The Paper: 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. The Company of Biologists Ltd, 00, 1–11. doi:10.1242/bio.012138.
Humans tend to be shortsighted: we often overlook what we cannot see with our naked eye. But the fact is, our “visible spectrum” makes up a very small portion of the full light spectrum (Fig. 1) and by ignoring those other important wavelengths, we are depriving ourselves of some pretty fantastic sights!
Ever been to a black light party or to a glow in the dark bowling alley? It is pretty mesmerizing to be surrounded by brightly glowing objects isn’t it? Well, turns out this same sort of glowing, or fluorescence, is fairly common in the animal kingdom (Fig. 2). Birds, fish, scorpions, shrimp, jellyfish, and corals are just a few of the animals that fluoresce when hit with the right type of light.
How does it work? Fluorescence occurs when a substance absorbs one type of light and emits a different wavelength of light. In animals, there may be some sort of chemical (chitin, cellulose, or GFPs) that absorbs high-energy wavelengths (like UV light- that is invisible to the human eye) but emits a lower energy light. As soon as the animal is removed from the source of UV light, however, the fluoresce ceases. Think of it this way: if you are in a dark room and wearing a white shirt, you cannot easily see the shirt. As soon as you pass a black light over the shirt, it starts to glow. Unlike the white shirt under the black light, many of the wavelengths emitted by fluorescent animals are still beyond our visible spectrum. But when we use special lenses, we are rewarded with some incredible sights.
So what is the point of all these pretty glowing colors? There are a few hypotheses. For some animals, the fluorescence is a mere byproduct of a chemical structure – for instance the keratin in our fingernails will fluoresce under the right wavelength. For other animals, however, the degree of fluorescence is so strong, that it is more likely used for some ecological function. In corals, or other photosynthetic organisms, it is thought that fluorescence may control excess absorption of harmful UV rays- basically acting like a sunscreen. But what about the large group of non-photosynthetic, fluorescent organisms? A recent paper has tested the hypothesis that in some cases, the fluorescence may be used to attract prey – and the results are pretty convincing.
The Study: Researchers from Monterey Bay Research Institution in CA and Brown University in RI set out to test the hypothesis that fluorescence may be used by predators to attract prey using the fluorescent predatory jellyfish, Olindias formosus (Fig. 3). The tentacles of the jelly contain GFP (or a green fluorescent protein) that fluoresces when hit with blue wavelengths (high wavelengths on the visual spectrum- these wavelengths are common in marine environments because low-energy wavelengths like red light scatter or are absorbed by the water leaving only blue wavelengths in the environment).
To test the capacity for these glowing tentacles to act as lures, the researchers placed an unsuspecting fish into a tank with the jellyfish. Don’t worry, no fish were harmed during the experiment- they were separated from the jellyfish by a clear barrier (Fig. 4). The tank was exposed to different wavelengths: white light (which may weakly induce fluorescence but is not visible above background light levels), yellow light (which does not induce fluorescence), and blue light (which strongly induces fluorescence, making it visible above background light levels). The scientists recorded the amount of time fish spent in the tank near the jellyfish and the number of times fish tried to strike at the jellyfish. For controls, the same procedure was conducted with a fake jelly (“blobject”) and without a jellyfish in the tank.
The scientists also took their research to the field- bringing a green laser pointer underwater and testing the responses elicited by reef fish when exposed to a bright, green light source. (Spoiler alert: scroll down to see the pretty comical results).
The Verdict: The fish in the tank spent significantly more time near the jellyfish and struck at the jellyfish significantly more times when the tank was illuminated under blue light (the only wavelength that would illicit the visible fluorescence) and exhibited different, more controlled attack behaviors only when exposed to the live jellyfish under blue lighting.
When the laser pointer was taken in situ, reef fish were attracted to the light it produced, chasing it and displaying aggression. Watch this (funny) video if you want to see how the fish responded to the laser pointer. Looks like cats aren’t the only ones to get excited with a laser pointer!
Conclusions and Significance: This study supports the hypothesis that some organisms use fluorescence to attract prey. It is important to note that there are numerous types of organisms which exhibit fluorescence and there are likely many different functions- this study only shows one of the potential uses of fluorescence.
It just goes to show how stunning nature can be when viewed through the right lens.