Perfect Little Killing Machines: Healing Factor

This long-time-coming installment of the Perfect Little Killing Machines series starts with a story about a shark I got to spend a fair amount of time with.  A couple years ago I was keeping a group of spiny dogfish in captivity as part of a study on their digestion rate, which can be used to calculate just how much food it takes to run a dogfish.  We captured all the dogfish with rod and reel gear, which is normally a fairly benign capture method for this species, but I noticed early on that one of the sharks had a nasty injury to her jaw.  It looked like the cartilage on the right hinge of her jaws had been damaged and the corner of her upper jaw was visibly protruding.  I’ve seen dogfish in the wild that had split lower jaws from encounters with longline gear, which had healed but likely forced the shark to change its diet for the rest of its life.  I kept a mental note that this particular dogfish should probably be donated to an aquarium rather than released at the end of the study.

About halfway through the study I started noticing that I couldn’t see the protruding bit of jaw cartilage anymore.  By the end of the shark’s three-week time in captivity there was no longer any trace of jaw damage at all and she was happily and normally eating pieces of menhaden.  I ended up releasing the dogfish with her schoolmates and she took off into the deep, leaving me with a profound appreciation for the capacity of these little sharks to recover from serious injuries.

A high capacity for wound healing is generally expected among sharks and rays, but a recent paper by researchers Andrew Chin, Johann Mourier, and Jodie Rummer provides some great examples of the almost Wolverine-like healing ability of sharks.  While conducting tagging and tracking studies on blacktip reef sharks (Carcharhinus melanopterus), the researchers had several opportunities to observe and document these sharks recovering from a variety of injuries.

The first type of injury that Chin et al. observed was umbilical scarring.  Many sharks, including both blacktip reef sharks and spiny dogfish, give live birth and recently-born sharks often have a sort of “belly button” wound between their pectoral fins.  This open wound is where either the shark once had an umbilical connection to its mother inside the womb (as is the case for reef sharks), or where the shark is finishing up the process of absorbing the yolk sac that fed it while it was developing inside its mother (as is the case for spiny dogfish).  Either way, the healing state of the umbilical scar is used as a way to determine how recently the shark was born.  Chin et al. found that for blacktip reef sharks the umbilical scar shrinks rapidly and is completely closed within about six days.  On this year’s winter tagging cruise, we happened upon a newborn-sized spiny dogfish with a still-open umbilical scar, showing that the little guy had likely been born less than a week before.

Another injury type observed by Chin et al. was the minor surgery used to deploy acoustic tags in a shark’s body cavity, an increasingly common research practice and something I’ve been doing on a regular basis to a variety of species.  All of their sharks were released within detection range of the receiver array and all were detected moving around for at least 30 days.  Those sharks that stopped being detected appeared either leave the area or get eaten by larger sharks rather than succumb to tagging injuries.  Four of the tagged sharks were recaptured, allowing the researchers to see how well the scar had healed in the days since tagging.  The shortest time at liberty was a shark recaptured after 29 days, which already showed a closed and almost totally healed surgery scar.  No scar or stitching was visible at all in the other sharks with longer times between captures.  This shows that sharks are pretty much back to normal less than a month after acoustic tagging and that after about four months there might not even be any external evidence that the shark was tagged at all.

The most interesting examples of wound healing found by Chin et al. were serious external injuries inflicted either by other sharks or boat strikes.  These wounds frankly looked awful when fresh; one of the boat-strike injuries was 2 inches deep and nearly 10 inches long.  Three days later, the injury was no longer bleeding but was still open.  When the shark was sighted again after 27 days the wound had completely closed, and a couple days later was visible only as some light scarring behind the pectoral fin.  Chin et al. also observed wounds inflicted by other sharks, including some nasty bites that were likely a result of territorial disputes or attempted predation.  These wounds also healed up to minor scarring within 40 days.

Healing progress of a serious boat strike injury to a blacktip reef shark. From Chin et al. (2015).

All of these observations illustrate the ability of sharks to recover from horrific-looking injuries that would probably kill or maim a lesser animal.  A high capacity for wound healing is a big evolutionary advantage for an animal that has to grab onto a mate with its teeth, not to mention eat prey that often fights back and survive attempts to eat them by other predators with even sharper teeth.  This also suggests that sharks might have a better chance of surviving injury from fishing gear than would be expected.  I probably shouldn’t be surprised that a spiny dogfish was able to snap its jaw back into place and heal it back to full functionality within weeks.  The real killer for sharks caught in fisheries may instead be capture stress, but the high post-release survival among spiny dogfish suggests they may have a way around that too.  But that’s a story for another post.

References

Chin, A., J. Mourier, and J. L. Rummer. 2015. Blacktip reef sharks (Carcharhinus melanopterus) show high capacity for wound healing and recovery following injury. Conservation Physiology 3: doi: 10.1093/conphys/cov062

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