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FLUORESCENCE: THE SECRET COLOR OF THE DEEP
Mikhail Matz - Whitney Laboratory for Marine Bioscience, University of Florida
Fluorescence is a subtle thing. It happens when a fraction of the light illuminating an object is absorbed and then
re-emitted as a different color, a two-step process that is necessarily inefficient. In the human visual environment
of white daylight, nearly all fluorescent signals get swamped and lost, so special lighting and viewing conditions are
required to study them (see more about fluorescence and ways to detect it
).
Because of this, fluorescence usually does not attract the
interest of visual ecologists. In the ocean, however, the illumination is
predominantly blue,
so there seems to be a natural opportunity to view fluorescence by unaided eyes: its green, yellow or red color
would stand out nicely against the background. We hypothesized that ocean inhabitants would be able to use fluorescence
for visual communication, and went out to look for evidence. It was also a very exciting opportunity to look at the ocean
environment with the fluorescence-detecting "alien eye", to see if we could detect features that thus far went unnoticed.
In addition, we are on the lookout for novel fluorescent compounds for biotechnology .
In animals, biologically functional fluorescence is usually bright, concentrated in certain body parts and shows one or
a few well-defined peaks in its spectrum suggesting involvement of specialized pigments. Thus far, we (as well as other
researchers
)
detected several examples of this in midwater creatures, which spend all their life suspended in the "big blue". In jellyfish
and their relatives, the fluorescence of tentacles and other feeding appendages seems to be related to prey attraction,
whereas bright fluorescence in copepods is clearly for the purpose of visual recognition. We take these observations as
support for our hypothesis, and are going to continue the search for fluorescence in midwater animals.
In contrast, fluorescence in the organisms living on the deep sea bottom in the majority of cases seems to be a mere by-product
of particular tissue biochemistry and is unlikely to play any particular adaptive role. Such fluorescence is usually weak, is not
localized to particular body regions and has a wide featureless spectrum. The most common example of it is green fluorescence
of the exoskeleton in crustaceans and other arthropods such as sea spiders (see sea spider video).
The reason for such a trend may be that the vision capabilities of bottom-dwellers are on average much less developed that in
midwater organisms, so there is simply nobody to see the super-weak fluorescent signals that might be spawned by the tiny
amounts of remaining downwelling light and bioluminescence of the local fauna. Note
that the situation may be quite different in shallow water where plenty of light is
available. In addition, last year we found two remarkable exceptions from the rule Í tube anemone and a fish, deep bottom-dwelling
animals that clearly possess fluorescence "on purpose" (video of the fish ).
The function of fluorescence in these creatures is a total mystery thus far, which we plan to address in the future studies.
Finally, just looking around with our "alien eye" resulted in one very exciting observation last year: methane hydrates
("methane ice" ), at least at the site where we observed
them, turned out to be extremely brightly fluorescent, being quite inconspicuous under the normal lighting conditions.
Methane hydrates, in addition to being a potential energy source, were suggested to have played a critical role in the events
of rapid climate change. Our observation may lead to a new approach for
mapping them on the seafloor; however, more observations are required to develop a proper technology.
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