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Backstabbed marine flatworms take no offense

There’s nothing like getting stabbed in the back, although for polyclad flatworms it’s one of the facts of life. Not just literally but figuratively, too, because stabbing is integral to the way they reproduce. Polyclads stab and get stabbed (called penis fencing) as a rite of reproduction that, while seemingly antisocial, is at least not fatal to either party. During a number of dives, a sudden appearance of these paper-thin worms provided me lots of opportunities to observe and photograph what it takes to make the next generation.

En garde!

Getting stabbed in the polyclad world is no one-sided affair because polyclads are a two-in-one male plus female. Over the course of a couple of weeks of diving at depths of about 65 feet, I came across many flatworms velcroed to the sandstone walls that line the canyon’s rim. Here I would see two flatworms sidle up to each other or several in a pile. Occasionally, one polyclad would raise the front half of its body to expose its double penises, which look like tiny cow udders, then deliberately lunge and stab a bundle of snow-white sperm into no particular location of its nearest neighbor.

The whole shot-in-the-dark stabbing routine works for polyclads because the testes and ovaries are not packaged separately in a specific body part but instead are scattered throughout the body. That seems reasonable when it comes to the ovaries because wherever sperm is injected it will find an egg. Yet it seems that sperm spread across the body would be diluted. Turns out that, at the ready, the sperm are shuttled into the penislike organ, which has spinelike structures akin to a hypodermic needle. En garde!


One polyclad may be the only one doing the stabbing or partners may simultaneously stab each other. The back-and-forth may result in multiple visible stab wounds. Even though I wasn’t privy to most of the fencing matches, I found plenty of individuals marked with sperm blisters.

Once the match concludes, the internal work begins. Foreign sperm set off on a journey through polyclad body tissues to locate an egg to fertilize. The worms do have a vagina but it exists only as an exit route for the fertilized egg capsules. As each fertilized egg is expelled, it passes through a ring of cement glands, which adhere to the capsule. The sticky eggs bind to the substrate so they won’t float away. After a few days, the embryos hatch as young flatworms to feed and develop into an adult.

Winning the bout

Penis fencing may not be glamorous but it does the job, as proved by my subsequent sightings of swimming mini-me polyclad flatworms the size of a flea. Though having both male and female parts suggests polyclads could just as well fertilize themselves, studies show that they rarely do. The strategy comes down to fatherhood demanding much less energy than that of a mother. And being able to father more offspring, even at the cost of having to be a mother, too, sees to it that more father DNA will be dispersed and mixed with diverse mother DNA. So, it’s a win-win for polyclads who skip the foreplay and go straight to sword play. 

— Judith Lea Garfield, naturalist and underwater photographer, has authored two natural history books about the underwater park off La Jolla Cove and La Jolla Shores. Send comments to