Many creatures pull out all the stops to attract the perfect mate. Some turn up the contrast on scales, pelts and plumage, while others build up muscle to impress and some sing their hearts out, but male tardigrades don't bother with any of those flourishes. ‘Very few tardigrade species have extreme male traits’, says Justine Chartrain from University of Jyväskylä, Finland. So how do tardigrades attract members of the opposite sex? One possibility was that the microscopic creatures might release alluring scents to attract a partner, but it wasn't clear whether tardigrades can even smell each other and whether suitors might be able to follow an attractive odour, let alone a scent trail, toward a potential mate. With very little known about the reproductive strategies of the ∼1500 tardigrade species currently identified, Chartrain and her colleagues Emily Knott and Sara Calhim (University of Jyväskylä), with tardigrade specialist Łukasz Michalczyk (Jagiellonian University, Poland), decided to find out how one tardigrade species, Macrobiotus polonicus, responds to members of the opposite sex.

Chartrain kept colonies of the tiny creatures in flooded Petri dishes; however, to make sure that the animals were keen to mate, Chartrain needed to be certain that the tardigrades had never encountered a suitor, so she collected individual eggs before allowing the animals to develop into adults in isolation. Then, she constructed a tardigrade-sized arena, with two adjacent circular chambers 8 mm apart, before submerging the arena in water and placing a male in one chamber and a female in the other. Once the scene was set, Chartrain collected a third tardigrade, placed it in the space between the two chambers and waited patiently to find out whether the animal could smell either of the tardigrades hidden in their respective chambers and, if it could, how the animal in the middle reacted.

After hours of watching the tiny creatures located between the two chambers, it was clear that the males could smell the female secreted in her chamber and were keen to reach her, showing no interest in the chamber containing the male tardigrade. However, the females placed in the space between the two tardigrade chambers were able to smell both sexes and meandered back and forth between the two. So, tardigrades can smell each other, but males are definitely attracted by the alluring scent of females, which made the team wonder whether tardigrades can follow the scent trails left by others as they saunter across surfaces?

This time, Chartrain prepared an arena coated in soft agar and gave a tardigrade a 5 min head start as it wandered on the surface before releasing a member of the opposite sex in the vicinity to find out whether it could locate the first tardigrade. ‘They moved slowly, but much more than expected’, she chuckles, recalling that the mini-beasts often explored the entire area. However, none of the tardigrades – male or female – succeeded in finding and following the trail of the pioneering tardigrade. Without water to carry the leader's aroma, the second tardigrade was unable to pursue it. However, when a male ran into a female as it strolled along, the male diverted course to associate with and move side by side with the female. But, when females collided with males, they did not deviate and continued on their solitary course.

So, tardigrades cannot follow each other's scent trails, but they can smell the aromas of other tardigrades when immersed in water, and even though male and female tardigrades are virtually indistinguishable by appearance, they clearly smell different and males are keen to locate females when they get the chance.

K. E.
First evidence of sex-specific responses to chemical cues in tardigrade mate searching behaviour
J. Exp. Biol.