Untitled

ADVANCES

14 Scientific American, July 2019

STEPHANIE WARE

Field Museum

ANIMAL 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 on

the 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 Daley

ment. 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 vast

underground 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 Hall

Genitals of the millipede Pseudopolydesmus

caddo glow brightly in UV light.