Sharks are powerful swimmers, built for constant movement and sharp reaction. But researchers noticed a strange thing: turn certain sharks upside down and they stop moving. You may think that this frozen state is unconsciousness, but it isn’t. In fact, it’s something that scientists call tonic immobility.
A Temporary Trance
You might be wondering what tonic immobility actually means for a shark. It’s a reflex that causes the body to go still, the muscles to relax, and actions to stop almost entirely. The shark keeps breathing in slow, steady rhythms, but everything else quiets down.
This reaction can be set off by flipping the shark onto its back or by touching its snout, where clusters of sensory receptors pick up electric signals in the water. Lemon sharks, great whites, and sand tiger sharks are known to respond this way.
Scientists have used the reflex to safely tag or examine sharks without sedation. Once the shark is upright again, the effect fades and normal activity returns.
The Reflex Triggers Internal Changes Too
Sharks in tonic immobility don’t just appear still on the outside. Internally, their bodies undergo several measurable changes. Heart rate slows down, blood pressure drops, and sensitivity to pain decreases. The entire nervous system shifts into a muted state, and the response happens automatically—without any conscious control or decision by the animal.
While this reflex has proved useful for research purposes, its origin has nothing to do with scientific convenience. Tonic immobility likely developed under natural conditions, though its original function remains unclear. Understanding why it evolved in the first place continues to be a central question in current shark behavior research.
Experts Tested Theories but Found Gaps
Image via Pexels/Diego Sandoval
A recent study from James Cook University looked at 13 species of sharks, rays, and ghost sharks. Seven entered tonic immobility when flipped, but the other six didn’t. That split gave professionals a chance to compare patterns across species and trace where the reflex shows up—or disappears—on the evolutionary timeline.
Several theories were considered. One suggested the behavior serves as a defense, like playing dead. Another pointed to mating, since male sharks sometimes invert females during copulation. A third linked it to sensory overload, where the body shuts down under intense stimulation.
Unfortunately, each idea that was presented came with its own set of flaws. Predators like orcas use tonic immobility against sharks and flip them over to immobilize and disembowel them. Meanwhile, both genders can enter the state, which undermines the mating hypothesis. And the sensory explanation remains untested to this day.
The Behavior Likely Came from Ancient Lineages
The most consistent explanation for this reflex points to history. Tonic immobility appears to be a plesiomorphic trait, which is something that passed down from early shark ancestors. As time passed, it faded out in some groups, possibly because it created more problems than benefits.
In coral reef systems, for example, small sharks often wedge themselves into narrow gaps to rest or feed. If one of them froze in a tight space, the result could be fatal. In that environment, it may be smart to lose the reflex and improve chances of survival.
Evolution doesn’t erase every outdated trait immediately. If something no longer helps but doesn’t cause major harm, it can stick around. That seems to be the case here.
