Undersea Penis Fencing! (That got your attention.)

This holiday season, millions of Americans will gush over freshly unwrapped Playstations and electric beard trimmers and other ribbon-festooned bits of electronic junk that will probably end up forgotten in a drawer by March. In all that exulting, one phrase that I doubt will be heard very often is: “Marine flatworms! No way! Oh boy oh boy oh boy oh boy!”

Yet those were the words I gleefully exclaimed last week when Elise’s mother presented me with Marine Flatworms: The World of Polyclads, a lushly pictorialized guidebook to some of my favorite invertebrates. I leafed through the sections on flatworm biology in a state of rapture. My girlfriend’s family looked on dubiously, no doubt wondering what kind of social deviant could possibly get so aroused by amorphous undersea blobs. I tried to persuade them of flatworms’ virtues, pointing fervently to the book’s most spectacular photographs: Check out this beauty, and this one, and this…

Marine flatworm, Thysanozoon sp. How is it that I have a million nudibranch pictures, but no flatties? Copyright Wolfgang Seifarth.

But convincing non-divers of flatworms’ merits isn’t easy. Sure, they’re colorful, but so are M&M’s. They may look pretty in the guidebook, but the page also renders them static, removes them from the context that makes them so spectacular: the reef.  It’s hard to explain how the vibrancy of the reef accentuates the worms’ gorgeous markings, how thrilling it is to find a species you’ve never before seen, how every dive becomes a joyful scavenger hunt for these brilliant invertebrates once you’re in the throes of flatworm mania. (I have equal love for the nudibranch, colloquially ‘sea slug,’ which looks quite similar to the flatworm but is taxonomically very different.)

Phrikoceros sp., copyright Michael D. Miller.

Only once have I seen a flatworm swimming, but I’ll never forget its supple grace, the way the margins of its body fluttered and undulated with heart-wrenching vulnerability: a soft-bodied little thing, detached from the reef that sheltered it, striking out in a vast predatory ocean, with only a thin coat of toxic mucus as protection.

Watch this video (flatworm begins swimming at 0:13) and tell me you’re not enthralled. Video courtesy of Youtube user memutic.

Anyway, I had to convince Elise’s family that flatworms are truly worthy of adulation. I flipped pages desperately, searching for the phrase that would catch their attention, and suddenly there it was – it might as well have been written in neon:

PENIS FENCING.

Say the book’s authors, Leslie Newman and Lester Cannon:

When two sexually aroused flatworms approach one another they slowly glide and touch, and then rear up, raising the front half of their body. The muscles around the copulatory structures are tensed and the penis, armed with a fine hypodermic-like hard stylet, is thrust out of the underside of the body and directed towards the partner. Then with deliberate motions each tries to stab its penis into the body of the partner – any part of the body. With short, stabbing motions each worm thrusts its penis stylet through the soft epidermis of its partner and injects a white bundle of sperm. They may do this repeatedly, until each bears multiple stab wounds and white blisters of foreign sperm. Then they lower their penis stylets and glide apart.

I nearly read this paragraph aloud, then realized that a description of a bizarre, rape-like sexual ritual was not the best excerpt to read to the family of one’s significant other. Good thinking, me.

Why do some flatworms engage in this strange behavior? (Only two groups actually penis fence; most flatworms care at least a little about consent.)  Worms are hermaphroditic, capable of being either father or mother, inseminator or inseminatee; so why would they rather impregnate than be pregnant?  The standard explanation is that becoming the mother is more energetically expensive – producing and carrying around a bundle of eggs is a serious drain on a flatworm’s resources. Although the eggs hatch after only a week, that week is still an exhausting and perilous one.

Video created by Leslie Newman – the very same Newman who wrote my guidebook. A strangely intense clip, complete with bombastic orchestral music and inane narration. Courtesy of http://www.theAwk.com

Also, fatherhood has an adaptive advantage: victorious father worms, which aren’t saddled with the egg burden, have the potential to inseminate other flatworms in battle and thereby spread their genes even further. Consequently, one would imagine, the entire flatworm population is steadily improving at penis fencing as good swordsmen propagate their genetic material. This strange mating behavior may well be producing a race of invertebrate Inigo Montoyas.

Finally, penis facing is also helpful in telling flatworms from their lookalikes, nudibranchs:

	Nudibranchs	Flatworms
Taxonomic Class	Gastropoda	Turbellaria
Breathing apparatus	External Gills	Respires through skin
Preferred Food	Sea Pens	Sea Squirts
Mode of Locomotion	Crawls with muscular foot	Undulates body to swim
Stance on penis fencing	No, does not engage in penis fencing	Yes, most definitely engages in penis fencing