Glass squid, illuminated

Scientists have discovered how two marine creatures are able to rapidly "switch" their colours - from transparent to reddish brown.
The species, an octopus and a squid, use their adaptable camouflage to cope with changing light conditions in the deep ocean...
The animals' skins contain light-sensitive cells called chromatophores, which contain pigments. When these cells detected the blue light of a bioluminescent predator, they immediately expanded, "dyeing" the animal a deep brown colour.
Dr Zylinski said the this dramatic colour change showed just how important camouflage was "in a habitat where there is nowhere to hide".
Neither transparency nor pigmentation is a complete solution to the hunting strategies used by predators in the deep ocean, she explained.
"By switching between these two forms, these cephalopods are able to optimise their camouflage in response to the optical conditions at that moment in time."

From BBC News.

The beautiful squid pictured above is named Leachia. It's one of many transparent sea creatures, along with jellies, crustaceans, and other cephalopods-- but as far as I know, none of those have the ability to change their opacity.