http://www.foxnews.com/story/0,2933,356883,00.html
Bizarre Crustaceans Undergo Evolution in Reverse
Thursday, May 22, 2008
Among the greatest mysteries in zoology for more than a century have
been vaguely shrimp-like creatures known as y-larvae.
Although these microscopic beasts are clearly young crustaceans, no
one knew what the adult forms looked like.
Now researchers may have solved this puzzle by dosing the y-larvae
with a hormone that forced them to go through a growth spurt.
The result — simple, pulsing, slug-like m***** of cells that were
"mind-blowing" to the scientists.
These surprisingly simple creatures — far simpler than their larval
stage — may be parasites found worldwide.
Dizzying diversity
Y-larvae, or facetotectans, were first discovered in 1899. There were
once x-larvae as well, the 'x' and 'y' both denoting something
mysterious.
Later on, the adult form of the x-larvae were found, but bafflingly,
even after intense searches, no one knew what y-larvae grew up to be,
so they kept their name.
These critters are just a few hundred microns large, or roughly the
size of the period at the end of this sentence. They occur with
dizzying diversity in coral reef areas, and are found in all oceans,
from the poles to the tropics. Their commonplace nature suggests the
adults play a major role in ecosystems around the globe.
To find out what these y-adults might be, an international team of
scientists used nets to collect more than 40 species of y-larvae from
a marine station at Sesoko Island near Okinawa, Japan. As they were
gathering the creatures, a cyclone approached.
"It was predicted to hit the marine station five days after the
arrival of our team," said researcher Henrik Glenner, a molecular
biologist at the University of Copenhagen in Denmark. "This put us
under substantial time pressure, because we knew that it was
impossible to catch the y-larvae after the cyclone had passed.
Therefore, we had to work during the nights."
Safe back in the lab
The researchers next exposed y-larvae to a crustacean hormone that
encouraged them to mature. The creatures metamorphosized into a
juvenile form, dubbed "ypsigons," unexpectedly shedding their
exoskeletons to become wriggling, eyeless, limbless creatures that
resemble parasitic crustaceans.
At first the researchers thought their eyes were tricking them, but
eventually "the juvenile literally crawled out of the old larval
carapace," Glenner recalled. "It was only after several repeated
experiments we actually believed what we saw. That feeling was a
mind-blowing experience."
The fact that ypsigons are vastly different and far simpler than
y-larvae might help explain why the adult versions of these creatures
have escaped detection for so long. These are so simple compared with
y-larvae that they even lack digestive tracts and nervous systems.
Ypsigons could make do without a digestive tract by directly absorbing
nutrients from their surroundings.
They might develop a nervous system later on in life, "but not
necessarily," Glenner said.
"I know it sounds strange, but in some adult parasitic barnacles —
rhizocephalans, which parasitize other crustaceans — there are no
traces of a nervous system either," Glenner told LiveScience. "This is
possible because their behavior as adults is restricted to certain
coordinated movements when they release their larvae."
Probably parasites
As the final adult stage of the y-larvae are probably parasites,
future efforts to uncover these y-adults — to solve this mystery once
and for all — will aim to identify their hosts by screening coral reef
animals for y-larvae DNA.
"These parasites could play a very im****tant role in the wild," said
researcher Jens Høeg, a marine zoologist and invertebrate morphologist
at the University of Copenhagen in Denmark. "They should not be seen
as evil or bad. Wherever these parasites are, be they in sea urchins
or sea stars or corals, they are probably im****tant in making up what
we consider a normal, healthy coral reef."
Høeg, Glenner and their colleagues Mark Grygier and Yo****hisa Fujita
detailed their findings May 19 in the journal BMC Biology. They were
sup****ted by the Carlsberg Foundation in Denmark and the Lake Biwa
Museum in Japan.
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