French scientists Nicolas Rode and Eva Lievens have found that two species of shrimp form swarms because they are being controlled by parasites in their bodies. Some of these need to get into birds to complete their life cycle. Others need to pass from one shrimp to another. They all achieve this by controlling their hosts and making them gather in large conspicuous swarms. The parasites can more easily jump into fresh hosts if the shrimps are swarming, and the swarms are more easily spotted and devoured by flamingos.

It’s a sinister twist on animal gatherings: Rather than finding safety in numbers, the shrimp are being collectively herded towards death’s door by unseen forces.

Rode and Lievens studied two species of brine shrimp—small crustaceans that looks like legs and eyes attached to a splinter. (You probably know them as sea monkeys.) Each one is just a centimetre long, but they gather in temporary swarms that can stretch for up to 2 metres. There are many possible explanations for these swarms, but none of them can fully explain what’s going on. The shrimp aren’t gathering in places rich in food or nutrients.  They’re not avoiding predators, because they’ll still swarm in water that’s too salty to support fish. And while some species gather to mate, others reproduce by cloning themselves. Sex cannot be the only explanation.

Rode and Lievens turned to parasites. If animals are behaving in odd ways, it’s always worth considering that parasites could be influencing them. Many of these body-snatchers are also pharmacists that commandeer the brains of their hosts to create head-banging caterpillarsdocile cockroachessuicidal crickets and fearless rats.