These parasites are not in tune with the lunar cycle

by Kyoto University
September 29, 2026

Like many parasites, nematomorphs are known to manipulate the behavior of their hosts 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 then reproduce. Some species have even been observed to induce positive phototaxis, attracting the host toward horizontally polarized light reflected on the water’s surface.

Given that some parasites, including nematomorphs, target nocturnal hosts, a team of researchers at Kyoto University wondered if parasites manipulate their nocturnal 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 may be more prevalent during the full moon, and sparse during the new moon.

From August to October during nights of the full and new moon, the researchers sampled camel crickets floating on the water in a small artificial pond in the Kiso River system in Nagano prefecture, where they also found large numbers of nematomorphs. In addition, 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.

Eventually, the scientists observed that camel crickets entered the water on both full and new moon dates. They did find that the number of crickets ingested by salmonid fish was significantly higher at night than during the day, but observed no significant difference in the number of crickets ingested on the full and new moon nights. These results indicate that nematomorphs do indeed manipulate their nocturnal hosts at night, but not in alignment with the lunar cycle.

Keep reading: https://www.kyoto-u.ac.jp/en/research-news/2026-09-29

Read the Ecology paper: https://esajournals.onlinelibrary.wiley.com/doi/10.1002/ecy.70475