WHAT IS RADIAL SYMMETRY? (WITH EXAMPLES) - BIOLOGY - 2025

The radial symmetry, also called actinomorphous, linear or regular, is symmetry that resembles a cone or a disk that is symmetrical about a central axis. Animals that present radial symmetry are symmetrical around an axis that goes from the center of the oral surface, where the mouth is located, to the center of the opposite or aboral end.

This symmetry is considered the primitive or ancestral state and is found in the first families of plants that appeared on the planet until the present. In modern plants, radial symmetry is observed in approximately 8% of all families.

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Radial symmetry manifests itself in sessile organisms (without a supporting or fixed organ) such as the sea anemone, floating organisms such as jellyfish, and slow-moving organisms such as starfish. Almost all jellyfish have four radial channels and are believed to have radial symmetry.

Radial symmetry is generally associated with the offer of rewards for pollination: the complete ring of nectary tissue around the base of the ovary or a series of separate nectaries related to the number of petals present, plus a mass of central anthers.

The radial flowers offer easy access to visitors and can serve as food for a variety of insects, including: beetles, lepidoptera and flies, which have a greater preference for this type of flower.

The way insects feed varies from species to species. Some do it in a disorganized way, they just land and feed. Others (the bees) are more organized and do careful and methodical work around the nectary ring: they make a complete circuit around all the petals in order, before retiring.

What does it consist of?

Radial symmetry is that which is observed when an imaginary line passes through any plane, through the central axis of a body, it is divided into two equal halves.

Animals that present this symmetry do not have a ventral, dorsal, head, tail or caudal region. In other words, in these beings there is no right side, no left side, no front or back, no upper or lower surface.

They are generally immobile: coelenterate (hydra), ctenophores and echinoderms. When an organism is radially symmetrical, it has the appearance of a cake that, when cut, presents almost identical parts.

The advantage that radial symmetry offers to organisms that possess it, is that they have an equal number of possibilities of finding food or predators in any direction.

Radial symmetry was used in the binomial animal taxonomy as a reference for the classification of the Radiata species (animals with radial symmetry). This class was part of the classification of the animal kingdom made by George Cuvier.

Special forms of radial symmetry

Tetramerism

It is the symmetry of four rays or channels in a radial body plane, presented by jellyfish.

Pentamerism, pentaradial or pentagonal symmetry

The individual is divided into five parts around a central axis, with separations of 72 ° between them.

Echinoderms, such as starfish, urchins, and sea lilies, are examples of pentamerism - five arms located around the mouth. In plants, pentameric or quintuple radial symmetry is appreciated in the arrangement of the petals and in fruits that have seeds.

Hexamerism or hexaradial symmetry

The structures of organisms have a body plan with six parts. In this group are the Hexacorallia corals, with polyps of internal symmetry of six times and tentacles in multiples of six, and the sea anemones Anthozoa.

Octamerism or octaradial symmetry

Division of the organism into eight parts. Corals of the Octocorallia subclass that have polyps with eight tentacles and octameric radial symmetry are located here. A separate case is the octopus that, despite having eight arms, presents bilateral symmetry.

Examples of radial symmetry

Actinomorphic flowers are those with radial symmetry and they look the same from any direction, facilitating pattern recognition. The petals and sepals are practically identical in shape and size, and when divided by any of their planes, equal parts will remain.

Many flowers, such as dandelions and daffodils, are radially symmetrical.

Animals belonging to the phylum Cnidaria and Echinodermata are radially symmetrical, although many sea anemones and some corals are defined with bilateral symmetry by the presence of a simple structure, the syphonoglyph.

Some of these specimens have non-radial parts, such as the slit-shaped throats of sea anemones, often present in some animals as well.

As a larva, a small starfish looks completely different from a star, resembling an alien spaceship with tentacle tips protruding from a central bell.

As adults, most starfish have five-sided symmetry (pentameric radial symmetry). It can move in different directions, guided by any of its five arms. If each of the five arms could be bent, each half would be positioned exactly on top of the other.

Case study: starfish

Studies by Chengcheng Ji and Liang Wu of the China Agricultural University have found that starfish may have hidden bilateral tendencies, which appear during times of stress.

In the larval stage, this species has a head and is clearly bilateral. Their five-sided symmetry only emerges when they grow up, but Ji and Wu believe that starfish never forget their bilateral beginnings.

In the experiment, the scientists exposed more than a thousand specimens to different situations, to observe their reaction. The first test consisted of moving the animals to a new space and observing which arms they used to move.

Another test consisted of turning the bodies and it was observed that, being upside down, the stars are pushed with two of their arms against the ground for support and then they are pushed with the opposite to turn and stay in position.

Finally, the stars were placed in a shallow space and an irritating liquid was poured on their back, immediately the animals moved away using their arms to move.

The tests showed that starfish have hidden bilateral symmetry and that they travel in chosen directions. This type of response is clearly evident when they are in stressful situations such as having to flee or turn to regain their position. If they have a preferred direction, they could make faster decisions in times of danger

Differences between radial and bilateral symmetry

In nature there are a great variety of flowers that are classified into two main forms: radial or actinomorphic symmetry flowers (jasmine, rose, carnation, lily) and bilateral or zygomorphic symmetry flowers (orchid).

Observations made on fossil flowers show that radial symmetry is a hereditary characteristic. On the contrary, bilateral symmetry is a product of the evolution of the species, even independently in different families of plants.

Some researchers have studied the fact that natural selection appears to favor the condition of bilateral symmetry over radial.

The observation of the evolution of the shape of the flowers indicates that pollinating insects prefer flowers with bilateral symmetry, therefore this type of symmetry is favored in terms of evolution.

Study with

José Gómez and his team, from the University of Granada Spain, used 300 plants of the species Erysimum mediohispanicum, typical of the southeastern mountains of Spain. This plant has a particular characteristic: in the same plant, flowers of radial symmetry and flowers of bilateral symmetry are produced.

A first step of the study was the identification of the pollinating insects, from a total of 2000 separate observations, each lasting one minute.

From these observations it was deduced that the most frequent visitor was a small beetle (Meligethes maurus) with an 80% frequency in relation to other species.

To determine which type of flower was preferred by insects, a technique known as geometric morphometry was used: measuring the three-dimensional shape of flowers to identify whether their symmetry is radial or bilateral.

The subsequent analysis of the results found that the beetles preferred the flowers of bilateral symmetry, manifesting their determining role in natural selection. In addition, it was observed that the bilateral symmetry flowers produced more seeds and more daughter plants.

Apparently, the preference for bilateral symmetry over radial symmetry has to do with the arrangement of the petals that facilitates the landing of insects on the flower.

References

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