ADVANCES
14 Scientific American, July 2019
STEPHANIE WAREField MuseumANIMAL PHYSIOLOGY
Glimmering
Gonopods
Millipedes’ genitalia fluoresce
under ultraviolet light
Millipedes are hard to tell apar t. Differ
ent species of the manylegged crea
tures often share the same dull colors
and tend to blend in with the gloom of
the forest floor. But under ultra violet
light, some millipedes display a striking
characteristic: their genitals glow brightly.
Stephanie Ware, a research assistant
at Chicago’s Field Museum, and her col
leagues have used this strange fluores
cence to help identif y the leggy ar thro
pods. Ware rigged up a cam era with inex
pensive UV flashlights to cap ture images
of millipedes’ glim mering “gonopods,”
specialized appen dages used for copula
tion. The camera took multiple pictures
that Ware stitched together to create a
composite image. In visiblelight photo
graphs, “it’s really hard to pick out differ
ent structures” on the millipedes, she says.
“But under UV, there were diff er ent pat
terns and colors that made them really
pop out.”
This technique makes it easier to dis
tinguish between similarlooking species,
according to Petra Sier wald, a zoologist
at the Field Museum. She and Ware and
their colleagues coauthored a study onthe topic, published online in April in
the Zoological Journal of the Linnean
Society. Using the UV technique, the
re searchers identified eight species—
which had previously been mis
categorized as 12—within the North
American genus Pseudo polydesmus.
Sier wald says this kind of imaging
could have applications in soil science
and conservation, help ing researchers
quickly assess whether certain milli
pede species are present in a habitat.
“Millipedes are very good indicators
for soil health because they recycle
rotting leaf litter,” she says.
Yet scientists still have no idea why
these animals’ genitals fluoresce. “The
order Polydesmida can’t even see—they
don’t have eyes,” Sier wald says. M. Gab ri
ela Lagorio, a chemist who studies photo
biology at the University of Buenos Aires
and was not involved in the study, says
the feature may or may not have an evo lu
tion ary purpose. She notes that it may
be “simply a nonfunctional consequence
of the chemical structure of a substance
present in the tissue.” — Jim Daleyment. It was proof that these organ isms
could survive both within the lake itself
and in the sediment below, where con di
tions are even more hostile. But Thomas
still thought it was unlikely that anything
other than archaea could survive there.
“I was thinking, ‘It’s an extreme environ
ment, and it’s only for the extreme guys,’ ”
he says.
The team’s most recent finding upends
that notion. Thomas and his colleagues
analyzed layers of gypsum (a mineral
left behind when saltwater evaporates)
that were deposited 12,000, 85,000 and
120,000 years ago. Entombed within them,
they discovered wax esters—energyrich
molecules that small organisms create and
store when food becomes scarce. Because
archaea cannot produce these molecules,
and multicellular organisms are very un
likely to survive such hostile conditions,
the team concludes that ancient bacteria
must have produced the compounds.
But how did these bacteria survive?
The wax esters carried traces of archaea
cell membranes, so the researchers
hypoth esize that the bacteria scavenged
remains of archaea. That survival mech
anism would explain how the community
managed to thrive in such seemingly deso
late con di tions. “Although we know there’s
a ton of diversity in the microbial biomass,
it’s al ways exciting to see what strategies
these microbial com munities use to sur
vive in diff erent environ ments,” says Yuki
Weber, a bio chemist at Harvard Universi
ty, who was not involved in the study.
“There’s still a lot that has to be learned
about the microbial metabolism.”
Fur thermore, Thomas and his col leag
ues found tantalizing hints that bacterial life
may exist in the Dead Sea ecosystem even
today. When they first opened a large vial
of contemporary sediments, for example,
they smelled rotten eggs—a telltale sign
of hydro gen sulfide gas, which is often pro
duced by bacteria. But the gas can also
have a non bio logical origin, such as geo
thermal activ ity (for which Yellowstone
National Park is famous), so the re search
ers are not certain that bacteria continue
to reside below the salty lake.
Even if they do not, bacteria most likely
live in similar conditions across Earth’s vastunderground biosphere, Weber argues.
And as scientists continue charting the
extreme environments in which life can
survive, they will better understand how
and where it arises on Earth and other
planets, he says.
Take Mars—in 2011 nasa’s Oppor tuni
ty rover stumbled on gypsum, the same
mineral that Thomas found in the Dead
Sea sediments. Its presence suggests that
as the Red Planet warmed, its oceans and
lakes evaporated. But before they did,
these bodies of water probably would have
looked a lot like the Dead Sea—maybe
even down to the biological processes,
says Tomaso Bontognali, a scientist at the
Space Exploration Institute in Switzerland,
who was not in volved in the Dead Sea
study. Bontognali works on the European
Space Agency’s ExoMars rover, which is
set to land in 2021 in an ancient ocean bed
on Mars. It will analyze sediment cores
with a simplified version of the method
used by Thomas’s team. The Dead Sea
evidence “makes the hypothesis that life
may have existed on Mars more plausible,”
Bontognali says. — Shannon HallGenitals of the millipede Pseudopolydesmus
caddo glow brightly in UV light.