Hammerhead Worms Are Toxic and Invasive, But Are They Dangerous?

But hammerhead worms, all members of the genus Bipalium, aren't something to get too fussed about as they're not a new thing in the U.S., or in most places in the world — they've been common residents of American gardens since the early 1900s. These predatory planarians are native to tropical and subtropical regions of the globe, so they love wet, warm spots. Over the course of the past couple of hundred years, global commerce has helped the hammerhead worm wriggle its way into most suitable habitats in the world — and there are a lot of suitable habitats out there, although you're unlikely to find them in a desert or at the top of a mountain.

"The majority of land flatworms in the U.S. are not native," says Matt Bertone, an entomologist at NC State University. "However, some have been here for over 100 years, so they are well established. They easily hide among objects and in soil where there's moisture, so moving any type of container or plants around the world has allowed them to colonize new areas. Thus, they are highly invasive and frequently show up in new regions."

Hammerhead worms are carnivorous and often even cannibalistic. They are sensitive to light and are active mostly at night, feeding on a variety of small, soft-bodied animals — snails, slugs and earthworms, although they occasionally feed on other small invertebrates like insects.

"They wrap around their prey with sticky mucus, and use a mouth located on their belly in the middle of the body to consume prey," says Bertone. "They use special enzymes to digest the prey outside of their body."

After the digestive juices have done their business, effectively turning prey into a puddle of goo, the hammerhead worm sucks its victim in with the help of a bunch of tiny hairlike structures on its underside, called cilia. The cilia also help the worms in locomotion, acting like hundreds of microscopic legs to pull them along on a thin film of slime the worms secrete.

Life cycles of flatworms are complex and differ from species to species. Hammerhead worms are hermaphroditic — they have both male and female reproductive organs — and can reproduce either sexually or asexually, though asexual reproduction is more common. For instance, Bipalium kewense — a species native to Southeast Asia but common worldwide — reproduce by fission, especially when they're outside their native range. In this process, a small portion of the body near the tail pinches off and becomes a new worm — a clone of its parent.

In other species, eggs are produced when they mate with other worms, self-fertilize or clone themselves.

"Species of Bipalium are the only known terrestrial invertebrates that produce tetrodotoxin, the poison that makes puffer fish deadly," says Bertone. "However, they do so only in small amounts and are not dangerous to humans unless eaten in large numbers. Thus, their danger is often over-exaggerated."

Hammerhead worms do pose a real threat to earthworm populations, however. For instance, researchers are concerned about the populations in France, where the presence of hammerhead worms had somehow gone undetected by scientists and gardeners alike for more than 20 years. The concern is that hammerhead worms — which don't aerate and fertilize the soil as earthworms do — have been eating earthworms and other helpful soil fauna into scarcity.