A tool for feeding and mating: The Swiss Army stylet of Gyratrix hermaphroditus

Free-living flatworms are famous for their intriguing copulatory strategies, yet microturbellarian reproductive biology remains poorly characterised. Through direct and histological observations, we further elaborate on the copulation mechanism within the rhabdocoel species complex Gyratrix hermaphroditus (Polycystididae, Kalyptorhynchia). Mating behavior is shown to involve a sequence of events, including precopulatory ‘sniffing’ and ‘rolling’, followed by simultaneous insertion of the stylet sheath in the bursa of the partner and stylet ejection. Respective durations of these events are measured and reported. As parts of the male system are also used in foraging, we also report on the feeding mechanism of these animals: the stylet is used to puncture the prey’s body wall, and the contents of the prostate vesicle are ejected, after which the worm uses its pharynx to feed on its prey’s body contents. We suspect that negative genetic correlations due to the twofold role of the male copulatory organ may constrain the evolution of this character and possibly explains the relative morphological stasis for this trait in G. hermaphroditus.