How Do Tardigrades Reproduce? Several Ways, It Turns Out

When it comes to reproduction, tardigrades demonstrate incredible adaptability. Some species can switch between sexual and asexual reproduction, based on environmental conditions.

Many tardigrade species reproduce sexually, with males releasing sperm into the female's body cavity where fertilization occurs. The female will then lay eggs that hatch into tiny new tardigrades.

Some hermaphrodite species — which have both male and female reproductive organs — are able to self-fertilize. This is an ideal trait for living in extreme environments, like the deep sea or sand dunes, where finding a mate is as rare as winning the lottery.

Both asexual and sexual reproduction occur in tardigrades. Parthenogenesis, the only confirmed asexual method, involves females producing offspring from unfertilized eggs, creating clones of themselves.

Studies in the Journal of Experimental Biology and Covich's Freshwater Invertebrates reveal more about their superpowers, showing how these microscopic marvels thrive in extreme conditions and make invertebrate reproduction look easy.

Mysterious Water Bears Caught in the Act

Video footage captured by a team of researchers at the Senckenberg Museum of Natural History in Görlitz, Germany, provides some insight into how the deed is done in one bisexual species of tardigrade called Isohypsibius dastychi.

And though it might not look like much to the untrained eye, the researchers assure us that things gets weird.

The animals mate right after the female has molted, something that happens several times each year. The researchers paired off 30 male-female couples (assessing the sex of a tardigrade is no easy task in itself!) and filmed them doing their thing.

They found that after the female's outer skin detaches, she lays eggs inside it. If, for whatever reason, no male happens to waddle by after she lays her eggs, no biggie; she just reabsorbs them back into her body.

But if there is a male around, he sort of wraps his body around her head, and the two engage in an hour-long mutual stimulation marathon, during which he might ejaculate several times into the space between her old skin and her new skin, fertilizing her eggs in the process.

Witnessing this unusual sex spectacle has raised almost as many questions with researchers as it's answered. For instance: How does the male direct his semen to the right place if he's essentially injecting it under her skin? And how does such a tiny beast last for so much foreplay?

Tardigrades, sometimes called water bears or moss piglets, are microscopic, water-dwelling organisms known for incredible resilience in extreme environmental conditions. These tiny creatures typically range from 0.012 to 0.02 inches (0.3 to 0.5 millimeters) in length.

They are one of the few species that can be found on every continent, in a wide range of environments — from deep ocean floors to the highest mountains to extreme conditions like hot springs and Antarctic ice.

Instance, it's unbelievably difficult to kill one of these microscopic creatures. You can dry them out, starve, freeze, burn, irradiate, poison or stick them down in the deepest, most pressurized ocean trench and they will survive. You can even expose them to the vacuum of space (more on that in a bit).

Anatomy of a Water Bear

Tardigrades have a plump, segmented body with eight legs, each tipped with tiny claws or suction pads, giving them their distinctive appearance.

They possess a simple digestive system, including a tubular mouth, pharynx and intestine, but lack a circulatory and respiratory system, relying instead on diffusion for gas exchange. Their nervous system comprises a brain and a ventral nerve cord. Unique cells called Malpighian tubules function in excretion.

Water bears have specialized structures like the buccal apparatus for feeding on plant cells, algae and small invertebrates. Their peculiar adaptability is partly due to cryptobiosis, allowing them to survive extreme conditions by entering a dormant state.

Chillin' in Cryptobiosis

In this state, the water bears retract their limbs, lose almost all body water and form a tun — a desiccated, lifeless form.

Tardigrades can remain dormant for years, even decades, in cryptobiosis. During this period, their metabolic activities drop to nearly undetectable levels. They can endure extreme temperatures, high radiation, vacuum conditions and lack of oxygen.

Researchers found that tardigrades produce unique proteins that protect their cells by forming a glass-like substance when they dry out, which helps them survive harsh conditions. This glassy material melts away upon rehydration, allowing tardigrades to return to their active state.

Even NASA has studied tardigrades, particularly their ability to survive in space. Tardigrades have been sent to the International Space Station to study how they adapt to microgravity.

These tiny titans of the microscopic world boast a diversity that would make even the most varied of ecosystems jealous. With over 1,300 species identified, these resilient creatures belong to the phylum Tardigrada, divided into several classes, orders and families.

1. Eutardigrada

First up, in the class Eutardigrada, we have the order Parachela, home to the family Hypsibiidae and its star player, Hypsibius dujardini.

This little guy is a lab favorite, known for its ability to survive conditions that would make a cockroach blush. Then there's Macrobiotus hufelandi from the family Macrobiotidae, often found lounging in moss and lichen, taking in the sights of its tiny world.

2. Heterotardigrada

In the class Heterotardigrada, the order Echiniscoidea brings us the family Echiniscidae with Echiniscus blumi, a freshwater and terrestrial habitat enthusiast. Meanwhile, Echiniscoides sigismundi from the family Echiniscoididae prefers the salty embrace of marine environments.

3. Arthrotardigrada

Not to be outdone, the order Arthrotardigrada introduces Batillipes mirus from the family Batillipedidae, a marine species known for its claw-like structures — perfect for the tardigrade fashion runway.

4. Mesotardigrada

Finally, the class Mesotardigrada keeps it exclusive with a single known species: Thermozodium esakii. Discovered in a hot spring in Japan, this elusive tardigrade is rarely studied, due to its limited specimens, making it the unicorn of the tardigrade world.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.