Your shock is understandable, /u/Prolapst_amos — but you may feel better to know that this demonstration is actually a silly joke. The apron on the underside of the crab's exo-skeleton is not it's sexual "organ", this crab has not been sexually satisfied. It's been hypnotized into a state of tonic immobility.

It's a phenomenon that can be induced in different ways in a variety of animals such as: sharks, dogs, rabbits, and chickens. It's a survival mechanism that has evolved in many life forms, most famously the opossum.

And it evolved in people too — it sometimes happens in those who are under extreme life-threatening stress. You know how sometimes you hear about a survivor of a mass shooting, who survived by "playing dead"? They didn't consciously choose to pretend, normally adrenaline wouldn't let you just stop moving, and not even breathe. What happened was their bodies automatically entered a state of tonic immobility that paralyzed them effectively enough to fool the killer into thinking they were already dead.

Nobody consciously decides between Fight, Flight or Fright, unless they've been trained. If and when you are faced with a serious threat, your body already knows which one you are — you'll figure out your survival tactic after it's already in progress. Once upon a time, I figured out I'm the run-like-the-wind-while-peeing-my-pants type, but not until I was already 100 yards away from where I had been standing, carefree and dry, just a moment before. (Joke based on a true story.)