With a predator beneath you, looking up to see your shadow,
It is good to be transparent, so the light just passes through
But a nearby light’s reflection makes you sparkle like a diamond,
So a darker pigmentation is the better thing to do.
Those are two competing strategies, and mutually exclusive
Each has fatal flaws, so choosing “clear or solid?” is a bitch;
But now Japatella heathi and Onychoteuthis banksii
Have evolved the best of both worlds—in an instant, they can switch!
Story, after the jump:
Ok, so you’re a squid or an octopus–a tasty blob of protein, no shell, no bones, and you hang out in the ocean–mesopelagic, so you have potential predators below you looking up at the dim (very dim at this depth) sunlight. Transparency is a decent strategy to avoid these particular predators; if the sunlight filters right through you, those predators will go after your more opaque peers.
But. At this depth, there are other predators, with bioluminescent flashlights that they can shine on you. That beautiful transparency works great for light shining through you, but it comes at the cost of light shining at you from nearby making you light up (if you’ve seen the videos from submarine exploration, you’ve seen how beautiful and jewel-like these “transparent” creatures are in direct light). If someone shines a light on you in the dark, you’d best be wearing a dark, ninja-appropriate outfit.
But. There’s a reason ninjas aren’t mesopelagic. (No, it isn’t that they can’t hold their breath that long, cos they can. They’re ninjas.) The real reason is those predators from two paragraphs back, looking up and seeing shadows. Tasty, tasty shadows.
What to do? Well, if you are a cephalopod, and by definition the coolest and smartest of all animals, you become a quick-change artist. The current issue of Current Biology reports on two species (the ones listed in the verse, above) that are transparent in ambient light or dark, but which immediately darken (by relaxing chromatophores) in response to LED lights approximating the bioluminescent light used by mesopelagic predators. The Beeb has very very cool video of the change; it is remarkably quick.
In this case, it makes perfect sense to be invisible only when no one is watching, and to be opaque when you are being viewed. A great strategy for the mesopelagic. Not a great strategy for fighting evil.