These animals’ ability to control light renders them almost invisible By Jasmin Fox-Skelly
We’ve all heard of squid and octopus using pigments to blend in with their surroundings, but what about becoming completely invisible? To become actually see through, and appear as if you aren’t there, you need to either allow light to travel through you unimpeded, or bend light around you - so that none reflects back at an observer. It’s a tricky task, but some animals are almost there.
In the ocean animals have two choices if they want to hide. Creatures that live in the deep ocean close to the seafloor can blend in with sand or rocks, or hide in coral. In the deep ocean it is often pitch black anyway and predators lack eyes, so being invisible is not necessary.
Animals that live close to the surface and want to hide can produce dazzling displays of light in a process known as bioluminescence, confusing predators below who think they are looking at dappled sunshine hitting the water’s surface. Animals that live in midwater though have neither of these options. This region is known as the pelagic zone, and it also happens to be where most invisible animals live.
Perhaps the easiest way of becoming invisible is by being transparent and letting light travel completely through you. In open oceans, which lack structures to hide behind, being transparent is a great way of hiding from all viewpoints and angles. It’s so popular in fact that transparency has independently evolved multiple times in completely unrelated animals.
One such animal, the glass octopus (Vitreledonella richardi) is so named because it is almost completely transparent. The gelatinous creature can grow up to 45cm (18in), if you include the tentacles. It lives 300-1000m below the surface in tropical and subtropical waters across the world, and is almost completely invisible to predators except for its digestive system, optic nerves and eyes.
But what’s the point in making your whole body transparent, if the eyes and guts are still visible? Even worse, these organs will cast shadows on the seafloor below, making them more visible to predators. Eyes need to absorb light to function, so it is not possible for them to be transparent. Guts betray their contents, so unless an animal feeds on transparent material, they will be visible. However the octopus, and all hosts of transparent creatures go to great lengths to disguise these opaque organs. The glass octopus (Vitreledonella richardi) for example has very elongated eyes which reduces its peripheral vision, but minimises the shadow it casts below - making it less likely to be detected by predators hunting from below. There is also some evidence that it orientates its body in such a way so as to minimise its shadow.
The glass octopus is not the only transparent animal to come up with an ingenious way of disguising its eyes. Many transparent molluscs camouflage their eyes with mirrors, as mirrors in the open ocean reflect only more ocean and so are invisible.
The glass family of squid, of which there are about 60 species, are almost entirely see through. They live, again in the pelagic region of oceans around the world, between 200 and 1000m below sea level.
Although their bodies are entirely transparent, their large eyes are opaque, which is a problem as predators swimming below can easily see the shadow they cast. However the glass squid (Cranchiidae) uses a clever form of camouflage to hide them. It uses photophores - organs beneath its eyes - to produce light in a trick called counter-illumination. This light looks very similar to the sunlight filtering down from above, so it makes the squid completely invisible to predators swimming below it. However the light could make the squid very conspicuous to viewers looking at it from other angles. Rather than an invisibility cloak, the glowing light could act like a beacon drawing predators to it.
Researchers from the University of Pennsylvania found that the squid’s photophores are amazingly able to match the amount of light they produce to that coming in from every direction, creating a sort of omnidirectional invisibility cloak.
This genus, or group of marine planktonic polychaete worms are almost completely transparent, making them very difficult for predators to see. Paradoxically at least 11 species in the group can also emit bright luminous colours. Most tomopteris worms glow blue, but one species, Tomopteris nisseni can produce yellow light and is one of only few such creatures on the planet to do so.
Some tomopteris worms can even distract predators by releasing a glowing part of their body called a parapodia, making the predator chase after the dispelled body part rather than the worm itself.
A salp is a completely transparent barrel shaped creature which swims and feeds at the same time by pumping water through its gelatinous body. They filter out the phytoplankton in the water to feed on. Although they look a bit like jellyfish they are actually more sophisticated and are closely related to fish and vertebrates - they have a heart and gills and can reproduce sexually.
Salps have a fascinating life cycle. For part of it they live by themselves, but they then clone themselves and form long strings and other shapes of connected organisms. Individual salps synchronise their swimming by communicating with one another via electrical signals.
Sometimes being transparent isn’t enough, and organisms need other tricks up their sleeve to remain invisible. This is certainly the case for the Hyperiid, a little crustacean bearing a resemblance to a shrimp. They are able to hide from predators by being transparent. However that only gets them so far. A plane of glass is also transparent but you can still see it if you shine a light on it, as the light is reflected back. This is a particular problem in the ocean because many predators use bioluminescence as a searchlight when hunting for prey.
A recent study suggests there is more to the hyperiid’s ability to hide than simple transparency. It turns out they are using a kind of nanotechnology to interfere with and bend light, cloaking themselves and almost rendering them invisible. The scientists used a scanning electron microscope to closely analyse seven species of hyperiids. They found that the legs of one species were covered in tiny nano sized hair-like protuberances.
The body of this species, and six others were also covered in nano sized bumps or spheres ranging in size from under 100 nanometers to around 300 nanometers. The tiny size of the bumps could minimise light scattering and the scientists found that a combination of both nanostructures - the bumps and the hairs could reduce reflectance by as much as 100 fold. The weird thing is that the researchers think these spheres could actually be bacteria.
Japetella heathi and Onychoteuthis banksii
The squid Japetella heathi and the octopus Onychoteuthis banksii also have a novel trick up their sleeves when it comes to invisibility - they can quickly switch from being transparent to a reddish brown colour.
They both live in the Pacific Ocean between 600-1000m deep – known as the mesopelagic zone. Although being transparent helps a lot with invisibility close to the water’s surface, as diffuse sunlight from the surface passes straight through transparent tissue, when you shine a light directly on something that is transparent, it suddenly becomes very visible.
Unfortunately this happens quite a lot in the deep sea, where predators use light-emitting organs called photophores like a searchlight when hunting. Prey at these depths are often red or black so that they reflect as little blue light as possible. Japetella heathi, an octopus, and Onychoteuthis banksii, a squid, are able to switch between both, but how do they do it? Both species’ skin contains light sensitive cells called chromatophores. The cells contain a dye, and when they detect light they immediately expand and release the pigment.
Sea Sapphires (Sapphirina) are ant size creatures that live in warm tropical and subtropical seas. They belong to a group of crustaceans called copepods. Different species emit a range of brilliant iridescent colours, from vivid blues to reds and golds.
What is remarkable about them is that one second they can shimmer brightly and the next they appear almost to disappear and the way they do this is fascinating. Their skin, or cuticle cells contain tiny crystal plates arranged in a hexagonal honeycomb pattern. The crystals contain guanine, one of the four bases that make up DNA. The crystal layers are separated from each other by a soup-like fluid called a cytosol.
A team of scientists found that the although the layers of guanine crystals are always exactly the same thickness – 70 nanometers, the thickness of the cytosol between the layers varies from 50 to 200 nanometers. It is this variety which determines the colour of the sea sapphire. Thicker layers of cytosol lead to longer wavelengths of light being reflected, which make the copepod look red or magenta.
The colour also depends on the angle of light which strikes them. As the angle becomes smaller and smaller, the wavelength of reflected light becomes shorter and the colour more violet. If the angle becomes small enough then the reflected light is in the UV spectrum, which means that we can’t see it and the sea sapphires disappear. The researchers found that light which hit the crustaceans at a 45° angle effectively caused them to become invisible.
All of the transparent animals discussed so far have lived in the sea, and there’s a good reason for that. To be transparent you need to be made up of stuff that neither absorbs nor reflects light. This is a difficult task for plants and animals that live on land because there is such a large difference between the refractive index of living tissues and air. The refractive index of a material describes how quickly light travels through it. Light travels fastest in a vacuum, and generally speaking the denser a material, the longer light takes to travel through it and the greater its refractive index will be.
As biological tissue is so much thicker and denser than air, when light waves go from travelling through air to body tissue, they slow down. This causes light to change directions and scatter, causing reflections that make the animal more visible.
In the sea there is less difference between the refractive index of water and biological tissues, so transparency is an easier task, hence why there are so many ‘almost’ invisible animals. Another reason you don’t find many see through animals on land is because organisms need pigments like melanin to protect them from UV radiation from the sun.
However there are some exceptions to the see through rule. One is the glasswing butterfly (Greta oto) which lives in Central America.
Although not all of its body is see through, its transparent wings make it difficult for predators to track it during flight. To look at how the butterfly achieves its transparency, scientists examined their wings under an electron microscope. They found tiny nano sized bumps called nanopillars which were scattered randomly and had different lengths. It seems that the random size and distribution of the nanoscale structures help the butterfly minimise reflections from its wings. The nanopillars interfere with rays of light hitting the wing, causing most to pass straight through rather than bouncing back.
Another exception to the rule is a translucent snail (Zospeum tholussum) that was discovered in the deepest cave in Croatia. Scientists from Goethe University, Frankfurt found the see through mollusc living 980m underground in the Lukina Jama-Trojama cave, in a chamber full of rocks and sand with a small stream running through it.
The snail belongs to a genus of miniature land snails that are found in dark, underground caves, and which are unable to move by themselves. Researchers believe they use running water from streams to transport themselves.
However even though it is translucent, the snail is still fairly visible, highlighting just how difficult it is for land animals to achieve what those in the ocean do.