See-through shrimp flex invisible muscle

Much of the body of a Pederson’s transparent shrimp looks like watery nothing, but it’s a superhero sort of nothing. The shrimp is transparent enough to read through, but it’s not some frail, filmy thing. It’s packed with invisible muscle.

Searching for Ancylomenes pedersoni shrimp has a touch of the summer-camp prank about it, being a hunt for something that’s mostly invisible. On a research trip to reefs in Belize, “everyone else would give up,” says Laura Bagge of Duke University. But she’d find an anemone, where the transparent shrimp hide among stinging tentacles, and then float in the water watching for the twitch of a tiny visible part, such as an antenna or claw. “It’s like a ‘Where’s Waldo’ thing, because you can’t see it at first and then all of a sudden it becomes visible.”

The clear shrimp look so delicate that Bagge was startled at how powerfully they can jet backward to escape. She practiced using a turkey baster as a squirt gun to maneuver shrimp into “escaping” into a glass jar. The shrimp voluntarily make little forays from their anemone homes to work as cleaners, darting into the mouths of fish and nipping up parasites and debris. To attract clients for these hygiene services, though, the shrimp have to violate their own superb camouflage. They sway and “do this little rocking dance” that gives away their location, Bagge says.

They’re fun to watch in the lab, too, she says. The brown or red food pellets that a shrimp eats show up as dots as they move into the stomach. “You can tell when they’re hungry,” she says: As the stomach clears, the dotted line disappears.

The digestive organs and most other visible structures are bunched toward the front of the body. Females, however, carry their clutch of yellowish eggs attached to the clear abdomen, somewhat ruining the camouflage.

Becoming transparent is more than a matter of not containing pigment. “The bigger issue is scattering,” Bagge says. Light shifts direction when it passes through one material and hits another with different refractive properties. “Think of a snowman,” Bagge says. It’s just water, but snow’s particles scatter so much light that it’s opaque. Now Bagge is looking at the muscles under extreme magnification to see if some quality of the fibers suppresses scattering. Whatever the trick turns out to be, it breaks down when the shrimp dies. A dead shrimp soon turns visible.