Understanding Tonic Immobility: The Frozen State of Sharks

Sharks, often portrayed as formidable predators of the ocean, exhibit an unusual behavior known as tonic immobility, where they become temporarily paralyzed when flipped upside down. This phenomenon raises questions about its evolutionary purpose and implications for shark behavior.

According to Joel Gayford, a doctoral candidate of marine ecology at James Cook University in Australia, tonic immobility is akin to hypnotizing a shark. "The animal completely stops swimming, and the only movement it's making is slow rhythmic breathing," Gayford stated in an email correspondence (Live Science, 2025). This trance-like state can be induced by flipping the shark over or stimulating its snout, which is rich in electroreceptors known as the ampullae of Lorenzini, as noted by Jillian Morris, founder of Sharks4Kids (Live Science, 2025).

During tonic immobility, a shark experiences a decrease in its sensory responsiveness, cardiac rate, and blood pressure, along with analgesia, or reduced sensitivity to pain. Abraham Miranda-Páez, a researcher at the nonprofit organization Pelagios Kakunjá, emphasized that this state allows scientists to safely conduct research on sharks with minimal stress to the animals (Live Science, 2025).

The evolutionary significance of tonic immobility remains a topic of debate among scientists. A 2023 study published in the *Environmental Biology of Fishes* suggests that this behavior may serve various functions, such as self-defense, courtship, or protection from sensory overload (Environmental Biology of Fishes, 2023). Miranda-Páez, a co-author of the study, posited that there is a consensus among researchers that tonic immobility is primarily a passive defensive response.

However, this theory is not universally accepted. Gayford argues that the idea of paralysis as a protective mechanism is flawed for sharks, as most of their natural predators, such as orcas, would not be deterred by a motionless prey. "In some instances, it is a clear disadvantage," he noted, pointing out that orcas have learned to incapacitate sharks (Live Science, 2025).

Interestingly, tonic immobility also occurs during mating. Morris observed that male sharks often flip females over, causing them to enter this state before copulation. However, she remarked that this behavior is not exclusive to females, indicating that mating alone does not fully explain the phenomenon (Live Science, 2025).

A 2025 study published in *Reviews in Fish Biology and Fisheries* suggests that tonic immobility may lack any significant purpose and could simply be an evolutionary remnant. Gayford, a co-author of this study, explained that tonic immobility has been independently lost in several shark species throughout their evolutionary history, suggesting that it may not be advantageous in all contexts (Reviews in Fish Biology and Fisheries, 2025).

The diversity of opinions regarding tonic immobility highlights the complexity of shark behavior and the need for further research. Currently, experts agree that while many theories exist, none are fully understood or conclusively supported. Morris emphasized the necessity for more data to unravel the purpose behind tonic immobility in sharks (Live Science, 2025).

In conclusion, while tonic immobility presents an intriguing aspect of shark behavior, its evolutionary significance and practical implications remain unclear. As researchers continue to explore this phenomenon, it may lead to new insights into the lives of these enigmatic marine creatures and their adaptations to the challenges of their environment.