Marine animals face lots of threats that could result in trauma to their body parts. A predatory animal may attempt to take a chunk out of them; they could be injured during mating behaviors; or those that live in more coastal, urban environments can come into contact with boats that can cause scars and injuries across the body, or even death. Now, however, a case of fin regeneration after a traumatic event has been seen in the silky shark species for the first time.
Silky sharks (Carcharhinus falciformis) are sleek ocean predators that grow to around 3.5 meters in length (11.5 feet). According to Save Our Seas, they used to be one of the most common shark species in the ocean, but are now one of the most consumed species in shark fin markets.
Off the coast of Florida in July 2022, a underwater photographer and diver captured images of a silky shark with a traumatic injury to its dorsal fin. The shark was an adult male, and the photos were looked at by marine biologist Chelsea Black, author of a new study, to help with ID – the shark was fitted with an tag with a number unique to that individual animal.
The researcher concluded that the most likely explanation for how the shark came to have such an injury was the purposeful removal of a satellite tracking tag from its dorsal fin with a sharp object. Off the coast of Florida it is illegal to retain silky sharks if they are caught by recreational fishing vessels, so this could offer one explanation as to why the tag was removed in such a violent manner.
The shark with the injured fin was not seen again in the year 2022 and was presumed to have left the area of Jupiter, Florida as part of the annual migration. In June of the following year, a male adult silky shark returned to the same area with an unusually shaped dorsal fin. Through photo ID and tag number matching from the previous year the same individual was identified, even though the shape of the dorsal fin had changed significantly.
Looking at the photographs from 2022, the shark was thought to have lost around 20.8 percent of the dorsal fin in the event. When the shark was again photographed 332 days later, it showed a rate of healing of 87 percent of the original size of the fin.
Exceptionally wound-healing abilities are not totally unknown in marine species. Prior to this example, dorsal fin regeneration had been documented in a whale shark (Rhincodon typus), the only other example of dorsal fin regeneration to date, but would healing for scratches and various other traumatic injuries has been seen in reef manta rays (Mobula alfredi), grey nurse sharks (Carcharias taurus) and a sicklefin lemon shark (Negaprion acutidens).
The wound to the silky shark was thought to have completely closed 42 days after the injury occurred, and measurements of the growth of the fin show an increase of 10.7 percent in fin area, suggesting that the fin was at least partially able to regenerate. This is in line with the expected wound-healing rates found in the other species.
Overall, these observations represent the first documented case of dorsal fin regeneration in a silky shark and only the second known example of dorsal fin regeneration ever observed. While the observations of this one individual are scientifically interesting, the author highlights the loss of knowledge that could have been used to better protect the entire species if the satellite tag had never been removed. They further stress that effective communication is needed between research and local communities for successful marine conservation action to take place.
The study is published in the Journal of Marine Sciences.