Birds, Broodsacs and Zombie Snails

Sometimes, when an Amber Snail has eaten poo, something spectacular and horrible happens.

Gilles San Martin.

How did the snail’s eyes get like that?

The bird poo it ate was infected with eggs of Leucochloridium, a parasitic flatworm. Once in the snail’s gut, they hatched into larvae. Some of the larvae reached the digestive gland where they grew into a cluster of pulsating, grub-like, embryo-packed, broodsacs - the parasite’s replication stage.

As the broodsacs got bigger they pushed their way from the snail’s body cavity into its eyes, stretching the eye-stalks to many times their normal size. As you can see in the video, through the transparent skin of the eye-stalks they look like wriggling caterpillars - a tempting snack for a hungry bird.

The parasite has another trick. It inhibits the snail’s light-avoiding instinct, making it more likely to stay in the open where it will be exposed to birds.

When birds see the fake caterpillars they eat them, destroying the snail’s eye-stalks in the process. A snail may survive this attack and regenerate its eyes but even so, it is doomed. The parasite will still be lodged in its digestive gland and when the eye-stalks have re-grown, new broodsacs will invade them.

In the bird’s gut, the broodsac spills its cargo of embryos. They will develop into adult flatworms that feed and reproduce sexually. Next time the bird takes a dump, its egg-filled poo will become an infected meal for the next unfortunate Amber Snail.

Thumbnail image: Gilles San Martin

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