Ever watched those horror movies where a parasite controls the thoughts and action of its human host, turning it into a mindless zombie in the process? This could be merely a work of fiction in the world of movies and horror tales but for snails from the genus Succinea found in Europe, the reality of turning into a mindless zombie is very real.
The green-banded broodsac,(Leucochloridium paradoxum) is a parasitic flatworm that uses gastropods such as snails as its host for part of its reproductive cycle. The snails ingest the eggs of the adult worms from bird droppings. As for how the eggs ended up in bird droppings, I will explain it further in my last paragraph. As the egg hatches, the larvae travels into the digestive system of a snail to develop into the next stage, known as the sporocyst, it grows into long tubes to form swollen “broodsacs” filled with tens to hundreds of cercariae .
These broodsacs invade the snail’s tentacles, generating a brilliant transformation, of the tentacles, into a swollen, pulsating, colourful display that mimics the appearance of a caterpillar or grub. The broodsacs pulsate according to the availability and intensity of sunlight. Uninfected snails generally prefer to seek dark areas to avoid predators but infected snails seem to have an impaired judgement when it comes to light detection. Snails in this case tend to move towards area of sunlight whereby predators such as birds would easily detect it. This is precisely the aim of the green-banded broodsac as it wants the bird to consume the swollen appendages which were formerly the snails’s tentacles.
Once consumed, the cercariae develop into adult distomes in the digestive system of the bird. These adult worms sexually reproduce and lay eggs that are released from the host via the bird’s excretory system. These droppings are then consumed by snails to complete the life cycle of this parasitic worm . Therefore, for the green-banded broodsac to reproduce, it must ironically be consumed in order for it to fufill its parasitic role. So much for zombie movies, green-banded broodsac really makes snails into mindless gastropods!
 DeLaCruz, D. 2003. “Leucochloridium paradoxum” (On-line), Animal Diversity Web. Accessed March 23, 2010 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Leucochloridium_paradoxum.html.
 Christian, Christian Fuchs . (2010, March 22). Distome (leucochloridium paradoxum). Retrieved from http://www.weichtiere.at/Mollusks/Schnecken/parasitismus/leucochloridium.html