“I set out to record the migration routes of silky sharks, named for their smooth skin, off the coast of Jupiter, Florida,” explained PhD Candidate Chelsea Black from the University of Miami. “Instead, in a story filled with twists and turns, I ended up documenting the rare phenomenon of a shark regenerating a dorsal fin.”
Silky sharks (Carcharhinus falciformis) grow up to 10 feet long and are known to congregate in South Florida each summer. However, the whereabouts of these enigmatic creatures for the rest of the year remained a puzzle that Black’s team wanted to solve. With the assistance of local boat captain John Moore, the researchers ventured to a known gathering site in 2022, tagging 10 silky sharks with GPS trackers to trace their elusive movements. These high-tech tags, attached to the dorsal fins, were programmed to fall off harmlessly after a few years. In that time, the tags transmitted GPS coordinates to overhead satellites, allowing the scientists to track the sharks’ journeys across the ocean.
Movement around the Gulf of Mexico? Expected. What was not expected was a tagged shark sporting a gnarly wound in its dorsal fin, shared by underwater photographer Josh Schellenberg who snapped the disturbing images. “The wound was the exact shape of a satellite tag and looked like a person had carefully cut around it and removed the tag,” Black told Phys.Org. “Each satellite-tagged shark also received a plastic ID tag that sits underneath the dorsal fin, which remained on this shark and allowed me to confirm the individual as one from the tagging study.” Identified as #409834 from the study, the pictures sparked a mixture of emotions in the team – relief that the shark survived, but sadness for the lost scientific data. Silky sharks are protected in Florida, making it illegal for anyone to kill or retain them.
That should’ve been the end of the story. “I never expected to see the shark again as silky sharks are only seen in Florida in the summer months, and I had now lost the ability to track him,” Black said. “[But] to my surprise, I did see this shark again.” A year later, the team received photos from John Moore and Josh Schellenberg, revealing the return of #409834 with a healing dorsal fin. Analyzing the images with specialized software led to an unexpected discovery: not only had the wound completely healed, but the dorsal fin had grown back an astounding 10.7% larger than its post-injury size in 2022. Within 332 days, the shark had regenerated enough tissue to restore almost 90% of its original dorsal fin size!
This remarkable feat poses intriguing questions about the healing capabilities of sharks, a phenomenon not fully understood by scientists. While limb regeneration is well-documented in other marine species, dorsal fin regeneration in sharks remains a rare occurrence. “Previously, dorsal fin regeneration has been documented only one other time in a shark—a whale shark in the Indian Ocean in 2006,” said Black. “While there may be other examples of dorsal fin regeneration in sharks, these cases are extremely difficult to monitor as it is unlikely to see the injured shark multiple times.”
Black and her team want to encourage collaboration with divers and photographers to actively participate in monitoring the healing process observed in tagging studies. This situation also underscores the importance of educating the public regarding satellite tagging studies. By doing so, perhaps researchers can avoid inadvertent interference from the public, driven by a lack of understanding of the importance of these studies.
