On the winner goes the Rough Periwinkle (Littorina saxatilis) from the North Atlantic (both sides of the pond)
On average, each clutch of 70 offspring had 19 fathers between them. Larger clutches had more…Males track females by following their mucus trails, and will attempt to mate with pretty much any snail they encounter, regardless of whether it is the correct sex or even the same species.
(2010) Extreme Female Promiscuity in a Non-Social Invertebrate Species. PLoS ONE 5(3):e9640. doi:10.1371/journal.pone.0009640