This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Like many parasites, nematomorphs are known to manipulate their hosts' behavior to complete their life cycles. Also known as horsehair worms, among other names, these creatures develop in their hosts' body cavities and lure their terrestrial arthropod victims into the water, where the parasites reproduce.
Some species have even been observed to induce positive phototaxis, attracting the host toward horizontally polarized light reflected on the water's surface. Because some parasites, including nematomorphs, target nocturnal hosts, a team of researchers at Kyoto University wondered whether the parasites manipulate their hosts in sync with the lunar cycle. If such a manipulation mechanism has been conserved across nematomorph lineages, the team hypothesized that water-entry behavior in nocturnal hosts infected by Gordius and Gordionus nematomorphs might be more prevalent during the full moon and less common during the new moon.
From August to October, on full- and new-moon nights, the researchers sampled camel crickets floating in a small artificial pond in the Kiso River system in Nagano Prefecture, where they also found large numbers of nematomorphs. They examined the stomach contents of salmonid fish in a nearby forested stream, treating the fish as fellow field researchers that continuously survey the river, day and night. Their findings are published in the journal Ecology.
The scientists observed that camel crickets entered the water on both full- and new-moon dates. They found that salmonid fish ingested significantly more crickets at night than during the day, but they observed no significant difference between full- and new-moon nights. These results indicate that nematomorphs do manipulate their nocturnal hosts at night, but not in alignment with the lunar cycle.
"We were surprised to find that the manipulation was not synchronized with the lunar cycle, but this may be adaptive," says first author Hinako Asakura. "If host manipulation depended heavily on inducing positive phototaxis, there would be nights—such as new-moon or cloudy nights—when light reflected from the water's surface would be insufficient to effectively lure nocturnal hosts." These findings suggest that nematomorphs may instead use manipulation strategies that are less dependent on light conditions and do not alter a nocturnal host's phototaxis. Two possible alternatives are altered geotaxis, an organism's response to gravity, and increased locomotor activity, which can be characterized as hyperactivity.
Next, the research team plans to investigate these possibilities to understand whether different nematomorph lineages have evolved various mechanisms that trigger the same behavior in hosts with contrasting biological rhythms. "It's important to recognize that many mysteries still remain about the mechanisms underlying host manipulation, and it is fascinating to think that parasites may have evolved different ways to achieve the same remarkable outcome: causing terrestrial arthropods to enter the water," says team leader Takuya Sato. Hinako Asakura et al, Nocturnal host manipulation by nematomorphs is not synchronized with the lunar cycle, Ecology (2026).
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