Fluorescence.

©Paul o’Dowd 2013

Fluorescence

You’ve heard the reactions of people who witness the Reef for the first time. “The colours are amazing!”, “The fish are so bright!”...

The question is; how do many marine creatures manage to appear brighter than the ambient light would seem to permit.

Pigments are chemicals which absorb some wavelengths of light and reflect others.

Many of the colours we see around us are the result of pigments reflecting selected wavelengths and absorbing others.

The energy associated with the absorbed light might then be used for other things.

Plants appear green because the green part of the spectrum is not absorbed by the pigments that harvest the energy of sunlight.

Instead, the wavelengths that make up the green part of the spectrum are reflected and that’s what we see.

Some pigments though, absorb certain wavelengths only to instantly release that energy as an emission of light of a different colour.

This is called, “fluorescence”.

In an environment rich in energetic but imperceptible wavelengths, like ultra violet for example, having a surface that converts those wavelengths to highly visible colours can make things seem to be lit from within.

The fluorescent object glows, compared to the more mundanely pigmented world around them. If you want to stand out, “fluoro” up.

Familiar examples include hi-viz vests and safety gear, “fluoro” highlighter pens, whiteners in laundry liquid, price tags, sale signs and many more.

On the reef, fluorescence takes on a range of very important roles.

Many familiar sea creatures use fluorescence, as you’ve no doubt guessed from their “illumination defying” colour displays.

It’s easy to imagine what exhibitionistic functions such displays may serve so I’ll gloss over the obvious; sex, territory, communications of various sorts.

Yellow, green, red and infra red fluorescence in coral is another thing again.

The bright colours seen in some corals are just some of the more dramatic displays of pigments which are still used, just not as boisterously, in less colourful corals.

Fluorescence involves an energy exchange which can be used to both amplify photosynthesis and protect the coral tissues from excessive solar radiation.

Some sections of the solar spectrum are not involved in photosynthesis.

These wavelengths still dump energy into the tissues of coral, which can produce heat and other damaging effects.

Fluorescent proteins absorb this extra energy and then convert it into wavelengths useful for photosynthesis.

This improves the efficiency of the algal partners by bathing them in a light environment more suited to their needs.

This also removes a significant source of stress on the coral, by reducing the amount of unutilized radiation that would otherwise cause problems.

Red fluorescent pigments have been found in a large range of fish including gobies, wrasse and others.

Red sponges, tunicates and many other invertebrates are now known to be red fluorescent, converting the ambient blue-purple light of their environment into a deep red emission that blends with other pigments to give us the dark browns, pinks, oranges, purples and rusty colours that we see if we’re looking, despite being below the “red zone”.

This red light is being generated by fluorescence from within the skin of these organisms, it is not ambient light reflecting from its surface.

Below the red zone, there has been little evolutionary pressure to develop the equipment for picking up the colour red.

It doesn’t travel well through water so it isn’t a part of the ambient light spectrum.

For the same reason, it’s useless for distance vision beyond a few meters even if a prey item was actively emitting bright red. Actually, many of them are fire-truck red and quite a few do emit it.

Predators on the lower reef slopes who need to see prey at a distance have very little use for that colour so they don’t perceive it. They are “dichromatic”, they see in two colours, unlike trichromats, like most of us, who see in three or the rare tetrachromat who can see in four colours.

Most other fish are dichromatic, and red isn’t on the list.

They see their green and blue world in the most useful area of the spectrum for that environment, green and blue.

Some fish do see red though, and I’m not talking about brooding Titan Triggers.

Over thirty species of fish from five different families are now known to fluoresce in red.

The fluorescent pigment is usually located around the heads and display fins of fish for whom close range communication is important.

An increasing number of these fish are being found to possess the ability to perceive the red displays of their mates and rivals.

These fish are able to use their displays as highly covert communication systems which are literally invisible to most other animals in their habitat.

Even to a predator capable of seeing red, should such an animal exist, the displays would only stand out at very close range in which case you’re already dinner.

More incredible still, is the role fluorescence plays in helping many sea creatures to disappear.

Chlorophyll fluoresces in the far red and infra red.

Algae are full of chlorophyll.

Coral are full of algae.

Therefore coral appear to fluoresce in the far and infra-red.

Many corals also use red fluorescent proteins for power management.

This explains why, in the absence of red in the illumination of the world below ten meters, we still see brown and occasionally even reddish corals.

Some grazing fish who spend their whole time in close proximity to coral also display a subtle red fluorescence that matches their habitat.

The fact that some grazers have camouflage to match this wavelength may suggest the capacity of some predators to be able to distinguish non-red prey from the background of the reef when in close.

We don’t know if any reef predators can do this yet but the ones that have been checked for the ability, can’t perceive red. It’s like red is the new black.

If some can see red, the red fluorescence would help the grazer blend in with the coral’s background emissions.

If not, then the slightly redder fish becomes simply more difficult to see.

The redder prey are the more difficult to see, but below the red zone, they need to make their own red to become invisible, and that is where fluorescence really shines.