Glass octopus (Vitreledonella richardi)
The glass octopus is one of the most mysterious creatures around. It is an elusive, deep-dwelling species that is very nearly invisible in its marine home. Only the digestive organs and eyes are opaque.
The eyes cast a much smaller shadow than the half-globe eyes of other octopuses
Unlike the large round eyes of many of its deep-dwelling fellows, the glass octopus has eyes that almost appear rectangular from the side. They are effectively tubes with a lens at one end and retina at the other. The eyes point upwards to gather the residual daylight from the sky far above.
The cylindrical eye shape may help to disguise the octopus in the open ocean, where there is nowhere to hide. From underneath, the eyes cast a much smaller shadow than the half-globe eyes of other octopuses.
Japetella octopuses also appear transparent. They have an extra trick: they can defend themselves from predators that have their own light source. In 2011 scientists found they switch to a darker red appearance if a predator shines a light on them.
Glass frogs (Centrolenidae)
Glass frogs are found in Central and South America, particularly the cloud forests. They are named for their see-through skin.
It is lime-green, resembles the muppet Kermit, and makes a long metallic whistling call
Many of the 100 species in the family have translucent bellies, so you can see the shape of their organs, bones and blood vessels. In some species the visible bones are green, while in others the organs are also translucent.
It's not clear why their undersides are see-through. Their backs are bright green, and this helps them to blend into the verdant leaf canopies they call home.
A new species called Hyalinobatrachium dianae was discovered in 2015 by Brian Kubicki of the Costa Rican Amphibian Research Center in Guayacán and his colleagues. It is lime-green, resembles the muppet Kermit, and makes a long metallic whistling call.
At the time, Kubicki told the BBC that glass frogs are "delicate" species.
Moon jellyfish (Aurelia aurita)
While the moon jellyfish appears like a luminous globe in the light of an underwater photographer's flash, its name actually relates to the four horseshoe-shaped organs visible through its upper section or "umbrella". These are the reproductive organs. They appear white in males and pale pink in females.
The tentacles are covered with stinging cells that can stun passing small fish
It is arguably one of the most recognisable transparent animals – especially for European beachgoers, as it frequently washes up on shorelines.
When not lying like a slimy plastic bag by your sandcastle, the moon jellyfish floats near the surface of the water. There it traps plankton on its mucus-covered bell, and moves them to its mouth to gobble up.
It also dangles tentacles below the water to trap bigger meals. The tentacles, and the fringed edging of its umbrella, are covered with stinging cells that can stun passing small fish.
They don't cause humans much pain but moon jellyfish still cause problems. Several nuclear plants have had to shut down their reactors after their cooling systems became inundated with the brainless creatures. Large blooms of jellyfish may become commonplace.
This fish hasn't been known long enough to earn a common name. Its Latin moniker means "blue-bellied night wanderer". This tells you something about its appearance and behaviour but leaves out the fact that, apart from the abdomen and head, it's almost completely transparent.
They had gone unnoticed for 30 years
Many larval fish are transparent, as this disguises them before they reach maturity. C. noctivaga just doesn't grow out of it, nor does it grow to a great size.
Discovered in Brazil's Rio Negro in 2011, at its biggest it measures just 17mm (0.7in) long. That's only 7mm (0.3in) longer than the world's smallest fish, Sumatra's Paedocypris progenetica.
It was a surprising find, as scientists had thought its home, the largest tributary of the Amazon, had been thoroughly explored. Afterwards, researchers found more specimens in the University of Sao Paolo's Museum of Zoology. They had gone unnoticed for 30 years.
Sea walnut (Mnemiopsis leidyi)
If you move slowly, have no eyes or brain, and basically look like a transparent blob, you get called a sea walnut. This animal's common name was inspired by its apparent lifelessness.
They were accidentally introduced to the Black Sea by cargo ships
It is not a jellyfish but a comb jelly. Rather than jet-propelling themselves through the water like jellyfish, comb jellies are powered by rows of microscopic, hair-like structures that vibrate. These look like combs, hence the name.
The sea walnut is transparent, but its combs scatter light into a rainbow of colours. In photographs this creates a web of pulsing neon rope lights. The animal also produces a blue-green bioluminescent glow, using specialised cells called photocytes.
They use their combs to trap prey, pushing water full of tiny plankton directly into the sea walnut's mouth. Sea walnuts are such voracious predators that they can dramatically affect food webs.
They were accidentally introduced to the Black Sea by cargo ships, and have eaten their way through much of its plankton, depriving animals like fish of food. Even the populations of large animals such as dolphins and seals have declined.
Glasswing butterfly (Greta oto)
Butterflies are known for standing out. Flashy, brightly coloured species like swallowtails rely on their eye-catching colour schemes to communicate and to secure mates.
The wings reflect so little light that not even a stray twinkle will give the butterflies away
Others do dress down, putting cryptic colours on the undersides of their wings to avoid predators. But none take it as far as the Central American glasswing butterfly. As you can guess, you can see straight through its wings.
The wild colours of butterflies are created by miniscule scales on their wings. But the glasswings have no scales on large portions of their wings, creating clear windows to the background behind them.
More impressively, the wings reflect so little light that not even a stray twinkle will give the butterflies away. This is a result of the wings' nanoscale structure.
A 2015 study described tiny pillar-like structures on the wings, the shapes and sizes of which are "chaotic". This arrangement resulted in substantially less reflection, and could be mimicked in future glare-free screens for computer monitors and smartphones.
Venus' flower basket (Euplectella aspergillum)
While plenty of see-through species get compared to glass, it's most accurate in the case of Venus' flower basket. Also known as the glass sponge, its skeleton is made of silica, the key material used to make glass.
The shrimp live imprisoned as monogamous pairs
The rigid body of the Venus' flower basket sticks out of the seabed into the cold waters of the western Pacific Ocean, at depths down to 1000m (328ft). It rises in a column of mesh to a height of 25cm (10in).
It looks like an intricate vase, hence the name. Water is sucked into the sponge's tissues and filtered for food particles.
The hollow body is occasionally home to shrimp, which enter as larvae but are trapped when they grow too large to escape. The shrimp then live imprisoned as monogamous pairs, making such sponges popular traditional wedding gifts in nearby Japan.
But once they leave the water, glass sponges literally pale in comparison with their former selves. They are lit by bioluminescence: in this case the glow is created by bacteria living on the sponges' surfaces.
Sea butterflies (Thecosomata)
Sea butterflies are actually marine snails that have adapted to life near the poles.
Sea butterflies feed by deploying a net of mucus over their wings to trap food particles
Rather than using its muscular foot to creep along the sea bed, a sea butterfly uses it to swim through open water. The modified foot is divided into two lobes and looks like a pair of transparent, flimsy wings. These "wings" flap, earning the animals their common name.
Many species of sea butterfly have lost their shells, and those that have kept them have made them completely clear.
Sea butterflies feed by deploying a net of mucus over their wings to trap food particles. This net can be five times the size of the actual snail. The sea butterfly sucks the nest back into its mouth afterwards, to recover the nutrients that went into making it.
Sea butterflies are said to make up more than 50% of the zooplankton in the polar seas, feeding a huge range of animals from herring to ringed seals.
Ghost shrimp (Palaemonetes)
See-through is a popular look among shrimp. These small crustaceans have evolved to become almost invisible to evade their predators. The term "ghost shrimp" is applied to several groups of animals, the largest being the genus Palaemonetes.
In some species, you can see the food in their translucent stomachs
There are more than 40 different species of Palaemonetes, living in fresh and brackish water around the world. They are sometimes called "glass shrimp", thanks to their translucent exoskeletons, or "grass shrimp" because they like to live among weeds.
Ghost shrimp make popular pets, because they clean up tanks by feeding on detritus. In some species, you can see the food in their translucent stomachs. The green-tinted eggs can also be visible inside females' bodies before they spawn.
Their eyes are among their few opaque features. The light-sensitive retinas can only work by capturing light, rather than letting it pass through. However, larval daggerblade grass shrimp have a layer of greenish-gold eyeshine. This may mask their conspicuous black eyes from predators.
Antarctic icefish (Notothenioidei)
Living in 10m (32ft) below the surface of the Southern Ocean, where temperatures hover around -2 °C (28.4 F), are fish that seem to be made of the ice they swim beneath. Antarctic icefish are so well-adapted to the frigid waters, they even have an antifreeze glycoprotein in their blood and body fluids to stop ice crystals forming.
Icefish dominate Antarctic waters
These fish are pale and many have translucent skin. One family, the crocodile icefish, do not even have red blood pumping through their veins. They are the only known backboned animals to lack haemoglobin, the protein that carries oxygen in the blood and makes it red.
At first this was thought to be another adaptation to their frosty home, but scientists now suspect it is an evolutionary error. With their cloudy white blood, the icefish can only transport 10% as much oxygen as red-blooded fish. To compensate they need large hearts, lots of blood and dense nets of blood vessels.
The waters beneath ice sheets were once considered inhospitable for fish, but recent surveys have revealed that icefish dominate Antarctic waters, comprising up to 35% of the biomass. However, sea temperatures are now rising rapidly, and their unique adaptations to cold waters could put them at risk of extinction.