 'left and right' emerges in the behavior of a radially symmetrical body. 35 36 Keywords 37 echinoderm, radial symmetry, limb number, moving direction, Bayesian statistical 38 modeling, WAIC 39 40 1881). Some studies have observed another locomotor mode, called as "paddling" 63 (Arshavskii et al., 1976a) or "reverse rowing" (Astley, 2012), where a backmost arm is 64 dragged while the other four actively row (Arshavskii et al., 1976a; Astley, 2012; 65 Glaser, 1907; Preyer, 1887; von Uexküll, 1905). The ophiuroid body creeps in a certain 66 direction with such bilaterally coordinated manners (Astley, 2012). Since the 'role' of 67 each arm switches when the body changes moving direction (Astley, 2012), brittle stars 68 do not have consistent antero-posterior and left-right axes in behavior. 69 Although the five-armed locomotion in common brittle stars and the individual 70 difference in specific species have been viewed in different contexts, none has 71 combined them to spotlight ophiuroid locomotion under the different numbers of arms. 72 4Referring to the human body, whether the body comprises five, six, or other numbers of 73 modules seems to bring a huge issue in individual function. How do these animals 74 manage the difference in the number of motile organs to realize adaptive locomotion 75 within a species? The aim of our study is to quantitatively describe the four-, five-six-, 76 Following the better model in terms of WAIC, we hereafter show the results on the 105 assumption of two distributions for all the cases. 106 The posterior medians of two distributions' means, which were calculated 107 separately for the negative and positive ranges, were ±17, ±29, ±46, and ±70 in four-to 108 seven-armed animals, respectively. These estimated values signify that the more arms a 109 brittle star had, the further two distributions of Θ were apart from each other (Fig. 2). In 110 other words, the average moving direction of individuals with more arms was more 111 angled from the opposite direction of the stimulated arm. Predictive distribution of Θ 112 indeed depicted this trend (Fig. 2). 113 114 126 direction angled from the midline of the stimulated arm. Two groups were here defined 127 by whether it angled clockwise (Θ sign = 0) or anticlockwise (Θ sign = 1). 128 The Θ sign -based grouping exhibited a common locomotor mode among four-, 129 five-, six-, and seven-armed animals in regards to B α 's posterior means. The directional 130 property of each arm could be explained by how many arms we count from the 131 stimulated arm. Primarily, one of the first neighboring arms to the stimulated arm 132 consistently took the largest or second largest |B α ...

A general model of locomotion of brittle stars with a variable number of arms

“…Data accessibility. The preprint of this article is available from the bioRxiv preprint server: https://doi.org/10.1101/616383 [40]. The datasets generated and/or analysed during the current study are available in the Figshare repository [30].…”