Echinoderms

Echinoderms Have Bilateral Tendencies
Chengcheng Ji1*., Liang Wu1., Wenchan Zhao1, Sishuo Wang1, Jianhao Lv2
1 College of Biological Sciences, China Agricultural University, Beijing, China, 2 College of Science, China Agricultural University, Beijing, China
Abstract
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, whichcontrols 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 behaviorstatistically 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 somebilateral 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.
Citation: Ji C, Wu L, Zhao W, Wang S, Lv J (2012) Echinoderms Have Bilateral Tendencies. PLoS ONE 7(1): e28978. doi:10.1371/journal.pone.0028978
Editor: Hector Escriva, Laboratoire Arago, France
ReceivedJune 19, 2011; Accepted November 18, 2011; Published January 11, 2012
Copyright: _ 2012 Ji et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted
use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This work was supported by the grant from the NationalNatural Science Foundation of China (Project J1103520) and National Basic Science
Foundation of China, (Education of Field Research, Project 0830630). The funders had no role in study design, data collection and analysis, decision to publish, or
preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: jichengcheng@yahoo.com.cn. These authors contributed equally to this work.
Introduction
There are six classes of echinoderms: Crinoidea, Asteroidea,
Ophiuroidea, Echinoidea, Holothurioidea and Concentricycloidea.
All are radial, except for the holothurians [1], which evolved
from a radial ancestor [2]. Among the actinomorphic echinoderms,
most are pentameral animals [1,2], and the other
symmetry forms are derivedfrom pentameral symmetry [3–5].
The ancestors of echinoderms, which originated from the
Cambrian era, were deuterostomes [6]. Because deuterostomes are all
bilateral, we can infer that the ancestors of echinoderms were
bilaterians [7,8]. To adapt to their benthonic habitat and planktonic
habitat niches, echinoderms evolved from bilateral symmetry first
to asymmetry, then to pentameralsymmetry [9–11].
Echinoderm larvae are bilateral during early development
[12,13]. They first change into asymmetry [14,15] then continue
to change into pentameral symmetry [11,16]. Larvae have an A/P
axis that is determined by the fixed developmental mode and the
absolute sequence in the embryonic development process [16].
During this process, tissues and organs match the corresponding...