For snorkellers in Mexico's Isla Mujeres, swimming with whale sharks can be a mesmerising experience. The mammoth fish are easily spotted here as they frequent the area in search of food – but as one tourist learned the hard way, what goes in, must eventually come out... 

Freelance videographer Julian Gunther is no stranger to diving with sharks, and in fact, this isn't even the first time he's found himself in a shark's faecal firing line. "I'd like to say it was," he says jokingly. "But it's not. It took a second to realise what was happening, but when it did finally click I just veered away and headed back up. I wanted to avoid getting any in my mouth or snorkel."

After watching the billowing blowout, the question on our minds is: How much krill could a whale shark shart, if a whale shark could shart krill?

The answer comes our way courtesy of marine biologist Dr Allistair Dove, who has done extensive work on the species. Back in 2010, he estimated one defecation to be about 30 feet (10m) long by 20 feet (6.6m) wide. A three-foot estimated thickness meant that particular plume would have been some 2,000 cubic feet (that's 12,457.67 gallons, or 56,633.68 litres) in its entirety. 

"It's unusual to see wild sharks in the act of pooping, but this group of animals was so numerous and feeding so heavily, that you could actually see several clouds like this at any given time," writes Dove about his research.

The shark Julian encountered wasn't quite so productive, but the world's biggest fish does everything on a giant scale. Even a small whale shark plume can be well over ten feet long. 

Wondering what that smells like? Just like many marine filter-feeders, whale sharks feed mostly on krill, small fish, larvae and other plankton. Add some digestive juices into the mix, and you're left with what has been described as "an unholy mingling of fart and fishiness", much like the smell produced by whale breath.

Interestingly, whale sharks don't always fully digest their meals. The going hypothesis is that these animals determine how much energy to put into digestion depending on how much food is available in the first place.

"Unlike mammals, which tend to have a relatively fixed gut passage time for food, a lot of cold-blooded critters can, well, sort of push it out the back end, simply by pushing more in the front end," says Dove. In places where food is abundant, whale sharks may trade efficiency for gluttony: absorbing fewer nutrients from each meal, but taking in more volume. 

Pooping events like these can teach us a lot about sharks, but capturing some of that majestic egesta is harder than you might think. For starters, these animals are fast swimmers, so their excrement disperses quickly. And not all sharks release clouds in as graceful a manner as this whale shark. Some species, like the great white, are especially spastic poopers (more on that here).

That up-down contortion of the body helps to facilitate faeces flow, and the gaping mouth (ferocious as it appears) is similar to what happens to our faces when we answer nature's call.

Final question: If krill and meat are largely reddish pink, why is shark poop DayGlo green? The vibrant colour comes from a combination of broken-down blood and muscle pigments from the shark's prey, as well as green bile and the yellow pigment bilirubin, both of which are produced by the shark. 

"Scientists can do all sorts of stuff with poo," says Dove. By analysing these underwater faecal tornadoes, we can check for parasites and pathogens, sequence the DNA of both shark and prey species, and even determine how much nutrition an animal is getting from its food. "It's a great way to learn a lot in a short time and do it in a totally non-invasive way." 


Top header image: Shutterstock

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