Tardigrades don't need any help in the weird department - the tiny creatures, also known as water bears, are only a few hundred micrometres long, but are almost impossible to kill. They can survive in the vacuum of space, endure total desiccation, and can even bounce back from being frozen for decades at a time.

Now we finally have footage of the strange creatures having sex, and researchers have shown it's even more complicated than expected (see below).

When it comes to mating, researchers knew that the some water bear species were bisexual (something that's not that uncommon in the animal world), and it was suspected that fertilisation happened outside the body.

But, on the whole, their sex lives have remained pretty mysterious.

Now a team of researchers from the Senckenberg Museum of Natural History in Görlitz, Germany, have finally filmed the process, which you can see in part below, providing new insight into how the creatures reproduce.

It might not look like much - this is the male ejecting his semen under the female's skin - but trust us, things get weird.

"In the present study we provide new insights into the mating behaviour of a bisexual tardigrade, Isohypsibius dastychi, revealing a process much more complex than expected," the team write in the Zoological Journal.

"Mating included mutual stimulation that preceded semen ejaculation and egg deposition."

As the team describes in the paper, the first step of the mating process is the female laying her eggs.

This happens during one of the animal's many moulting periods - when they shed their outer cuticle.

The eggs are laid inside that outer layer of cuticle, and then a male approaches and gets into position around her, a process that can take several minutes.

Once he's in the right spot, the water bears engage in mutual stimulation, until the male finally ejaculates his semen via an opening above his anus into the female's outer layer of skin - which is what you can see happening in the footage above.

This confirms that fertilisation actually occurs outside the female's body - although the researchers still aren't entirely sure how the semen gets to her eggs.

The team also found that if no mating occurred, the females reabsorbed their eggs. And the temperature that the animals were on determined how quickly the offspring ended up becoming adults. 

There's still a lot to learn, such as why the animals take part in this foreplay, and how exactly sperm is directed to the correct location.

Hopefully further study of this creatures will reveal more insight into their reproduction mechanisms. And knowing water bears, we're sure there are more weird surprises in store.

The research has been published in the Zoological Journal, and you can see more footage of the act over at BBC Earth.