Shark Spa Services: Why fish deliberately rub sharks

Black tip shark.

Source: Luc Viatour / Wikimedia Commons. Distributed under CC BY-SA 3.0 license.

In oceans around the world, fish have been observed rubbing shoulders with sharks. Today, researchers collecting these observations have concluded that irritation between fish and sharks is more frequent and widespread than previously believed, suggesting that the behavior plays an important ecological role and raises more questions for scientists to explore.

Curious observations

University of Miami graduate student Lacey Williams first noticed the behavior while using a drone to track a great white shark in South Africa’s Plettenberg Bay. While filming the predator, a school of liches actively approached it in order to rub their bodies against its tail.

During the field season, Williams and fellow graduate student Alexandra Anstett observed the behavior over and over again, sparking their interest.

Frictional behavior, where an animal bends its body to rub against a rough surface such as sand or rocks, is well documented in marine animals. The presumed goal is to get rid of skin irritants like parasites and bacteria. But choosing a potentially dangerous predator to get angry with seemed particularly intriguing to Williams and Anstett.

When they returned home from their fieldwork, Williams and Anstett asked if other researchers and divers had noticed rubbing behavior between fish and sharks. With the help of collaborators from the Atlantic White Shark Conservancy, they began to realize that this behavior had been reported in different species around the world.

However, when the researchers went to review the existing scientific literature, they arrived almost empty-handed.

“We were shocked to see that there is virtually no literature on interspecies irritation, and irritation between prey and predators, in particular,” Williams said. “It was surprising that, despite the frequency and extent of this behavior, there was nothing in the literature that explored it more.”

Singular shark skin

Williams and Anstett and their collaborators compiled all the records they could find of irritation between fish and sharks, examining underwater photos, drone footage, video feeds from divers and reports of first hand to find 47 cases of behavior.

The incidents, which have been documented in 13 locations around the world, involved eight different shark species irritated by 13 species of fish (including a case of silky sharks rubbing against a whale shark). The duration of the irritations ranged from eight seconds to over five minutes. Sometimes a single fish rubbed against a shark, while at other times a school of over 100 fish participated.

    Pascal Deynat / Odontobase / Wikimedia Commons.  Distributed under CC BY-SA 3.0 license.

Microscopic view of dermal denticles.

Source: Pascal Deynat / Odontobase / Wikimedia Commons. Distributed under CC BY-SA 3.0 license.

Given the difficulties in observing underwater behavior, the prevalence of irritation reports between fish and sharks raises more questions than answers.

The first question is why fish apparently risk their lives to get angry with sharks rather than rough sand or rocks.

Williams and Anstett say this appears to be the only phenomenon where prey actively seeks and rubs against a predator.

“This behavior is uniquely aquatic,” says Anstett. “You don’t see gazelles going up and rubbing shoulders with lions.”

While they can’t say for sure what is going on, the researchers have a few assumptions. The first is that shark skin might be a more effective surface for removing parasites and other annoying hitchhikers compared to other surfaces. Shark skin is covered with small, tooth-like scales called dermal denticles. Their orientation and texture can provide a sandpaper-like surface for fish to rub against. However, more research is needed to determine the effectiveness of shark skin in removing parasites compared to other substrates.

Marine mysteries

Another question raised by this article is how sharks feel when fish irritate them. Most of the time, sharks seemed indifferent to serving as back scrapers for fish. But in about a third of the interactions analyzed, sharks appeared to react negatively by wiggling their bodies or corkscrew diving, apparently trying to shake the fish.

“The percentage of sharks that reacted negatively was pretty low, but it was still interesting to see that clearly some individuals didn’t like it,” says Williams. “So are the fish getting it at no cost to the sharks, or is it at the expense of the sharks?” We do not know yet.

    Elias Levy / Wikimedia Commons.  Distributed under CC BY 2.0 license.

Great White shark.

Source: Elias Levy / Wikimedia Commons. Distributed under CC BY 2.0 license.

Anstett points out that it is possible for fish to transfer their bacteria and parasites to sharks during rubbing. However, whether this type of interaction is harmful or neutral from a shark perspective remains a mystery.

“Future research could test the skin of the chafer and the chafer,” says Anstett. “We know very little about terrestrial microbiomes, let alone marine microbiomes. Now, there is even more momentum to fill some of these fundamental knowledge gaps, because there is something worth exploring here. “

Ultimately, Williams and Anstett hope their study will inspire other researchers to delve deeper into these questions.

“We know so little about sharks, including their behavior and interactions within marine communities,” says Williams.

“But we know that marine species are being wiped out. How could this affect marine communities, including microbial communities? There is so much we still don’t know.

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