Silky Sharks Find New Ways to Adapt to Injuries
This article is about fin regeneration in silky sharks. It is very difficult to study regrowth of tissue and response to injuries in sharks because these injuries are often seen and observed by chance and, generally, the sharks cannot be observed in the long-term.
Recently, however, researchers have been presented with a unique opportunity. They were able to document a silky shark almost one entire year following a traumatic injury to its dorsal fin. The injury caused the shark to lose 20.8% of its first dorsal fin; 332 days following this event, it had been observed that the fin had regrown to 87% of its original size.
What Was Known Before This Discovery?
It was known that sharks and other elasmobranches (skates and rays) are very good at wound healing; however, it is difficult to mirror the healing and calculate healing rates in these individuals, as they are so mobile.
Sharks can sustain injuries externally through many different ways – mating, attempted predation, and aggressive behavior from other sharks, as well as human-related injuries such as marine debris (getting tangled in rope and plastic), getting hit by boat propellers, and some fishing practices. Generally, there is a very low amount of samples that show wound healing because sharks are negatively buoyant, meaning they sink after they die, limiting opportunities for study.
It was known previously that sharks can regenerate the top layer of their skin after 3 weeks and complete dermal scales can be formed 4 months following injury. Prior to this study, the only instance of dorsal fin regeneration had been seen in a whale shark who had regenerated 6% of the fin tissue over 5 years.
What Are The Functions Of The Shark’s Anatomy?
- Dorsal Fin: Helps the shark swim in a straight line and stabilize its body, preventing the shark from rolling to one side or the other.
- Pectoral Fins: Located behind the head and move outward from the sides of the body. Sharks use these to steer, lift themselves in the water column, and for stability while moving. They may angle downwards if the sharks are making quick turns or moving lower in the water column.
- Pelvic Fins: Located further back on the body closer to the tail and are mainly used for stability and help with up/down movement in the water column. In male sharks, the pelvic fins act as claspers to hold on when mating with a partner.
- Anal Fins: Located between the tail and the pelvic fins. They help stabilize the shark in the water and keep the shark’s body upright.
- Tail: Mainly used to push the shark forward through the water. In some species, the tail may also be used to hit and stun their prey for hunting.
New Discovery Of Fin Regeneration In The Silky Shark
This study is focused on healing of an adult male silky shark in Jupiter, Florida; healing was noted 332 days after the initial injury to its first dorsal fin. The injury was documented in July of 2022 by an underwater photographer and diver who noticed the unusually shaped cuts to the shark’s dorsal fin. This diver knew the author was working on a study satellite tagging silky sharks in the area, and sent the photos over to her to be identified. It is believed that the injury was sustained from the satellite tag purposely being removed with a sharp object. Throughout the rest of the year, the shark was not spotted again, performing its normal seasonal migrations. However it was spotted again in 2023 by multiple divers.
How was the study performed?
In many research studies, sharks get satellite tags attached to them to monitor their movement and activity. All of these tags have unique tag numbers that help identify the shark. They were able to determine the time of the injury based on when the satellite tag stopped sending out signals because it was ripped off; it was determined that the injury occurred sometime between 19-32 days prior to being photographed. There is a specific type of computer software that allows researchers to calculate the percentage of fin loss based on photos they receive. The fin’s wound was traced 10 times, then averaged together to eliminate error as much as possible.
The tags allowed divers to spot the same injured silky shark 332 days later; divers described the shark as appearing very healthy and swimming normally. They took photos and observed that the top part of the fin used with the bottom of the wound before tissue filled in the middle portion. They then calculated a new total area of the dorsal fin after healing.
What were the results?
Their calculations showed that 10.7% of the dorsal fin grew back after the injury. Since the original wound affected 20.8% of the fin, we can conclude that half of the fin regenerated over that year. The shark’s wounds had reached 90% closure by day 42 and the dorsal fin eventually healed to 87.6% of its original size. It is believed that there is potential for the shark to recover completely later this year.
What can we do with this information?
This study expanded scientists’ current knowledge of dorsal fin regeneration in sharks; prior to this, it had only been documented in a whale shark. It is known, however, that this information may not be applicable to all species of shark, and it may be dependent on the conditions. Silky sharks spend most of their time in warm waters, which is believed to be better than cold waters for healing; additionally, there was enough fin remaining to serve as a base for regrowth, while in other cases the fin might not be able to recover at all.
Overall, this study should emphasize how important it is to leave satellite tags alone and not forcibly remove them. In addition to causing extreme damage to the shark, it also causes loss of large amounts of data for scientists to learn more about the species. This kind of data is often used to create changes to policy and conservation status of species; we need as much of this data as possible. Even though the fin was able to partially regrow, it still cannot return to 100% of its original size, causing the shark to have to adapt and get used to new ways of moving.
By: Greer Jessie, BS Undergraduate Student, University of Florida and Shark Angels Intern
Scientific Study: Resilience in the Depths: First Example of Fin Regeneration in a Silky Shark (Carcharhinus falciformis) following Traumatic Injury by Chelsea Black, 2023.