As they swim through the ocean away from human eyes, much of a shark’s life is a mystery. But if you look closely, you can find clues about what they’ve been through – in particular, through injuries, wounds and scars.
In a new study, published in the journal Frontiers in Marine Science, researchers have proposed a system to categorise what causes scars on great white sharks (Carcharodon carcharias).
"The scars and wounds seen on the sharks tell us about their interactions with each other, their environment, their prey and humans,” says study author Scot Anderson from the California White Shark Project.
"Many of the scars are distinctive enough so they can be easily classified, such as cookiecutter shark (Isistius brasiliensis) bite scars that are round full circle or a crescent moon mark when they don’t remove the bite.”
The California White Shark Project team shared a video of a 16ft female great white shark with an unusual scar on the top of her head. As the shark swims towards the camera, a perfect circle becomes visible on her head, almost like a dolphin's blowhole. That's the iconic plug of perfectly circular missing flesh that shows she was snacked on by a cookiecutter shark.
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“Cookiecutter wounds can tell us the white sharks get these marks during the 'offshore phase' off their migration as that is when they overlap habitat with cookiecutter sharks,” says Anderson.
Other types of scars might indicate boat strikes, defence wounds or a scrape from the rocks. It could even tell us if the sharks have been mating. Females might have "light impressions” from bites from other sharks on their heads and pectoral fins from a mating encounter.
"In many shark species, males must bite the female in order to hold on while copulating,” he says. By the time his team observes these ‘hold bites’, they are “always mostly or completely healed”, Anderson says, suggesting that white sharks mate offshore – giving scientists another clue about where and how great whites reproduce.
“It is a line of evidence and a piece of the puzzle to try and determine where and when white sharks are mating, as we still do not know for sure,” Anderson adds.
But to be able to interpret these clues – written in the scars on a shark’s body – we need to be able to determine how the injury might have occurred. For example, the study suggests that “a pattern of spaced dots” shows where parasitic copepods used to be attached to the animal, “a series of parallel cuts with even spacing” are the mark of a boat propeller, and a white “rope burn” could be from entanglement in fishing gear.
The new paper proposes a "systematic classification system” to help scientists determine the source of different types of scars, wounds, and injuries.
This information, says Anderson, helps us learn even more about these magnificent animals: “It's another puzzle piece that tells a story of what these white sharks experience.”
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Image and video credit: California White Shark Project
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