6Dinoflagellates Practice Mutually Assured Destruction
We don't like to criticize dinoflagellates (their fans get vicious in the comments) but their defense mechanism appears to be the worst idea in the world.
Any sharp motion in the vicinity will immediately trigger a near-blinding flash of light that shines through their entire body. In the presence of a plankton-eating creature, this bioluminescence would seem to serve all the survival utility of bringing a glowstick-twirling raver on a big cat safari. So why do it?
The light may well betray their position, but it's also their way of screaming "Eat me!" in the most derogatory sense possible. The thing about predators of dinoflagellates is that they themselves are prey to larger predators, and so on. So the dinoflagellates predator may devour them, but with the sudden flash of light announcing it, the predators have now been illuminated to larger predators in the vicinity.
Even after the dinoflagellate has been consumed, the predator's motion will continue to usher forth bursts of light. For largely transparent creatures like certain shrimp, this is like slapping a road flare on its chest and attaching dinner bells to each foot.
It's a Mutually Assured Destruction scheme that's simultaneously brilliant and dickish.
5The Vampire Squid is Terrifying and Lazy
The vampire squid lives way down in the parts of the sea where barely any light can reach, as a creature called a vampire squid would. That's why the black variety of this squid are only visible in our nightmares, and navigate via the terrified screams of approaching fish.
The more common, and almost as horrifying, red vampire squid calls upon defense techniques that are equal parts cunning and bizarre.
First, it pulls the webbing connecting each tentacle over its head, revealing a hood of formidable looking spines. These fleshy protrusions are actually about as imposing as a koosh ball, not unlike the vampires of the Twilight series, but it's a good bluff.
In the poorly lit surroundings this exposed black underbelly also cloaks the squid for a retreat, which it would totally do if hadn't just eaten. It doesn't want to get cramps, you know? So instead of running, it illuminates photophores set behind its eyes and slowly contracts them, giving the illusion of shrinking into the distance.
Let us emphasize that it doesn't actually run away. It is so profoundly lazy that it is already conserving energy for all of the floating it will be doing later. That's like evading a knife-wielding maniac by doing one of those "walking down fake stairs" tricks before curling up and taking a nap behind the couch.