Peer into the mouth of a 20-ton whale shark, and you’ll find row upon row of teeny, hooked teeth. You might think you’d be better off staring down the barrel of a gun. But in reality, whale sharks mean you no harm, and these sharp-tipped structures don’t seem to play a role in whale shark feeding at all. Despite their looks, whale sharks—much like their mammalian namesakes—are filter feeders that nosh primarily on tiny plankton.
Or, at least, when they can get their hands on it. According to a study published today in the journal Ecological Monographs, the blood and tissue of wild whale sharks reveal that they might go without food for months at a time as they traverse the seas. The results come from a team of scientists at the University of Tokyo that has developed new techniques to take accurate, efficient molecular snapshots of these gentle giants’ wellbeing.
“These are massive, charismatic creatures—the biggest fish in the sea,” says Clare Prebble, a whale shark researcher at Southampton University who was not involved in the study. “And yet they’re so elusive. There’s so much basic information we know nothing about.”
One thing, however, is clear: Their numbers are dwindling. And scientists are now scrambling faster than ever to pull back the curtain on these mysterious, bus-sized beasts before the clock runs out.
Though researchers know enough about whale sharks’ diets to keep them relatively healthy in captivity, the full range of what enters their gaping (and deceptively toothsome) mouths—and when—remains incomplete. “We really don’t know what they’re eating beyond what we see them doing on the surface,” says Jennifer Schmidt, a whale shark expert at the Shark Research Institute who was not involved in the study. “We know they make dives, so they may be eating other prey at deeper levels. But that’s almost impossible to visualize.”
To get a more complete picture on whale shark feeding habits, a research team led by Alex Wyatt, a marine biologist at the University of Tokyo’s Atmosphere and Ocean Research Institute, decided to take a different approach. Their technique capitalized on a simple principle: You are what you eat.
After an animal consumes food, the molecular components of that meal get broken down and redistributed throughout the body. But certain chemical signatures remain intact long after they enter the bloodstream, and some are repurposed into the base materials of muscle tissue, cartilage, and bone. These chemical signatures are powerful enough to reveal not only what types of food an animal’s been eating, but also where it’s been eating—creating something Prebble calls a “biological passport” through both time and space.
To get the most out of this technique, however, Wyatt first needed to know what these signatures looked like and how long they lasted in typical whale shark bodies. Blood and tissues eventually turn over, but the rate at which this happens depends on a multitude factors, including an animal’s metabolism, age, and growth rate. Obtaining this sort of information would require tracking individuals over long periods of time, which wasn’t a viable option for wild whale sharks. So Wyatt took his first set of measurements in five captive sharks living in either an aquarium or sea cages.
“With aquarium sharks, we know exactly where they’ve been the entire time,” Prebble says. “This is data that’s impossible to collect in the wild.”
For years, Wyatt and his colleagues meticulously monitored the captive sharks’ growth and diet, and took regular samples of their blood and cartilage through sickness and health. Though the sharks ate up to 100 pounds of food a day, each individual went through at least one period of not eating at all. Ultimately, one of the captive sharks died after almost five months of fasting, despite numerous attempts to nurse it back to health.
In the end, the researchers found that chemical signatures from food were fairly slow to disappear from the whale sharks’ blood and cartilage—on the order of months and years, respectively. This also meant, however, that a single sample from a shark at sea might yield a fairly accurate readout of its long-term feeding habits.
Armed with this baseline knowledge, the researchers next studied samples collected from eight wild whale sharks that had been accidentally entangled in fishing nets off the coast of Okinawa between 1979 and 2006. By comparing the data from the wild sharks to that of their captive counterparts, the team was able to approximate the health of free-living sharks—even without collecting samples over multiple months.
The wild whale sharks’ blood and tissue revealed that they appeared to be dining on plankton and other sea life from two consistent locales: one that was coastal, and another in the mid-Pacific Ocean. But Wyatt was unsettled to see that several of the sharks seemed to be starving, yielding chemical readouts that closely resembled the dying specimen that had gone without food for months before its death.
“[Whale sharks] can probably survive on their bodies’ reserves for quite a long period of time,” Wyatt says. “But it may be that these sharks aren’t getting enough food. They’re potentially starving out there.”
According to Schmidt, however, this could be par for the course. Even filter feeding—a relatively passive pursuit compared to ambushing and eviscerating large, fleshy prey—has an energetic cost. “These sharks have to swim, and that huge mouth being open creates a lot of drag,” she says. “They’re working and burning energy… It’s not like popping bonbons in your mouth.” In other words, if whale sharks are going to bother opening their mouths, it has to be worth the effort. As these sharks migrate between coastal feeding sites, they may spend a decent bit of time going hungry.
For now, it’s unclear if this fasting is motivated by necessity or choice. Whale sharks have been known to engage in some pretty odd behaviors at mealtime—including ingesting what looks like plant matter (which Wyatt notes was also true of the sharks in this study). It’s possible that this is just a byproduct of algae and seaweed getting sucked up in the catchall of filter feeding. But Wyatt theorizes that some of this nibbling may not have been entirely accidental.
Schmidt and Prebble are a bit more hesitant to embrace the notion of whale shark herbivory. At the same time, though, whale sharks are known to eat “pretty much anything that will fit down their throats,” Schmidt says.
Besides, Prebble adds, “there’s an entire ocean of things they can eat.” We landlocked humans probably don’t know the half of it.