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If you uncover a bunch of rolly pollies under a log, you don't expect to find a bright blue one crawling among all the usual grays and browns. But it turns out your fun surprise is some very bad luck for that terrestrial isopod.

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You might call them woodlice, pillbugs, potato bugs, or roly pollies. But they aren't lice or bugs.

They're not even insects. They're terrestrial isopods: the only fully land-dwelling crustaceans. And they come in all sorts of colors—there's brown, light brown, black, /dark/ brown… okay, they form a pretty limited rainbow.

Every once in a while, though, you might get lucky and find a brilliant blue roly polly crawling around the dirt. But your good luck is their bad luck because that blue comes from a virus. And when it makes them colorful like that, the virus is almost always lethal.

The viruses that turn roly pollies into roly bluies are part of a larger family known as iridoviruses. Iridoviruses can infect all sorts of ectotherms — animals whose body heat primarily comes from their environment — including crustaceans like our friends the pillbugs, true insects, reptiles, and fish. And, at least among the invertebrates on that list, the most obvious symptom is a noticeable color change both inside and outside of the animal.

Infected animals can turn blue, green, yellow, red, or just be kind of shimmery, depending on the virus. Now, on its own, that isn't too remarkable. Plenty of illnesses and infections change an organism's color.

But most infection-related color changes come from some sort of pigment: a chemical that reflects certain colors and not others. Iridoviruses don't use pigments. You can't put iridoviruses in a test tube and extract a blue substance from them.

That's because the blue of infected pillbugs is an example of structural color— it's the arrangement of the viruses inside an animal's cell that determines what colors the cell appears to be. And ultimately, it all comes down to the wavelike properties of light. Waves can interfere with each other, reinforcing or cancelling out depending on the way that crests and troughs line up with one another.

And that interference can alter what color is perceived. For example, red light's wavelength is about 700 nanometers. So if you had two semi-reflective layers of something -- like layers of glass -- separated by about 700 nanometers, the crests of the red light reflected by one would overlap with the crests of the red light reflected by the other, making it seem like the thing is tinted red.

But if the layers were separated by about 350 nanometers instead, the crests reflected by one would overlap with the troughs reflected by the other. No red light would come out at all, even though both layers are reflecting red. And the same goes if you have individual atoms doing the reflecting instead of layers of glass.

So that's structural color. t just depend on the colors that are reflected; it depends on the physical arrangement, separation, and reflective properties of whatever's doing the reflecting. You can find structural color in butterfly wings, birds, beetles, and plenty of other species. And you can find it in animals tinted by iridoviruses.

That's because the viruses don't just spread all over the place when they reproduce inside a host animal's cells; they arrange into rows and larger structures, a lot like atoms do in a crystal. Smaller viruses tend to pack closer together, leading to the blue tint seen in animals like roly pollies. Larger viruses tend to spread out more, giving us redder animals, instead.

Iridoviruses have one more curveball to throw, though. Sometimes they cause this kind of characteristic color change, but usually, they don't. Two animals of the same species can even get infected by the same kind of iridovirus, and one might change colors while the other doesn't.

These are called covert infections, and scientists still aren't sure what makes the virus go down one path or the other — partly because they're not sure why iridoviruses color animals in the first place. Iridescent infections are easy for other potential hosts to avoid, which could potentially make it harder for the virus to spread. However, a bright bug is more likely to get eaten than one that's well camouflaged, and a predator that eats an infected bug in one place could carry the infection elsewhere.

So, some scientists think that obvious colors help the viruses spread far and wide, while covert infections let the viruses move around within a single population. It's also possible that the colorfulness of these viruses isn't an adaptation in itself. Patent infections — the ones that cause color changes — happen when the virus reproduces really quickly inside a host's cells, while covert infections tend to be when the virus reproduces more slowly.

And it could just be that when lots of viruses cram into a cell, it takes less energy to arrange like a crystal than it would to do something else. What we /do/ know is that patent and covert infections tend to have different symptoms. Patent infections are just about universally lethal, but they don't usually go full-on pandemic and kill whole animal communities.

They just consistently infect small percentages of certain species — anywhere from one in a hundred individuals to one in a million. Covert infections are much more common— estimates range from ten to thousands of times as common, depending on the host species. And they're not nearly as lethal as patent infections.

They usually impact things like an animal's ability to move around or reproduce. Whether patent or covert, though, iridoviruses can have some pretty big ecological effects. They've killed off whole populations of tiger salamanders and bass, and they've been found in bee colonies that have suddenly died, suggesting they might play a role in colony collapse disorder, too.

Understanding the epidemiology of these viruses could help researchers predict and prevent outbreaks in species we don't want to lose, like bees. And it might even allow scientists to get the viruses to do some good, like control invasive pests. In the meantime, they give physicists and biologists something to talk about at dinner party.

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