Being a snack-sized animal in the open ocean is tough. Some have it easier than others. Creatures on the bottom can blend in with stones and sand. Stands of kelp and coral provide hiding places in other ocean habitats.
But in midwater, there is no place to hide. There, creatures can get eaten pretty quickly by something unless they can find a way to disappear. Laura Bagge, a graduate student at Duke University, thinks she knows how to make that happen—at least in a group of tiny, shrimp-like crustaceans called hyperiids.
Bagge, along with biologist Sönke Johnsen and Smithsonian zoologist Karen Osborn, recently published a paper in the journal Current Biology, describing how hyperiid amphipods use nanotechnology to cloak themselves with invisibility.
The discovery was made by Bagge, the paper's lead author, who worked with Osborn at the Smithsonian's National Museum of Natural History in Washington, D.C. “She was interested in the transparency of these animals. Transparency has been looked at in other animals and they do it in known ways so far but nobody had looked at this in these guys."
Bagge examined the surfaces of the animal's exoskeleton to study their structure. "She found these bumps and thought they were interesting,” says Osborn.
The bumps turned out to be microscopic spheres. In some cases she found a nano-sized shag carpet and on others, a layer of tightly packed nano-spheres. They were sized just right to dampen light in a manner similar to the sound-proof foam insulation that decreases noise in a recording studio. Hyperiids seem to have two possible ways to make their surfaces not reflect light—nano protuberances on their cuticle (a shag carpet essentially) or a microfilm layer of tiny spheres. The closer that they looked, the more those little spheres seemed to be bacteria.
“Every indication is that they are bacteria but. . . they are extremely small for bacteria,” says Osborn. "There is a possibility that these are some strange excretions, but it's a pretty microscopic chance.” She adds that Bagge is now working on exploring that possibility with microbiologists.
Animals living in the midwater habitat of the ocean adapt different camouflage methods to deal with light coming from different directions. Light from the sun becomes dimmer and changes color as it penetrates deeper water. To deal with this, fish and other sea creatures hide from predators stalking them from above by adapting dark colors on the top parts of their bodies as a disguise to blend in with the dark depths below.
At the same time, to hide themselves from predators lurking beneath them, they may be shaded underneath their bodies with lighter colors, or even glow, in order to blend in with the light from above. Mirroring on the sides of some fishes is another way to hide.
The hyperiids start out with a big advantage: They are transparent. But that only gets them so far. A pane of glass is also transparent, but when you shine a light at it from certain angles, it will flash and become visible.
Bioluminescence is an important part of the strategies of many creatures that are both predators and prey in the ocean. By flashing lights from various directions, a predator can see the flash back from its transparent prey. To avoid detection, a free-swimming hyperiid with no place to hide needs a way of dampening the light and keeping it from flashing back.
This is what the bacteria seem to be doing for their hosts. These cells are small as bacteria go, ranging from under 100 nanometers to around 300 nanometers (100 nanometers is less than the diameter of a single strand of hair). The ideal size for dampening flashes is 110 nanometers in diameter, but anything up to around 300 nanometers can help reduce visibility.
“Hyperiids are really tough little buggers,” says Osborn. They were relatively easy to work with, she says, because they stay alive in a laboratory setting. “They are happy in a bucket, happy if you leave them alone.”
The scientists plan to sequence at least parts of the genomes of the bacteria in order to learn more about them. Do all species of hyperiid host the same species of bacteria? Do the bacteria also live in the water without a host? Sequencing DNA is an important step towards answering these and other questions.
Bagge initially concentrated on only two species of hyperiids, but Osborn encouraged her to branch out and see if these nanotechnologies were common among more of the 350 known species in the sub-order. Osborn was able to find her more samples, both living and long dead.
“It was really interesting to compare the fresh specimens to the things we have in the collections at the National Museum of Natural History that are over 100 years old,” says Osborn. “We found the microfilm consistently on the specimens we looked at . . . It gives us the diversity that you can't get from anywhere else. Smithsonian's historical collections come into play for a lot of studies.”