An enigmatic intruder stealthily infiltrates the undergrowth of backyards, forests, gardens in Ontario. This unique creature – the only one of its kind in Ontario – is a member of the Bipalium genus and is commonly known as the wandering broadhead planarian, Bipalium adventitial – or hammerhead worm. With its distinctive shovel-shaped head and diet consisting primarily of invasive earthworms, snails and slugs, this creature is a fascinating mystery, silently shaping the soil beneath our feet.
Read on to find out what to do if you encounter a hammerhead worm in Ontario.
How to Recognize a Hammerhead Worm
Hammerhead worms, growing to about five to eight centimetres long, are easily distinguishable from earthworms. Their fan-shaped, yellow-to-tan-coloured body with a long dark dorsal stripe that does not extend over their heads is a clear identifier. Recognizing these worms is important given the threat that they pose to ecosystems in Ontario.
Hammerhead worms are likely native to Asia. Their bodies tend to move like a snake, with their heads shifting from side to side. Like any earthworm species, they can be found in gardens, woodlands, under objects such as leaf litter, and where the soil is moist.
These worms were introduced in the U.S. within horticultural plants and have been regularly found in greenhouses or in or around newly purchased plants from nurseries since they were first discovered in California in 1943. In Ontario, the wandering broadhead planarian was first discovered around 2017.
Hermaphrodites – A Single Worm Can Start a New Population
Hammerhead worms are hermaphroditic, meaning they have both male and female reproductive organs. Sexual reproduction has not been observed in a laboratory setting or the wild.
This species can also reproduce through a process called “binary fission,” where an individual flatworm spontaneously breaks in half. Each half regrows, then in about a week, the missing half yields two flatworms instead of one – the head regrows the tail, and the tail regrows a head. Both reproductive strategies mean hammerhead worms are not just unique, but also potentially threatening. They can easily establish in new areas because it only takes one or two individuals to start a new population.
Are Hammerhead Worms Harmful to Humans?
All Bipalium species secrete chemicals through their skin to make themselves noxious to predators and aid in the digestion of their prey. Hammerhead worms carry a neurotoxin called Tetrodotoxin that may be used as a defence mechanism but is a highly potent poison.
In humans, Tetrodotoxin primarily affects the nervous system. Symptoms may include numbness and tingling, especially around the mouth and extremities. Paralysis can occur, starting with the face and spreading to other body parts. Muscle weakness and difficulty speaking or swallowing may also be observed. These chemicals can cause skin irritation in humans if they hold the flatworm and domestic mammals if they consume it.
Many flatworm species can carry parasitic nematodes within them. Although exposure to these chemicals is likely low when handling them, caution should still be exercised to prevent any potential harm. People should not hold these worms without wearing gloves.
Hammerhead worms are known predators of earthworms and sometimes will also consume slugs and snails. The worm usually follows a chemical trail left behind its prey, and when it finds it, it crawls over the top, wraps itself around it, and begins engulfing it. Usually, hammerhead worms feed predominately on non-native earthworms and sometimes on slugs and snails in urban and suburban areas. This species may be a beneficial biocontrol agent in areas where invasive jumping worms have been introduced.
Management Techniques
Management of land planaria is complicated as many species are well established. In Ontario, there is only one species known to science, the wandering broadhead planarian, Bipalium adventitium, which at time of writing have been found in many different cities from Montreal to London.
This species has been spreading rapidly, with new populations being discovered in various parts of North America. Understanding and managing this species can empower people to protect local ecosystems and prevent further spread.
If you encounter individual hammerhead worms, there are a few effective methods for dealing with them. If you choose to eradicate them, they can be killed by placing them in alcohol or freezing them in a bag. Salt can also be applied to individual planaria to avoid touching them altogether. If you use salt, be careful when applying it around plants to prevent harming vegetation.
Do not to cut up land planaria to control them. As mentioned, terrestrial flatworms can regenerate from fragments, so cutting them into pieces creates more planaria and simply exacerbates the problem.
When implemented correctly, these management strategies can help control the population of the hammerhead worm and mitigate its potential impact on local ecosystems. Please visit the Invasive Species Centre for more information on managing invasive species.
Bipin is an enthusiastic explorer of the natural world, driven by a deep-seated sense of wonder and curiosity. He holds a Master’s degree in Entomology and a Master's degree in Biology. Bipin has had the privilege of serving as a past President of the Toronto Entomologist Association. In addition, he is honoured to serve on the boards of several environmental and wildlife organizations across Ontario.
2 Replies to “What You Should Know About Hammerhead Worms in Ontario”
Great read, very informative. It is scary that they can reproduce on their own though! scary stuff
Very informative article. Thank you so much.