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...
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