The sarcastic fringehead (Neoclinus blanchardi) is a footlong fish that lives off the Pacific coast of North America. When unthreatened, the fringehead's visage is fairly unremarkable. But if a marine predator (or stray human hand) attempts to dislodge a fringehead from its crevasse, the fish will race into action, brandishing its frightening Predator-like mouth at the interloper.
And things get even stranger when two sarcastic fringeheads get embroiled in a turf war. How strange? Two words: "mouth wrestling." Are you brave enough to behold the angriest make-out party in the animal kingdom?
As the University of Michigan explains of this temperamental fish's guest policy:
As suggested by their name, sarcastic fringeheads are extremely temperamental. They are fiercely territorial creatures that aggressively protect their homes from all intruders, regardless of size. The majority of the time, sarcastic fringeheads are at rest in their homes — only their heads protrude. However, upon the first sign of danger, they will employ their enormous mouths and needlelike teeth for defense. Initially, they emit only a warning accomplished by the flexing and snapping of their jaws. If the intruder ignores the warning, they will use their ferocious teeth to attack.
The comedy ramps up even further when two fringehead compete over prime real estate. If one sarcastic fringehead desires the other's pile of ocean detritus or found seashell, the mouth wrestling match will unfold.
The fringeheads have poor eyesight, so even if they're outclassed by a foe, they won't know until they're dancing cheek-in-cheek.
Sarcastic fringeheads don't present a threat to humans, with the exception of the obnoxious diver whose unprotected fingers go poking around their homes. Here are some videos of the sarcastic fringehead flaunting its dentures at any creature unlucky enough to get in its way.
Semi-related: The predatory tunicate, another horrendous mouth at the bottom of the ocean. And let's not forget the cookiecutter shark!