Mating behaviour of the marine turbellarian Macrostomum sp. : these worms; suck

Simultaneous hermaphrodites experience unique conflicts of interest during reproduction, some of which are reflected in their complex mating behaviours. We here provide the first detailed description of the mating behaviour of a marine flatworm of the genus Macrostomum, a cosmopolitan group of microturbellaria. Mating in this species is usually initiated by the precopulatory behaviours circling and reeling, then leads to reciprocal copulation where worms mutually insert their copulatory stylet, and often ends in an intriguing postcopulatory sucking behaviour. We provide detailed data on the frequencies and durations of the different behaviours, and examine some biotic and abiotic factors that could influence the mating rate. We further speculate on the function of sucking and suggest that it could be an adaptation for the digestion of sperm and/or the removal of seminal components, which may function as allohormones.