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Tonic Immobility in Sharks
Tonic immobility (TI) is an unlearned reflex, characterised by a state of immobility and torpor. This behaviour has been reported to occur in a variety of animals – including insects, amphibians, reptiles, birds, mammals and fish.
TI has been induced in sharks such as the Spiny Dogfish, Lemon Shark, Sandbar Shark, Leopard Shark, Whitetip Reef Shark, Blacktip Reef Shark, California Round Ray, Clearnose Skate, Cownose Ray and Southern Stingray by inverting the subject. This unnatural posture is thought to alter the animal’s sensi-motor interchange with the environment, causing a ‘limp’ response. The animal displays relaxation in muscle tone and deep rhythmic respiration. Sharks usually enter TI in less than a minute and, if left alone in tonic immobility, they can remain in this state for up to 15 minutes before eventually righting themselves and swimming away.
Some sharks, however, are induced into TI in other ways. For example in Tiger Sharks it can be attained by placing hands lightly on the side of the animal’s snout approximate to the area surrounding its eyes, where the ampullae of Lorenzini are located. It is though that this may alter the electro-magnetic field sensing ability of the animal and cause the TI response.
TI is extensively used as an aid in shark husbandry and when handling wild sharks in the field to minimize struggling by the animal and reduce the possibility of injury. Likewise, many anglers make use of tonic immobility when removing hooks to improve the chances of the shark’s survival after release. Also, because a strong stimulus such as a repellent can terminate tonic immobility, the behaviour has been used to test the effectiveness of various chemical shark repellents on Lemon Sharks.
There is evidence suggesting that some cetacean predators may use TI to prey on elasmobranchs. In 1997, in the waters around the Farallon Islands, a female Orca was observed holding a White Shark upside down for 15 minutes, causing it to suffocate. Whether intentional or not, the Orca likely caused the shark to enter a tonic immobility state. In New Zealand, Orcas have also been observed turning upside down before attacking stingrays, holding them in their mouths, and quickly righting themselves – flipping the prey over, inducing TI in the stingray.
The hypothesis that TI is used as an anti-predatory behaviour is acceptable when considering other animals but why would sharks, apex marine predators; evolve such mechanisms as a defence strategy? Also, sharks do not appear to enter tonic immobility as a fear response, as other animals do. Some scientists have suggested that tonic immobility in sharks may in fact be related to mating. Sharks employ internal fertilisation and numerous reports indicate an immobile state often associated with reproductive activities. This is circumstantial evidence however, and no study has investigated the possible role of TI in mating yet.
➤ Shark Reproduction
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