After 400 million years on this planet, sharks have evolved an arsenal of adaptations for survival. Among the most impressive is their ability to heal after traumatic injury, and one lemon shark recently displayed those recovery skills in a way we've never seen before.

The feat of resilience began when the shark ingested a piece of stainless steel in the form of a "fish stringer", a looped piece of equipment used by spear-fishermen to hold their catch. The metal perforated the animal's stomach wall – and that's when things took an extraordinary turn. The foreign object began to inch its way around the shark's body cavity, past vital organs (perhaps even through them), until it finally popped out through the animal's skin.

It took over a year, but the lemon shark survived with only some scar tissue to show for its ordeal ... like it ain't no thing.


Exactly how the shark came to swallow the fish stringer is unknown, but the collaborative team of divers and scientists who documented this unusual case in a recent paper suspect scavenging gone awry: if the shark raided a fishermen's cache, it could easily have swallowed the stringer along with it.

To be clear, a fish stringer isn't a projectile bit of kit, so it wasn't shot into the shark's flank. What's more, shark skin is extremely tough, so it's unlikely that anyone could have stabbed the object in by hand without causing a telltale entrance wound.

Divers first spotted this animal in 2014 near a recreational shark-diving site known as "Lemon Drop", located near Juno Beach off the coast of Florida. Initially, the stringer only just protruded behind the shark's right pectoral fin, and no one could figure out exactly what the object was.

Over time, more and more of the metal began to push through: by January 2016, the free end of the open stringer had also breached the skin, this time through a puncture wound on the animal's underbelly.

This certainly isn't the first time a shark has sucked up a piece metal, but problematic meals are typically expelled by everting the stomach, or by passing the indigestible tidbits through the cloaca, an all-purpose opening used for urinary, intestinal and reproductive functions.

The case of this lemon shark is believed to be the first fully documented incident of "transcoelomic expulsion" (meaning "through the coelom", the shark's body cavity).

This sequence of sightings at "Lemon Drop" dive site shows the progression of the shark's injury and healing.

"Ingestion of foreign objects by sharks has long been established in both the literature and popular culture," writes the team, which was led by conservation biologist Dr Steven Kessel, director of marine research at Chicago's Shedd Aquarium. "This has largely been the result of a long-standing human fascination with cutting open shark stomachs to reveal strange contents, such as bibles, unexploded bombs and ticking watches."

Tiger sharks are especially infamous for snacking on man-made objects: from tin cans and tile, to condoms and chamois leather.

For Kessel and the rest of the team, however, it's important to focus not only on the "wow factor" of such mealtime oddities, but also on the potential risks they pose to marine life. A whale shark in Thailand, for example, was killed by just a single rigid plastic straw.

Luckily for this metal-munching lemon shark, the process of expelling the steel had only a temporary effect on its health."The individual appeared progressively more emaciated over the course of our observations [until February, 2016]," explains the team.

A bulge visible on the animal's belly began to worsen, probably the result of some kind of blockage as the free end of the stringer pushed against the lower muscle wall. It's also plausible that the stringer nicked the animal's liver, which takes up a large portion of the body cavity. But amazingly, by the following December, the shark returned to Lemon Drop without its shiny "accessory". Its wounds had closed, and the animal looked healthy once again.

Initial observations showing the metallic foreign object protruding from the right side of the lemon shark: (a) profile view of right side on 6 December 2014; (b) ventral view on 21 December 2014. At this time, there was no protrusion from the underside of the animal's body. Images: Joanne Fraser, Kessel et al./Marine and Freshwater Research
Observations taken on 13 February 2016 show both the stringer's free end, which popped out of the ventral (under) side of the body, and the initial structure protruding on the right. Images: Joanne Fraser, Kessel et al./Marine and Freshwater Research
Final observations on 14 December 2016. The foreign body is completely expelled from the lemon shark and the wounds are healed. Images: Joanne Fraser, Kessel et al./Marine and Freshwater Research

"We identified the individual by the scar tissue that was evident, in addition to its size, sex and specific pectoral fin notching," notes the team. "We can only speculate on the specific event that led to the final removal of the object. Given the shape and structure of the stringer, it most probably exited through the opening on the right side of the shark."

The researchers suspect the predator had a helping hand during the – literal – final push: perhaps a rock snagged the dangling stringer out, or a diver pulled it free. The entire process, however, took at least 435 days, so regardless of any assistance at the end, this is an impressive demonstration of resilience.

We've seen unbelievable tissue regeneration in sharks and their kin before, most notably in white sharks like "Prop" and "Chopper", whose bodies were severely injured by propeller blades, or "Gums", who lost all of his teeth in a dive cage-related incident. And who could forget the amputee sawfish in Western Australia that managed to survive for several months with its brain cavity exposed to seawater.

Drawing of a closed and open stringer. The section of the structure that was protruding from the right side of the shark, and the free end protruding from the underbelly, are circled in red. Image: Kessel et al./Marine and Freshwater Research

But it is this lemon shark's endurance display through internal trauma that makes the encounter so interesting. We're talking about an animal that kicked around for over a year with a perforated stomach lining, as well as progressive muscle and body-cavity damage.

"The most surprising thing for me was the sheer size of the object," says Kessel. "I have seen several sharks with fishing gear poking through their body walls, but never anything this large and never such a detailed account as our photographer and coauthor, Joanne Fraser, managed, and had the insight, to capture."

Resilience of this kind is no doubt useful in predators who regularly tackle risky prey – species like porcupine fish and stingrays, known for spiking the enemy quicker than "Game of Thrones" characters. If nothing else, these abilities could allow an injured male shark to mate one more time before death, securing his genetic legacy.

MORE: Lemon shark asphyxiated by porcupinefish in the Maldives

We're only beginning to scratch the surface on how sharks manage such speedy and successful healing, but coauthor and aquatic animal medicine specialist Dr. Bill Van Bonn notes that the ability likely relies – at least in part – on relationships with the ocean's tiniest inhabitants.

"We believe now that [sharks] actually co-exist with many marine microbes in ways that are foreign to land animals," he says. "For example, it is not unusual for us to culture a bacteria from a blood sample collected from a shark that would be very troubling for almost any land animal."

It may be that sharks tolerate foreign bacteria and other microbes in their bodies instead of attacking them as invaders. Moreover, tiny openings in the body cavity called "coelomic pores" may enable beneficial microbes to pass from water into their body cavities and out again.

While we still don't know how common such feats of recovery might be, or how many species can pull them off, there are important takeaways here for how humans interact with the ocean's top predators.

What to do when a fish swallows a hook (which can be far more hazardous than a hook in the jaw) is an issue that has been much debated over the years. For conservation-minded fishermen, the prospect of releasing an animal with potentially deadly gut "cargo" can pit the inclination to help against upholding best-practice guidelines.

But there's a reason why many shark biologists and wildlife officials support line cutting: any time spent fighting on the hook puts an immense amount of stress on a shark's body. That stress inhibits the animal's reflexes, and its effects can prove fatal even after the shark is released in seemingly good condition.

"There has been emerging evidence with fish that cutting leaders and leaving the hook in a deep/gut hooked fish, rather than digging around and trying to remove it, can result in greater survival potential," explains Kessel. 

If lemon sharks are capable of expelling hooks and other foreign bodies from the digestive tract, and healing the resulting internal damage, perhaps the number of gut-hooked sharks that die as a result of their injuries is lower than we think.

Of course, this is only one example, so the jury's still out on that hypothesis. Interestingly, in the days since this observation was published online, several anecdotal reports of similar body-cavity expulsion incidents have emerged for other fishes, including sandbar sharks and catfish.

"I encourage anyone who has a similar account, especially if supported by photographic or video evidence to get it into the literature, or contact me if they feel it could be incorporated into a collective account," says Kessel.   


Top header image: Doug Finney/Flickr