Buzz
In most cases, the term 'single-celled organism' is closely associated with microscopic size, and this is for good reason. The overwhelming majority of unicellular organisms never exceed a length of one tenth of a millimeter. Their size is constrained by various factors: Larger cells struggle to maintain structural stability; the movement of food and waste within the cell becomes more challenging. In numerous instances, growing larger doesn’t offer enough evolutionary advantages to justify investing energy into further growth. These and other influences keep microorganisms predominantly microscopic. However, in a vast, ancient, and diverse microbial world, there are certainly exceptions. This list highlights a few of those single-celled organisms that break the mold.
10. Stentor
Reaching up to 2 millimeters in length, the trumpet-shaped freshwater protozoa of the genus Stentor are visible to the naked eye and renowned among microbe enthusiasts for their impressive size. While 2 millimeters may not seem large, it's important to note that this size surpasses that of many multicellular invertebrates. Among unicellular organisms, Stentor is a true giant.
One reason Stentor can grow so large is due to its unique internal structure. Unlike typical cells, Stentors (like most of the organisms on this list) possess multiple nuclei, the cellular components that house DNA and control the cell. Having more than one nucleus appears to assist larger cells in managing their extensive cellular structure. Specifically, Stentor has numerous small micronuclei that oversee reproduction, while a single, large, string-like macronucleus controls its routine functions.
Stentors are classified as ciliates, meaning they are covered in tiny, hair-like structures known as cilia. Stentors and other ciliates use these cilia to swim, coordinating their movements to propel themselves. But cilia serve more purposes. While Stentors obtain some nutrients from the algae living inside them, they are primarily filter feeders. To gather food, Stentors attach themselves to floating debris or sediment, extend their trumpet-shaped “mouth,” and employ a ring of modified feeding cilia to generate a current that draws in bacteria, smaller protists, and the occasional unsuspecting water flea.
In other words, not only is the single-celled Stentor larger than several multicellular creatures, but it sometimes feeds on them.
9. Spirostomum
The largest species of the worm-like Spirostomum genus can reach lengths of up to 4 millimeters, making them far larger than their Stentor cousins. Found in both freshwater and saltwater, they are often mistaken for tiny worms. However, under a microscope, it becomes clear that these creatures are actually long single cells.
Despite its length, Spirostomum is also famous in the microbial world for its extraordinary ability to shrink. When threatened, it can shrink to a quarter of its original size in less than a hundredth of a second. This is the fastest-known contraction of any cell.
Like Stentor, Spirostomum is a ciliate. Its cilia are arranged in a spiral pattern, which helps propel it forward and funnels bacteria into its small 'mouth' along the side of its body. Similarly to Stentor, Spirostomum has one large macronucleus and multiple smaller micronuclei—a configuration unique to ciliates.
However, Spirostomum differs from Stentor in terms of its prey. While Stentors are capable of hunting small multicellular creatures, Spirostomum primarily feeds on bacteria.
8. Chaos Carolinensis
Imagine an amoeba, then magnify it to the size of a sesame seed. That’s Chaos carolinensis. While its size fluctuates with its shape, the largest individuals can stretch to 5 millimeters in length. It’s so large that placing a cover slip on it under a microscope can actually damage it.
Even though C. carolinensis is quite large, it behaves much like a smaller amoeba. It moves around using temporary, gelatinous extensions known as pseudopods (which means 'false foot' in Latin). These pseudopods are also used for feeding. When C. carolinensis encounters prey, it surrounds it with its pseudopods and engulfs the prey into an internal, temporary compartment called a vacuole. Inside the vacuole, the prey is digested alive, and the remnants are eventually expelled as waste. C. carolinensis consumes other microbes as well as tiny invertebrates, such as water fleas or rotifers. It continues feeding until it's ready to reproduce.
Similar to Stentor and Spirostomum, C. carolinensis possesses multiple nuclei, though unlike the other two, these are not specialized or organized. A single nucleus simply cannot control a cell of this magnitude. In fact, depending on its size, C. carolinensis can harbor up to 1,000 nuclei.
The naming of Chaos carolinensis was subject to a prolonged debate among scientists following its discovery, as they struggled to categorize it. As a result, older references often used various names such as Pelomyxa carolinensis and Chaos chaos. To simplify matters, some authors resorted to calling it “the giant amoeba.”
7. Gromia Sphaerica
When scientists from the University of Texas explored the seafloor near the Bahamas, they were perplexed to find several peculiar, grape-sized spheres. Although they appeared to be stationary, they clearly left trails behind in the sand. Initial theories ranged from a strange new type of snail to oddly-shaped feces. But a closer look revealed a far stranger reality. The objects were actually massive, 3-centimeter (1.2 in) wide spherical protists slowly rolling across the seabed at a nearly glacial speed.
Gromia sphaerica, also known as the Bahamian Gromia, is a type of testate amoeba, meaning it is an amoeba-like organism that protects itself with a soft, porous shell called a test. By extending its delicate pseudopods through openings in the test and securing itself to the seafloor, the cell can gradually move along, feeding on organic material in the sediment.
The discovery of this slow-moving giant protist had significant consequences for scientists’ understanding of evolutionary history. The oldest clear evidence for multicellular life dates back 580 million years, but the finding of fossilized tracks as old as 1.8 billion years has prompted some researchers to suggest that the origins of multicellular life go much further back. Initially, it was assumed that no microbe could have left such tracks. However, those ancient traces closely resemble those left by G. sphaerica, suggesting that its ancient relatives could have produced them. As a result, the timeline for multicellular life now appears to be much later than previously thought.
Unfortunately, very little is known about these rolling blobs of cytoplasm because it’s difficult to gather live samples. Despite their shell, they are soft and fragile by human standards. In fact, researchers have described them as being even softer than a grape.
6. Sailor’s Eyeball
Until now, all the creatures on this list have been “animal-like” protozoa, but an entire list could easily be dedicated to the giant single-celled algae. Known as bubble algae, Sailor’s Eyeball (Valonia ventricosa) can grow to 4 centimeters (1.6 in) in diameter or more. Found in shallow tropical waters around the globe, this marble-like protist is mostly solitary, although it sometimes forms small clusters. Younger individuals are a translucent green, but older ones are often coated in smaller algae and creatures. Essentially, Sailor’s Eyeball is so large that it provides a home for other multicellular life.
While some people are fascinated by Sailor’s Eyeball’s unique biology and its striking, gemstone-like appearance, it’s most notorious as a persistent pest for aquarium owners. Frequently introduced into tanks via “live rocks” taken from the ocean, this algae rapidly spreads and is remarkably hard to eliminate. Killing or removing it proves tricky, and popping them does little to help since that’s actually how they reproduce.
5. Spiculosiphon Oceana
Reaching a maximum length of 5 centimeters (2 in), this peculiar aquatic protozoan has astonished scientists since its discovery. Divers first encountered it in 2013 in an underwater cave off the coast of Spain and initially thought it was a carnivorous sponge. (Indeed, such sponges do exist.) However, closer inspection revealed a far stranger truth.
Spiculosiphon oceana is part of a group of test-building amoebas known as Foraminifera, though its similarities to the closely related Gromia sphaerica are minimal. Unlike the rolling, detritus-feeding sea-grape, this species remains anchored in place and feeds by filtering. To gather food, S. oceana extends its long, tentacle-like pseudopods through the pores of its test, letting them float freely in the water. As plankton get trapped, it digests them. This feeding method is remarkably similar to that of many marine invertebrates, including carnivorous sponges.
For its remarkable achievement as a 5-centimeter-long single-celled organism, scientists recognized S. oceana as one of the top 10 new species discovered in 2013.
4. Acetabularia
Commonly known as Mermaid’s Wineglass, Acetabularia is a fascinating genus of algae that takes on a mushroom-like form and can grow up to 10 centimeters (4 in) tall. These algae are typically found in clusters in shallow, rocky waters, populating subtropical regions worldwide. They sometimes form vast carpets on the seabed, their light green caps creating a stunning underwater landscape.
Acetabularia stands out from the other organisms on this list due to its unique internal structure. While most large unicellular organisms feature multiple nuclei that increase in number with size, Acetabularia operates differently. It typically has just one large nucleus located at the base of its “stem” throughout its life. The only time it deviates from this pattern is during reproduction, when the nucleus divides several times, and the resulting daughter nuclei travel to the top frond. There, they form spore-like reproductive cysts, which will disperse and generate new Acetabularia.
The size of the cell combined with its singular nucleus made Acetabularia a key figure in the field of cellular biology. In experiments conducted in the 1930s and 1940s, German scientist Joachim Hammerling (whose research was funded by the Nazis) demonstrated that the nucleus functions as the cell’s control center. By grafting the caps and nuclei of two different Acetabularia species, he found that the cell would adopt the traits of the species from which the nucleus originated.
3. Caulerpa Taxifolia (Aquarium Strain)
This type of unicellular algae, with its long string of fern-like fronds, is a giant even within its own group of macroscopic unicellular algae. In the Mediterranean, its preferred habitat, it can grow to an impressive length of nearly 3 meters (10 ft). Caulerpa taxifolia is so enormous, structurally complex, and resembling multicellular organisms that some sources fail to mention that it’s actually one incredibly long cell, filled with countless nuclei and other parts floating within.
However, C. taxifolia is not originally from the Mediterranean, nor does it naturally grow to such large sizes in its tropical homeland. Instead, the massive Mediterranean variant is the result of human intervention, similar to the case of the Africanized killer bee. With its attractive appearance and ease of care, C. taxifolia was initially introduced to aquariums in the 1970s. A German aquarium sought to breed it for display purposes, exposing it to harsh chemicals and UV light to induce mutations, making it more resilient, faster growing, and able to survive in colder waters. By 1980, they had created the desired strain and shared it with aquariums across Europe.
In 1984, disaster struck. The cold-water strain “escaped” from an aquarium in Monaco. Within a few years, it had taken over the Mediterranean. Compared to its natural form, the mutant strain grows larger, faster, and more aggressively, tolerates pollution, and can regenerate from fragments as small as 1 centimeter (2.1 in). It’s also toxic. Despite numerous eradication attempts, the spread continues, and the main concern now is how to contain it.
Due to the environmental havoc it has caused, C. taxifolia has earned the nickname “killer algae” and a spot on the Global Invasive Species Specialist Group’s list of the 100 most dangerous invasive species.
In the end, there you have it—a unicellular organism that’s bigger than you.
2. Plasmodial Slime Molds
Initially thought to be a type of fungus, plasmodial slime molds, or Myxomycetes, are a fascinating type of unicellular life that blur the lines between individual organisms and collective ones. Starting life as small, amoeba-like creatures living in the dirt, much like any other single-celled organism, they feast on bacteria. However, under certain conditions, something remarkable happens. These individual cells begin to group together, merging into one enormous mass. While most slime molds remain relatively small in this form, some can grow to over 1 meter (3 feet) in diameter or even larger.
Now functioning as a single organism, the slime mold moves across the ground at a slow pace, consuming anything in its path—be it food or unfortunate bacteria. Essentially, it behaves like a massive amoeba, navigating around obstacles and even detecting the best food sources from a distance. This process continues until it has devoured enough. Once that happens, the slime mold halts, creates fruiting bodies, and releases spores to start the cycle all over again.
But hold on. If the slime mold originates from individual cells coming together, doesn’t that mean it’s not technically unicellular? The answer is no. Plasmodial slime molds are indeed unicellular. Unlike the “cellular slime molds,” where the cells keep their separate membranes, plasmodial slime mold cells completely fuse, dissolving the membranes between them and forming a single, massive cell with millions of nuclei.
1. Syringammina Fragilissima
The largest member of the Xenophyophore class (example pictured above), which is already known for harboring unicellular giants, this immense amoeboid creature resides on the ocean floor and can grow up to 20 centimeters (8 inches) in diameter. Like many of its relatives, it doesn’t create its own test but instead assembles it from the remains of smaller microorganisms and sponges. It glues these fragments together with a slimy secretion, forming an intricate network of delicate tubes that serve as its home.
Sadly, there’s still much we don’t know about Syringammina fragilissima. While scientists believe it feeds on bacteria, the exact method remains a mystery. Theories range from filter feeding to cultivating bacteria within its shell. Even its reproductive process is unclear. This is partly due to its deep-sea habitat, but also because of its extremely fragile nature. Its scientific name translates to “very fragile sand pipe.”
