The allure of fluorescence in the ocean

Why do so many marine animals have bright fluorescent pigments? This video describes how one function was demonstrated experimentally.

Fluorescence is a process where high-energy light temporarily excites electrons in a molecule. When the molecule relaxes, the energy is re-emitted as a lower-energy photon with a longer wavelength. For example, blue or violet light is often used to excite green, yellow, or red fluorescent emission. Fluorescence is a passive physical property of many molecules, and unlike bioluminescence it is not something that animals can actively turn on and off.

One of the most famous molecules in all of science is Green Fluorescent Protein (GFP). It was first discovered in a bioluminescent jellyfish, and later found in non-luminous corals, sea anemones, and other organisms. These proteins have proven so useful in laboratory and clinical settings that in 2008, the original GFP researchers were awarded the Nobel Prize in Chemistry. Yet despite the many ways that humans use the fluorescent proteins, we don’t really know the ways that the animals use them.

To test how animals might use fluorescence we conducted predator-prey experiments with the flower-hat jellyfish. We found that the fluorescent tentacle tips, when excited by ambient blue light from the environment, were an irresistible attractant to potential prey (a supernormal stimulus; see Tinbergen 1948). We also found evidence for fluorescent structures in a range of other predators.

You might have heard about fluorescent sharks and turtles, but just because something is fluorescent doesn’t mean it’s serving a useful function (see Mazel 2017). Even your own teeth and fingernails are fluorescent, but that is just because of their chemical composition.

For this video, we filmed the fluorescence of animals by shining a blue light on them and putting a yellow filter in front of the lens. The filter blocks out the excitation light but lets the fluorescent light be recorded. You can try it at home with a piece of yellow plastic and a blue LED flashlight. You might be surprised what you find!

Script/Narration/Animations: Steve Haddock
Editing: Susan von Thun, Kyra Schlining
Music: Gently Onwards, Tape Deck, My Achy Heart (YouTube Audio Library)
Additional Music: Steve Haddock

Shallow water reef footage was filmed by Steve Haddock in Indonesia, Australia, and at the Monterey Bay Aquarium.

ADDITIONAL READING

Reference publication:
Haddock, S. H. D., & Dunn, C. W. (2015). Fluorescent proteins function as a prey attractant: experimental evidence from the hydromedusa Olindias formosus and other marine organisms. Biology Open. 4:1094-1104


More information about determining whether fluorescence is serving a function:
Mazel, C. (2017). Method for determining the contribution of fluorescence to an optical signature, with implications for postulating a visual function. Front. Mar. Sci. 4:266.


More information about supernormal stimulus:
Tinbergen, N. (1948). Social releasers and the experimental method required for their study. The Wilson Bulletin. 60:6-51.


Cormier and McCann (2014). The amazing green-fluorescent protein.

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