Sharks, alongside many other aquatic and terrestrial animals exhibit a phenomenon known as tonic immobility or ‘apparent death’, a reflex that causes the animal to enter a hypnotic or trance like state. Whilst the behaviour can occur naturally, it can also be induced in some animals including sharks and rays.
Tonic immobility in sharks
Tonic immobility can be induced in sharks by pushing down on and stimulating tiny sensory pores (termed Ampullae of Lorenzini) on the animal’s snout. This behaviour is particularly useful when handling sharks either in the wild or in a captive setting, and has enabled researchers to test the effectiveness of chemical repellents. By inducing this state, which can also be triggered by turning sharks onto their back, handlers can interact with sharks with reduced chance of injury either to themselves or the animal. If undisturbed, sharks can remain in this state of inactivity for up to 15 minutes, before snapping back into a normalized state. Alongside muscular relaxation, sharks in tonic immobility exhibit deeper, more rhythmic breathing patterns.
Purpose of tonic immobility
It is not clear why some animals exhibit tonic immobility. It is thought that in some cases it may be a technique to avoid predation, as the animal can ‘play dead.’ This does not really explain the occurrence of tonic mobility in sharks however as they are an apex predator. It has been suggested that this may have more to do with mating behaviour in sharks though this has not been proved.
Disadvantages of tonic immobility
As already mentioned, tonic immobility can be induced in one animal by another. This leaves the subject in a vulnerable position and appears to be a hunting tactic for some species. Orcas for example, have been observed to predate both sharks and sting rays by inducing tonic immobility in them first. In the late 1990s a female orca was observed to kill a shark by holding it upside down until it suffocated. It is likely that at least some of this time was spent in tonic immobility where the shark was unable to struggle away. Orcas have been also observed to induce tonic immobility in sting rays by turning them upside down, though in this case orcas are thought to first capture rays whilst they themselves are upside down, before turning onto their front to turn invert the ray.
An orca showing possible tonic mobility induction behaviour.
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