The Deep Sea Creatures That Hide from Predators By Turning Almost Fully Invisible

Some amphipods use a layer of small protuberances on their surface to reduce the amount of light reflected off of them. But what these protuberances are made of could be even more fascinating…

Images of creatures in the wild using camouflage to blend in their surroundings never cease to amaze. Whether it’s a gecko in slumber hiding from a predator, or a stone flounder parked on the sea bed waiting to ambush its prey, camouflage in the wild is an evolutionary arms race. In environments like forest floors and sea beds that are laden with distinguishing features, animals blur the outline between their bodies and surroundings by resorting to a variety of techniques.

Although this is easier said than done, there is an obvious reliance on external factors like the presence of a tree bark or other flora. In the waters of the open ocean – the pelagic zone – traditional strategies are useless, forcing its inhabitants to adopt innovative approaches.

Amphipods, which look like sand fleas but can get closer in size to a pocket beagle, reside in such waters. A special type of amphipod, of the suborder Hyperiidea, have been successful in avoiding predators thanks to their transparent bodies that make them almost invisible. However, transparency alone may not be enough to ensure their survival.

When light passes from one medium into another, it undergoes refraction – even as a part of it is reflected back. Think of a clear, transparent empty glass in a dark room and how its contour is exposed by a single flash of light. The reason we can glimpse the shape of a transparent medium is because of this reflected light. In the pelagic zone, where there is an abundance of light in the blue-green region of the visible spectrum, even a small amount of reflection from the surface of a transparent creature can lead to its detection. A searchlight from a predatory dragonfish can easily enhance the contrast of the creature, which has otherwise almost maximised its cryptic ability. So what then do such transparent creatures do?

According to a recent study in the journal Current Biology, the hyperiids resort to a phenomenon first explained by Lord Rayleigh in 1880. Tiny projections of the order of a few nanometers adorn the surface of such creatures, specifically Cystisoma spinosum, the largest of the transparent amphipods. The arrangement of these projections, or nano-protuberances, changes the refractive index that the incoming light experiences. This is critical because an abrupt change in refractive index – such as that observed at the water-cuticle interface – leads to a larger amount of reflection.

This single of nano-protuberances smoothens the change in refractive indices, as a result increasing the transmission of, and reducing the reflectance of, light. This optical effect, called the Rayleigh effect, has been used to develop anti-reflective coatings for camera lenses, solar cells and spectacles. But here, too, nature beat us to this idea. The eye of moths and the wings of cicadas and hawkeye butterflies have similar structures. A seminal study dating from 1960 found that moths in particular have nano-protuberances in their compound eyes to reduce glare and enhance transmission.

So, what lends novelty to this study on hyperiids?

A large majority of the hyperiids observed were peppered with a layer of tiny spheres across their appendages. This layer formed a thin film serving a function similar to that of the nano-protuberances seen in C. spinosum.

A lot of animals have biofilms on their surfaces, but to our knowledge, nobody has ever observed such a thin, homogenous layer before. It is unusual that on each animal, the spheres are all the same size, and they are all nano-sized (smaller than about 300 nm and smaller than any known bacteria),” according to Laura Bagge, the lead author of the study. “The size range that we saw works very well as a thin-film for anti-reflection.”

The layer, again functioning according to the Rayleigh effect, has been shown to reduce the amount of light reflected from the cuticular surface by a factor ranging from four to 250, depending on the density and diameters of the spheres chosen in the modelling study. This is a remarkable feat, akin to adopting an invisibility cloak. What is more remarkable and perhaps the most intriguing hypothesis to result from the study is that this anti-reflective film may, in fact, be a living thing.

The authors mention that preliminary analyses of this layer indicate that the spheres may be a type of bacteria – possibly an undiscovered kind. Studies suggest that these are not mere projections arising from the cuticle of the host but are structures attached to the host by what could be fimbriae, tiny appendages by which bacteria often attach to their surroundings.

They also observe a binary fission plate, which is a furrow-like feature along which cells divide, on the surface of these nanospheres. This further strengthens the idea that these structures could be bacteria. If the layer of spheres indeed turns out to be bacteria, not only would it be a fascinating example of symbiosis but also a singularly unique one. As Bagge points out, “This would be the first example of a symbiosis that functions like an anti-reflective cloak.

The hyperiids genera observed as part of the study are not only prey for other larger oceanic creatures but also aggressive predators themselves. Having an anti-reflective coating of sorts can serve a dual function – to hunt and to avoid detection. Then again, they could also serve some entirely different functions as well. The authors note that the study is a work in progress. Bagge remarks that future work will include studies to determine how these animals make their entire body volume clear and how internal light scattering may affect their visibility.

A better understanding of such symbiosis can unravel many such previously undiscovered relationships. If the unique structural features possessed by hyperiids are to evade detection, then maybe that has lead to some interesting coevolutionary patterns in its predators as well. While scientists have been trying to use an array of materials for obtaining efficient anti-reflective properties in the hopes of designing an invisibility cloak, maybe we must turn our lenses to nature, which has often provided us with the easiest answers.

Hat-tip to Mohammad Rafiuddin for discussions on scattering and reflection.

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