The Cruel Life of Snail Helminths

However there’s one group of predators who would love to make a quick meal of the snails: birds. For this reason snails are equipped with excellent olfactory organs (smelling) and also light sensors in their tentacles (those long, worm-like structures sticking out of their heads). The light sensors enable them to detect light and dark. Snails and slugs shy away from light and are thus nocturnal creatures, while their predators, most birds, are diurnal (come out in the daytime). That seems like a great arrangement. However the snail helminth, Leucochloridium paradoxum (commonly called the green-banded boodsac) drastically changes that set up.

Helminths are parasitic worms that grow inside of another organism. The eggs of L. paradoxum are found in bird droppings, a food source for snails. While the snail thinks he’s getting a good meal, he’s actually ingesting the eggs. These eggs are very hearty and are not digested in the gut of the snail. The eggs hatch into baby parasites and eventually grow in one of the internal organs, similar to a human’s liver and pancreas. Here the parasite spews out hundreds of parasite embryos into a broodsac in the tentacles (usually preferring the left tentacle) of the snail. This sac of embryos is brightly colored and also pulsates in response to light. Since the light sensors are also located in the tentacles, the infecting parasite and its progeny interfere with the snail’s brain and sense of light detection, forcing the snail to venture out in the day rather than the night…not a good thing since their natural predators, birds, are also out in the day.

The colorful, throbbing tentacles look like juicy caterpillars to the birds in the sky, and before long, a snail’s tentacles are ripped off and eaten. The parasitic embryos have just now entered into the bird’s digestive system where they grow, reproduce, and lay more eggs in the rectum of the bird. The next “bird bomb” that occurs will contain the parasite eggs, ready to be eaten by some unsuspecting snail and therefore repeating the infection process. It’s a cruel, but successful life process of the L. paradoxum helminth, transforming a part of an intermediate host (the snail) into what looks like the prey (caterpillar) of its main host (birds).