Invertebrates
Hammerhead Flatworm
Bipalium · also called hammerhead worm, shovel-headed garden worm, land planarian
Find a long, flat, striped ribbon of a creature with a head shaped like a spade stretched across your garden path after rain, and you have found a hammerhead flatworm: an invasive earthworm hunter that carries pufferfish poison and grows back if you cut it.
Overview
Bipalium and its relatives are land planarians native to Southeast Asia, now spread worldwide in the soil of potted plants. The body is a flat striped strap 5 to 20 cm long, with a half-moon, shovel-shaped head. It hunts earthworms, following their slime trails and digesting them externally.
Poison and regeneration
Its skin and slime contain tetrodotoxin, the same nerve poison as a pufferfish, used to subdue worms and deter predators. It mostly reproduces by dropping a fragment from its tail that grows into a whole new worm, so cutting one up multiplies it.
A garden pest
By eating earthworms in numbers it can reduce local earthworm populations, which harms soil health, and it is treated as an invasive pest wherever it establishes. It needs constant moisture and emerges on humid nights and after rain.
Status
Not Evaluated — common in its native range and increasing as an introduced pest across the warm and temperate world. Handle only with a tool or bag, and never cut it.
Hammerhead Flatworm: common questions
Are hammerhead flatworms dangerous to people or pets?
They are not a serious danger, but they are best not handled with bare hands. Hammerhead flatworms secrete tetrodotoxin, a strong nerve poison, in their skin and slime, and while the amount is tiny, contact can cause skin irritation, and a dog or cat that ate one could feel unwell. There are no records of the toxin seriously harming a person through skin contact. The practical advice is to pick one up only with a tool or a gloved hand or a plastic bag, wash your hands afterward, and keep curious pets away from any you find.
How do I get rid of a hammerhead flatworm without making more of them?
Do not cut it, crush it into pieces, or chop it with a spade, because each fragment can regenerate into a new worm. The recommended method is to pick it up with a stick, a leaf, or a gloved hand, seal it in a plastic bag, and either add salt or vinegar to the bag or put it in the freezer for a day, then throw it away. Some people drop them into soapy or salty water. The key is to keep the whole animal contained so no piece is left in the soil.
Why does a flatworm have a poison that comes from pufferfish?
Tetrodotoxin is not made only by pufferfish. It is actually produced by certain bacteria, and a range of unrelated animals, including some newts, frogs, octopuses, sea stars, ribbon worms, and these land planarians, accumulate it, in several cases from bacteria living in or on them. In the hammerhead flatworm it appears to serve two purposes at once. It helps immobilise earthworms, which are large and muscular relative to the worm, and it makes the flatworm unpalatable to would-be predators such as birds and ground beetles. The flatworm was one of the first land invertebrates confirmed to carry it.
Is it true you should never touch a hammerhead worm?
The advice not to touch one with bare skin is sensible, for two reasons. First, the tetrodotoxin and irritant mucus on its surface can cause a rash or, at worst, a mild reaction if it gets into a cut or your eyes. Second, and more practically, if you try to remove one by pulling or squashing it, you risk leaving fragments behind, each of which can grow into a new worm, so handling it carelessly can make an infestation worse. Observing one is completely safe; just use a tool or a bag if you need to move or dispose of it.