by I made an accidental and amazing discovery while studying the movements of sharks off the coast of Jupiter, Florida. I set out to record the migration routes of the silky sharks, named for their smooth skin. Instead, in a story full of twists and turns, I documented the rare phenomenon of a shark regenerating a dorsal fin.
Tagging, then trauma
It all started in the summer of 2022, when my team and I tagged a silky shark (Carcharhinus falciformis) as part of my Ph.D. research. Silky sharks are commonly found in the open ocean and grow to be 10 feet long. Scientists know that these sharks congregate in South Florida every summer, but where they go the rest of the year is a mystery – one that I hoped to solve.
Local boat captain John Moore took us to a site known to gather sharks. We carefully caught and attached GPS trackers to the back, or top, of 10 silky sharks.
The tags, attached like large earrings, do not interfere with swimming and are designed to fall off after a few years. When the tag’s antenna breaks the surface of the water, satellites overhead collect its GPS location, hopefully revealing details about the shark’s secret life.
I went home to track their journey from my laptop.
The story took an unexpected turn a few weeks later, when I received disturbing photos from an avid diver and underwater photographer, Josh Schellenberg, who I knew about my work.
The photos showed a male silky shark with a large gaping wound in his dorsal fin, as if someone had taken a cookie cutter in the shape of a satellite tag and punched through it. Josh wondered if this person was one of the sharks from my study.
While placing the GPS tags, I also place a second tag under the dorsal fin of each shark that displays a unique identification number, so I was able to confirm that the injured shark was one from my study, #409834.
I felt a mixture of relief and sadness. Relief that the shark survived this ordeal; it’s a pity the scientific data is not collected now.
Silky sharks are often caught in this area by local fishermen but are protected in Florida and illegal to kill or keep. Josh’s photos of #409834 showed several hooks in his mouth, so I knew this animal had been caught several times since my team tagged him.
The way the satellite tag attaches means that it cannot naturally tear off the fin and leave a wound of this shape. Why someone cut off the shark’s satellite tag is still a mystery, but maybe they thought they could resell it or maybe they wanted to interfere with research. I never expected to see that shark again.
Return #409834
Flash forward to a year later, the summer of 2023. I received some photos of silky sharks from John Moore, our boat captain, who is also a keen diver. Sean was on the lookout for any sharks that were returning seasonally to Jupiter. In the many shark photos he sent, I noticed a silky shark with an oddly shaped dorsal fin.
I knew right away it was going to be #409834 from the previous summer. A few days later, John was able to get close enough to photograph the ID tag to confirm my case. Josh Schellenberg #409834 was also seen and taken. With both John and Josh’s photos, I was able to compare the healed dorsal fin to the brand new one.
I did not expect to make a breakthrough discovery. Simple curiosity made me start analyzing the photos. But the revelation was amazing: Not only was the wound completely healed, but the dorsal fin of 2023 was 10.7% larger in size than it had been after the injury in 2022. New fin tissue had regenerated.
My analysis found that within 332 days, the shark had regenerated enough tissue that its dorsal fin had almost grown back to 90% of its original size, growing back more than half of what was cut off in 2022.
The dorsal fin, which is critical for balance, steering and hydrodynamics, is critical to a shark’s ability to hunt and survive. Seeing any infection or signs of malnutrition in #409834 indicates an extraordinary feat of endurance.
Scientists know that sharks have incredible healing abilities – but the mechanisms behind these observations are still poorly understood. While limb regeneration is widely documented in other marine animals such as starfish and crabs, there is only one other documented case of dorsal fin regeneration in a shark – a whale shark in the Indian Ocean that regrew its dorsal fin after a boating accident in 2006.
400 million years of resilience
There is a reason that sharks on Earth are longer than trees and have survived multiple extinction events that wiped out other species. They are the product of 400 million years of evolutionary adaptations that show their amazing resilience and have managed to survive.
Finding an ability that helps make them resilient is a major scientific advance – especially as scientists still question where silky sharks spend most of their time in the Atlantic.
In the end one person’s effort to undermine shark science and harm sharks was not worth it. Instead, the intensity of the shark prevailed and led to an amazing discovery about this species. This story also shows that there are many individuals, including scientists like me and shark enthusiasts like Josh and John, who truly love and respect these animals.
While I won’t know for sure where #409834 spends the rest of the year, I hope it continues to return to Jupiter each summer so we can further assess its progress. Based on the healing rate calculated in my study, we may see his dorsal fin grow to 100% of its original size.
This article is republished from The Conversation, a non-profit, independent news organization that brings you facts and analysis to help you make sense of our complex world.
Written by: Chelsea Black, University of Miami.
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Chelsea Black does not work for, consult with, or receive funding from any company or organization that would benefit from this article, and has not disclosed any relevant interests beyond their academic appointment.
