Researchers have discovered that parasitic hairworms infected praying mantids (or maantids) are attracted by horizontally polarized light. This causes them to penetrate the water. These research results are a first in the world and show that parasites can alter the host's light perception system to their benefit, making the host behave abnormally.An international research team, including Graduate student OBAYASHI Nasono and Associate Professor SAKURA Miori and Associate Professor SATO Takuya of Kobe University's Graduate School of Science and Technology, Associate Professor IWATANI Yasushi of Kobe University's Graduate School of Science and Technology), Professor TAMOTSU (KYOUSEI Science Centre for Life and Nature Nara Women's University) and Professor CHIU MingChung (National Changhua University of Education Taiwan)These research results were published in Current Biology, June 21st 2021.The Main PointsThe hairworm parasite lives in the body of an insect host, such as crickets or mantids. It is usually found in forests and grasslands. The hairworm matures when the host enters water (including rivers and ponds) where it breeds.Researchers have been trying to figure out how the hairworm gets into the water for more than 100 years.Two-choice experiments in a laboratory showed that horizontally polarized light (*1) attracted a greater percentage of hairworm infected mantids than uninfected.Outdoor experiments revealed that more infected mantids were found in the deep pool, which strongly reflected horizontal sunlight than the shallow pool, which reflected weakly-polarized light.Hairworms may induce water ingestion by manipulating the perception of horizontally-polarized light.Research BackgroundNormal animal behavior and morphology are controlled to ensure survival and reproduction. However, parasites make up approximately 40% of terrestrial organisms. It is believed that every wild animal has at least one parasite. Parasites could influence the behavior and anatomical changes of wild animals. It is amazing to note that many parasite species can manipulate the behavior and morphology of their hosts (host manipulation) for their own advantage (i.e. To increase parasite fitness. Many biologists are fascinated by parasites that manipulate hosts.AdvertisementHairworms are an example of a parasite manipulating the behavior of its host. Hairworms, nematomorph parasites, live in insects like mantids or camel crickets. They are also known as the 'host'. Hairworms can reproduce in rivers and lakes, so they manipulate the host to make it jump into the water. Research has shown that the bright reflected light (light intensity), on the surface of water attracts the host and causes it to fall in. Apart from rivers and ponds' surfaces, there are other bright environments and instances of light in nature. These include forest openings, bright sandy habitats, and grasslands that reflect sunlight or moonlight. The host manipulation would be impossible if the host was attracted to all occurrences of bright light in the natural world. This behavior of entering water cannot be explained by mere attraction to light.Polarized light (*3) refers to a type light in which the electric field of the light waves oscillates only in one direction. Horizontally polarized light is reflected off water surfaces. Recent research has shown that many arthropods use this light to seek out water or avoid it.Researchers speculate that the horizontally polarized light attracts the host and draws it into the water.Research Methodology and ResultsThe researchers started by testing the hypothesis in laboratory experiments. They wanted to determine if mantids, which are members of the Asian praying mantis, Hierodula Patellifera, were infected with a hairworm parasite (Chordodes.sp.). Horizontally polarized light would attract both infected and uninfected mantids from the same species. The experiment was conducted with a three-sectioned cylinder, which had a polarized section at one end and an unsaturated section at the opposite end. After 10 minutes, the mantid was placed into the middle section. It was then recorded (i.e. It was then recorded after 10 minutes (i.e.,. whether it had moved into unpolarized or polarized sections). These two-choice tests were conducted with four different intensities of light (twilight: 150 lux, cloudy and 6000lux, and sunshine: 15,000lux). This was done to determine whether brightness had an effect on the proportion of people attracted to the unpolarized light.AdvertisementResults showed that infected mantids opted for polarized light at a higher rate than uninfected people. There was a high preference for polarized lights with more than 2000 lux. Mantids were less likely to choose the polarized section of light if the angle was adjusted to vertical regardless of how strong the light is or their infection status. These results show that mantids with hairworm parasites attract horizontally polarized light.The research group then conducted outdoor experiments to determine if infected mantids would leap into a pool that reflected strong, horizontally-polarized light. The experiment was conducted in a cultivated area of the Food Resources Education and Research Center. A mesh enclosure was constructed that contained two pools: Pool A which was strongly reflective horizontally but dimly lit (low light intensity) and Pool B where the surface reflection was stronger (strong light intensity but weakly-polarized).Researchers released infected mantids to a tree in the middle of the two pools. They then observed the insects enter the water using video footage from stationary cameras.14 of the 16 infected mantids displayed this behavior. They entered Pool A which strongly reflected horizontally-polarized light. Based on both laboratory and outdoor tests, it is clear that infected mantids were induced to jump into the pool by horizontally polarized sunlight.This study also revealed that infected mantids enter the water around noon. The laboratory experiments measured the distance that mantids walked and found that they walked more at noon. This suggests that parasites or hosts may have a circadian rhythm, increasing the host's attraction horizontally polarized light as well as the host's activity level. This could lead to water-entry behavior at a particular time of the day.Research Significance and Future DevelopmentsAnimals have many abilities to sense the intensity, color and shade of light. These results demonstrate that parasites are able to manipulate these abilities to make the host animal exhibit behavior that is beneficial to them.The research group is currently investigating the mechanism that uninfected mantids use to perceive horizontally polarized sunlight. They also want to understand how hairworms work with this mechanism. These aspects will help us understand parasitic behavior manipulation and could lead to the development of new methods to control animal behavior.Glossary1. Horizontally Polarized Light: Linear Polarization refers to when electromagnetic waves of light only travel in one direction (for instance, vertically or horizontally). Horizontally polarized light is reflected from water's surface. The deeper and darker the water's bottom, the more horizontally polarized light is reflected by the bodies of water.2. Extended phenotype is: This biological concept states that genes have a greater impact than just on an organism's morphology or expressed behavior. They also have an extended effect on the environment and other people. A parasite's genes can be altered by host manipulation to affect its morphology and expressed behavior. This is an example a extended phenotype.3. Unpolarized light: The sun's rays are unpolarized. When this light is reflected, for example by molecules or objects above the atmosphere, or the surface of a body water, some of the electromagnetic waves become linearly polarized, i.e. They run in one direction, resulting in light being polarized to some degree.Video: https://www.youtube.com/watch/ci42nMom988This research was funded by the Japan Society for the Promotion of Science KAKENHI grants (19K22457 and 19H04925 respectively) and Kobe University's Fund for the Promotion of Joint International Research.
