It is undeniably an incredible sight.... One of the world's most iconic and fearsome predators, limp, flipped on their back or on the tip of their nose, immobile, putty in someone's hands. "Tonic immobility" is used by scientists and veterinarians, and also by SCUBA divers and social media influencers to show that sharks are not always voracious, terrIfying predators... But is it ethical? Does "tonic" cause harm to sharks? In short... should we not be doing it!?
Playing Dead
"Tonic immobility" (or "tonic" for short) is a reversible, coma-like state that many different types of animals - lizards, chickens, birds, guinea pigs and sharks - can enter. The condition is also known as "animal hypnosis", "catalepsy" or "death feigning" because a creature in tonic will be unresponsive, with a motionless posture, like they are dead. Sharks and other fishes also go limp and relax all their muscles. It is described as "innate" because it is a natural response, not a behaviour that is learned from watching other animals (Brooks et al, 2011; Watsky & Gruber, 1990).
Whilst we don't really understand the underlying physiological mechanisms of tonic immobility, we do know that it's onset is linked to disruption of the sensory systems (Watsky & Gruber, 1990; Brooks et al, 2011; Williamson et al, 2018; Mukharror et al, 2019).
Tonic can be induced by several methods. Most commonly by dorso-ventral inversion (meaning the shark is flipped over), but also by applying pressure to the snout or the tail, or by pumping water into the mouth and out of the gills (this sounds barbaric, but is actually an essential method to assist a shark to breath during medical or scientific procedures) (Watsky & Gruber, 1990; Brooks et al, 2011; Williamson et al, 2018; Mukharror et al, 2019).
Dead Weight
In sharks the onset of tonic is fairly rapid and can last anywhere from a few minutes to several hours depending on the species (Watsky & Gruber, 1990; Brooks et al, 2011; Williamson et al, 2018; Mukharror et al, 2019).
The best method and how quickly a shark enters tonic immobility varies between species. For example, the Hamelhera walking shark aka swell shark (Hemiscyllium halmahera) is not at all susceptible and can only be induced into tonic by directly rubbing the electrosensory organs (known as the "Ampullae of Lorenzini") in their snout (Henningsen, 1994; Mukharror et al, 2019).
Comparatively, reef sharks like the blacktip (Carcharhinus melanopterus), Caribbean reef shark (C. perezi) and whitetip (Triaenodon obesus) are all very susceptible to tonic immobility (Henningsen, 1994).
A general rule is that the smaller the species of shark, the shorter the duration they remain in tonic immobility. For instance, lemon sharks (Negaprion brevirostris) are especially susceptible and they generally remain in a stable state of tonic immobility for a longer period of time compared to other, smaller sharks (Watsky & Gruber, 1990; Henningsen, 1994).
Dead Sexy
We don't really know why tonic immobility has evolved or what it's function actually is. Some scientists think it has some function in mating. For example, in zebra sharks (Stegostoma tigrinum), tonic can be induced by applying pressure to the tip of their tails. During courtship, a male zebra shark will follow the female and then bite down on her tail - inducing tonic in order to make her susceptible to mating (Williamson et al, 2018; Páez et al, 2023).
Playing Dead
However, as tonic occurs in both male and female sharks, it seems unlikely that it's only function is in a mating setting. Some experts wonder whether tonic is part of an anti-predator response. In this case it seems likely that tonic is a kind of state of hyperawareness that decreases the likelihood of being detected by a threatening predator (Brooks et al, 2011; Williamson et al, 2018; Páez et al, 2023).
A Medical Tonic?
Tonic immobility is widely used in animal husbandry and by scientists and veterinarians, as it means the shark is relatively placid and easy to move around, with limited risk of injury to the handler. So tonic can be used as an alternative to chemical anaesthetics - reducing the risk of overdose or side-effects, whilst ensuring the shark is relaxed for transportation, for administering medicines and fluids, taking blood samples or when fitting electronic monitoring tags for scientific research (Henningsen, 1994; Williamson et al, 2018; Mukharror et al, 2019; Páez et al, 2023).
But can we be certain that tonic isn't harmful!?
Scientists studying the blood chemistry of young lemon sharks undergoing tonic have learned that it is actually a very stressful experience for the shark! By taking blood samples and measuring blood pressure and ventilation (breathing rates) of sharks undergoing an extended state of tonic, experts have discovered that, far from being a relaxed state, tonic is actually more akin to a hyperaware paralysis (Brooks et al, 2011).
The notable changes to the shark's breathing and significant perturbations to multiple blood chemistry markers associated with stress, has caused researchers to conclude that during tonic a shark's body is ramping itself up to prepare for an escape, but it is catatonic and unable to move (Brooks et al, 2011).
"Tonic Immobility is an inherently stressful experience"
In-tonic-ated
As contemporary research indicates that tonic immobility is not a pleasant experience for the shark, this should drastically change our perspective on whether inducing tonic is unethical. Whilst there may be certain situations where it is necessary to use tonic immobility - medical procedures for a shark that lives in an aquarium for instance - but sharks are sentient beings capable of pain and deserve to be treated with respect. If we are causing a shark serious stress by putting them into tonic immobility for no reason, it is certainly NOT acceptable to be inducing tonic in sharks for the sake fun or for boosting social media followings. Is it really any different to trying to eat a live octopus or kicking a cat on film?
References
Brooks EJ, Sloman KA, Liss S, Hassan-Hassanein L, Danylchuk AD, Cooke SJ, Mandelman JW, Skomal GB, Sims DW & Suski CD (2011). The stress physiology of extended duration tonic immobility in the juvenile lemon shark, Negaprion brevirostris (Poey 1868). Journal of Experimental Marine Biology and Ecology, DOI: 10.1016/j.jembe.2011.09.017. Access online.
Henningsen AD (1994). Tonic immobility in 12 elasmobranchs: Use as an aid in animal husbandry. Zoo Biology, 13, 325-332. Access online.
Mukharror DA, Susiloningtyas D & Ichsan M (2020). Tonic immobility induction and duration on halmahera walking shark (Hemischyllium halmahera). IOP Conference Series: Earth and Environmental Science, 404:012080. Access online.
Páez AM, Padilla EMH & Klimley APA (2023) review of tonic immobility as an adaptive behavior in sharks. Environmental Biology of Fishes, 106, 1455–1462. Access online.
Watsky MA & Gruber SH (1990). Induction and duration of tonic immobility in the lemon shark, Negaprion brevirostris. Fish Physiology and Biochemistry, 8:3, 207-210. Access online.
Williamson MJ, Dudgeon C & Slade R (2018). Tonic immobility in the zebra shark, Stegostoma fasciatum, and its use for capture methodology. Environmental Biology of Fishes, 101, 741–748. Access online.
By Sophie A. Maycock for SharkSpeak
