Do parasites really wait for a full Moon to manipulate their hosts?

These parasites control their insect hosts' behaviour to reproduce. Scientists wanted to know whether moonlight was part of the strange strategy. Now they have the answer.

In Short

• Horsehair worms control hosts to enter water for reproduction
• Researchers studied if this behaviour follows Moon phases
• Parasites may use non-light cues like gravity to manipulate hosts

Some parasites can do something straight out of a science-fiction story; they can take control of their hosts' behaviour to complete their own life cycle.

Among them are nematomorphs, better known as horsehair worms. These parasites grow inside the bodies of terrestrial insects and other arthropods.

When they are ready to reproduce, they manipulate their hosts, mostly nocturnal, into entering water, where the worms emerge and reproduce.

This made researchers at Kyoto University wonder if parasites manipulate their hosts in sync with the phases of the Moon, and have now found that this strange behaviour does not appear to follow the Moon's cycle.

Their study, published in the journal Ecology on September 10, found that camel crickets entered water on both full-moon and new-moon nights.

DO PARASITES WAIT FOR A FULL MOON?

The researchers wanted to know whether nematomorphs manipulate nocturnal hosts differently depending on how much moonlight is available.

The question arose from earlier observations involving nematomorphs and daytime insects.

Some species can make their hosts move towards horizontally polarised light reflected from the surface of water.

Since moonlight changes between a full and new moon, the researchers wondered whether infected nocturnal insects might be more likely to enter water under a full Moon.

To test this, Hinako Asakura and Takuya Sato's team conducted field observations from August to October at a small artificial pond in the Kiso River system in Japan's Nagano prefecture, where large numbers of nematomorphs were also found.

They also examined the stomach contents of salmonid fish in a nearby forest stream.

The fish effectively provided another way to track what was happening in the ecosystem, as they feed in the stream both day and night, their stomach contents offered more insights about when crickets were entering the water.

MYSTERY OF PARASITE MANIPULATION

The researchers found that salmonoid fish consumed significantly more crickets at night than during the day.

But there was no significant difference between the number of crickets consumed on full-moon and new-moon nights.

That suggests nematomorphs manipulate nocturnal hosts at night, but not according to the lunar cycle.

"We were surprised to find that the manipulation was not synchronised with the lunar cycle, but this may be adaptive," Asakura said.

She added that relying heavily on reflected light would make the strategy less effective on new moon or cloudy nights.

The researchers now suspect that the parasites may use mechanisms that do not depend on light, such as changing how the host responds to gravity or increasing its movement.

"Many mysteries still remain about the mechanisms underlying host manipulation," said Sato, adding that different nematomorph lineages may have evolved different ways of achieving their motive of getting their hosts to jump into water.

- Ends