For the first time, scientists have figured out how tardigrades — some of the toughest creatures on the planet — mate.
These tiny, hardy critters have few obvious differences between males and females, which made it unlikely that they found mates by sight alone. It turns out, females may release a chemical cue that lures the males, researchers found.
The males strongly responded, moving toward the females in water environments. The females didn't seem to have the same compulsion, researchers reported in the November issue of the Journal of Experimental Biology.
Related: Tardigrades survive being dried out thanks to proteins found in no other animals on Earth
Tardigrades — also known as "water bears," thanks to their chubby appearance — can tolerate extreme conditions. For example, they can survive a combined exposure to the vacuum of space, cosmic radiation and UV radiation. Unlike some animals, male and female tardigrades are hard to distinguish. There are size differences but no obvious secondary traits.
As a result, it wasn't clear how most of the 1,300 tardigrade species found mates. One theory is that these microscopic animals release a chemical signal to find a mate. To test that theory, Justine Chartrain, a doctoral researcher at the University of Jyväskylä in Finland, and colleagues performed a series of experiments with the species Macrobiotus polonicus to see how individuals would react when exposed to members of the opposite sex.
They placed a female tardigrade in one sealed "arena" and a male in another, with another tardigrade in the middle. Then, they recorded the behavior of the middle water bear.
"In the water environment, males were spending more time next to females than next to males," Chartrain told Live Science in an email. This suggested that the males could smell the females in their chamber and were lured by it.
Based on these results, the researchers wondered whether the tardigrades could follow a chemical trail that worked in a medium other than water. So they tested the chubby creatures in a Jell-O-like substance called agar. When one water bear was released, it was given a head start to wander across the agar before another tardigrade was released.
"We wanted to know whether tardigrades could deposit chemical cues on the agar and follow this path,” Chartrain said.
Neither sex followed a path created by other tardigrades, but in the agar, "we saw that sometimes males followed females after randomly encountering them," Chartrain said. The females basically ignored the males, while the males often changed course to move alongside the females.
The study suggests that the tardigrades can only locate opposite-sex mates in water environments and that only the males actively seek out females for mating.
