My friend sent me this video:
It’s pretty interesting, if not a little misleading. While it looks like the shark is getting all snuggle bunny, this response is called tonic immobility. Apparently this is a reflex reaction akin to “playing dead” that can be seen in multiple types of organisms. There are two ways an animal can “play dead”, with the first involving actual temporary paralysis and the second simply embracing the ruse of being stone cold (acting). The actual paralysis is called tonic immobility and the second is called thanatosis.
Other animals that may show this response are rabbits (alot of bunny owners refer to this as putting said furball in a “trance”), possums, guinea pigs, sheep, etc. It’s seen both in examples of invertebrates and vertebrates.
Ugly Overload had an interesting entry that talked about a study done with flour beetles in spider webs. A researcher bred those with the response and those without. Those that played dead in the spider’s lair tended to run away and live another day so to speak. This makes sense from the perspective of a predator that hunts live prey as dead prey are more likely to carry disease and rot which tends to be where scavengers come in.
The understanding has been that you can sort of self-induce this state in certain animals by flipping them onto their backs, sometimes with the nose pointed downwards. Shark researchers have used this response to their advantage in completing their research on shark behavior. However, the response seems to vary among species and its effectiveness can be debated:
Check out this BBC video. It refers to this state as a trance or hypnosis, but these are really more appropriately labeled examples of tonic immobility
A Bit of Stardust 
[…] it appears the orca may have exploited the phenomenon of tonic immobility, which I wrote a blog entry on previously. I’ve also written on the differing hunting behavior of orcas. Nature likes to […]