Becky Crew is a Sydney-based science communicator with a love for weird and wonderful animals. From strange behaviours and special adaptations to newly discovered species and the researchers who find them, her topics celebrate how alien yet relatable so many of the creatures that live amongst us can be.
WHO'S GOT NO respiratory system, no circulatory system, no skeleton, and a mouth for an anus? This guy.
The aptly named hammerhead worm belongs to one of the most primitive animal groups in the world - the Platyhelminthes, otherwise known as flatworms. Predatory and large, with some species capable of growing to half a metre long, hammerhead worms are a global menace, but not because they pose any threat to humans.
A number of hammerhead worm species feed exclusively on earthworms, literally tearing them apart before dissolving them in enzymes and drinking them up. And as anyone who's ever had a garden will know, earthworms are the good guys.
Native to the tropic and temperate zones of Asia and Australasia, hammerhead worms of the Bipalium genus have invaded almost every corner of Europe and the United States. They love anywhere that's dark, cool, and moist, like piles of humus, or under rocks, logs or shrubs, and while they hate anywhere dry, they can withstand brief periods of desiccation by coiling themselves into a tight ball and enveloping themselves in mucus.
Hammerhead worms glide across the ground on a single mucus-fuelled 'creeping sole', holding their half-moon-shaped heads aloft while moving them back and forth and side to side. Like an old man combing the beach with a metal detector, hammerhead worms use the chemoreceptors on the lower surface of their heads to sense earthworm mucus and body secretions on the substrate.
Once an earthworm has been located, it will be subdued in a coat of mucus and cut into several pieces by a hungry hammerhead. To feed, a hammerhead worm will extend its pharynx - or throat - out of its mouth, secrete earthworm-dissolving enzymes onto its prey, and digest the softened flesh. Once the earthworm has been processed for nutrients, it will be excreted out of the same orifice it came in.
Hammerhead worm can regenerate itself
You might think that the perfect revenge on the earthworm-ravaging hammerhead worm is to cut it into pieces, but that's actually the worst thing you could possibly do to a hammerhead worm if you want to get rid of it.
If a hammerhead worm is cut into bits, either lengthwise or across its body, each piece will become a new, perfectly functional worm over the course of two or three weeks. During this time, the fragments will grow longer and narrower, and a flattened, spade-like head will form on one end.
Some hammerhead worms, like the Southeast Asian species Bipalium kewense, will even split themselves into multiple pieces deliberately as part of an asexual reproductive strategy known as fragmentation.
By sticking the tail-end of their bodies to the ground and pulling away from it, hammerhead worms can break off a piece of themselves that will become a younger version. These immediately mobile fragments only take a couple of weeks to reach adulthood, and a single hammerhead worm can release one to two fragments for reproduction each month.