Do Sharks Really ‘Play Dead'? The Complicated Truth About Tonic Immobility.

Forbes

6 days ago

The team also documented the first-ever data on a chimaera species (Callorhinchus milii), which ... More showed no TLR response, adding an important piece to the evolutionary puzzle of this behavior in early vertebrates.
Tonic immobility (TI) is a strange and fascinating behavior that turns up across the animal kingdom. From insects to fish to mammals, at its core, it's a sudden and temporary stop in movement where animals appear to freeze in place. In some, this looks like muscles locking up. In others, like many fish, the body goes limp. The causes vary why TI happens vary: pressure to a certain body part, flipping the animal upside down, or even sensory overload. And while this reaction has been seen as a defense mechanism — sort of like a opossum 'playing dead' to avoid predators — that explanation doesn't fit all species or situations. For sharks, rays, and chimaeras (collectively known as chondrichthyans), TI takes the form of muscle relaxation when inverted, something researchers call the 'tonic limp response,' or TLR. Marine biologists often take advantage of TLR during data collection, temporarily flipping these predators to keep them still, but its evolutionary role remains… well, a mystery.
To explore this, scientists Joel H. Gayford and Dr. Jodie L. Rummer from James Cook University conducted a combination of hands-on experimentation and a broad literature review to better understand how widespread and variable the tonic limp response (TLR) is across cartilaginous fishes. In their experimental trials, they carefully inverted individuals from 13 different shark and ray species to see if this common method of inducing tonic immobility would cause the animals to relax, stop struggling, and begin deep rhythmic breathing, all hallmarks of TLR. Their approach was consistent, gentle, and quick to avoid stress, mimicking the standard protocol used in field research. Of the 13 species tested, just over half (seven species, to be exact) displayed the expected TLR when flipped onto their backs, while the remaining species showed no response at all. For those that did respond, the time it took for the behavior to kick in ranged from just 7 seconds in the common smooth-hound (Mustelus mustelus) to 25 seconds in the blacktip reef shark (Carcharhinus melanopterus). Once immobile, the duration of stillness varied dramatically too, lasting anywhere from a brief 12 seconds to over two minutes (131 seconds) in the Atlantic guitarfish (Rhinobatos lentiginosus). Importantly, the behavior was remarkably consistent within species. Every individual of a species either consistently entered a tonic state or consistently didn't, suggesting that TLR is a fixed trait at the species level, not influenced by individual variability or environmental context in the short term. Looking at all the data, the duo found no connection between TLR and body size, habitat depth, geographic range, or whether the species was a predator or not. This suggests that ecological factors (i.e. like where a shark lives or how it feeds) don't strongly predict whether it has this trait. Instead, the presence or absence of TLR seems to be deeply rooted in evolutionary history.
This line of thinking would make sense with the results from the very first empirical test of TLR in a chimaera species, Callorhinchus milii, also known as the elephant fish. This cartilaginous species showed no signs of TLR when inverted, which opens up new questions about whether this trait existed in their common ancestor, was lost in chimaeras, or perhaps never evolved in this group at all. In fact, the study's models suggest TLR was likely present in the common ancestor of all chondrichthyans and has since been lost at least five times across the group (and no evidence suggests it evolved anew in any lineages). This pattern hints that TLR may be a plesiomorphic trait, or something inherited from a distant ancestor that has stuck around in some species, even if it no longer serves a clear function.
Scientists tested 13 species for their TLR response and reviewed published studies of 29 additional ... More species. Some sharks froze when flipped. Others didn't. The study found that seven of the 13 species tested exhibited TLR, while six did not.
There are a few ideas about what TLR might have evolved for. One is predator avoidance (that playing dead to escape danger scenario). But in the case of sharks, there's no solid evidence this works, and the mechanics of predator attacks on sharks (like suction or tearing) make it unlikely that flipping over and going limp helps. Another theory suggests TLR plays a role in mating, especially since sharks have been seen inverting females during copulation. But again, this theory has issues. TLR doesn't differ between males and females, and if going limp made females more vulnerable to unwanted mating, natural selection would likely weed that behavior out. A third idea suggests TLR might protect animals from sensory overload, like hitting a reset button. But no one's tested that theory in sharks. Taken together, the explanations don't hold up well.
What's perhaps most interesting finding in this study is that among the sharks and rays that lack TLR, all are small-bodied species that live in shallow, complex habitats like coral reefs or kelp forests. These environments are 'complex' in that they are full of tight spaces and tangled structures. So, if a shark goes limp in one of these spots, especially upside down, it could get stuck or injured. That could be a strong enough risk for evolution to get rid of the trait in those environments. On the other hand, larger sharks or those living in open water wouldn't face the same risk and could hang onto TLR without much cost.
As highlighted in this new research, there's still much we don't know. The chimaera tested in this study didn't exhibit TLR, but with just one species sampled, it's hard to say whether that holds true for the whole group. And some species may respond to other triggers besides inversion, like touch to sensory organs, but this study didn't test for that. Ultimately, they Australian team cautions against one-size-fits-all assumptions when it comes to evolutionary biology. Just because a trait looks similar across species doesn't mean it evolved for the same reason — or even that it's still useful today. TLR in sharks and rays might once have served a purpose that's long gone.
Or it might still matter in ways we haven't figured out yet.
Either way, these animals hold clues to their deep evolutionary past. And it's this very behavior that will possibly unlock more answers.