The sexual battles of flatworms: barbed sperm, mating rings, traumatic insemination, and going down on yourself

In the wet sands of Italy’s beaches, an epic battle of the sexes is playing out. The combatants are flatworms and though their bodies are simple, their sexual habits are anything but. Take the common species Macrostomum lignano. After sex, if a female has been filled with unwanted sperm from an undesirable partner, it can double over, put its mouth over its genital opening, and suck the sperm back out. It’s an easy fix that gives the female control over who fertilises her eggs. But the sperm can put up a fight. It has a very different shape to the tadpole-like figure of human sperm. At its front, it has a sinuous ‘feeler’ that is uses to anchor itself into the walls of the female genitals. Further down, two large backwards-pointing bristles also help to hold the sperm in place. These barbed sperm can’t be sucked out easily. Even after ...