What Causes a Shark to Slide Into a Trance When Upside Down?

Some sharks go into tonic immobility when they're upside down. This condition can be confusing because they often look dead as it's happening. Some of the specific types of sharks that are known to be susceptible to tonic immobility are whitetip reef sharks (Triaenodon obesus), blacktip reef sharks (Carcharhinus melanopterus), lemon sharks (Negaprion brevirostris), sandbar sharks (Carcharhinus plumbeus), silky sharks (Carcharhinus falciformis) and tiger sharks (Galeocerdo cuvier).

Sharks go into trances with the loosening of their muscle and respiratory processes. When they're upside down, their dorsal fins become straighter and more streamlined. The atypical stance is believed to have an effect on reciprocity with their surroundings, both through their motor and sensory reactions. Some believe tonic immobility might be a defense mechanism for sharks. If they stay completely still, they can be inconspicuous to others. Many doubt that theory because of sharks' impressive predation powers, however. Tonic immobility is also believed to be a breeding behavior in sharks, as rubbing them sometimes causes immobility.

Once sharks are upside down, it doesn't take long for tonic immobility to kick in. It usually takes no more than 60 seconds to render a shark motionless. If nothing interferes with their tonic immobility, it can last for a maximum of 15 minutes or so. Once sharks "snap out of it," they revert to their normal positions, and then promptly resume their typical activities. Tonic immobility in and of itself is not believed to cause any permanent detrimental effects on sharks.

Some predators take advantage of tonic immobility in sharks. It's often a quick way to make normally tough opponents weak and temporarily subdued. Killer whales (Orcinus orca) are specific examples of this. Killer whales have been spotted forcing sharks into upside-down positions for many minutes at a time. Forcing sharks to remain in this position can sometimes\ suffocate them. This makes it simple for predators to dine on sharks. When researchers handle sharks for observation or treatment, they often rely on tonic immobility to subdue them temporarily. This prevents the sharks from biting them.