All You Need To Know About Radiolarians

Radiolarians are members of the Radiolaria subclass and they are commonly found in marine environments around the world.

All You Need To Know About Radiolarians

In this article, we will discuss everything you need to know about radiolarians, including their characteristics, ecology, and how to study them. Let’s begin by defining what they are.

What Are Radiolarians?

Radiolarians are eukaryotes, so despite being single-celled, they still have a clearly defined nucleus and a nuclear membrane.

Although they are found in major oceanic ecosystems worldwide, some species are only found in specific regions.

They are very similar to amoebas, but unlike amoebas, the majority of radiolarians are not capable of moving and are instead an immotile species.

One feature that all radiolarians share is globular shells that are usually made from silica and have several holes.

Radiolarians feed on a wide variety of different food sources but have been known to form symbiotic relationships with other organisms in their environment if food is scarce.

Radiolarians have known to exist on Earth for at least 550 million years and have been discovered in every ocean, as well as several other bodies of saltwater around the world.

They can also be found at various depths, from the seafloor to the surface. They were first discovered in the 19th century and since then, over 15,000 different living and fossil species have been identified.

It is estimated that around 800 to 1,000 different species currently inhabit the world’s oceans.

Classification Of Radiolarians

Formally, radiolarians belong to the following classes.

Kingdom: Protista

This covers organisms that are simple, eukaryotic organisms that don’t have enough characteristics to be classified as plants, fungi, or animals.

As they’re eukaryotes, they do have a nucleus and they are found everywhere in the world around us, but especially in aquatic environments. They can exist as either single-cell organisms or grouped together in colonies. 

Phylum: Sarcomastigophora

Sarcomastigophora is a phylum of the Protista kingdom. This phylum covers a variety of different organisms, including both unicellular and colonial ones.

Organisms in this phylum can move by using either flagella or pseudopods or even both. Many species in this group are free-living and are found throughout the world.

There are even some organisms in this phylum that are parasites and capable of causing diseases in animals. 

Subphylum: Sarcodina 

This can be classified as either a subphylum or even a subclass. It’s one of the largest phyla and covers organisms such as amebas and others that are closely related to amebas.

Organisms in this subphylum can exist as either single cells or in colonies and some are parasites. They have temporary cell extensions, called pseudopods, that are used for functions such as movement and feeding.

Class: Actinopoda 

This class consists of planktic marine organisms, but it is possible to find some in freshwater. Members of this group have tentacles that originate from their radial ambulacral vessels.

They’re similar to the class Foraminifera, as the majority of actinopods are protozoans. This distinguishes them from the other naked members of the phylum sarcomastigophora.

They’re also different from many other organisms under the subphylum sarcodina as actinopods are freely floating and depend on water currents to move from one place to another.

Subclass Radiolaria

This is their specific subclass.

The Ecology Of Radiolarians

As we stated earlier, radiolarians are found in major oceans all around the world. They have been discovered at a wide variety of depths, from the ocean surface to the bathypelagic depths, which are around 3,000 to 13,100 from the surface.

The majority of radiolarians are exclusively found in marine waters, but there are some that can be found elsewhere. For example, Lophophaena rioplatensis have been found to thrive in brackish waters.

These are waters that are in between marine and freshwater as they still have some saline, but not enough to be considered true marine environments.

To survive in brackish waters, however, the conditions have to be right. The saline levels need to be around 15.4 PSU (practical salinity unit) and they are completely absent from areas that have a salinity level below 30 PSU.

Marine environments are not the same worldwide. They can have a variety of different conditions due to variations in circumstances such as their salinity levels and temperature.

All You Need To Know About Radiolarians

This means that some radiolarian species are only found in specific regions and cannot be found worldwide. As many radiolarians are specific to certain conditions, their presence can be indicators of the properties of those waters and environments.

Another factor that depends on the environment and geographic location is the symbiotic relationships that can form between radiolarian species and algae. 

Radiolarians Characteristics

Let’s take a closer look at some of the characteristics of radiolarians. 

Cell Structure

As we stated earlier, radiolarians are single-celled organisms that have a nucleus. They have a spherical body with a large and complex nucleus located in the center.

Radiolarians are microzooplankton and as such, they are very small in size at only around 30 to 300 micrometers/microns (μm.)  In comparison, this is around the same size as the length of a human skin cell.

Compared to similar organisms, the cytoplasm within the cell membrane is highly organized. An organic wall separates the cytoplasm from the outer part of the cell.

Radiolarians have a highly complex outer cell layer that has a variety of different cell organelles, inclusions, and radiating pseudopods.

These pseudopods can differ from radiolarian species to species to form axopods instead. The axopods have microtubules and a surrounding envelope of cytoplasm. 

Radiolarians have shells, but these can also differ in their structure from species to species. In some, the shells are partially embedded in the cell substance so they don’t completely cover the cell.

However, in the majority of species, the shell does extend to cover the entire cell body and is often covered by a thin layer of protoplasm.

As the skeleton of the radiolarian forms, amorphous opaline silica is secreted into the intracellular vesicles, and this is how the shell is gradually formed. 

The shells, or exoskeletons, of radiolarians, have numerous spines that extend outwards which distinguishes them from organisms such as Foraminifera.

The outermost skeleton is known as the cortical shell and this is formed by the fusion of these spines. Elongated spines and beams connect this shell to the inner shells and they also give support to the entire structure and keep it solid.

You will also find small pores, or perforated holes, on the shell which allow pseudopods to extend when the radiolarian is feeding.

Feeding can also occur through axopodia, which are narrow pseudopodia that have microtubules enveloped in cytoplasm.

These long and slender axopodia are more rigid than the regular pseudopods because of the microtubule shafts inside.

The increased rigidity means that axopodia contribute in an important way to supporting the pseudopodia when feeding.

Sources Of Food

Radiolarians can feed on a wide variety of materials and it all depends on what is available in their environment.

By using their pseudopods and axopods, they can trap and feed on several different organisms such as bacteria, diatoms (algae photosynthetic organisms), and small protists that exist amongst other organisms.

Some radiolarians are omnivorous and will feed on both plant life and tiny organisms in their environment. However, many other radiolarian species are more specific in their diets and will only feed on certain types of food.

It is also possible for radiolarian species to form a symbiotic relationship with types of algal instead of purely relying on the food sources available in their environment.

This is more common when food is scarce and the radiolarians are unable to find enough food themselves. 

One of the most common circumstances where this occurs is with radiolarian species that live in the upper layers of the ocean. These layers receive more sunlight making photosynthesis possible.

This results in algal symbionts coming into contact with various Radiolarian organisms in the water and residing in the peripheral cytoplasm that surrounds the central bodies of the cells.

As they’re exposed to sunlight, the symbionts can make their own food by using the water and carbon dioxide that supports their Radiolarian host. This helps support the algae when other types of food are not available.

Radiolarians can be a very important part of the food chain in their environment. Not only can they form these symbiotic relationships with algae, but they can also serve as a food source for many other organisms.

For example, they can provide nutrition for salps.

Life Cycle And Reproduction

There have been studies that have discovered the production of gametes (reproductive cells) by radiolarian cells. However, there is not enough evidence to fully understand exactly how radiolarian cells reproduce as yet. 

In polycystines, which were classed as radiolarians under an old classification, reproduction occurred with the contraction of the extracapsular cytoplasm.

While this occurs, waste matter and symbionts are also ejected. Within the cell itself, the centrally located nucleus goes through numerous cycles of division.

This results in several nuclei, now known as swarmers, filling the intracapsular cytoplasm. These swarmers have two main components, which are an organic envelope and a vacuolar-bound strontium sulfate.

The swarmers escape the cell through several ruptures on the capsule wall.

However, although this nuclei division and creation of swarmers have been observed, the fusion between the swarmers have not, so the later stages of this process and what happens to the swarmers are not yet known. 

When it comes to the lifespan of radiolarian species, most of them are expected to only live for a few weeks or around a month. 

All You Need To Know About Radiolarians

Colonies And Size

As we have previously stated, radiolarians can exist as single cells or grouped together in colonies.

These colonies consist of numerous cells and are interconnected by a network of cytoplasmic strands. The cells are also wrapped in a gelatinous envelope that is produced by the cells. 

The size of the colonies can vary greatly. They can be reasonably small and measure only a few centimeters, but some colonies have been observed to reach sizes of over a meter. Colonies also come in a variety of shapes.

They’re commonly found in either spherical or ellipsoidal shapes, but some species instead form elongated colonies that are ribbon-shaped.

Single-cell radiolarians also differ in size. Some of the larger species can grow up to be between one and two millimeters in diameter. The species that reach this size are less common than the smaller ones and are usually found on the surface of the water.

It’s possible for single-celled, non-colonial radiolarian species to not have the outer skeleton that we discussed earlier and will instead be enclosed by a gelatinous coat. Some of them also release siliceous spicules in the peripheral cytoplasm. 

Comparing Radiolarians To Phaeodaria

In the past, the subclass radiolaria was further divided into two main groups which included polycystines and Phaeodarians.

However, after further research and genetic studies, it was decided that Phaeodarea did not belong to this subclass.

Instead, they were reclassified under the class Thecofilosea and discovered to be closely related to radiolaria, instead of part of their subclass.

It’s easy to see why they were originally included as a radiolarian subclass as radiolarians and Phaeodarians share a number of characteristics.

For example, Phaeodarians also have elaborate skeletons that are made from silica, although they can also be made from other materials. However, they do not have any axopodia, as radiolarians do.

Instead, Phaeodarians produce a network of fine pseudopodia that are tightly interconnected. These pseudopodia originate from the protoplasmic strands that are found in the central capsules of the Phaeodarians.

When compared, you will see that Phaeodarians are larger in size than radiolarians. However, it is very rare for Phaeodarians to form colonies as radiolarians often do.

Both radiolarians and Phaeodarians are found in marine environments, but while radiolarians can be found on the surface, Phaeodarians are often found in levels deeper than radiolarians inhabit. 

Phaeodarians differ in a number of other ways, also. Their capsular walls lack the fusules that are found in radiolarians and some species can have skeletal frameworks that are made from hollow tubes.

Although the reproduction of radiolarians isn’t fully understood yet, parts of it have been observed. In Phaeodarians, no sexual reproduction has been observed at all yet. 

How To Collect Samples Of Radiolarians

Thankfully, this is a relatively easy process that requires either a jar or a plankton tow net. You can purchase tow nets online, but they can also be made at home.

You should be able to collect some radiolarians from any area that has a high concentration of radiolarian shells or where these organisms are known to be present.

How To Make Your Own Tow Net

Simply cut a soda bottle into thirds and dispose of the bottom third. Keep the cap on your top third and punch three evenly placed holes around one of the edges of the middle piece.

Take a nylon stocking and cut a small hole in the tow. Slide the top of the bottle into the nylon and arrange it so that the top of the bottle, with the cap, pokes through the hole. You can tape it in place to ensure it doesn’t move.

The middle of your bottle should then be attached to the end of the nylon with the end with the holes facing away. Cut three lengths of string that are around three feet in length and attach one to each of the holes you made in the middle section of the bottle. 

Radiolarians Microscopy 

Once you have collected some radiolarians, you can prepare them for viewing under a microscope. This is also pretty easy to do.

All You Need To Know About Radiolarians


Take the water you’ve collected and run it through a sieve to separate the water from the specimens you’ve collected. Once you’ve removed the water, you might be able to see some larger radiolarians with the naked eye, but others might be too difficult to see.

If you’re unable to find any, use a dropper for smaller organisms to collect some water before you completely finish sieving all of the water.

You can then drop some of the water onto a glass slide. It may take a few tries to find a radiolarian, but it will work eventually. We would advise you to observe your sample at x100 magnification first before switching to x400 for an even closer look.

How To Isolate Radiolarians From Indurated Sediments

You might find that your radiolarian skeletons are trapped in sediments. Luckily, there is a way to release them so you can observe them under your microscope. 

First, you will need to boil your sediment sample in water as this will help to disintegrate it. Once it is disintegrated, sieve the sediments through a 45-micrometer screen in order to remove other material and organisms.

Take the sieved residue and boil it again, only this time add some hydrogen peroxide so that it doesn’t boil over. Sieve one more time, using the same screen as before.

There will be some carbonate left behind, so use some dilute hydrochloric acid to dissolve this. This process should be enough to isolate the radiolarians. 

You will need to wait for the sample to dry before you can observe it. While it is still wet, place some of the residue onto a glass slide and leave it until it dries.

Once dry, you will need to embed the sample using ​​Canada Balsam and then cover the slide with a coverslip. As we suggested before, begin with an x100 magnification and then move up to x400.

The x100 magnification will allow you to see the spines, patterns, and skeleton bodies that surround the cell body.

The higher magnification should be enough to see the cell body itself. Using both of these magnifications will let you observe everything that makes radiolarians so unique and interesting.

Final Thoughts

Radiolarians have existed for millions of years but our understanding of them is still in its infancy. Over 15,000 different species have been discovered in either living or fossilized records and an estimated 800 to 1,000 still live in the world today.

They’re commonly found in marine environments across the world but some can also survive in brackish water.

They are single-celled organisms that have their own nucleus but can also be found in large colonies that can grow to be over a meter in length. Each radiolarian only lives for a few weeks to a month and their reproduction is not yet fully researched and understood. 

If you want to learn more about radiolarians, it is relatively easy to collect your own samples from marine waters using only a jar or homemade tow line.

A good microscope will allow you to observe their cell structure and the intricate patterns of their spines and skeletons.

Jennifer Dawkins

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