Behold the humble water flea, locked in a battle of mythological
proportions

Date:
September 20, 2021
Source:
Washington University in St. Louis
Summary:
Biologists sized up an unlikely natural phenomenon: when parasitism
actually causes the number of hosts to increase, an effect known
as a hydra effect. A study of common water fleas and their fungal
parasites includes laboratory components and an analysis of 13
fungal epidemics in nature. The scientists use consumer-resource
theory to explain why -- and in what types of systems -- the hydra
effect can occur.



FULL STORY ==========================================================================
In Greek mythology, Hydra was a monstrous water serpent that lived in
a swamp and terrorized nearby residents. When intrepid Hercules sliced
off one of Hydra's multiple heads, two more heads grew back in its place.


==========================================================================
This counterintuitive result -- when an action taken to reduce a problem actually multiplies it -- is known as a hydra effect. Scientists propose
that ecological systems exhibit something akin to a hydra effect when
a higher death rate in a species ultimately increases the size of its population.

In other words, what does kill you makes you ... more abundant.

New research from Rachel Penczykowski, assistant professor of biology
in Arts & Sciences at Washington University in St. Louis, investigates
why and in what types of systems this hydra effect can occur. The study includes laboratory components and an analysis of 13 fungal epidemics
in nature.

"Disease epidemics can drive declines in host populations, trigger
conservation crises for wildlife and sometimes even drive hosts extinct," Penczykowski said.

"Typically we predict that parasites that increase host mortality
rate should decrease host density. But this is not always the case." Penczykowski's study in the journal American Naturaliststarts with the
common water flea. These tiny crustaceans with antler-like antennae
and rounded bellies twitch their way through freshwater lakes across
North America.



==========================================================================
But the story is even larger than these adorable algae-eaters.

Food web interrupted Water fleas are filter feeders that play a key role
in local food webs.

"If you want a nice, clear lake, you want a lot of these zooplankton
swimming around gobbling up all the algae," Penczykowski said.

Water fleas typically range in size from 1-5 mm. They grow larger as
they eat and then become food themselves, hunted by midge larvae and
larval fish.



==========================================================================
But it's that gobbling mouth that gets water fleas into trouble. While
gliding through the water eating little floating plants, water fleas accidentally swallow spores of a fungal parasite.

The parasite soon takes hold -- an infection that seems all the more
dramatic because it's easy to see the fungus spores spread through the
water flea's see- through body.

Ultimately, the infection kills the water flea in a matter of days.

"You see these little needle-shaped things accumulating," Penczykowski
said.

"Then it starts to grow in their head and everywhere." Heavily infected
water fleas become more sluggish.

In the new study, Penczykowski and her collaborators conducted feeding experiments and determined that water fleas tend to eat significantly
less once infected, even during the early days of their infections.

Cue the hydra: Here is the opportunity for other water fleas to step up.

Water fleas are normally such important algae "grazers" that their
presence can mean the difference between a mucky marsh and a really nice swimming hole. But once some individuals in a lake become infected and
stop foraging as much, there is more algae available for as-yet-uninfected water fleas to eat.

Key conditions Penczykowski discovered that this is one of the key
conditions for a hydra effect. The parasite must target a host -- in
this case, water fleas -- that would otherwise substantially consume
all available food resources.

The new study uses consumer-resource theory to explain why -- and in
what types of systems -- the hydra effect can occur. (The hydra effect
cannot be used to describe the spread of coronavirus or effects in human systems, Penczykowski noted.) In addition to the laboratory feeding experiments, Penczykowski and her collaborators investigated 13 fungal epidemics in nature. All of their samples came from freshwater lakes,
located primarily in the U.S. Midwest. The scientists counted infected
and uninfected water fleas and figured out how much algae was available
for the water fleas to eat.

The model they had developed in the laboratory was accurate in predicting
the results they saw in the wild.

"We found evidence for a foraging-mediated hydra effect: large outbreaks depressed foraging rate and correlated with increased densities of both
algal resources and Daphnia hosts," Penczykowski said.

"There is a balance between the mortality imposed by the
parasite and the impact on the feeding rate," she said. "If
the parasite is too virulent and poses too high of a death
rate, you would be less likely to see this hydra effect. But
also if there's no increased death rate, you don't expect it." ========================================================================== Story Source: Materials provided by
Washington_University_in_St._Louis. Original written by Talia
Ogliore. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Rachel Penczykowski, Marta S. Shocket, Jessica Housley Ochs,
Brian C. P.

Lemanski, Hema Sundar, Meghan A. Duffy, Spencer R. Hall. Virulent
disease epidemics can increase host density by depressing foraging
of hosts. The American Naturalist, 2021; DOI: 10.1086/717175 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210920121803.htm

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