Are These Horrifying Parasites Manipulating Snail Minds for Survival?

Clearly, something had to spark the inspiration behind the 'Alien' films.

Approximately 40 percent of all known animal species are parasites. These range from 30-foot-long fish tapeworms to the tiny Orthione griffenis, a crustacean no larger than a cough drop that survives by feeding on shrimp blood.

Numerous parasites have adapted to target highly specific hosts. Consider Strigiphilus garylarsoni, a chewing louse named after Gary Larson, the cartoonist behind 'The Far Side.' This louse spends its entire life on owls, feeding exclusively on feathers and organic matter found on their skin. No other animals are known to host this unique louse.

However, some parasites require more than one host. For certain species, reproducing and completing their life cycle depends on transitioning through multiple carriers.

This is true for Leucochloridium worms, peculiar organisms often accused of transforming snails into 'zombies.'

This behavior is believed to be part of a complex strategy involving birds and their droppings. If successful, the worms' plan results in snails losing their eyes to hungry birds.

Leucochloridium Parasites Are Flukes

The Leucochloridium parasites are not only bizarre but also classified as flukes.

Flukes, or trematodes, belong to the Trematoda class of flatworms. These organisms are characterized by their suckers, which they use to attach to various surfaces.

Every fluke is parasitic. According to the Animal Diversity Web database, most trematodes have intricate life cycles, with larval stages often infecting different hosts than their adult forms.

For a trematode, securing a suitable snail host is often essential.

There are approximately 18,000 to 24,000 fluke species. Typically, these parasites spend a portion of their life cycle inhabiting mollusks, a group that includes octopuses, mussels, and snails.

Depending on the species, flukes may reside in a host mollusk's kidneys, digestive system, or even its reproductive organs. Snails are frequent hosts for trematodes, and without them, the zombifying Leucochloridium flukes would be unable to reproduce.

Early Life Stages of the Leucochloridium

Adult Leucochloridium are elongated, flat parasites that infect insect-eating birds. They prefer the cloaca, the opening through which birds excrete and lay eggs. There's no need to judge their choice of habitat.

An adult Leucochloridium can live inside its bird host for weeks or even months before dying, though the exact timeline remains uncertain. Eventually, the parasite lays eggs, which are expelled through the bird's droppings.

Snails that live on the ground often consume bird droppings. If the right type of snail, typically an amber snail from the Succinea genus, ingests feces containing fluke eggs, the situation becomes quite bizarre.

Once a snail consumes the eggs, they hatch into transparent larvae. During the sporocyst stage, these larvae develop broodsacs—vibrant, banded tubes filled with more larvae. These sacs bear a striking resemblance to wriggling caterpillars.

Perhaps that resemblance is intentional.

How Do the Snails' Eyestalks Change?

Leucochloridium broodsacs don't appear randomly. Snails perceive their surroundings through light-sensitive eyespots, located at the tips of their tentacles, or eyestalks, which extend from their heads.

A healthy snail can retract its tentacles into its head at will, a behavior you might have observed if you've ever handled one.

However, when a snail is infected by these flatworms, its eyestalks become compromised. The growing broodsacs invade the tentacles, preventing the snail from retracting them.

To make matters worse, the broodsacs begin to pulsate. In a rhythmic motion reminiscent of a psychedelic rock video, the sacs expand and contract rapidly, sometimes dozens of times per minute. The snail's thin skin makes this spectacle fully visible to onlookers.

The broodsacs' vibrant color patterns add to their allure. They feature striking bands of green, orange, yellow, white, black, or brown, making them highly noticeable.

This bizarre display may have an evolutionary purpose.

Since the 1800s, scientists have speculated that the pulsating broodsacs are a ruse to deceive birds into thinking they are caterpillars. Birds that eat these sacs ingest larvae, which then migrate to the cloaca, mature into adult flukes, and restart the cycle.

How the Worm Influences the Snails' Behavior

This is where the concept of 'zombie snails' originates.

In the 1920s and 1930s, some researchers suggested that Leucochloridium actively alters snail behavior.

The parasites supposedly compel infected snails to abandon their usual habits. Under the flukes' influence, the snails move into open, brightly lit areas, such as the tops of leaves, where they become easy prey. Birds attracted to caterpillar-like movements spot the pulsating broodsacs and peck at them, often removing the eyestalks entirely.

However, this theory remains unproven, as no one has observed such behavior in the wild.

Experiments from 1874 showed that captive birds readily attacked the pulsating broodsacs of infected snails. Yet, this doesn't confirm the behavior occurs naturally, as animals often act differently in captivity. Just ask any wolf researcher.

New Research and Unanswered Questions

In 2013, researchers Wanda Weslowska and Tomasz Weslowski revisited the host manipulation hypothesis.

Their study, published in the Journal of Zoology, involved observing wild snails in Poland's Białowieża National Park. Some snails carried Leucochloridium broodsacs, while others were parasite-free.

The researchers noted, 'Infected snails with pulsating broodsacs exhibited different behavior compared to their uninfected counterparts. They traveled greater distances, occupied more exposed and well-lit areas, and positioned themselves higher in the vegetation.'

This increased visibility likely makes the parasites more susceptible to bird predation. Additionally, by choosing elevated spots, the snails and their broodsacs become more accessible to birds flying above.

A study published in the Journal of Environmental Biology in September 2022 revealed that Leucochloridium broodsacs pulsate more rapidly in daylight than in darkness. Since many birds that consume their larvae are daytime hunters, this increased activity during daylight may help the parasites attract avian predators.

Despite these findings, much remains unknown about the dynamic between these flukes and their snail hosts.

If the parasites truly manipulate snail behavior, as evidence suggests, how do they achieve this? Is it a form of mind control, or is another mechanism at work? Do the broodsacs genuinely deceive birds into mistaking them for caterpillars? And if not, how do adult Leucochloridium worms reach the cloacas of their bird hosts?

Perhaps future research will provide definitive answers. Until then, this phenomenon offers ample inspiration for a new series of 'Alien' movies. Over to you, Ridley Scott.