SCIENCE science.org 29 OCTOBER 2021 • VOL 374 ISSUE 6567 529they detect microalgae and stop to graze.
They slow down at night, notes Bernd
Schierwater, an evolutionary biologist at
the University of Veterinary Medicine Han-
nover. He thinks that slowdown represents
“the first [evolutionary] step toward sleep—
getting a rhythm for rest” to recharge for
the next feeding cycle. That may be enough
of a respite for an animal that lacks energy-
hungry nerve cells, he says.
Smith thought the idea of a sleeping
placozoan was silly—until the jellyfish
and hydra studies drove home that sleep
was not just for brainy creatures. At times,
placozoans rotate in place, which Smith
suspects may also represent a kind of
sleep. Because these creatures cringe when
exposed to UV light, it might be possible
to test whether they become unresponsive
to UV light in this state, she notes. She
provided Raizen with some animals to run
such tests. But he couldn’t keep them alive
in the lab—they are very finicky eaters—
and eventually he gave up.
Joiner had similar problems with his
puffball sponges. He and his collaborator,
marine biologist Greg Rouse of the Scripps
Institution of Oceanography, babied the
sensitive creatures, which require a con-
stant supply of fresh seawater laden with
the microbes they eat. Joiner stopped bythe ocean daily on his way to work to the
collect seawater. The researchers stuck
tubs housing the sponges in an incubator
to control light levels and temperature.
Then they mounted a digital camera over
the tubs to watch for subtle contractions in
the sponges’ bodies as they pumped water
through their chambers to filter out food.
Eventually, with the help of magnetic stir-
rers in the tubs, they were able to keep the
sponges healthy enough that they started to
contract—about once every 3 hours. That
was exciting, Joiner says, because it gave
the researchers a reliable behavior they
could monitor for sleep-related changes.
Studies in another sponge species have
suggested the animals have cycles of rest
that may allow them to reorganize and
rejuvenate their cells after
pumping water equivalent
to 1000 times their volume
daily. Marine biologists
Sandie Degnan and Bernard
Degnan at the University of
Queensland, St. Lucia, saw
hints of daily rhythms in the
contractions in Amphimedon
queenslandica, found on the
Great Barrier Reef. And af-
ter sequencing its genome—
the first for a sponge—they
reported in 2017 that a few
genes associated with circa-
dian rhythm in other organisms turned on
and off in 24-hour cycles. In unpublished
work, graduate student Davide Poli has ob-
served that select regions of the sponges’
bodies seem to stop pumping throughout
the day, as if working in shifts. That might
be “behavior that can be approximated to
sleep,” he says.
Future studies could use glutamate or
other substances that stimulate contrac-
tions to keep sponges pumping nonstop for
hours or even weeks to test whether their
health declines, says Sally Leys, a marine
biologist at the University of Alberta, Ed-
monton. “That could indicate that by being a
multicellular animal, you depend on having
periods during which the tissues can repair
and regenerate.”IF CELLS THROUGHOUT the body benefit
from sleep, it makes sense that those cells
would have some say in when sleep hap-
pens. And the search for sleep’s far-flung
control switches could lead researchers to
new treatments for sleep disorders, which
affect 60 million people in the United
States alone.
Paul Ketema, a neuroscientist at UC Los
Angeles, has studied Bmal1, a ubiquitous
protein that regulates gene expression and
is known to help sleep-deprived mice stayawake. Until now, researchers had assumed
the brain made and used Bmal1 for that
task. But Ketema and his colleagues discov-
ered sleep-deprived mice depend instead on
Bmal1 produced in the muscles. He suspects
the protein may be part of a pathway that
somehow helps link muscle exertion to levels
of sleepiness. And he’s hopeful a Bmal1 drug
targeted to muscles may one day counter the
negative effects of all-nighters. “I am one
of those people who thought sleep was all
about the brain,” he laughs. “That is not only
an erroneous viewpoint,” it’s also one many
of his colleagues still hold, he says.
Other mouse studies have shown the
gastrointestinal tract, the pancreas, and
fat tissue generate signaling molecules
called neurohormones that appear to af-
fect the onset and duration
of sleep. Understanding
feedback from these organs
to the brain “could sug-
gest new pharmacological
approaches, with drugs that
target organs other than the
brain,” Sehgal says.
Mourrain’s team at Stanford
is developing a way to watch
that feedback process play out
cell by cell in the thumbnail-
size, transparent fish Dani-
onella translucida. By using
fluorescent tags and other
markers that track the activity of certain mol-
ecules in the fish’s brain and body, his team
will observe how different types of cells con-
trol sleep—and benefit from it—over time.
When Mourrain started to study sleep in
fish 15 years ago, “there was a lot of push-
back that fish don’t sleep,” he recalls. That
changed about 2 years ago, when he and
his colleagues developed polysomnography
techniques for fish and discovered that, like
people, they cycle between quiet and active
sleep states. “It was a tipping point for our
field,” Mourrain says, convincing skeptics
that fish could be good stand-ins for mam-
mals in sleep studies.
Will the orange sponges be the next crea-
tures to stun the sleep skeptics? Maybe not
right away. Joiner and Rouse couldn’t keep
their sponges healthy long enough to carry
out reliable experiments. After months of fits
and starts, they paused the work to rethink
the set up. Then, COVID-19 hit and shut the
experiment down. Joiner doesn’t have the
staff to start it back up.
But another sponge may step in instead
to show that even the simplest animals
truly sleep. In emails, Itoh coyly referred
to his lab’s work on these simple creatures.
Mum’s the word for now, it seems. “These
are ongoing projects,” he wrote. “Please
PHOTO: WOLFGANG JAKOB/SCHIERWATER LAB look forward to them.” j
Reptile Bird Marine mammal HumanYes Not tested yet
No or not applicable InconclusiveResearchers want to
know whether early-evolving
placozoans sleep.