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Behavior

Colorful adaptations: the use of fluorescence as a lure

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.

Introduction:

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!

 

Figure 1: The spectrum of wavelengths showing that our visible spectrum makes up a very small portion on the scale (between infrared and ultraviolet wavelengths)

Figure 1: The spectrum of wavelengths showing that our visible spectrum makes up a very small portion on the scale (between infrared and ultraviolet wavelengths)

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.

Figure 2: fluorescence is widespread in the animal kingdom. A) cockatiel (image from www.backyardchickens.com) B) coral polyps (image from www.reefland.com) C) mantis shrimp (image from arthropoda.wordpress.com) D)  scorpion (image from www.jasonsteelwildlifephotography.yolasite.com) E) fireworm (image fromwww.divefriendsbonaire.com) F) jellyfish (image from nature.ca)

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.

Figure 3: Olindias formosus when photographed under normal “white” light (A-C). C) shows the close up of the tenticles with faintly visible fluorescent region (the yellow dot below the pink tips). D) the tentacles under blue light showing the fluorescence. E) the tentacles under blue light with a long pass filter so that blue light is removed and only the fluorescence is seen.

Figure 3: Olindias formosus when photographed under normal “white” light (A-C). C) shows the close up of the tentacles with faintly visible fluorescent region (the yellow dot below the pink tips). D) the tentacles under blue light showing the fluorescence. E) the tentacles under blue light with a long pass filter so that blue light is removed and only the fluorescence is seen.

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.

Figure 4: The experimental set up. A fish is placed in the tank with either a real jellyfish (or “medusa”), a fake jellyfish (or “blobject”), or an empty tank- separated by a transparent barrier.

Figure 4: The experimental set up. A fish is placed in the tank with either a real jellyfish (or “medusa”), a fake jellyfish (or “blobject”), or an empty tank- separated by a transparent barrier.

 

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.

Ashley Marranzino
I recently graduated with my MS from the University of Rhode Island. I specialize in studying fish biology/ecology, and deep-sea biology but love all things to do with marine biology.

Discussion

3 Responses to “Colorful adaptations: the use of fluorescence as a lure”

  1. When reading this article, we were really fascinated by the fluorescent lights the animals illuminated. From this article, we learned that many animals use fluorescent light. Many animals such as birds, scorpions, fish, jellyfish, and corals use fluorescent light. For example, corals and other marine plants use their fluorescent light to control the amount of harmful UV rays that they are absorbing. We learned so much about the studies that were done in the Monterey Bay Research center, and at Brown University. For example, we found out that the marine researchers conducted an experiment testing if they use fluorescent lights, and if so, how? In conclusion to the experiment, we found out that predators tend to use their fluorescent lights for luring in prey. However, we were wondering, is there any way to control their own lights? Also, how have they evolved to have mirror-like proteins? Thank you for your time, it is much appreciated.

    -Elise Rosenthal
    Esther Nguyen

    Posted by Esther Nguyen & Elise Rosenthal | May 24, 2016, 2:05 pm
    • Ashley Marranzino

      Hi Elise and Esther,

      Thanks for reading the post and I am glad you were able to learn so much from it!

      Different predators have different ways to control their lights. Animals that use bacteria to help them bioluminesce usually store the glowing bacteria in some area in their body that they can cover up (for example, with a some dark skin). They can control their bioluminescence almost like closing or opening a blind on a window, allowing more light in or out of their body. Other animals control their own bioluminescence with their own nervous system. They can produce different hormones or other chemicals that can turn their lights on or off.

      As for the mirror like proteins- great question. I do not know the specifics of how these proteins arose, but likely some animal had a mutation in their genes that resulted in a mirror-like tissue. This was a beneficial adaptation that allowed that animal to reproduce and spread the genes for mirror-like tissue to their offspring.

      Let me know if you have more questions I can answer for you. Thanks for your interest.
      -Ashley

      Posted by Ashley Marranzino | May 25, 2016, 3:02 pm

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  1. […] study new compounds or proteins, green fluorescent protein (GFP)–previously on oceanbites, here–which comes from the jellyfish species Aequorea victoria  can be used. GFP is particularly […]

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