I made an accidental and surprising discovery while studying the movements of sharks off the coast of Jupiter, Florida. I set out to record the migration routes of silky sharks, named for their smooth skin. Instead, in a story full of twists and turns, I managed to document the rare event of a shark regenerating its dorsal fin.
Labeling, followed by trauma
It all started when my team and I tagged silky sharks in the summer of 2022 (Carcharhinus falciformis) as part of my PhD study. research. Silky sharks are usually found in the open ocean and grow up to 3 meters long. Scientists know that these sharks congregate in South Florida each summer, but where they go the rest of the year remains a mystery; I was hoping to figure it out too.
Local boat captain John Moore took us to an area where sharks are known to congregate. We carefully captured and gently placed GPS trackers on the dorsal fin or upper fin of 10 silky sharks.
Tags worn like large earrings do not interfere with swimming and are designed to fall off after a few years. Once the tag’s antenna breaks the water’s surface, its GPS location is detected by overhead satellites, hopefully revealing details of the shark’s secret life.
I went home to follow his travels on my laptop.
The story took an unexpected turn a few weeks later when I received disturbing photographs from Josh Schellenberg, an avid diver and underwater photographer who became aware of my work.
The photos showed a male silky shark with a large, gaping wound on its dorsal fin, as if someone had taken a cookie cutter shaped like a satellite tag and pierced it through. Josh wondered if this person was one of the sharks in my study.
While placing the GPS tags, I also placed a second tag under each shark’s dorsal fin that displayed a unique identification number, so I could verify that the injured shark was from my study #409834.
I felt a mixture of relief and sadness. The shark is relieved to have survived this ordeal; sadness for scientific data that can no longer be collected.
Silky sharks are often caught by local fishermen in this area, but they are protected in Florida and are illegal to kill or keep. Josh’s photo #409834 showed several hooks in his mouth, so I knew this animal had been captured several times since my team tagged him.
The way the satellite tag is attached means it would be impossible for it to naturally detach from its fin and leave a scar like that. Why someone cut off the shark’s satellite tag remains a mystery, but perhaps they thought they could resell it or wanted to interfere with research. I never expected to see that shark again.
Return of #409834
Let’s go one year later, to the summer of 2023. I received a few photos of the silky sharks from our boat captain, John Moore, who is also an avid diver. John was looking for any of our sharks making their seasonal return to Jupiter. In the many shark photos he sent, I noticed a silky shark with an oddly shaped dorsal fin.
I immediately realized it had to be #409834 from the previous summer. A few days later, John managed to get close enough to photograph the ID tag to confirm my hunch. Josh Schellenberg also spotted and photographed #409834. With photos of both John and Josh, I was able to compare the healed dorsal fin to the newly injured one.
I didn’t expect to make a groundbreaking discovery. Simple curiosity led me to analyze the photographs. But the result was surprising: Not only was the wound completely healed, but his dorsal fin in 2023 was 10.7% larger than it was after the injury in 2022. New fin tissue had been regenerated.
My analysis determined that the shark regenerated enough tissue in 332 days for its dorsal fin to grow back to nearly 90% of its original size and for more than half of what was cut to regrow by 2022.
Vital for balance, orientation and hydrodynamics, the dorsal fin is vital to a shark’s ability to hunt and survive. #409834 showed no signs of infection or malnutrition, indicating an extraordinary feat of endurance.
Scientists know that sharks have an incredible ability to heal, but the mechanisms behind these observations are still not fully understood. While limb regeneration has been widely documented in other marine animals such as starfish and crabs, there is only one documented case of dorsal fin regeneration in a shark: a whale shark that regrew its dorsal fin after a 2006 boating accident in the Indian Ocean.
400 million years of durability
There’s a reason why sharks have remained on Earth longer than trees and have survived numerous mass extinction events that wiped out other species. They are the product of 400 million years of evolutionary adaptations that demonstrate their extraordinary resilience and prepare them for survival.
Being able to pinpoint the ability that makes them so resilient is a major scientific advance; especially considering that scientists still question where silky sharks spend most of their time in the Atlantic.
One person’s attempt to undermine shark science and harm a shark was ultimately fruitless. Instead, the shark’s endurance won out and led to an incredible discovery about this species. This story also shows that there are countless individual people with a genuine love and respect for these animals, including scientists like me and shark enthusiasts like Josh and John.
I’ll never know for sure where #409834 spent the rest of the year, but I hope it continues to return to Jupiter each summer so we can further evaluate its progress. Based on the recovery rate calculated in my study, we can see that the dorsal fin has grown 100% to its original size.
This article is republished from The Conversation, an independent, nonprofit news organization providing facts and analysis to help you understand our complex world.
Written by: Chelsea Black, University of Miami.
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Chelsea Black does not work for, consult for, own shares in, or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond her academic duties.