Rubbing against a shark is annoying. Their skin is composed of hard scales – which look like microscopic teeth – and often leaves people scratching like a ‘carpet burn’ if they get too close and are scratched by one of these predators. Pour some salt water on the wound and depending on the extent of the ‘shark burn’ you have, you will wince and possibly cry out in pain.

Unless you are a fish.

If you’ve spent any time observing a shark in an aquarium or out in the wild, you’ve likely seen instances of fish (and other animals) rubbing against sharks. A collaborative research team led by the University of Miami (UM) Shark Research and Conservation Program at the Rosenstiel School of Marine and Atmospheric Sciences has found that this behavior is frequent, widespread, and may play a previously underappreciated ecological role for aquatic animals. “While sharks and inanimate objects, such as sand or rock, the phenomenon of shark scales has been well documented as the only instance in the wild in which sharks are found,” said UM Rosenstiel School. prey actively seek and rub against predators. PhD student Lacey Williams, who co-led the study now published in the journal Ecology, The Scientific Naturalist, along with graduate student Alexandra Anstett.

The global team – with authors from the US, South Africa, the Galapagos and Mexio – examined underwater photos, videos, drone footage and eyewitness reports to find 47 case of fish rubbing against shark skin. These “rubbing” or “swapping” events were recorded in 13 locations around the world and ranged from eight seconds (a quick turn) to more than five minutes (a full turn). Footage captures 12 finfish fighting with eight different shark species, including the great white! In some cases, the team found that fish actively changed their behavior to chase nearby sharks to quickly scratch – they appeared 19 times when a fish lurched or garrick (Lichia ami) chasing a passing white shark.

“While we don’t know exactly why it happened, we do have a few theories. Shark skin is covered with tiny tooth-like scales known as the stratum corneum, which gives the shark a rough surface,” said UM Rosenstiel School of Research associate professor and study co-author Neil Hammerschlag. . “We suspect that scaling the shark’s skin could play an important role in removing parasites or other skin irritants, thereby improving fish health and fitness.”

The number of fish confronting sharks ranges from one to more than 100 individuals at a time. But, there is definitely a risk to these smaller animals even if there is safety in numbers, right? Yes, and the scientists hypothesized that “fish feed on specific areas of the shark because dentures in these regions are more effective at removing parasites due to changes in dentin morphology.” on the shark’s body. However, it could also be because those regions have the lowest predation risk.” It could also be that the shark the animal is rubbing against poses no threat at all, with footage showing silky sharks (Carcharhinus falciformis) chafing on the head of a whale shark (Rhyncodon typus), a filter eater is not interested in trying to digest a large shark.

The frequency with which scientists have seen this behavior leads them to believe that sharks can act as portable cleaning stations for fish with a wide range of life histories, including reef, coastal and marine species. floating fish. “For pelagic species that do not regularly clean up or have access to other cleaning mechanisms in coastal ecosystems, sharks can play an important role in removing ectoparasites or other contaminants. cause other skin irritations, improving the health and fitness of pelagic fish,” the new study states. “If parasite transfer is possible, another potential ecological impact could be parasitism or disease transmission. […] If transfer takes place, predated sharks, particularly pelagic fish, could be vectors for the transmission of microorganisms or parasites as they migrate through ecosystems. However, because the current literature has only just begun to discuss the potential for transfer of microorganisms and parasites between organisms, no studies have yet explored the potential broader ecological consequences. “.

The shark’s unique skin morphology may be why this type of behavior is only found in marine ecosystems (to our knowledge). “If sharks act as mediators to remove or transfer parasites between systems, they could play a role in the transmission of pathogens or parasites,” the researcher states. “Therefore, spatial shifts or shifts in shark populations can disrupt vector pathways and have implications for species and systems.”

Funding for the study was provided in part by grants from the Isermann Family Foundation.

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