Species Spotlight

This Week: Raving About Radiolarians!

Credit: Flickr: Microscopy

There are a lot of fascinating creatures in paleontology. Some of them were ginormous, such as the ground sloth, Giganotosaurus, or Megatherium. Some were also extremely small, so small they are only visible with a Scanning Electron Microscope. Smaller creatures tend to get less media attention, and a lot of them are underrated. One of these amazing creatures that lies undetected by popular media is called a radiolarian.

What is a radiolarian, you may ask? Radiolarians are a kind of protozoa (a single-celled organism) known for their intricate shells. They are closely related to amoebas and ciliates, the blobby and hairy looking cells from biology class. Radiolarians are marine organisms that individually are small, but they can live in colonies that in total measure up to one meter in length. The shells of radiolarians are what they are most known for. The shells are made of silicate minerals and easily fossilize. There are also living relatives of the fossilized radiolarians, which have made them an interesting study, as most paleontologists have to work with more distant relatives when studying other creatures.

Where are they in the fossil record? Radiolarians are found consistently throughout the fossil record, with findings as far back as the Cambrian to as recent as the Quaternary period in the Cenozoic (because they have living relatives). They display an array of adaptations in this large span of time, which can make for interesting studies to find out why and how they adapted through geologic time. Because of how far back they extend in time, they are also good index fossils. Index fossils are used to help correlate rock strata from different countries, and even different continents by identifying the same fossils in different rock layers.  

What do radiolarians look like? At their largest, radiolarians are millimeters in length, with the average radiolarian only being a few micrometers long. They are most known for their silicate shells, which have beautiful geometric shapes (some even have radial symmetry). They have two shell layers, and it hasn’t been extensively researched as to why they are structured this way. Their external shell may have spikes or other patterned textures.The second layer houses its organelles; radiolarians have an internal nucleus, Golgi apparatuses, and mitochondria (its powerhouse!) to help them function. They also have vacuoles and food reserves inside their tiny bodies. Radiolarians also have small alveoli that are theorized to act like reinflatable life vests that fill up in shallower waters and deflate in deeper waters to allow them to access a variety of depths (“Radiolarians”). The internal structure and alveoli structure has been theorized through observation of extant species of radiolarians.

There are two groups of radiolarians: the extinct species in the fossil record, and the extant (living) group in the oceans today. The extant group has two categories: the Polycystines and the Phaeodarians. The Polycystine group is made of an opal silicate, whereas the Phaeodarian group has a different silicate shell that hasn’t been researched as much.

What did extinct radiolarians contribute to their environment? Extant (or living) radiolarians prey on phytoplankton, like dinoflagellates and diatoms. It can be inferred, based on structural similarities between extant and extinct radiolarians, that the extinct radiolarians had similar diets. In a lab environment, radiolarians have been observed to cling to foreign objects, thought this has not yet been observed in nature. This ability could have been used to cling a colony to coral, rock, or onto the seafloor.  There is also experimental evidence indicating that Polycystines share a symbiotic relationship with some algae forms. The radiolarians absorb organic carbon that is produced by the algae, and together they create an extended mechanism that moves similar to an amoeba cytoplasm (it looks like stretching silly putty). It’s pretty amazing to think of what these small creatures can do – tiny but mighty!

While it is fascinating to learn about how radiolarians function, why is it important to study them? Radiolarians can contribute to knowledge about climate change, especially throughout geologic time. Radiolarians of both extant kinds live in different oceanic latitudes – some in deeper water and some in shallow waters. These waters are divided by oceanic stratification, the tendency for waters to be divided by their different salt levels or densities. Scientific studies have been conducted to hypothesize reasons for some of the adaptations seen in radiolarians over time. The main factor does not seem to be a length of time, but rather changes within silicate content within the ecosystem of radiolarians. More specifically, the amounts of silicate available to the radiolarians in their respective “layers” in the ocean may have affected their efficiency in using silicate to make their shells. The amount of silicate available to radiolarians greatly affects how they construct their shell. By observing the silicate concentration of the shells, observations of water content variability in ocean stratification can be made, revealing how the ecosystems changed over time. Changes within the Cenozoic oceans have been theorized to be a factor in a significant decline in silicate use – from .18% 35 million years ago to .06% today (Lazarus et al)! This drastic change occurred only within shallow-water radiolarians and not deep-water specimens. This change in silicate availability seems to be a combination of competition with other organisms and a swelling of nutrients being moved from one place to another (Lazarus et al).

In summary, radiolarians are impressive organisms that functioned as single-celled organisms that also lived in large colonies – found in both rock strata and modern-day ecosystems. They play an important role as a food source for larger marine organisms, but also manage population control for several kinds of phytoplankton. Their unique structure and composition also make them useful for studying the health and changes within marine ecosystems over time. Who knew that such a small organism could make such an impact on paleoecology?!


Lazarus, David B, et al. “Radiolarians Decreased Silicification as an Evolutionary Response to Reduced Cenozoic Ocean Silica Availability.” Proceedings of the National Academy of Sciences, 14 Apr. 2009, https://www.pnas.org/doi/epdf/10.1073/pnas.0812979106. Accessed 22 Mar. 2022.

“Radiolaria: Fossil Record.” Fossil Record of the Radiolaria, University of California Museum of Paleontology , 1993, https://ucmp.berkeley.edu/protista/radiolaria/radfr.html#:~:text=Radiolaria%20are%20present%20in%20the%20fossil%20record%20from,Ordovician%20limestones%20of%20Spitsbergen%20%28Fortey%20and%20Holdworth%2C%201971%29.

“Radiolarians.” The Paleontology Portal, National Science Foundation, 2009, http://paleoportal.org/index.php?globalnav=fossil_gallery§ionnav=taxon&taxon_id=44.