A tool for feeding and mating: the Swiss Army stylet of Gyratrix hermaphroditus (Rhabdocoela, Platyhelminthes)

Abstract

Free-living flatworms are famous for their intriguing copulatory strategies, yet microturbellarian reproductive biology remains poorly characterised. Through direct and histological observations, we here elucidate the copulation mechanism within the cosmopolitan rhabdocoel species complex Gyratrix hermaphroditus Ehrenberg, 1831 (Polycystididae, Kalyptorhynchia). The sclerotized reproductive organs of these animals were visualized through Differential Interference Microscopy and Scanning Electron Microscopy. Mating behaviour is shown to constitute a sequence of events, including precopulatory ‘sniffing’ and ‘rolling’, followed by simultaneous insertion of the stylet sheath in the copulatory bursa of the partner and stylet ejection. Respective durations of these events were measured. As the copulatory organ is 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 it pharynx to feed on its prey’s body fluids. We suggest that the twofold role of the male copulatory organ may confine the evolutionary plasticity of this character trait, possibly explaining the morphological stasis in G. hermaphroditus.