A Passing Glance

5 August 2007

Iridescence is one of those curious optical games that light loves to play. From pearl earrings to tropical butterflies that appear to be made out of cellophane, iridescence is a source of luminous beauty across the natural world. Like most magic however, it has its dark, deadly side too.

In animals, iridescence is often termed structural coloration since it derives from the physical structure of the object rather than pigmented chemical compounds. All smoke and mirrors, or rather, mirrors and mirrors, iridescence happens when light bounces off different reflective surfaces in a semi-transparent substance. As the light rays exit, they interfere with each other, sometimes destructively muting each other other times resonating into an impossible burst of color. Because the final appearance depends on the exact path taken by the light before reaching the eye, iridescent objects change their hue depending on your point of view giving them a magical, animated quality.

Perhaps the simplest example of iridescence is a soap bubble, which gets its colors from the combined actions of light reflecting of the internal and external surfaces of the film. As the membrane warps and attenuates, tugged and mangled by gravity, wind and surface tension, the thickness of the film changes resulting in the familiar psychedelic swirl of colors.

For all of it’s magical aesthetic, iridescence is relatively simple to achieve and is found frequently in the inanimate world from the sheen of a puddle contaminated with a thin layer of petroleum to prized gems and minerals like labradorite and opal. Iridescence is also seen across the spectrum of living creatures: the flashy gorget of a hummingbird, the pearly glow of a mollusc shell, a freshly deposited dog turd studded with blue-bottle flies.

Daring Jumping Spider (Phidippus audax) with iridescent fangs and a male Rufous Hummingbird (Selsaphorus rufus) with iridescent gorget.

The isopod I’m holding in the photo at top caught my eye as it rooted around among the tomatoes, doing the things that isopods do, you know, eating feces, venting toxic ammonia gas and drinking through its anus. These ubiquitous little soil crustaceans are some of the most familiar animals of childhood, and go by an astonishing variety of common names, you may know them as wood lice, pill bugs, sow bugs, slaters, potato bugs, roly polys or cheese logs(!) depending on where you grew up.

Like most adults, I don’t pay isopods much notice although they are one of the most commonly collected creatures when I’m leading nature investigations with kids and I often stumble upon them myself when I’m looking for more exotic fare…

I’d seen blue isopods before, but in keeping with my neglectful attitude hadn’t really given them a second thought. This one however seemed especially marvelous with its metallic ultramarine shine and I actually darted inside to get the camera, surely the first time a roly-poly has inspired quick action on my part. I half expected the little crustacean to have crawled out of sight, but just a little hunting turned it up and I snapped a few pictures then let it go. I slapped the photo up on my Flickr site and promptly forgot about the whole thing.

And that surely would have been the end of it but for one of those happy connections that is the magic of “Web 2.0.” Another Davis-based Flickr user saw my photo and left a brief comment informing me that the source of the magical color was, get this, a virus.

With their strange crystalline “bodies” it’s not at all surprising that some viruses too can exhibit iridescence. In fact, there is a whole family of these shiny viruses named, appropriately, Iridoviridae. Occasionally, these viruses build up in the tissues of their host in such density that they actually cause the unlucky victim to glow with an ominous luster. The culprit here is likely to be the again aptly named Isopod Iridescent Virus (IIV) first described from Berkeley isopods in 1980. Other iridoviruses affect insects (especially flies), fish and even frogs. Parasitic nematodes that live inside isopods can even contract the virus from their host!

The infection is apparently inevitably fatal, which makes it ironic (or at least tragic) that a co-worker of mine told me that when she was a kid “the blue ones were good luck!”

So much hidden beauty and terror in the world, and I don’t think I’ll ever look at a roly-poly the same way again.

        1 tatccaagtc taatccaagc atataattta gactactcta ccaatgttgt ggataaaaca
       61 atacccgacg ggctctgtca cggtatgaaa tgggaggatc atgtgtcatg tgaacatgat
      121 ccaaaaatca tcagaaaaaa agaattaaca gtcaaaatag acgaattgaa aagaaagatt
      181 aaggatgaaa aggacagggg taggaaggat aaggtgaaaa atagacttgc gtccttaaca
      241 aaagaaaggt ctaacataac aaaaaaactt agtaaaagtg ttatgtgtgc taaaagaagt
      301 tatagatttt taaaagagcc tcgaggagtt cttccgacaa tcataaagga tcttctggac
      361 gccagaaaaa acactcgtgc ggaaataaaa atatggcaaa ataaacttaa atcagcaggt
      421 ggtcttgaag aacaagaaat cgcaaaatca atgatccaaa ttctggacca aagacaaaat
      481 tcgtacaaag tcagtgctaa ctcaatgtat ggagcaaccg gtgttagagt tggagcgtta
      541 ccgtttatgc caatcgcaat gtgcacaaca tacatgggaa gaaaaagtat tcttgaagtt
      601 gtagaacacg tgaaagatta tggaggaaca gtggtgtacg gggacacgga c

DNA sequence from an Isopod Iridescent Virus.

9 Responses to “A Passing Glance”

  1. Kudos! I’m going to forward this to my brother, who swears that there’s a fish that gets infected by the “disco worm” which makes its eyeballs turn rainbow colors and then go insane.

  2. neil Says:

    Thanks Chelsea. Sounds like hes’ talking about an iridescent virus.

  3. Kevin Z Says:

    Nice post! I just found your blog through your comments on another. Had a look around and I like what I see. Nice photos. What do you shoot with? I’ve been trying to find a good macro camera, especially one to use under the microscope. You might be interested in my deep-sea isopod post when I was writing with deep sea news (not an iridescent one though).

  4. Neil Says:

    Thanks Kevin! I use a Canon PowerShot SD400 (actually my girlfriend’s camera) it’s not an SLR but it’s a very compact fun little camera with awesome digital macro capabilities. I’m not sure if it’s scope compatible though. Someday I’d like to get around to outfitting my old Canon EOS 35mm with a nice macro lens so I can actually control composition a little more.

    And, thanks for the giant isopod link very cool stuff!

  5. […] finds so much more in just a Passing Glance, that it is well worth a second […]

  6. […] The isopods off to the right (or if you’d rather, rollie-pollies, sowbugs, pillbugs, woodlice etc.) belong to a predominately marine group, the crustaceans. In fact, they still have gills! This makes them one of the most reliable denizens of moist microclimates, logs, underneath rocks, leaf-litter etc. Hence their place of honor on the IRF logo at top. I’ve written more about terrestrial isopods and the bizarre color-changing infection they get in A Passing Glance. […]

  7. Tedmadman Says:

    Nice post. I’m actually doing a project with this very virus, in order to discover a way to break it apart as part of a school project.

  8. Great article! I’m loving your website;

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