attack of the zombie snails

The semester's begun, teaching has started, admin isn't letting up any time soon, & there are days when I feel like a zombie by home-time. So it seems entirely appropriate to revivify a post I wrote 3 years ago, on that very subject.

Honestly, sometimes I think the zombie apocalypse is already here. Certainly zombies seem to be flavour of the month (& whatever friends say, I still can't bring myself to watch Walking Dead). And I've written about them myself: well, the insect variety, anyway.

But our developing understanding of how parasites 'zombify' their hosts has been developing since well before the latest iteration of human zombies grabbed the popular imagination. I was reminded of this when I saw the video below (in all its over-the-top hyperbolic glory), for I was first introduced to the concept of zombie snails years & years ago by one of David Attenborough's TV programs**. (According to my aging memory, it would have been an episode of Life on Earth.) 

The parasite involved here is a flatworm (strictly speaking, a member of the branch of Platyhelminthes known as flukes) called Leucochloridium paradoxum. It has the delightful common name "green-banded broodsac", which is a pretty accurate description of its appearance. 

Flukes have a fairly complicated life cycle involving multiple hosts and L,paradoxum is no exception: eggs hatch into miracidia, and each miracidium subequently develops into a sporocyst. Each sporocyst contains large numbers of cercariae, which is where the ‘broodsac’ name comes from. In this state they move through the snail’s body to its ocular tentacles, where their bright colours & movement show through the thin skin of the eyestalks. Apparently, if you’re a bird, this looks like a caterpillar… Anyway, once ingested by a bird, the cercariae mature into adults, which reproduce and the whole cycle begins once more.

Where does the mind control part come in? Well, your average snail doesn’t usually spend a lot of time out in the open – such behaviour can make one rather too visible to predators. But instead of their normal photophobic behaviour, infected snails come out in the open, often climbing up grass stems or out onto branches. Combined with the flashy tentacular display – which doesn’t occur in the dark – this makes them easily visible, & easy prey. (Having said that, I do wonder whether this is truly mind control: after all, having a parasite stuffed up your eyestalk must impair one’s ability to detect ambient light intensity.)

** And in ‘reading’ up for this post, I see that the wonderful Sir David also covered the zombie ants: