December 19, 2005 | David F. Coppedge

Undersea Christmas Lights Explained

There is a marine animal like a jellyfish that puts on one of the most dazzling light shows in nature.  Some ctenophores, or comb jellies, can send multi-colored pulses of light that radiate down their sides in a rainbow of colors.  If you’ve ever seen one of these on a TV nature show, you were probably stunned and asked, How does it do that?  Three scientists from Oxford and Paris were intrigued by the “spectacular iridescence” of these comb jellies and decided to find out.  The first thing they found out was that nobody had ever explained it before.
    Their investigation, published in Current Biology,1 concluded that the effect is part photochemical and part structural.  The animals have rows of specialized cilia associated with light-producing (bioluminescent) organs.  The cilia are arranged in precise formations such that they act as “photonic crystals” that can concentrate particular wavelengths of light (this mechanism also operates in bird feathers and butterfly wings; see 11/18/2005).  The color seen by an observer depends on the angle of the photonic crystals along the line of sight.  As the cilia beat in synchronized patterns down the sides, this angle changes, causing the colors to change in wavy patterns down their sides.  The result is a dazzling undersea light show:

Our results show that the observed colouration of the ctenophore Beroë cucumis can be explained by the structure described, which operates as a photonic crystal.  This is the first time a photonic crystal composed of cilia has been reported.  The parallelogrammatic cilial packing is also new: the two-dimensional photonic crystals previously described have had hexagonally, squarely or rectangularly packed components.
    Remarkably, our results indicate that this structure is optimised not only for reflection of ambient light to generate bright colouration across the visible spectrum, but also to transmit light of wavelengths around that of the organism’s bioluminescence. (Emphasis added in all quotes.)

    Since ctenophores lack eyes, the scientists figure these function as a deterrent to predators.  The researchers hope by learning about the design of these light-producing structures, inventors might find useful applications: “A photonic crystal is a rare type of colour-producing structure,” they said, “composed of a regularly repeating structure with dimensions a fraction of the wavelength of light, complex optical properties and large commercial potential.”

1Welch, Vigneron and Parker, “The cause of colouration in the ctenophore Beroë cucumis,” Current Biology, Volume 15, Issue 24, 24 December 2005, Pages R985-R986.

Watch for these on the next undersea nature program on TV; the light show truly is “spectacular” as the authors describe it (click here for an image gallery, but to be fully appreciated, the lights must be seen in action).  Ctenophore lighting is another “useless” wonder of nature that appears to be overkill simply for evading predators.  The world is a vast treasure chest of ingenious solutions to physical problems that challenge our own intelligence to understand.  Approaching these phenomena from a design perspective not only helps explain them, but also leads to what Francis Bacon called experimenta fructifera, fruitful experiments that can improve our lives.  For more thoughts by William Dembski on intelligent design and biomimetics, particularly regarding the Vorticella spring (12/13/2005), see Uncommon Descent.

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