Hammerhead "Slug"

Worm measuring over 3 WGPBC units

Our friend Pat found this while walking her dog.  It left a slime trail in the morning light and she had the forethought to photograph it with her friend Erin's snack pack for a size comparison.  Converting WGPBC units, (Whole Grain Peanut Butter Crackers) to inches (5.2") we can say the critter is over 7.3" long!

Her story is a great example of how an untrained naturalist approached the unknown.

"It almost looked like a small snake with a dark stripe down center back and lighter stripes on either side of that. It had a ginkgo leaf shaped ‘head’ and seemed to be de-scaling the curb like a snail as it crept along. It also moved differently than your standard worm.  When we looped by the area again on our walk the live one had its tail smashed and succumbed to its injury. I did collect the dried out dead specimen, don’t ask me why.

I saw another one a few days later that was at least 12 inches long, and the flat half-moon ‘head’ was proportionally larger as well.  After the sightings last week I Googled ‘worm-like slug’ and got an immediate hit on Hammerhead Slug.   The site I viewed did not describe them as being in this area, but it sure looked like what I was seeing."

The Hammerhead Slug, aka. Greenhouse Planarian (Bipalium kewense, BK) is the "Worlds Largest Flatworm" and one of the few flatworm species that live on land.  It is unrelated to slugs which are in a totally different phylum, but it leaves a slime trail like slugs and snails do.  It was probably originally an Asian species that has been imported along with potted plants.  It occurs throughout the southern US as well as world wide.  Pat's is the first one reported in Missouri on Inaturalist.org.

Bipalium kewense capturing an earthworm  -Pierre Gros

Here is a description of the feeding process.

"The flatworm initiates here the process of ‘capping’ the anterior end of the earthworm. Observed reactions of the prey suggest that it is at this stage that the planarian secretes a toxin to reduce prey mobility. The planarian also produces secretions from its headplate and body that adhere it to the prey, despite often sudden violent movements of the latter during this stage of capture. Pierre Gros.

Posterior fragment

Although all Bipalium sp.  are hemaphroditic, their usual reproduction is by asexual fragmentation.  They attach their tail to the ground, then pull it off to leave the segment to move on its own.  Over the next few weeks it regenerates a head and pharynx and becomes a fully functional if somewhat shortened adult.  Fun fact in bad taste: Its pharynx is also its anus!

BK feeds primarily on earthworms which it tracks down, then pins down by its slime.   It also produces a neurotoxin (tetrodotoxin) that results in paralysis that may help it mobilize the worm.  Dinner spoiler alert!  Then its pharynx spreads out and releases an enzyme that partially digests the worm outside its body.  Later the same pharynx will evacuate its waste.  You can watch it attack a worm in this video although you may want to mute the sound.

The BK produces very little toxin and it is not harmful to humans or their pets.  One source says it is a pest for farmers because it kills worms but it is hard to imagine there are enough of them to be a problem.  I am thinking it might make a nice pet for a Nature Nerd.  Pat, are you game?
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Much more detailed information on Bipalium kewense can be found at  this Wikipedia link and Entnemdept.ufl.edu.