Something special for photography buffs
I have mentioned fluorescence a few times in my blogs, have written an item about GFP (the Green Fluorescent Protein that won O. Shimomura, R.Tsien and M. Chalfie in 2008 the Nobel prize in Chemistry) and explained how a scientist had erroneously described a bioluminescent cockroach from South America that turned out a misinterpretation, because the insect was not bioluminescent (= creating its own light), but fluorescent – just like it has recently been reported from the platypus, some other mammals and even the hawksbill sea turtle (namely emitting visible light only when illuminated with light of shorter wavelengths like, for instance, ultraviolet radiation).
To distinguish the two types of responses in the natural environment is actually quite simple: when struck by a beam of light, say by UV radiation, and the animal “glows”, it will only glow as long as it is illuminated by the light source. The moment the UV-source is taken away or switched off, the fluorescence will also disappear. Bioluminescence, however, whether induced as a response to illumination or, more commonly, a physical disturbance or due to an exposure to a chemical, the light emitted by the aroused animal will linger on for a while. Therefore, fluorescence and bioluminescence are two quite different phenomena based on totally different chemicals and reactions. For the fluorescence so-called fluorophore chemicals need to be housed in the fluorescent tissue, but the bioluminescence requires a substrate known as luciferin and an enzyme known as luciferase and oxygen. Different luminescent animals may have chemically different luciferins and luciferases.
What causes some animals to fluoresce when in the beam of a UV-light are various chemicals like fluoresceine that gives off green light and similar molecules that emit other colours. Fluorescein is a widely used fluorophore substance used to label antibodies with, so that certain intracellular components like, for instance, actin filaments stand out in bright green when illuminated under a fluorescence microscope with UV-light. I used this kind of microscope and technique in conjunction with a study on chromatophores of the skin (and therefore the body) of fish.
Why some animals should be fluorescent is really a bit of a mystery. I was amazed at the extraordinarily bright fluorescence that came from newly hatched millipedes of a Paraspilobolus species. Incidentally a species that is not only fluorescent, but known to be also truly bioluminescent when forcefully touched or physically attacked. However, what (if any) function the fluorescence observed in these millipedes which spend most of their lives under loose layers of soil or hidden amongst leaf litter could possibly have, is not yet clear. All that is known is that fluorescence in millipedes, generally, is not a rare phenomenon. With spiders and scorpions, both having wonderfully luminescent species, it is similar: nobody so far has come up with a convincing explanation why so many species of them are fluorescent.
For a recently discovered truly fluorescent tree frog from Argentina, Carlos Taboada et al. suggested that it would help the nocturnally active frogs to recognize each other, as the eyes of these frogs may be more sensitive to longer than shorter wavelengths. Fluorescence, as mentioned above, emits light of longer wavelengths (e.g. in the green) than the UV-or short wavelength blue lights that might have been falling on the frog in its natural environment. However, the eyes of the frog and their spectral sensitivity have not been measured yet and secondly, the amount of UV-light during the day under the foliage of trees is not exactly high and at night it is absent. For creatures in the sea, some of which also fluorescent, the same problem of little or no UV would occur. The frog’s fluorescence, therefore, may have other functions, perhaps aimed at predators making the frogs less visible or advertising their distastefulness. What is, however, totally true is that one can take fantastically beautiful photos of fluorescing animals: see this article and gallery.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2021.
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