Scientists, including one of Indian origin, have found that
jellyfish go into a sleep-like state, making them the first animals without a
brain or central nervous system to do so. The findings push the origin of sleep
further down the evolutionary tree of life - to before the emergence of a
centralised nervous system. "It is the first example of sleep in animals
without a brain," said Howard Hughes, from California Institute of
Technology in the US.
Researchers studied Cassiopea, a mostly stationary jellyfish
native to mudflats, mangrove swamps, and other warm, shallow waters. The
jellfish were kept in tanks with artificial seawater in labs. Cassiopea do not
look like typical jellyfish - they are silver dollar-sized, splotched with
black pigment, and rest upside-down on the sea floor, researchers said. With
tentacles curled above bell-shaped bodies, the jellies resemble miniature heads
of cauliflower. However, they pulse like other jellyfish, contracting and
relaxing in a steady rhythm, and researchers wondered if the behaviour was
continuous.
"We went in at night and videotaped them with an
iPhone," said Ravi Nath, graduate student at Caltech. After recording the
jellies, the team got their first clue that Cassiopea might be sleeping: They
pulsed less frequently at night. Researchers then created an image processing
programme to count the pulses of 23 jellies over six consecutive days and
nights. They observed, as earlier, that pulsing activity ebbed at night but a
little food dropped in the tanks could quickly wake the jellies up again.
Researchers noted a second sign of sleep after dropping the
floor out from dozing jellies. The team placed Cassiopea inside a pipe with a
mesh bottom and then dipped the pipe into the tank - so submerged jellies
rested on the mesh instead of the tank floor. Then they lowered the pipe deeper
into the tank, forcing the jellies to lift off the mesh and float in open
water.
During the day, Cassiopea will quickly pulse their bells and
swim down to rest on the mesh again. However, at night, it takes them about three
times longer to start pulsing. It is like the jellyfish are a little groggy.
This delayed response to stimulation is typical for sleeping animals,
researchers said. The study was published in the journal Current Biology.
