A young staghorn coral that fluoresce redder is less likely to settle and develop into coral polyps than young staghorn corals that fluoresce greener. This surprising discovery was made by assistant professor of biology Mikhail “Misha” Matz and his colleagues* at the Austin branch of the University of Texas.

“By simply looking at the color of a larval population, we may soon be able to say which larvae are going to be long-range dispersers and which will be short-range dispersers,” says Matz. “Under global warming, we expect a lot of evolution of this particular life history trait.”

The less likely a coral larvae is to settle, the more likely it is to disperse from its native reef and end up settling somewhere else. If global warming forces coral species to move to cooler regions in order to survive, this will naturally be an important trait.

In their study, Matz and his colleagues crossed different color morphs of the coral species Acropora millepora and subjected the offspring larvae to ground-up calcareous red algae. The ground-up algae is clue that tells the larvae that this is a place where it could settle.

The experiment showed that the larvae that displayed the redder fluorescent color of their parents were less likely to settle and turn into reef-building polyps than their greener siblings.

According to Matz, the correlation between settlement and fluorescence could be completely random. The genes that determine fluorescent color and the genes that control the organisms response to the ground-up red algae may simply be located next to each other in the chromosome and therefore be inherited together.

It is on the other hand possible that the color for some reason have a function to fill as this coral disperse and settle. Matz and hits colleagues will now proceed to investigate if this is the case or not.

Even if the correlation is completely random, the finding is still important since researchers can use the fluorescence as a marker when studying Acropora millepora larvae.

What is fluorescence and why are corals fluorescent?

Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. The emitted light is usually of a longer wavelength than the absorbed light. From a human perspective, really striking examples of florescence occur when the absorbed radiation is invisible to the human eye while the emitted light can be seen.

Why corals are fluorescent and if it serves any particular purpose for them remains unknown.

“Bright, multicolored fluorescence of reef-building corals is one of the most spectacular and least understood visual phenomena in the ocean, and we still have no idea what purpose it serves”, says Matz. “But our discovery is a really good lead towards determining the function of fluorescence.”

The paper “Fluorescence of coral larvae predicts their settlement response to crustose coralline algae and reflects stress” has been published in the journal Proceedings of the Royal Society B.

http://rspb.royalsocietypublishing.org/content/early/2011/01/22/rspb.2010.2344.short?rss=1#aff-1
* C. D. Kenkel, Integrative Biology Section, University of Texas at Austin, Austin, TX, USA
M. V. Matz, Integrative Biology Section, University of Texas at Austin, Austin, TX, USA
M. R. Traylor, Integrative Biology Section, University of Texas at Austin, Austin, TX, USA
J. Wiedenmann, National Oceanography Centre, University of Southampton, Southampton, UK
A. Salih, School of Natural Sciences, University of Western Sydney, Penrith, New South Wales 1797, Australia

As you probably know already, many sea living creatures are capable of emitting their own fluorescent light. Turning yourself into a living light bulb comes in handy when you live at depths where no sunlight or only very little sun light is capable of reaching you, and the glow can for instance be used for communication, as camouflage, or to lure in prey.

Up until now, most fish experts have assumed that marine fish living below a depth of 10 metres (30 feet) could not be red since the type of sunlight necessary for the colour red to be visible to the eye isn’t capable of travelling so far down into the ocean, and why would an animal develop a red pigmentation that nobody could see in its natural habitat?

New light has now been shed on the situation and – according to a study published on September 15 by researchers at the University of Tubingen in Germany – fish living at these depths have managed to circumvent the problem of light scarcity by emitting their own red fluorescent light instead of relying on sun beams to display their colours. According to the study, a lot of marine species are capable of emitting a fluorescent red light which can be seen even at depths below 10 meters.

“The general consensus, which dominated fish literature for 20 or 30 years, was that fish don’t see red very well or at all,” says Nico Michiels, one of the researchers behind the study. “We have been blinded, literally, by the blue-green light that is available on reefs in the daytime.”

The scuba diving research team made their discovery when looking through a filter that blocked out the brighter green and blue light waves. While using the filter – which leaves only red light waves – the scientists realised that their dive spot was inhabited by a long row if different marine creatures capable of emitting their own red light. In addition to fish, they saw fluorescent red coral, algae and other small organisms.

Further investigation revealed that the red glowing organisms use guanine crystals to produce their light. Guanine is one of the five main nucleobases found in DNA and RNA and guanine crystals are commonly used by the cosmetics industry to give products such as shampoo, eye shadows and nail polish a shimmering lustre. As early as 1656, a Parisian rosary maker named François Jaquin extracted crystalline guanine forming G-quadruplexes from fish scales – so called pearl essence. Guanine crystals are rhombic platelets composed of multiple transparent layers and the pearly lustre appears when light is partially reflected and transmitted from layer to layer.

The red fluorescent light emitted by the organisms studied by Michiels and his colleagues is only visible at a close distance, at least to us humans. More research is now needed to investigate why so many sea dwellers have developed this capacity and how the red colour benefits them in their daily life.