Symmetry, Locomotion, and the Evolution of an Anterior End: A Lesson From Sea Urchins

Abstract

Bilaterally symmetrical, "regular" sea urchins in the Family Echinometridae (Class Echinoidea; Phylum Echinodermata) were found to lack a locomotor anterior. Heterocentrotus mammillatus and Echinometra mathaei were observed while locomoting. Members of both ellipsoidal species were found to proceed with their short or long axis foremost with statistically equivalent frequencies. This finding demonstrates that the evolution of bilateral symmetry is not always accompanied by the evolution of a locomotor "anterior" end. The elliptical echinometrid sea urchins provide a particularly appropriate study group for investigating the relationship between the evolution of body form and locomotor behavior. Although the radially symmetrical regular sea urchins, from which the echinometrids sprang, lack a locomotor anterior, all "irregular" echinoids, which are also derived from a regular ancestor but are bilaterally symmetrical, possess an "obligate" locomotor anterior. The symmetry and behavior exhibited by the elliptical echinometrid sea urchins therefore demonstrates that the first irregular echinoids (which exhibit bilateral symmetry by definition) need not have possessed a locomotor anterior as they do today.

Fluctuating Asymmetry: Methods, Theory, and Applications

“…Symmetry is often adaptive. Bilateral symmetry, for example, usually occurs in actively moving organisms, such as most animals, where efficient locomotion is important [27,28]. It influences flight efficiency of birds [29,30] and improves internal circulation [31].…”

Getting around when you’re round: quantitative analysis of the locomotion of the blunt-spined brittle star,Ophiocoma echinata

“…Additionally, early fossil metazoans display a wide array of body plans, including both radial and bilateral symmetry (Knoll and Carroll, 1999). The factors that led to the near-total dominance of the bilaterally symmetrical body plan are unclear and strongly debated, but a common hypothesis is that bilateral symmetry confers a locomotor advantage (Barnes, 1974;Beklemishev, 1969;Buchsbaum, 1976;Meglitsch and Schram, 1972;Willmer, 1990; but see Finnerty, 2005;Grabowsky, 1994). This hypothesis is supported by theoretical arguments on the mechanical advantages of anatomical specialization for movement in one direction (Beklemishev, 1969), in silico experimental evolution (Bongard and Paul, 2000) and the relative frequency of various body plans in sessile versus locomotory organisms (Beklemishev, 1969;Collins, 2001).…”

Asymmetry of Early Paleozoic trilobites

“…Bilateral symmetry is a fundamental characteristic of many organisms. Numerous functional reasons make symmetry important, including hydrodynamic stability, locomotive efficiency (Wainwright et al 1976;Vogel 1981;Grabowsky 1994 and references therein), and sensory acuity (van den Berg & Brenner 1994). In addition, an increasing volume of biological literature documents other important evolutionary aspects of bilateral symmetry (Polak & Trivers 1994).…”