Tonic Immobility in Sharks

Tonic Immobility in Sharks

Sharks are a misunderstood but fascinating species of fish that are not well liked but are a necessary component to maintaining a complete ecosystem. Many things are known about sharks but so much more is not. Sharks are difficult to study since many species cannot survive in captivity; particularly the larger species, such as bull sharks and tiger sharks. These sharks have to be studied in their habitat. As one can imagine, attempting to study sharks in the ocean can be somewhat intimidating and dangerous. But an interesting trait of sharks makes this task a little simpler. This trait is known as tonic immobility. Tonic immobility is essentially a state of paralysis or hypnosis that animals can enter into voluntarily or be entered into involuntarily. In the case of sharks, tonic immobility occurs when the sharks are placed in an inverted position or are rubbed on the sides of their head (Whitman et al 1986). While it is a state that does not occur in all species of sharks, it does affect most types. It has also been noted that it only occurs in late juvenile to adult species (Whitman). Based on studies, this state appears to be related to a neurological condition, but this theory has yet to be proven (Klemm 1976). As noted by the studies referenced above, tonic immobility is used by biologists to study sharks. Tonic immobility is used by biologists to tag sharks in the ocean. It has also been used by biologists for intrusive surgeries on sharks, as placing sharks in tonic immobility acts as an anesthetic. In addition to these science studies, tonic immobility is used to test shark repellents (“Tonic Immobility”). Fishermen have also found this to be a convenient method to remove hooks after catching sharks, making a potentially stressful moment for the shark less distressing. While this state of paralysis is of great benefit to marine biologists and fishermen, the primary function of tonic immobility may be for mating purposes. Mating between sharks is often quite violent. As noted by Johnson and Nelson (1978), male sharks appear to bite the females in order to hold onto them. After biting the female, they seem to purposely turn them upside down to copulate. It is surmised that since the female is in a tonic immobility state during this copulation then she does not feel the pain associated with the bite and therefore makes a compliant mate. Immediately after copulation, the male shark releases the female and she returns to her normal state as soon as she returns to an upright position. Unfortunately for sharks, tonic immobility can be dangerous. In this state of paralysis, sharks are in a vulnerable state and can be harmed or even killed without being able to defend themselves. There are at least two documented cases of such attacks on sharks by killer whales. In Costa Rica, witnesses observed killer whales feeding on a bull or lemon shark (Anon. 1996). Another witness account took place off the San Francisco coast and again witnesses observed killer whales feeding on a shark, this time a great white shark (Anon. 1999). In both cases, witnesses noticed that the sharks were held upside down in the mouth of a killer whale until the shark stopped moving. Killer whales have apparently learned of this vulnerability in sharks and are now using it to their advantage. It is speculated that killer whales have discovered that by holding sharks upside down they incapacitate them and suffocate them after a period of time, making for an easy meal. While this method of attack does not bode well for sharks, it may be that this learned behavior by killer whales is limited only to these feeding grounds. Regardless, it seems logical that the benefits of tonic immobility in mating far outweigh the dangers. And while the cause may continue to be a mystery of science, it certainly has found a way to use it to advantage. Hopefully this state will continue to help biologists in their study of sharks. The knowledge gained from these studies may be all that stands between sharks surviving or total extinction. The more that is learned and passed on about shark behavior may hopefully convince those that hunt and kill them in ignorant fear to stop the destruction of a species that has much to offer in the circle of life. Works Cited

Johnson, Richard H., and Donald R. Nelson. "Copulation and Possible Olfaction-Mediated Pair Formation in Two Species of Carcharhinid Sharks." Copeia 1978.3 (1978): 539-42. Print.

Klemm, W. R. "Identity of Sensory and Motor Systems That Are Critical to the Immobility Reflex (“Animal Hypnosis”)." Journal of Neuroscience Research 2 (1976): 57-69. Print.

"Predation on a White Shark (Carcharodon Carcharias) by a Killer Whale (Orcinus Orca) and a Possible Case of Competitive Displacement." Marine Mammal Science 15.2 (1999): 563-68. Print.

"A Report of Killer Whales (Orcinus Orca) Feeding on a Carcharhinid Shark in Costa Rica." Marine Mammal Science 12.4 (1996): 606-11. Print.

"Tonic Immobility." SharkDefense Technologies. Web. 12 July 2010. .

Whitman, Pamela A., Joseph A. Marshall, and E. C. Keller, Jr. "Tonic Immobility in the Smooth Dogfish Shark, Mustelus Canis (Pisces, Carcharhinidae)." Copeia 1986.3 (1986): 829-32. Print.

Cited: Johnson, Richard H., and Donald R. Nelson. "Copulation and Possible Olfaction-Mediated Pair Formation in Two Species of Carcharhinid Sharks." Copeia 1978.3 (1978): 539-42. Print. Klemm, W. R. "Identity of Sensory and Motor Systems That Are Critical to the Immobility Reflex (“Animal Hypnosis”)." Journal of Neuroscience Research 2 (1976): 57-69. Print. "Predation on a White Shark (Carcharodon Carcharias) by a Killer Whale (Orcinus Orca) and a Possible Case of Competitive Displacement." Marine Mammal Science 15.2 (1999): 563-68. Print. "A Report of Killer Whales (Orcinus Orca) Feeding on a Carcharhinid Shark in Costa Rica." Marine Mammal Science 12.4 (1996): 606-11. Print. "Tonic Immobility." SharkDefense Technologies. Web. 12 July 2010. . Whitman, Pamela A., Joseph A. Marshall, and E. C. Keller, Jr. "Tonic Immobility in the Smooth Dogfish Shark, Mustelus Canis (Pisces, Carcharhinidae)." Copeia 1986.3 (1986): 829-32. Print.