Parasites manipulate praying mantis's polarized-light perception,
causing it to jump into water

Date:
June 29, 2021
Source:
Kobe University
Summary:
Researchers have revealed that praying mantis (mantids) infected
with parasitic hairworms are attracted to horizontally polarized
light that is strongly reflected off the surface of water, which
causes them to enter the water. These results demonstrate that
parasites can manipulate the host's specific light perception
system to their advantage, causing the host to behave in an
abnormal manner.



FULL STORY ========================================================================== Researchers have revealed that praying mantis (mantids) infected with
parasitic hairworms are attracted to horizontally polarized light that
is strongly reflected off the surface of water, which causes them to
enter the water. In a world-first, these research results demonstrate
that parasites can manipulate the host's specific light perception system
to their advantage, causing the host to behave in an abnormal manner.


==========================================================================
This discovery was made by an international research group consisting
of Graduate student OBAYASHI Nasono, Associate Professor SAKURA Midori
and Associate Professor SATO Takuya of Kobe University's Graduate School
of Science, Associate Professor IWATANI Yasushi (Faculty of Science and Technology, Hirosaki University), Professor TAMOTSU Satoshi (KYOUSEI
Science Center for Life and Nature, Nara Women's University) and Dr. CHIU Ming-Chung (National Changhua University of Education, Taiwan).

These research findings were published in the scientific journal Current Biology on June 21, 2021.

Main Points
* The hairworm parasite lives inside the body of an insect host
(such as
mantids and crickets) that typically inhabits forests and
grasslands.

When the hairworm reaches maturity, it induces the host to enter
water (including ponds and rivers), where the hairworm breeds,
thus completing its life cycle.

* How the hairworm causes its host to enter the water has mystified
researchers for over 100 years.

* A two-choice experiment conducted in a laboratory revealed that
a higher
percentage of mantids infected by a hairworm were attracted
by horizontally polarized light (*1) compared to uninfected
individuals.

* In the outdoor experiment, a higher number of infected mantids
entered
the deep pool that strongly reflected horizontal light than the
shallow pool that reflected weakly polarized light.

* There is a strong possibility that hairworms induce the host to
enter the
water by manipulating the host's perception of horizontally
polarized light.

Research Background Normally, an animal's morphology and behavior
are regulated in order to benefit the individual's survival and
reproduction. However, approximately 40% of terrestrial organisms are
parasites and it is said that all wild animals have at least one type
of parasite. In other words, various anatomical changes and behaviors
observed in wild animals could be strongly influenced by parasites.

Remarkably, there are many species of parasite that can alter aspects
of their host's morphology and behavior (host manipulation) for their
own benefit (i.e.

to increase the parasites' fitness). Parasites that manipulate their
hosts are a good example of an extended phenotype (*2) and have come to fascinate many biologists.



==========================================================================
A known phenomenon whereby the parasite manipulates the host's behavior
can be seen in hairworms. Hairworms are nematomorph parasites that live
inside insects such as mantids and camel crickets (referred to as the
'host'). However, hairworms reproduce in rivers and ponds, so in order
to move themselves into these environments, they manipulate the host so
that it jumps into the water.

Previous research has suggested that the brightness of reflected light
(light intensity) on the surface of the water attracts the host, causing
it to fall in. However, aside from the surfaces of rivers and ponds, there
are many other luminous environments and instances in nature, including
forest openings, bright sandy habitats and grasslands reflecting sunlight
or moonlight. If the host were attracted to every single occurrence of
bright light in nature, then the host manipulation would fail. Therefore
this behavior of entering the water cannot be sufficiently explained in
terms of mere attraction to the light.

Polarized light (*3) is a type of light where the electric field of the
light wave oscillates in only one direction. The light reflected off
the surface of a body of water contains a lot of horizontally polarized
light, and it has been shown in recent years that many arthropods use
this horizontally polarized light to either seek out or avoid water.

The researchers hypothesized that host, manipulated by the hairworm,
is attracted by the horizontally polarized light and enters the water.

Research Methodology and Results To investigate this hypothesis, the researchers began by conducting laboratory experiments to see whether
or not mantids (the Asian praying mantis species Hierodula patellifera, hereafter referred to as 'infected mantids') infected by a hairworm
parasite (Chordodes sp.) and uninfected mantids of the same species
would be attracted to horizontally polarized light. The experiments were conducted using a cylinder divided into three sections, with a polarized
light section at one end and an unpolarized light section at the other
end. The mantid was placed in the middle section, and its location
was recorded after 10 minutes had elapsed (i.e. whether or not it had
moved into the polarized or unpolarized light sections). Over the course
of these two-choice tests, four different strengths of light were used
(these corresponded to twilight: ~150 lux, cloudy weather: 2000 and 6000
lux, and sunny weather: 15,000 lux) to investigate whether the brightness affected the percentage of individuals attracted to the polarized light.



==========================================================================
The results revealed that a higher percentage of infected mantids
chose the polarized light compared to uninfected individuals. Among
the polarized light choices, there was a particularly high tendency for polarized light of over 2000 lux to be chosen. Conversely, mantids did not
tend to choose the polarized light section if the angle of polarization
was changed to vertical, regardless of the strength of the light and
their infection status. From these results, it can be concluded that
mantids infected by hairworm parasites are attracted to horizontally
polarized light.

Next, the research group conducted outdoor experiments to investigate
whether or not infected mantids would jump into a pool reflecting
strong, horizontally polarized light. The experiments were conducted in
a cultivated area belonging to the Food Resources Education and Research Center, Graduate School of Agricultural Science, Kobe University. A mesh enclosure was set up containing two pools: Pool A, strongly reflecting horizontally polarized but dim light (low light intensity), and Pool
B, where the surface reflection was brighter (strong light intensity)
but weakly polarized.

The researchers released the infected mantids into a tree located
in-between the two pools and observed the insects' entry into the water
via video footage captured by stationary cameras.

Among the 16 infected mantids that exhibited this behavior, 14 individuals entered Pool A, which strongly reflected horizontally polarized light.

Therefore based on the results of both the laboratory and outdoor
experiments, it can be concluded that infected mantids are induced by
the horizontally polarized light to jump into the water.

Another interesting discovery made by this study is that many infected
mantids entered the water at around midday. In the laboratory experiments,
the distance walked by mantids was measured and it was found that infected mantids walked more during noon. This presents a new possibility that
the circadian rhythm of the host or the parasite may be involved in
increasing both the host's attraction to horizontally polarized light
and the host's level of activity, which ultimately induce water-entering behavior at a specific time of day.

Research Significance and Further Developments In nature, animals have
evolved diverse abilities to perceive the intensity, color, shade and polarization of light. These research results show, for the first time
in the world, that parasites can skillfully manipulate these abilities
to cause the host animal to exhibit behaviors that benefit the parasite.

This research group has recently begun investigating what kind of
mechanism uninfected mantids have for perceiving horizontally polarized
light. They are also trying to work out how hairworms manipulate
this mechanism. Illuminating these aspects will not only enable us to understand parasitic behavioral manipulation but could also contribute
towards the development of new methods for controlling animal behavior.

Glossary 1. Horizontally polarized light: Linear polarization is when
the electromagnetic waves of light only go in one direction (for example, horizontal or vertical). Light reflected off the surface of water contains horizontally polarized light. Bodies of water reflect a greater level
of horizontally polarized light the deeper and darker the bottom is.

2. Extended phenotype: The main idea of this biological concept is
that an individual organism's genes do not only effect the individual's morphology and expressed behavior, but have an extended impact on other individuals and the surrounding environment. Through host manipulation, a parasite's genes affect the host's morphology or expressed behavior. This
is an example of an extended phenotype.

3. Polarized light: The light that radiates from The Sun is unpolarized.

However, when this light is reflected (for example, by molecules and
objects above the atmosphere or by the surface of a body of water), the direction of some of the electromagnetic waves becomes linearly polarized
(i.e. they run in one direction), resulting in the light being polarized
to a degree.

Video: https://www.youtube.com/watch?v=ci42nMom988 This research was
carried out with funding from Japan Society for the Promotion of Science
(JSPS) KAKENHI grants (19K22457, 19H04925, and 19H04934) and Kobe
University's Fund for the Promotion of Joint International Research.

========================================================================== Story Source: Materials provided by Kobe_University. Note: Content may
be edited for style and length.


========================================================================== Journal Reference:
1. Nasono Obayashi, Yasushi Iwatani, Midori Sakura, Satoshi Tamotsu,
Ming-
Chung Chiu, Takuya Sato. Enhanced polarotaxis can explain
water-entry behaviour of mantids infected with nematomorph
parasites. Current Biology, 2021; 31 (12): R777 DOI:
10.1016/j.cub.2021.05.001 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/06/210629120804.htm

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