What’s That Weird Worm?

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What does 2020 have to offer us next? From COVID-19 to murder hornets, this year has been full of challenges and media headlines. Most recently, images and articles have come out on the invasive hammerhead worm. While the name sounds menacing, don’t fret, N.C. Cooperative Extension, Currituck County Center is here to provide research-based information on this new viral story.

Hammerhead worms are a member of the genus Bipalium, a group of predatory flatworms. These worms are invasive to the United States and originate in tropical environments in southeast Asia¹. Reports of hammerhead worms in the U.S. started emerging as early as 1901², and were prevalent enough to be used in classroom experiments in Louisiana by the 1930s³. Growing up to a foot long, Bipalium are characterized by their shovel-shaped head where they get their characteristic hammerhead name4. Similar to the shark, hammerhead worms are predators and feed on soft-bodied planarian such as slugs, earthworms, and other Bipalium5. These worms feed heavily on earthworms and have been reported to destroy entire populations of earthworm farms1. On top of that, hammerhead worms produce a tetrodotoxin, similar to the toxins found in pufferfish, that is thought to be used defensively as well as to subdue earthworms6.

hammerhead worm

While all that may seem bleak and just another reason to curse 2020, everything is going to be fine. Hammerhead worms have been here for over 100 years and haven’t been a problem yet. That’s due in part to their biology. Bipalium require humid, moist environments. This limits the areas that they can spread to, and explains why images and social media posts come out after heavy downpours. Another factor that aids in invasive species thriving and spreading is lack of predation. While hammerhead worms don’t have any natural predators, they are cannibalistic and anecdotal reports have come out about amphibians that will feed on the worms. Although it’s not much, these two predators can help keep populations from growing unchecked.

In the end, the hammerhead worm is an unusual planarian that you may start noticing on rainy days. These worms are not something to be worried about. If you find a worm you think might be a hammerhead worm, send a picture to Adam Formella, Currituck County Agriculture Extension Agent at adam_formella@ncsu.edu. Just be careful handling the worms as there is little information on their tetrodotoxin’s effects on humans. And please, don’t eat the worms!

If you have any questions or concerns, please contact Adam Formella, Currituck County Agriculture Extension Agent at (252) 232-2261 or by email at adam_formella@ncsu.edu. If you’d like to know more about invasive species, sign up for our Invasive Species Zoom class on October 15, 2020, from 9 to 10 a.m. Register for the free class online


(1) Choate P.M., and Dunn R. A. 2015. Featured Creatures: Bipalium kewense Moseley and Dolichoplana striata. Web.

(2) Esser R. P. 1981. Land Planarians (Tricladida: Terricola). Contribution no. 227, Bureau of Nematology, Florida Department of Agricultural and Consumer Services, Division of Plant Industry, Gainesville, Fl.

(3) Dundee D.S., and Dundee H. A. 1963. Observations on the land planarian Bipalium kewense Moseley in the Gulf Coast. Systematic Zoology 12: 36-37. 

(4) McClure M. 2014. Invasive Species: Hammerhead Flatworm/Hammerhead Slug. Web.

(5) Ducey P. K., McCormick M., and Davidson E. 2007. “Natural History Observations on Bipalium cf. vagum Jones and Sterrer (Platyhelminthes: Tricladida), a Terrestrial Broadhead Planarian New to North America,” Southeastern Naturalist 6(3), 449-460).

(6) Stokes A. N., Ducey P. K., Neuman-Lee L., et al. 2014. Confirmation and distribution of tetrodotoxin for the first time in terrestrial invertebrates: two terrestrial flatworm species (Bipalium adventitium and Bipalium kewense). PLoS One. 2014;9(6):e100718.