If you don’t know what Fluorescence diving is, check out the previous article in the Fluorescence Night Diving series or check out the video:
Fluorescence is not only observed in corals, but also in many other marine organisms such as e.g. tunicates, barnacles, sponges, anemones, jellyfish, clams, nudibranchs, cephalopods, shrimp, crabs, worms and fish, to name just a few. There are also fluorescent organisms in fresh water, such as for instance cave olms in Eastern Europe . The green chlorophyll responsible for photosynthesis in plants is also fluorescent, it fluoresces in red or in some algae in orange.
By-product or a necessary biology
Given the enormous spread of unrelated species showing the capability to fluoresce, it is extremely unlikely that this phenomenon is a mere by-product (as it appears to be in the case of chlorophyll). This capability must provide some benefit to all these animals. Unfortunately research into what benefit this might be is still in its infancy.
However, there are some preliminary hypotheses and findings:
- There are studies which suggest that fluorescence in corals may act as a sunscreen, protecting the corals (especially in shallow parts of the reef) and in particular their symbiotic algae (which live inside the coral's tissues) against harmful UV radiation.
- Another hypothesis states that fluorescence allows corals to transform the only light available to them, namely blue light, into such wavelengths as can be used by their symbiotic algae for photosynthesis, allowing the corals to dwell successfully at greater depths than their competitors without such a capability can, giving them an evolutionary advantage to survival.
- It is also speculated that fluorescence in fish might help them to visually merge with the background of e.g. fluorescent corals, in order to make these fish less conspicuous to predators.
Some new findings by Prof. Nico K. Michiels, suggest that some fish are using red fluorescence on their bodies for communication among themselves.
It was already known that fish can willfully change their body's coloration (similar to octopuses, squid and cuttlefish), and that they use this ability for communication. Such as to attract mates, intimidate competitors and discourage or hide from predators.
However, it is a new discovery that they can also willfully change their fluorescence.
Just like divers need a way of communicating underwater, apparently so does the life down there.
It is also surprising that they use red fluorescence, because it is the color most quickly absorbed by water, and because many fish do not even have the ability to see red light. It seems that these fish have developed a secret and private communication channel of their own, allowing them to be conspicuous towards their peers and unconspicuous towards predators at the same time.
A faster Reef Conservation
Fluorescence torches using high power blue LEDs allow to illuminate greater areas of a coral reef at once. This has the potential to replace or at least complement the traditional, tedious and time-consuming Reef Check.
Thanks to these strong fluorescence torches, corals which are affected by disease or which fell victim to predation, dead or broken off corals as well as zones of conflict between competing corals can be detected from a great distance. This is usually through color changes of their fluorescence, the lack of fluorescence altogether (of dead corals) or intense red fluorescence from algal overgrow.
Health state of corals does indeed correlate with their fluorescence, which means that the latter can be used as a measure of the former: „Effects of cold stress and heat stress on coral fluorescence in reef-building corals“.
Spot new corals in the build
Another important benefit of using high power fluorescence torches is that they make finding coral recruits freely swimming coral larvae which have settled down to become coral adults, much easier and allow to do so from a much greater distance, provided the coral recruits in question are fluorescent, in which case they will stick out like little beacons.
Coral recruits are essential to assess the dynamic balance between destruction and construction of a coral reef, in order to determine whether the coral reef is able to recover from present damages or succumbing to them, and in order to measure the effectiveness of conservation efforts.
Factors of destruction are predation, such as from parrot fish, crown-of-thorns starfish and snails such as Drupella and Coralliophila, detrimental environmental factors such as global warming, pollution and of cause careless snorkelers and divers.