My first encounter with a body snatcher was at university. While my friends were jostling over summer research positions involving seals, penguins and other cute cuddlies, I was drawn by the last proposal on the zoology bulletin board: “Does more than one trematode species manipulate the behaviour of its host?” I was intrigued. What was a trematode? And could it really control the actions of another animal? I quickly scrawled my name in the empty space. I didn’t know it then, but I had just joined the search for a master manipulator unknown to science.

The mystery organism was believed to live inside a shellfish that I had often collected and eaten for dinner: the humble, yet delicious, New Zealand cockle. In some coastal areas these bivalves are loaded with parasites, specifically the trematode flatworm Curtuteria australis. Regular cockles like to bury themselves in sediment to avoid being eaten by shore birds. But under the spell of the Curtuteria parasite, which has been studied for several years, this protective mechanism ceases to function—the cockle lies hopelessly on the surface, easy prey for oystercatchers. It’s bad news for the cockle, but for Curtuteria, the oystercatcher is the last stage of its life cycle—a long campaign of treachery that unfolds like this: first the parasite possesses a whelk—a dark-shelled sea snail commonly found on mudflats. There it reproduces asexually, building up an army of larval worms—more correctly known as cercariae—which seek out a suitable second intermediate host, a cockle. The cercariae get sucked through the cockle’s siphon, and once inside, they migrate to the mollusc’s foot—the fleshy bit in seafood chowder. Here they form cysts—tiny, gooey dwellings. When enough cysts have become embedded in the foot of the unfortunate cockle, the atrophying muscle tissue shrivels to such an extent that it becomes useless as a crawling and digging tool. At low tide, infected cockles have no choice but to remain on the surface, where they are vulnerable to oystercatchers, the last dance in this microscopic game of musical chairs. Sexual reproduction occurs in the belly of the oystercatcher, and parasite eggs eventually pass in bird faeces, hatch, and seek out their first host, a whelk, where the cycle begins again.

For the already ill-fated cockles, the plot thickened when a visiting researcher noted that the cysts lodged in cockle feet were of two distinct sizes. Was there possibly a second parasite species involved? And was it, too, actively manipulating cockles, or simply hitching a ride and letting Curtuteria do all the work? It was my mission to find out.

A quick flick through parasitology texts told me that the Curtuteria parasite belongs to a group of worm-like organisms called trematodes which are usually no more than a few centimetres in length, although species as small as one millimetre and as large as seven metres have been recorded. Almost all of the estimated 18,000 to 24,000 trematode species are parasites. Their most distinctive external feature is the presence of two suckers, one close to the mouth and the other on the underside of the animal.

However, most parasites do not cause the death of their host—it’s not generally in their best interest to drive their living larder to an early grave. So why would Curtuteria seek to be devoured by oystercatchers?

“The answer lies in the parasite’s life cycle, or the places these guys go to develop and reproduce,” explained Robert Poulin, the Southern hemisphere’s leading authority on parasite manipulation. “Let me show you what I mean.”