One of the most incredible things about sharks is their remarkable ability to heal from seemingly very nasty injuries. Violent mating rituals, bites from rivals and propellor strikes are no match for these hardy critters. Their wounds heal impressively quickly. But scientists are only just beginning to understand how this is possible. So how quickly do sharks recover from these injuries? And how on Earth do they do it? Why are they able to heal so much more quickly and seamlessly than we can?
Fin-jured!
Sharks can incur injuries a whole host of different ways; they may cut themselves on sharp rocks or corals, they can receive wounds from some scrappy prey animal that refuses to go down without a fight, or they may be bitten from other sharks; either from aggression, predation by larger sharks or as part of their mating rituals. Females can also tear as they are giving birth (Chin et al, 2015; Black, 2023).
Humans may also hurt sharks (intentionally or accidentally), as they might be hit by boat propellors, snare themselves on marine debris, be injured during research studies or be injured by harpoons, hooks, nets and other equipment used by fishers. As we expand further into their habitats, these types of injuries are increasingly on the rise (Chin et al, 2015; Black, 2023).
Yet despite all this harm, sharks are able to recover and they have been observed healing from minor injuries incredibly quickly. When scientists looks at sharks' cuts under a microscope, we can see that their skin (known as "epidermis") can heal in three weeks and the plate-like scales that cover the skin (called "dermal denticles") can regenerate in just four months (Reif, 1978; Black, 2023).
Bite Me!
A common injury female sharks suffer is bite wounds on their flanks and gills after mating with males (to learn more about this, check out Fifty Shades of White). These can look quite nasty, but female blacktip reef sharks (Carcharhinus melanopterus) have been observed completely healing from these love bites in as little as three to five weeks (Chin et al, 2015).
Sharks are also capable of healing from much more serious wounds. For example, there have been incidents where sharks have been seen to heal from major harpoon strikes. Blacktip reef sharks have recovered from significant lacerations and even survived when nearly an entire fin has been sheared off (Chin et al, 2015).
It is remarkable that these cuts can heal up so quickly, but also leaving little to no scar in some cases. For instance, when injured sicklefin lemon sharks (Negaprion acutidens) have been photographed over the period of a year, even large lacerations have been witnessed completely disappearing, to the point they are almost indistinguishable (Buray at al, 2009; Black, 2023).
It's a Stitch Up
In some cases, sharks have been able to not only heal, but to regenerate large portions of their injured fins. When clips are taken off of the tops of whale sharks' (Rhincodon typus) dorsal fins, they can sometimes almost completely grow back - looking somewhat back to normal five years later (Womersley et al, 2021).
In a significant case, where a scientific tag had been cut off of the fin of a silky shark (Carcharhinus falciformis), scientists were astounded to see that the fin almost completely regrew. Not only did this major wound close almost completely within 42 days, but just 332 days after the trauma, the fin had grown back to more than 87% of its original size. This means that the fin had regenerated in less than a year (Black, 2023).
If human beings were to suffer an injury as severe as this, we would require stitches, painkillers, bandages, and likely even bed-rest in order to heal, but sharks can seal themselves up whilst continuing to swim, socialise and hunt for prey (Buray at al, 2009; Chin et al, 2015; Womersley et al, 2021; Black, 2023).
Skin and Bones
Sharks have evolved specialised physiological mechanisms that allow them to heal their wounds so miraculously. On a molecular level, they have evolved rapid anti-inflammatory responses, which means that their wounds begin to calm down almost immediately. They have also evolved to host a unique "microbiome" of friendly bacteria on their skin that aid in healing. Scientists suspect that high concentrations of omega-3 polyunsaturated fatty acids may play some role in the regeneration of skeletal muscles, but we do not completely understand the cellular and molecular mechanisms behind their remarkable healing abilities (Black, 2023).
It seems that sharks generally heal more rapidly in warmer waters, as their metabolic rates increase in warmer temperatures. For example, fin injuries on whale sharks are thought to heal so quickly because they live in shallow, tropical regions. Comparatively, skin abrasions on great white sharks (Carcharodon carcharias) heal noticeably more slowly when they are spending time in more temperate regions (Domeier & Nasby-Lucas, 2007; Chin et al, 2015).
Size Matters
Whilst they are not 100% of all the mechanisms of how it all actually works, geneticists analysing shark DNA have discovered that they possess a lot of genes that are related to wound healing and genetic repair (Marra et al, 2019).
There is also some speculation about how the size of their genome might factor into sharks' remarkable ability to heal. The great white's genome is 4.63Gbp long - that is double the size of a human being's. The whale shark also has more DNA, with a genome of 3.44Gbp in length. Brownbanded bamboo sharks (Chiloscyllium punctatum) and cloudy catsharks (Scyliorhinus torazame) have even bigger genomes; coming in at 4.7 and 6.7 Gbp respectively (Marra et al, 2019).
Now, a bigger genome is not necessarily better, as a lot of DNA doesn't actually have a function. This "junk DNA" might arise because it was accidentally inserted or because a stretch of genes duplicated themselves mistakenly, and it sticks around because it doesn't cause any serious damage to the animal or affect its "fitness". As a result, every organism has some "repeat sequences" in their DNA that may or may not do anything. Yet in sharks there is some evidence that their high proportions of DNA repeats and large genomes play some kind of role in healing (Marra et al, 2019).
However they do it, it is clear that being able to recover quickly, even after very traumatic injuries, is a massive evolutionary advantage to sharks. Not only does it reduce their exposure to infections, but recovering so fast also means that sharks can minimise how long they need to make necessary adjustments to their behaviour - swimming more slowly, limiting their body movements, avoiding wriggling into tight spaces etc. - whilst they are recooperating. Scientists are studying their remarkable abilities in many different fields, and the secrets of their regeneration will have applications for all manner of human diseases from dementia to cancer... So maybe the magic of sharks may hold the key to saving human lives (Chin et al, 2015; Black, 2023).
References
Black C (2023). Resilience in the depths: First example of fin regeneration in a silky shark (Carcharhinus falciformis) following traumatic injury. Journal of Marine Sciences.
Buray N, Mourier J, Planes S & Clua E (2009). Underwater photo-identification of sicklefin lemon sharks, Negaprion acutidens, at Moorea (French Polynesia),” Cybium, 33:1. Access online.
Chin A, Mourier J & Rummer JL (2015). Blacktip reef sharks (Carcharhinus melanopterus) show high capacity for wound healing and recovery following injury. Conservation Physiology, 3:1, cov062. Access online.
Domeier ML& Nasby-Lucas N (2007). Annual re-sightings of photographically identified white sharks (Carcharodon carcharias) at an eastern Pacific aggregation site (Guadalupe Island, Mexico). Marine Biology, 150. Access online.
Heim V, Grubbs RD, Smukall MJ, Frazier BS, Carlson JK & Guttridge TL (2023). Observations of fin injury closure in Great Hammerheads and implications for the use of fin‐mounted geolocators. Journal of Aquatic Animal Health. Access online.
Marra NJ, Stanhope MJ, Jue NK, Wang M, Sun Q, Pavinski Bitar P, Richards CP, Komissarov A, Rayko M, Kliver S, Stanhope BJ, Winkler C, O’Brien SJ, Antunes A, Jorgensen S & Shivji MS (2019). White shark genome reveals ancient elasmobranch adaptations associated with wound healing and the maintenance of genome stability. Proceedings of the National Academy of Sciences, 116:10. Access online.
Penketh L, Schleimer A, Labaja J, Snow S, Ponzo A & Araujo G (2020). Scarring patterns of whale sharks, Rhincodon typus, at a provisioning site in the Philippines. Aquatic Conservation Marine and Freshwater Ecosystems, 1-13. Access online.
Reif WE (1978). Wound healing in sharks, Zoomorphologie, 90:22. Access online.
Womersley F, Hancock J, Perry CT & Rowat D (2021). Wound-healing capabilities of whale sharks (Rhincodon typus) and implications for conservation management. Conservation Physiology, 9:1, coaa120. Access online.
By Sophie A Maycock for SharkSpeak
