10 Marvellously Microscopic Critters

We are lucky to live on a planet that is covered in fascinating organisms. We can be inspired by a soaring eagle, marvel at a leaping whale, or be moved to help the environment by the plight of the panda. But these big animals soak up most of the attention we pay to the natural world. If we look closer, we find that, beyond the range of human vision, the microscopic world is just as packed with weird and wonderful creatures if only we pause to examine them.

Here are ten organisms you can see under a microscope to fill you with wonder or, occasionally, horror.

Related: Top 10 Creepy Creatures That Might Exist

10 Gastrotrichs

The gastrotrichs are some of the most elegant of the creatures in the microscopic realm. They glide around and slightly shimmer as they are covered in minute spines. This anatomical feature is how they got their name—gastrotrich is derived from Greek words meaning “hairy stomach.” This has led researchers to call them hairy-backs or hairybellies. Not the most flattering titles.

These creatures are part of their own phylum, one of the largest divisions in taxonomy. All animals with a backbone are members of the phylum Chordata, for instance, while there are millions of species in the phylum Arthropoda, including all insects and crustaceans. The gastrotrichs get much less attention than those other groups but are well worth a look.

They have some interesting reproductive techniques. Some gastrotrichs are true hermaphrodites—the same individual can produce both sperm and eggs and mate with others of their species. Some gastrotrichs go it alone, however, and only reproduce by parthenogenesis—not needing to mate to give birth.^[1]

9 Demodex

Try not to get itchy, but you have mites living in your face right now. Demodex mites like to burrow down into hair follicles and eat the skin cells nearby, as well as the oils that we naturally produce. They have eight legs as an adult and are related to spiders and ticks. They are obligate parasites, meaning that they can only survive on their hosts. Each species of Demodex has only a single species of host that they live on. Two species live on humans: Demodex folliculorum and Demodex brevis, both frequently referred to as eyelash mites, alternatively face mites or skin mites.

Despite most humans having a Demodex population living on them, these unexpected guests cause few problems. We are not born with these mites and pick them up from contact with our parents. They were considered tidy guests, too—it was thought that because they don’t have an anus, they never poop on your face. They were supposed to just store up all their waste inside themselves for their entire lives. A study in 2022, however, found that Demodex do have anuses after all, which means your face is their toilet.

Demodex die after a few weeks, which makes your face something like a mite graveyard, too.^[2]

8 Amoeba

Amoebas are not a taxonomic group like mammals or jellyfish. Instead, amoebas come from all sorts of groups, including bacteria and fungi. What makes an amoeba an amoeba, then? The way they move.

Sometimes, when you are looking at a sample under a microscope, you will spot a blob of jelly that seems to be stretching out. This is probably a single-celled amoeba. Amoebas do not have cell walls, so they are free to change their shape. This allows them to push out bulges from their body, known as a pseudopod. These are used for motion and to envelope their food. Once surrounded, the amoeba takes its food inside its cell and begins to digest it. It is not unusual to see an amoeba moving around with the remains of its prey still inside it.

In such a diverse group of organisms, there are some that buck the trend of what being an amoeba is usually like. Testate amoeba surround themselves with a shell. Some grow their own, but others make theirs by sticking the shells of other microscopic creatures to their cell membrane.^[3]

7 Rotifers

Rotifers are everywhere. If there is a patch of water that has not been chemically treated, then you are likely to find a rotifer lurking there. They can even sometimes be found in raindrops. When you look at them under the microscope, you might at first mistake them for a minuscule maggot. But when you watch some of them feed, you will understand why they are called “wheel animals.”

The bedelloid rotifers are instantly recognizable when they open up the corona that surrounds their mouths. These body parts are covered in cilia, little mobile hair-like parts that actively move. When viewed under the microscope, it looks like two wheels covered in spikes have started to rotate. This motion creates a vortex that pulls in the bacteria and algae that the rotifers feed on.

Rotifers are also pretty tough. Some can survive being dried out. Others were known to survive up to 10 years in a freezer. That’s nothing, though. A sample of Arctic permafrost that had been frozen for 24,000 years was defrosted, and still-living rotifers emerged from the ice.^[4]

6 Ciliates

Moving through the water is pretty easy, right? Well, it is for us. When you are microscopic, the material properties of water make it much more difficult. It is more like burrowing through treacle. Ciliates have found that the best way to get around in water is to have a thick coating of those moving cilia on the edge of their single cell.

The ciliates are a diverse group of organisms and come in many shapes and sizes. Perhaps the most famous ciliate is the Paramecium. These organisms use their cilia to spiral through the water while they search for bacteria and algae to ingest. Vorticella are ciliates that anchor themselves in place with a long stalk and use cilia to pull their food toward themselves. Some lose their useful cilia over the course of their lives, however.

Suctoria use cilia to move about when young, but when they settle into a suitable position, the cilia are lost, and instead, they produce long spikes tipped with toxic chemicals. When another single-celled organism touches the spike, it is trapped, and the suctoria sucks its insides out. A minuscule vampire.^[5]

5 Stentors

Stentors are perhaps the most impressive of the ciliates. Sometimes, they get so large that they are no longer really microscopic—it is possible to see some with the naked eye. This makes them unusually large for single-celled organisms.

The stentors get their name from their trumpet-like appearance. In Homer’s epic The Iliad, the herald for the Greek army was called Stentor and was said to have a voice as loud as fifty normal men shouting. The microscopic stentors do not, as far as we know, make much noise. Their trumpet-like shape is lined with cilia that sweep particles of food and microorganisms down into its interior, where they can be consumed.

Intriguingly, some stentors are capable of learning. This was thought to be impossible for single-celled organisms. If you poke a stentor, it reacts by jerking away. If you do this frequently in a short period, it stops reacting as strongly for several hours as if it remembers you are just playing with it.^[6]

4 Diatoms

Diatoms are the jewels of the microscopic world. They come in a wide variety of shapes that are created by their cell wall made from a shell of silicon dioxide, essentially glass. These cell walls can take the form of simple rods, but others make flat circles or triangles, while some are complex three-dimensional constructions.

Diatoms are some of the most successful organisms on the planet. They make up a sizeable fraction of the biomass of all living things on the planet. Each year, they take several billion tons of silicon out of the water they live in as they construct their beautiful walls. They are so numerous that when they die, they can reshape the planet. Some parts of the ocean are covered in a layer of dead diatoms many hundreds of meters thick. More than 30 million tons of dust, made mostly from ancient diatom remains in the Sahara, are carried each year by winds over the Atlantic Ocean and help to fertilize the Amazon rainforest.^[7]

3 Lacrymaria

Lacrymaria olor has one of the longest necks of any organism in the world—if a single cell can be said to have a neck. It feeds by extending its “neck” to up to seven times the length of its body and searching around for prey that strays too near. When the “head” on the end collides with another single-celled organism, it either pulls it toward the body or, if the prey is too large to pull in, rips a part of the prey off and feeds on that.

The Lacrymaria olor (Latin for “swan tear”) is a popular organism for scientific research. This is not always pretty. One researcher used a micro-scalpel to sever the “head” from Lacrymaria. This resulted in the head zooming off and swimming away. The neck closed up quickly, however, on the main part of the organism, and a new head began to swiftly regenerate. Alas, the decapitated head soon stopped its escape into independence and disintegrated.^[8]

2 Hydras

The Lernaean Hydra of Greek mythology was a huge beast with multiple heads. When one was cut off, two more would grow in its place. You don’t have to be Hercules, however, to deal with a hydra of your own. The microscopic hydra is nowhere near as threatening as the legendary one. Most are smaller than one centimeter. But they are no less impressive.

They do bear a superficial resemblance to the mythological beast with their multiple tentacles. If you cut one of them off, a new one does grow. If you cut up a hydra just right, they can regenerate their bodies—you can get several copies of the hydra from just one individual. Being relatives of the jellyfish, they have stinging cells, which they use to paralyze their prey before feeding on them. Even this is not the most amazing thing about the hydra, however.

It seems that, unless injured or starved, the hydra are immortal. They never die from old age thanks to some clever biochemical and genetic machinery. They are biologically immortal.^[9]

1 Tardigrades

Tardigrades are the darlings of the microscopic community. Their name means “slow stepper,” but they are more popularly known as water bears or moss piglets. With their chubby bodies and wiggling legs, they are pretty cute. What has made them famous, however, is their “immortality.”

Unlike the hydra, the tardigrade is not biologically immortal. It is, however, one of the toughest animals in existence—in the right circumstances. Because tardigrades live in water, they are acutely sensitive to drying out. Going without water is deadly for most animals, but tardigrades have a trick to deal with it. When they sense they are running out of water, they pull in their legs and produce special proteins to protect their cells. In this state, known as a tun, tardigrades can survive a surprising amount of otherwise deadly damage.

You can put the tuns under 6,000 times the normal atmospheric pressure or into a vacuum, and when placed back in water, the tardigrades will emerge as if nothing happened. Even 1,000 times the normal lethal dose of radiation does not bother them. They were even exposed to the harsh conditions of space by being sent into low Earth orbit and left there for 10 days. Within half an hour of being put into water on their return, many had survived the ordeal.^[10]