The ‘zombie’ snail in the image has been infected by the green-banded broodsac (Leucochloridium paradoxum), which is a parasitic flatworm (platyhelminthes) that uses terrestrial snails of the genus Succinea as intermediate hosts (with their primary host being birds). In its larval (miracidial) life stage a flatworm will be consumed by a snail and then develop into a sporocyst (almost like an egg sac). From there the sporocyst will enlarge and swell to form the swollen “broodsacs” which are now filled with heaps of cercariae (the next larval stage for the flatworm). The broodsacs themselves move into the tentacles of the snail and begin to pulsate (look up a video – it’s totally psychedelic!) in an amazing display that is meant to mimic a tasty caterpillar. The flatworm even alters the behavior of the snail in such a way as to make itself more detectable to potential predators – remember, the flatworm WANTS to be eaten, that’s how it will continue its development. At this point, if an insectivorous bird sees it and eats it, the cercariae will develop to their terminal / adult stage, which will then reside in the digestive tract of the birds. Male and female flatworms will then reproduce and lay eggs, which are then released via the host’s digestive tract and left for the next unwitting snail host to pop by for a snack.
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