Mind-Controlling Parasite Turns Snails Into Disco Zombies

The natural world is always fascinating, with millions of unique species for us to explore and research. But now and again, nature presents us with something so strange that it is, frankly, bizarre. The scenario playing out in this extraordinary YouTube clip is one of them. It features a parasitic worm that takes over a snail and turns it into a pulsating, zombie-like version of itself. Let’s unpack exactly what has happened to this poor snail.

Pulsating Parasite

The animal causing this startling spectacle is the green-banded broodsac (Leucochloridium paradoxum). It is a parasitic flatworm (fluke) found in the Northern Hemisphere with reported sightings in several countries across Europe and Asia, including Germany, Belgium, Sweden, the UK, and Japan. It lives in moist areas such as marshes, but only in places where snails are present, as it needs them for part of its lifecycle. This is when things start to get weird.

Green-Banded Broodsac Lifecycle

Let’s start with the eggs of this fluke. They are found in bird poop, deposited on the ground. This is ingested by the amber snail (Succinea putris), and because the eggs have now come into contact with water, they hatch into the next life stage called miracidia. These tiny, elliptically shaped lifeforms only have around 10 cells but can move.  They make their way to part of the snail’s digestive system called the hepatopancreas and develop into a sporocyst with many branches. At the end of some of these branches is a swollen broodsac containing many larvae. Either one or both of the snail’s tentacles (eyestalks) are occupied by broodsacs. It can no longer retract them.

Inside the eyestalks, the larvae (each covered in protective mucus) run out of space. As they grow, the skin stretches thin, making the activity inside visible. The disco is about to start!

Pulsating Disco Beat

The brightly colored bands of the brood sac pulsate at a frequency of 40 to 80 pulses a minute. As you can see in the clip, they are mesmerizing, but you are not the intended audience. Green-banded broodsacs are manipulative neuroparasites that, in a rather sinister twist, also take over the snail’s brain. They force their hosts to move to a well-lit, open area where it is most likely to be spotted by an insectivorous bird. The pulsating movement of the poor snail’s eyestalks looks just like a caterpillar moving.

The bird approaches and takes a bite. The larvae inside the broodsac (metacercariae) pass into the bird’s intestines, where they attach to the gut wall using their suckers. Before long, they mature and begin producing eggs, which are expelled with the bird’s poop to complete the cycle. If the snail is lucky, it loses only one tentacle and survives, but it remains at risk of being infected again.

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