The humble garden snail has a nemesis. No, it’s not a bird, or a clumsy gardener, or even the French. It’s not even something that the snail can see (assuming, of course, that they actually had decent vision) because this threat lives inside its body. It’s called a trematode, or bird fluke, and it’s a tiny parasitic, mind-controlling flatworm.
While grazing in the garden, snails must be careful what they are eating. They sometimes feed upon tasty bird droppings, but unbeknownst to them they may contain hidden horrors: bird fluke eggs. The eggs were shed by an adult fluke, which lived inside the stomach of a bird, and exited the animal along with its excrement. Although the unborn flukes ultimately want to end up inside the body of a bird like their parent, they must first spend their youth inside a snail. They will not hatch if they end up inside another animal.
The snail cannot digest the fluke eggs and so they hatch within its body. The parasites grow and take on a sac-like form, quickly taking over the majority of the snail’s internal body cavity. They now replicate themselves, producing lots of tiny embryos. These embryos move into the brood sac in the snail’s head and mature into yet another form, which look a bit like tadpoles. The flukes have already come a long way, but they cannot stay in the head of the snail forever – they must get into the intestinal tracts of a bird if they are to mature and reproduce. The only way of transferring from snail to bird, however, is if their current host gets eaten.
It is likely that sometime in its life any one snail will get eaten by a bird, but the flukes cannot wait that long. To ensure that it happens sooner rather than later, the flukes attempt to control the snail’s behaviour. The parasites, which have now spread to the snail’s eye stalks, start to swell and change colour (see picture above), pulsating to make the snail appear more conspicuous to predators. Then, as they invade the snail’s rudimentary brain, the flukes somehow cause the snail to venture out into the open, its stalk eyes flashing like beacons.
The throbbing coloured bands within the swollen tentacles of the snail quickly attract the attention of foraging birds. Perhaps it looks like a juicy caterpillar. Or maybe it just intrigues the birds. It isn’t long before a bird swoops down and either eats the entire snail or, at the very least, rips its tentacles off.
In the end, the flukes are the only winners. The snail has probably died and although the bird thinks it has obtained an easy meal, it has unknowingly allowed the flukes to reach their ultimate destination. Once inside the bird’s gut, they will take up permanent residence and start stealing much of their host’s nourishment. When they are ready, they will produce eggs that pass through the bird’s digestive system and start the whole horrible cycle all over again.