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, 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.
Highlights
There are six classes of echinoderms: Crinoidea, Asteroidea, Ophiuroidea, Echinoidea, Holothurioidea and Concentricycloidea
The center of gravity in a bilateral animal is supposed to lie on the plane of bilateral symmetry
In other words, during their evolution from the Cambrian era to present, some bilateral symmetry has persisted in adult echinoderms
Summary
There are six classes of echinoderms: Crinoidea, Asteroidea, Ophiuroidea, Echinoidea, Holothurioidea and Concentricycloidea. Except for the holothurians [1], which evolved from a radial ancestor [2]. Most are pentameral animals [1,2], and the other symmetry forms are derived from pentameral symmetry [3,4,5]. 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, to pentameral symmetry [9,10,11]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
