Experimental investigations on the partner-finding behaviour of Isohypsibius dastychi (Isohypsibiidae: Tardigrada)

The present study gives the first detailed results on the partner-finding behaviour of tardigrades. We investigated whether the finding of mating partners by Isohypsibius dastychi followed certain criteria that are expected if pheromone signals are involved. For this purpose, tardigrade pairs were positioned in an arena, at a range of distances from each other. Only those females that were ready for mating attracted the male. The smaller the initial distance between them, the more directly the male headed for the female. The route travelled and time taken increased exponentially with increasing initial distance between partners, which means that the male lost orientation with respect to the female the larger the distance between the pair became. The findings indirectly suggest a yet undetermined female pheromone, which produces a concentration gradient in the water and loses strength quickly in terms of distance. Our assumption is supported by rare observations of sperm release without direct contact between partners that nevertheless resulted in fertile offspring. The observation that the female partners in this no-contact mating behaviour then deposited their eggs (a behaviour that was never seen when males were absent) suggests that they, likewise, can sense a fresh ejaculate of spermatozoa in their direct vicinity.