Two-part barbed penises, a physical struggle and 20 minutes of penetration. That's how some sea slugs do it. But the real shocker is that, for one species at least, those in the female role seem to engage in these bizarre, violent sexual encounters more often than might be biologically necessary.

Nothing about sea-slug sex sounds like fun. A new peer-reviewed paper, published online on August 22 in PLoS ONE, reads almost like an Everything You’ve Always Wanted to Know about Sea Slug Sex (But Where Afraid to Ask) or a Fifty Shades of Sea Slug Sex. To procreate, one of these sea slugs pierces the skin of its partner with "a syringe-like penile stylet that injects prostate fluids," wrote the researchers, led by Rolanda Lange, of the University of Tuebingen in Germany. The slug then injects a “spiny penis” into its partner's genital pore to pass along sperm. The whole ordeal takes about 20 minutes in the wild.

Copulation this dramatic fashion can be dangerous, particularly for the one on the female end. The "stylet injection and penile spines can result in tissue rupture and wounding," the report says. There’s also the "vigorous precopulatory struggle" and prolonged penetration with a penis that "bears four to five large hook-shaped spines at its base and a crown of 20 to 30 minute fine pointed spines at its tip, which are spread like an anchor in the female genital tract during mating. "

But perhaps the strangest thing is that the sea slugs in question, Siphopteron quadrispinosum, are hermaphroditic, which means that any slug can assume the position of a male—or a female—in an encounter. In just over half of observed instances (54 percent), they both choose to be both, simultaneously piercing and being pierced in what is called reciprocal copulation. Despite the travails of acting as the female, however, these sea slugs assume only the female role once or twice a day.

To find out more about these strange liaisons and their unexpected frequency, the researchers collected specimen sea slugs from shallow water algae near Lizard Island in Australia and moved them into individual containers. They created dozens of "mating groups" of four sea slugs, and randomized each to either a low, medium- or high-frequency for mating opportunities. The low-frequency group only got to rendezvous about once every three days. The intermediate-frequency group was put together about once a day. And the high-frequency group got three meet-ups each day. Each mating opportunity lasted for one hour. Slugs didn't always take advantage of each other's company (but if any pairs were still going at it after their time was up, they were allowed to finish and then returned to their individual containers). The mating frequency pattern continued for six days.

The researchers collected eggs from spawning individuals after mating took place. They found that the spawn size for these copulating slugs actually maxed out under the moderate mating condition. But those that got more opportunities to mate (in the high-frequency group) continued to go for it even though their total egg count was about 23 percent less the intermediate group's total (and mean spawn size was about 16 percent lower). The group with low-frequency mating also had a lower rate of fecundity, with 10 percent fewer eggs and a 9 percent mean lower spawn size. This goldilocks fertility sweet spot so far is the first one found in an animal that engages in this sort of traumatic mating strategy.

Perhaps it's not surprising that under the frequent mating condition, the sea slugs seemed to lose some of their reproductive fitness. Those in the male role also started spending less time penetrating their partners (perhaps conserving limited amounts of sperm). And those in the female roles might have been diverting energy from egg production to repair wounds inflicted by mating.

Why would these slugs continue to pair off more often than necessary? The researchers aren't entirely certain. Theories borrowed from studying other organisms include the possibility that injected sperm could also be absorbed by the body as additional nutrients or that receiving sperm from more than one mate might stimulate egg production.

Or perhaps they are just evolved to get while the getting's good. These slug populations, the authors reported, "usually appear and disappear within a month," so the six-day "experimental period is likely close to cover lifetime reproduction."

There are plenty of other examples of traumatic copulation—bedbugs and some squid have been known to play hard. The study suggets that S. quadrispinosum is one of the more interesting subjects for studying this sort of sex.