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For polyclad worm, sex and swordplay are synonymous

Tape worms, round worms, heart worms. Are gummy worms the only worms we like? If you’re a diver, the answer is no because of polyclad worms, a group of free-living (nonparasitic) marine flatworms that are beautiful, delicate and elusive. Polyclad means “many branches” and designates how the gut is divided into many branches as it radiates away from the mouth, which is located on the body’s underside. The earliest living animal survivors to hunt prey, polyclads typically engulf their prey whole, which includes invertebrates such as mollusks and crustaceans. Having a mouth part located on the body’s underside makes catching prey convenient because all they have to do is slide over the top of the victim and suck it in.

Local flatworms I see in the La Jolla Ecological Reserve display an exaggerated, ruffly edge and belong to the genus Pseudobiceros. This polyclad genus has the only members that actively swim to notify predators, like fish, that they taste nasty. Studies have shown that the worms harbor a toxic mucus coating that closely resembles ciguatara toxin. This is the powerful nerve poison famously found in puffer fish and the Japanese delicacy fugu. For those who dare, bon appetit. When it comes to breathing, polyclads don’t fret their lack of gills because they can exchange gases across their entire surface.

Living life on all things solid and often out in the open means that temperate polyclads like ours mostly run the color and pattern gamut, from vanilla to chocolate, to match the rocks and sediment over which they slither. They get around by flattening an already-flattened body against a substrate as if vacuum sealed. When they find a crack or tiny hole, they suck themselves in like smoke drawn into a fan. Some Pseudobiceros sp. may be separate species or simply variants of one species. If they are the same species, there must be some advantage to the color shift, but this is unknown.

Polyclads may be among the simplest of all animals but their reproductive system is among the most complex in the animal kingdom, and not only because they are hermaphrodites. Unlike most animals, where male or female gonads are sequestered in an egg or sac of sorts, polyclads have both kinds of reproductive cells scattered throughout their body. Being hermaphroditic suggests they have difficulty finding each other since having both male and female gonads is useful when one’s sexual options are severely limited, like when encounters with potential mates are extremely rare. I know I rarely find them. However, even for marine flatworms, some Pseudobiceros sp. take the cake when it comes to external genital parts because they have one female pore and two male ports. Male ports are used like hypodermic needles to inject sperm into the opposing worm’s tissues. A couple of Pseudobiceros species are known to “penis fence,” a dueling behavior that involves stabbing your opponent with sperm. Some researchers view it as the flatworms’ way to get a leg up on fertilizing each other’s eggs while minimizing the sperm received.

But why have a two-to-one ratio of male to female parts? Throughout the animal kingdom, males generally invest far less energy by fathering offspring than females do by nurturing the brood, such that strategies for populating the future gene pool aren’t equal for both sexes. In general, being male means spreading seed as widely as possible (quantity), while being female means being particular (quality). This conflict assures a healthy mixture of genetic traits. However, as hermaphrodites, Pseudobiceros species don’t have these checks and balances. Having two male ports balances out the energy investment against having a female pore, thereby assuring a robust population. At least this is one thought.

Polyclads are so fragile, the researchers who study them must be gluttons for punishment. Besides being no more than a few millimeters thick (or should I say thin), when polyclads become stressed, which readily happens, they self-destruct by dissolving in their own enzymes. How depressing to get back to the lab and find your day of difficult field work disintegrating into mucus before your eyes. If these challenges aren’t daunting enough, they are very hard to find. Polyclads themselves can relate. En garde!

— 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