Ji C et al. (2012), Echinoderms have bilateral tendencies.

ECB-ART-42299

PLoS One 2012 Jan 01;71:e28978. doi: 10.1371/journal.pone.0028978.

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Echinoderms have bilateral tendencies.

Ji C , Wu L , Zhao W , Wang S , Lv J .

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Echinoderms take many forms of symmetry. Pentameral symmetry is the major form and the other forms are derived from it. However, the ancestors of echinoderms, which originated from Cambrian period, were believed to be bilaterians. Echinoderm larvae are bilateral during their early development. During embryonic development of starfish and sea urchins, the position and the developmental sequence of each arm are fixed, implying an auxological anterior/posterior axis. Starfish also possess the Hox gene cluster, which controls symmetrical development. Overall, echinoderms are thought to have a bilateral developmental mechanism and process. In this article, we focused on adult starfish behaviors to corroborate its bilateral tendency. We weighed their central disk and each arm to measure the position of the center of gravity. We then studied their turning-over behavior, crawling behavior and fleeing behavior statistically to obtain the center of frequency of each behavior. By joining the center of gravity and each center of frequency, we obtained three behavioral symmetric planes. These behavioral bilateral tendencies might be related to the A/P axis during the embryonic development of the starfish. It is very likely that the adult starfish is, to some extent, bilaterian because it displays some bilateral propensity and has a definite behavioral symmetric plane. The remainder of bilateral symmetry may have benefited echinoderms during their evolution from the Cambrian period to the present.

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Genes referenced: LOC100887844

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	Figure 1. The madreporite is shown by Point A.Arm 1 is the arm opposite to the madreporite, and the other arms follow clockwise successively in aboral view. The coordinate system is as shown in the figure. Point O, the origin, is located at the center. Arm 2 lies on the positive x-axis.
	Figure 2. The five-pointed star conforms to the real starfish's shape.Line h, the distance between the center and bottom of the arm, is 1. Line H, the length of an arm, is 2.985. Line L is the cutting line. When weighing, the arms were cut off along Line L. The central disk is the light-colored regular pentagon. We considered the arms as cones, so Point G, the center of gravity of an arm, lay at H/4 of the triangle. We assumed the central disk to be homogeneous, so Point O, the center of gravity of the central disk, lay in the center of the regular pentagon. The unit of action is the shaded part. A unit of action contains one arm and 1/5 of the central disk near the arm. Point U is the center of gravity in the plane unit, and we assigned the frequency of action of each arm to it. Point U is located at Uâ€Š=â€Š[(hÃ—hÃ—2/3)+HÃ—(h+H/3)]/(h+H)â€Š=â€Š1.6617.
	Figure 3. The turning-over process is shown from Step 1 to 6.Generally, the starfish firstly extended its arms upwards, then bent two adjacent arms against the ground for support, stamped the ground with the opposite arm and lifted the other two arms upwards on each side, finally the opposite arm lost its contact with the substrate and the starfish turned over.
	Figure 4. Crawling action of the starfish.Generally, the starfish crawled with two arms forward as the leading arms, two on the side/rear and one backward. The arrow indicates the direction of movement.
	Figure 5. The coordinate system is the same as in Figure 1 .The blue, yellow, green and red planes represent the symmetric planes of turning-over, crawling, fleeing and average, respectively. Anterior and posterior directions are as shown.

References [+] :

Arenas-Mena, Spatial expression of Hox cluster genes in the ontogeny of a sea urchin. 2000, Pubmed, Echinobase