Four years
after falling in love with the disco clam -- a cute little mollusk known
for its underwater light show -- Lindsey Dougherty has discovered the
secret of its mirrored lips.
Disco
clams are named for the rippling, rhythmic light show they put on with
their mirrored lips, visible even in the dim blue depths. Underwater
video by Lindsey Dougherty; production by Roxanne Makasdjian & Phil
Ebiner, UC Berkeley.
A dive instructor and University of
California, Berkeley, graduate student, Dougherty first encountered the
two-inch clam in 2010 while diving with her mother and sister in
Wakatobi, Indonesia. She and her sister even did a bit of underwater
disco dancing to the clam's flashing beat.
"I've dived with
humpback whales and great white sharks," said Dougherty, who first
learned to dive at age 14 and taught diving in Zanzibar. "But when I saw
the disco clam, I was enamored. I said then, 'I'm going to do a Ph.D.
on the disco clam.'"
It didn't take long for her to confirm that
the flashing was not, as most people assumed, a form of bioluminescence
-- a chemical reaction inside animals like plankton that produces light
similar to that of a glow stick. Instead, she found, the edge of the
clam's mantle lip is highly reflective on one side. When the clam
unfurls its lip -- typically twice a second -- the millimeter-wide
mirror is revealed and reflects the ambient light, like a disco ball.
In a cover story in this week's issue of the British Journal of the Royal Society Interface, Dougherty reports the unusual and perhaps unique structure of this mirrored lip.
Tiny packed silica balls act as mirrors
The
inside of the clam's lip is packed with tiny spheres of silica, only
340 nanometers in diameter, that are ideal reflectors, particularly of
the blue light that penetrates deeper into seawater than does red light.
The outside of the lip contains no silica nanospheres, so when the lip
is furled, no light is reflected.
By repeatedly unfurling and
furling the lip, the clam produces a continual rippling light show. The
non-reflective back of the lip strongly absorbs blue light, so it
appears dark and makes the contrast between the sides even more
striking.
Dougherty used high speed video, transmission electron
microscopy, spectrometry, energy dispersive x-ray spectroscopy and
computer modeling to study the detailed internal structure of the margin
of the clam's lip. She was assisted by colleagues Roy Caldwell, UC
Berkeley professor of integrative biology; Sönke Johnsen of Duke
University; and N. Justin Marshall of the University of Queensland,
Brisbane, Australia. She could find no other instance of animals using
silica nanospheres as flashing reflectors, though the white color of
several insects apparently comes from a layer of silica that reflects
white light.
The big question, Dougherty said, is why the clam flashes at all.
Called Ctenoides ales
and sometimes referred to as the electric clam, disco clams are found
in tropical areas of the Pacific Ocean, living in crevices in reefs and
typically in clusters of two or more. Light is dim and blue-green at the
clams' typical depth, which ranges from 3 to 50 meters (10-150 feet),
but their rippling mirrored lips are visible even without artificial
illumination. Dougherty said the question she is exploring is whether
the clam is trying to attract prey, mostly plankton, or other clams and
potential breeding partners; or if it is trying to scare away predators.
In
ongoing experiments in Caldwell's lab, she is studying the structure of
the clam's eyes -- all 40 of them -- to see whether they can even see
the disco light. She also is raising clams in tanks to determine if they
signal one another visually or chemically, and is testing their
responses to fake predators.
The field work was conducted at
Lizard Island Research Station in Australia and the Raja Ampat Research
and Conservation Centre and Lembeh Resort in Indonesia.
Story Source:
http://www.sciencedaily.com
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