Explore biradial symmetry. Learn the definition of biradial symmetry and understand its various advantages. Discover examples of biradial symmetry in animals.
Updated: 11/21/2023
Biradial Symmetry | Overview, Advantages & Examples
Table of Contents
- Symmetry in Animals
- What is Biradial Symmetry?
- Biradial Symmetry Features
- Advantages of Biradial Symmetry
- Examples of Biradial Symmetry Animals
- Lesson Summary
Frequently Asked Questions
Are bilateral symmetry and biradial symmetry the same?
Bilateral symmetry refers to the ability to divide an organism into two equal parts. By contrast, biradial symmetry occurs when an animals can be divided into four parts about a central axis. Each opposing segment is identical to each other, while adjacent segments are dissimilar.
What organisms have biradial symmetry?
Cnidarians represent the primary group of organisms that have biradial symmetry. Examples of cnidarians include jellyfish, hydrozoans, corals, and sea anemones.
Table of Contents
- Symmetry in Animals
- What is Biradial Symmetry?
- Biradial Symmetry Features
- Advantages of Biradial Symmetry
- Examples of Biradial Symmetry Animals
- Lesson Summary
Scientists describe animals based on the type of symmetry present in an organism. Symmetry refers to the presence of two or more equally divisible parts of an organism. In animals, there are three basic types of symmetry:
- Asymmetry
- Bilateral symmetry
- Radial symmetry
Asymmetry refers to the absence of symmetry in an animal. Sponges represent one type of animal that has an asymmetrical body plan.
Bilateral symmetry, the second type of body plan, occurs when an organism is divisible into two equal halves. Many vertebrate animals, such as the bird shown in Figure 1, have bilateral symmetry. Indeed, bilateral symmetry in animals represents a common characteristic found in vertebrates like humans, fish, birds, reptiles, and mammals, with roughly 99% of all animals exhibiting bilateral symmetry.
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Radial symmetry represents the third type of body plan found in animals. What is radial symmetry? Radial symmetry in animals refers to the division of an organism's body into two or more equal pieces about a central axis. Common food items such as pies, cakes, and pizzas exhibit radial symmetry because they can be cut into two or more equal slices or wedges. In much the same manner, some animals can be divided into multiple wedges.
There are two main types of radial symmetry in animals: pentaradial and biradial symmetry. Pentaradial symmetry is characteristic of animals from the phylum Echinodermata, such as sea stars, sea lilies, and sand dollars. These animals all have pentaradial symmetry because they are divisible into five equal pieces.
The second type of radial symmetry is biradial symmetry. This lesson covers the main characteristics of biradial symmetry and provides examples of animals exhibiting this unique body plan.
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Biradial symmetry represents a less common form of radial symmetry that involves the division of the body into four quadrants through the placement of two planes at right angles to each other. However, unlike the standard radial body plan, in which each segment is identical to the other, biradial symmetry is characterized by a different pattern.
As seen in Figure 2, showing a circle divided into four parts, only opposite sections are identical. This means that:
- Segments A and C are identical to each other
- Segments B and D are identical to each other
- Segments A and C are different from segments B and D
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This type of oppositional symmetry reflects the internal and external arrangement of an organism's structures, in which the same organs are found in segments A and C, while a different set of organs are found in B and D. In essence, this means that biradial organisms have two sets of bilateral symmetry arranged about a central axis. Due to this characteristic, scientists argue that biradial symmetry represents an intermediate form lying between radial and bilateral symmetry.
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Some of the main features of biradial symmetry include:
- Division of the organism into only two planes
- Opposite sections are identical, however, adjacent sections are not
- Reflects aspects of both bilateral and radial symmetry
- Represents an intermediate between bilateral and radial symmetry
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For organisms organized using a biradial body plan, there is one primary adaptive advantage of this type of anatomical arrangement:
- The ability to sense danger in a spherical radius about the body
Although scientists have argued in the past that radial symmetry evolved to enable organisms to move in any direction when needed, recent studies have demonstrated otherwise. For instance, a majority of animals with radial and biradial symmetry are either sessile (immobile) or move primarily by floating in the water. Moreover, studies show that organisms with bilateral symmetry have greater maneuverability than animals that exhibit biradial symmetry. Instead, the main purpose of biradial symmetry is to enable the animal to sense danger from predation from all around its body.
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There are two main groups of animals that exhibit biradial symmetry:
- Ctenophores, or comb jellies
- Certain cnidarians such as freshwater polyp hydras
Ctenophores, often referred to as comb jellies, represent a group of 80 different species of carnivorous invertebrate marine animals. Invertebrate refers to the absence of a spinal column, or vertebrae, in an animal. Similar in appearance to jellyfish, comb jellies have a spherical main body from which two tentacles emerge. Instead of using muscles to move through the water, comb jellies rely upon eight columns of ctenes or comb rows formed by cilia that have been fused together.
The mouth and anus are located at opposite ends of the comb jelly's body, with the mouth found on top and the anus situated at the base of the animal. This north-south orientation represents the central axis that gives the animal its biradial symmetry. The internal location of the tentacle sheath supporting the two tentacles provides the comb jelly with its first axis of division. The second axis is located at a right angle to the tentacle sheath. As a result, adjacent segments of the animal are not identical, while opposite segments of the comb jelly are identical.
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Another example of biradial symmetry in animals is in the phylum Cnidaria, which includes animals like corals, sea anemones, and hydrozoans. While all cnidarians have radial symmetry, only a small subset exhibit biradial symmetry; freshwater polyp hydras. The lifecycle of a majority of cnidarians consists of alternating medusae and polyp forms, in which the medusae utilize sexual reproduction and the polyps rely upon asexual reproduction.
Unlike the majority of animals in the phylum Cnidaria, hydras only occur in the polyp form and rely upon budding for reproduction. Budding represents a type of asexual reproduction where the parent organism creates an identical clone of itself by pinching off a small section of its own body. This section grows larger in size until it breaks away from the parent. While adult hydras only exhibit radial symmetry, the budding pattern often occurs in a biradial pattern. This means that buds form on opposite parts of the hydra's body so that opposing buds are identical to each but different from adjacent buds.
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Animals are often described in terms of their body plan, or the type of symmetry exhibited by an organism. There are three main types of body plans: asymmetry, bilateral symmetry, and radial symmetry. Animals that are not divisible into two or more equal parts are referred to as asymmetrical. By contrast, bilaterial symmetry is observed in animals such as birds that are divisible into two identical parts. Radial symmetry occurs when an organism can be divided into two or more identical parts about a central axis.
A special type of radial symmetry is known as biradial symmetry, in which the organism can be divided up into only two planes. The main advantage of radial and biradial symmetry is the ability of an organism to sense danger from all around its body. Since it has characteristics of bilateral and radial symmetry, biradial symmetry is believed to be an intermediate type of body plan between radial and bilateral organization in animals. Biradial symmetry occurs primarily in comb jellies and freshwater hydra polyps, in which opposing body segments are identical, yet different from adjacent body segments.
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Video Transcript
Types of Symmetry
To understand what biradial symmetry is, it helps to know the more common types, as biradial symmetry is kind of a bilateral and radial symmetry intermediate. Even the name is an intermediate: bi+radial.
Bilateral symmetry is a type of symmetry where the organism can be divided down the middle, and the two halves are mirror images of each other. The word ''bilateral'' means ''two sides.'' In bilateral symmetry one plane divides the organism into two parts. You, by the way, have bilateral symmetry.
Radial symmetry occurs when the symmetry exists around a center and the organism can be divided into similar pieces (like a cake). More specifically, more than two planes will divide the organism into identical parts. Sea anemones are animals with this body plan.
Biradial symmetry is when the organism can be divided up into equal parts, but only in two planes. It is different than radial symmetry, because two planes divide the organism, but not more than two. Comb jellies are an example of an organism with biradial symmetry. Think of a cake with tentacles coming out the bottom.
Comb Jellies and Biradial Symmetry
There are many species within the Ctenophora phylum, the comb jellies, all with varying body shapes and sizes. If you were to divide the comb jelly in half from mouth to their sensory organ (called the statocyst), they would show radial symmetry. However, the lower portions of the comb jelly have tentacles that negate radial symmetry (i.e., the animal could not be divided up like a pie with similar pieces).
If you had to choose a side for comb jellies (either radial or bilateral), they are probably more like the radially symmetrical animals over the bilateral animals.
Evolution & Advantages
Some scientists believe that animals with bilateral symmetry evolved from those with radial symmetry. Why? Radial symmetry is great for animals that don't move a whole lot, or depend on water to help them move. But being bilateral is better for organisms that want to move in a specific direction. As organisms evolved, certain environments selected for a bilateral body plan over a radial plan. Today around 99% of animals have bilateral symmetry, so it's assumed it provides advantages that radial symmetry does not provide (i.e., being able to go where you want to go).
Some scientists believe that the biradial animals are an intermediate held over from the transition from radial to bilateral symmetry. Ctenophores live in marine environments and propel themselves using cilia combs, which are tiny hair-like structures usually associated with bacteria and not animals. While some can swim at five centimeters per second, they also depend upon the water current to move them.
Biradial symmetry allows for one of the advantages of radial symmetry: the ability to have sensory organs all over instead of crammed into the head. This can help them to sense danger or food in all directions. Like animals with radial symmetry, comb jellies do not have a brain, but they have webs of nerves that are located throughout the jelly, including near the mouth, cilia, and tentacles. This allows them to sense food and danger all around them.
Lesson Summary
Let's review. In bilateral symmetry, the organism is divided down the middle and the two halves are mirror images. In radial symmetry, the organism can be divided into more than two planes with identical pieces. Biradial symmetry is an intermediate where the organism can be divided, like radial symmetry, but in two planes only.
The Ctenophora phylum, or comb jellies, have a biradial body plan and many believe that this body plan is an evolutionary stepping-stone from radial to bilateral symmetry. Radial symmetry allows for much more movement. Bilateral symmetry for none. Biradial allows for some, if limited, movement that still relies primarily upon the ocean current. Like the other animals with radial symmetry, the comb jelly has sensory structures around its body (and not focused in the head like many bilateral animals).
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Biradial Symmetry | Overview, Advantages & Examples
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