There's a species of octopi that live in groups. On nights with bright moon, predators can identify them because they cast a shadow. They have evolved an organ to intake particular bacteria that use what's called quorum sensing to detect when there's enough of them around. When the organ is full of these bacteria, the bacteria glow and therefore stop the shadow of the octopi. It's a great example of symbiosis, more specifically mutualism of two species. The octopi get saved and the bacteria get a safe place to live.
That is unless it doesn't literally kill you that literally now actually means figuratively too.
I said, "No; six should be enough."
(copypasted from http://itre.cis.upenn.edu/~myl/languagelog/archives/002938.h...)
What's popular is correct by definition, that's why dictionaries reflect usage, they don't prescribe it. Language is about communication, not syntax, people do use "literally" when they mean figuratively and you know it, I know it, so they are communicating, the word has obtained new meaning. Words are not static things.
In short: The dictionary isn't the "law", it lags behind the language as it is actually used. Once the "reverse" definition of "literally" became common in usage it became the new definition - not what a dictionary used to define it as.
It seems weird because the animal itself would be a lot more visible.
This wikipedia article  about firefly squid says that the animal uses it's ability to match the brightness and colour of its underside to the light coming from the surface, making it difficult for predators to detect it from below
Is that it?
Here's the wiki on the octopus species though. https://en.wikipedia.org/wiki/Euprymna_scolopes
Here's some dolphins for you (daytime though): https://www.youtube.com/watch?v=6lWmOhS83kU
It's OK -- I didn't know either, until right now. Thanks for the tip!