Radial Symmetry

Radial Symmetry Definition and Examples

Feather star
Feather star, which exhibits pentaradial symmetry. Jeff Rotman/The Image Bank/Getty Images

Radial symmetry is the regular arrangement of body parts around a central axis.

Definition of Symmetry

First, we should define symmetry.  Symmetry is the arrangement of body parts so they can be divided equally along an imaginary line or axis. In marine life, the two main types of symmetry are bilateral symmetry and radial symmetry, although there are some organisms that exhibit biradial symmetry (e.g., ctenophores) or asymmetry (e.g., sponges).

Definition of Radial Symmetry

When an organism is radially symmetrical, you could cut from one side of the organism through the center to the other side, anywhere on the organism, and this cut would produce two equal halves. Think of a pie: no matter which way you slice it, if you slice from one side to the other through the center, you'll end up with equal halves. You can continue slicing the pie to end up with any number of equal-sized pieces. Thus, the pieces of this pie radiate out from the central point. 

You can apply the same slicing demonstration to a sea anemone. If you draw an imaginary line across the top of a sea anemone starting at any one point, that would divide it into roughly equal halves.

Pentaradial Symmetry

Echinoderms like sea stars, sand dollars and sea urchins exhibit a five-part symmetry called pentaradial symmetry. With pentaradial symmetry, the body can be divided into 5 equal parts, so any one of five "slices" taken out of the organism would be equal.

  In the feather star shown in the image, you can see five distinctive "branches" radiating from the star's central disk.

Biradial Symmetry

Animals with biradial symmetry show a combination of radial and bilaterial symmetry. A biradially symmetrical organism can be divided into four parts along a central plane but each of the parts is equal to the part on the opposite side but not the part on its adjacent side.

Characteristics of Radially Symmetrical Animals

Radially symmetrical animals have a top and bottom, but don't have a front or back or distinctive left and right sides. 

They also have a side with a mouth, called the oral side, and a side without the mouth, called the aboral side. 

These animals typically can move in all directions.  You can contrast this to bilaterally symmetrical organisms like humans, seals or whales, who usually move forward or backward and have a well-define front, back and right and left sides.

While radially symmetrical organisms can move easily in all directions, they may move slowly, if at all. Jellyfish primarily drift with waves and currents, sea stars move relatively slowly compared to most bilaterally symmetrical animals, and sea anemones barely move at all. 

Rather than a centralized nervous system, radially symmetrical organisms have sensory structures scattered around their body.  Sea stars, for example, have eye spots at the end of each of their arms, rather than in a "head" region.

One advantage of radial symmetry is that it may make it easier for organisms to regenerate lost body parts. Sea stars, for example, can regenerate a lost arm or even an entirely new body as long as a portion of their central disk is still present.

 

Examples of Marine Animals with Radial Symmetry

Marine animals that exhibit radial symmetry include:

References and Further Information:

  • Morrissey, J.F. and J.L. Sumich. 2012. Introduction to the Biology of Marine Life (10th Edition). Jones & Bartlett Learning. 467pp.
  • University of California Museum of Paleontology. Bilateral (left/right) Symmetry. Understanding Evolution. Accessed February 28, 2016.