Species Spotlight · 30 Nov 2021
It's often said that the whale shark's maximum length from end to end is about 45 feet, but this is a bit of a fish story. The longest verified measurements of live (or recently deceased) specimens are in the 40-foot range.
How heavy can they get? That's a difficult question to answer. Weighing such huge marine animals is no easy task, so many scientists simply estimate instead. Still, researchers at the Okinawa Churaumi Aquarium have managed to weigh several individuals over the years. The heaviest they've encountered was a captured 30-footer that weighed just above 7 tons. In the wild, longer ones probably weigh a good deal more.
For years, there was much debate over how baby whale sharks come into the world. Then, in 1995, a pregnant female was fatally harpooned near Taiwan. An autopsy revealed that her uteri (all sharks have two) contained around 300 unborn embryos. This discovery confirmed that the species is ovoviviparous — in other words, whale sharks, like certain snakes, hatch from eggs that are stored inside their mother's body, and the young then exit her womb fully formed.
Thirty or 40-foot adult whale sharks have few natural enemies. Juveniles, on the other hand, are easy pickings for predators like blue sharks and marlins. These youngsters have every incentive to get big quickly — which is exactly what they do.
Consider this: in just three years and 68 days, one newborn whale shark at Japan's Oita Ecological Aquarium went from weighing 1.7 to 333.4 pounds. Another infant showed an astonishing growth rate of 18 inches per year over 630 days.
But, like human babies, young whale sharks don't keep growing at the same speed forever. Once the fish reach a certain size, scientists theorise that their growth rate slows down considerably — by then the creatures have, in theory, become big enough to scare away would-be attackers.
Most sharks have 20 to 30 rows of teeth, but whale sharks have more than 300 rows — that means a whale shark has 3,000 individual teeth, each one about the size of a match head.
All those teeth don't do very much, though. Despite their enormous size, whale sharks exclusively dine on very small life forms like plankton, krill, fish eggs, and small fish, usually swallowing their food whole. Attached to the gills is a mesh-like network of long, cartilaginous bars known as "gill rakers", which allow water to escape but prevent even millimetre-sized prey from doing so. Eventually the meal is forced down the shark's narrow throat and digested.
Sometimes a whale shark will lazily swim with its mouth agape, a low-energy feeding technique that lets it passively swallow any food items in its path. But when it spots a dense cluster of potential targets, the shark changes tactics, creating suction by rapidly opening and closing its jaws to pull dinner into its cavernous maw.
Generally, whale sharks — which are tropical and sub-tropical fish — are encountered from latitudes 30° N to 35° S. Within this range they move around a lot: in three years, a single whale shark can travel 8,000 miles or more. Though scientists don't fully understand their migration habits, we know the fish tend to gather en masse in specific places at specific times. Huge schools visit places like the Galapagos Islands and the Yucatan Peninsula every summer to gorge on plankton.
Covered in hard, tooth-like scales called denticles, the hide on a whale shark's back can be up to 4 inches thick, and the shark can toughen it still further by clenching the muscles just beneath it. Their underbellies, by contrast, are relatively soft and vulnerable — so when approached by divers, a whale shark will often turn its belly away from them.
Most experts agree that whale sharks reach sexual maturity around age 30, but their total life expectancy is unknown, and estimates are all over the map. Some researchers think the big fish probably die in their sixties; others speculate that whale sharks can live to be 100 or even 150 years old.
A study tracking six different whale sharks near western Australia found that the sharks mostly avoid deep plunges — the surveyed animals spent over half of their time within 100 feet of the surface.
With that said, whale sharks occasionally travel much, much further down. One tracked shark spent at least 12 uninterrupted hours at a depth of 3,215 feet, and tagged whale sharks off the coast of India have been recorded hitting depths of 2,200 to 3,200 feet. Why the fish embark on such extreme dives is still a mystery, though the answer likely has something to do with either keeping cool or gathering food.
No two whale sharks share the exact same pattern. Just behind their gills, every single one has a totally unique arrangement of pale, white spots. Today this is helping biologists keep tabs on individual sharks, using pattern-recognition software originally developed for mapping star fields.
Conservation groups now run whale shark identification programs open to the public — anyone who films or photographs a wild whale shark can submit the footage along with basic details of when and where the encounter happened. The software then checks whether the pattern matches a shark already on record, and if it does, contributors get a summary of that individual's migration history.