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August 12, 2014

Six Surprising Facts About Whale Sharks

A whale shark. Photo credit: Wikimedia

Whale sharks are beautiful, gentle creatures. Mostly solitary and nomadic, they spend much of their lives wandering the high seas alone. This makes whale sharks hard to study, so very little is known about them. The lack of information is surprising, considering that they stand out as the largest fish in existence. Of course you already knew that, but here are six other facts that you might not have known about whale sharks, the largest non-mammal vertebrate in the world (we won’t count that fact either).


1. Whale sharks can stand on their tails.
A whale shark feeding vertically at the surface.  Photo credit: Louisiana Department of Wildlife and Fisheries


Whale sharks may be the largest fish in the world, but they subsist on the tiniest prey in the ocean. Like the baleen whales they are filter feeders, and they consume great quantities of zooplankton and small fish such as sardines and anchovies.

All three species of filter-feeding sharks—the other two are basking sharks and megamouth sharks—feed by swimming with their mouths open, which forces water into the mouth and out through the gills. But only whale sharks can feed while stationary. They do this by suctioning water into their mouths, and they often hover vertically in the water column when doing this, as if standing on their tails.


2. The skin of an adult whale shark is up to 5 inches thick.
Thick skin, along with unmatched size, is an adaptation that protects whale sharks from predators. This does little to protect them from humans, however. Although most whale shark fisheries around the world have been officially shut down, illegal, unregulated and unreported fishing continues in places. 

Perhaps a bigger threat is ship strikes. Like those other oceangoing giants, the great whales, whale sharks spend a lot of time near the surface, and ship strikes are a significant cause of mortality.


3. Although mostly solitary, whale sharks sometimes congregate in great numbers.

More than 100 whale sharks were observed in the Northern Gulf of Mexico in June, 2010. Photo credit: Jennifer McKinney

Unlike most baleen whales, which feed in rich, cold water near the poles, whale sharks live in the tropics and sub-tropics. Warm seas tend to produce relatively little food, and whale sharks migrate great distances through these warm water "food deserts" to find oases of productivity.

These oases tend to be near the coasts, where upwelling brings nutrients to the surface and rivers deposit nutrients from the land. One of the most important areas for whale sharks in the United States is off the Louisiana coast, where nutrients from the Mississippi River fuel seasonal phytoplankton blooms. Although whale sharks are generally solitary, hundreds of them sometimes congregate there and elsewhere to feed.

These bursts of productivity are seasonal, and whale sharks migrate great distances to take advantage of them. How they know exactly where and when to find food is one of the great mysteries of the species.


4. Whale sharks often travel with tuna.
If you see a whale shark in the water, there's a good chance a school of tuna is swimming beneath them. In some parts of the world, fishermen locate tuna by spotting whale sharks, which are often visible due to their conspicuous size and habit of swimming near the surface.Tuna are among the fastest-swimming fish in the world, and it is odd that they associate with such slow-moving giants. Perhaps tuna follow in the hope that the whale shark will lead them to food. Whatever the reason, tuna do provide a valuable service to the sharks.

When tuna feed on small fish like sardines or anchovies, they circle their prey, and the small fish react defensively by forming a dense, spherical swarm called a bait ball. When they do this, it's not uncommon to see a whale shark standing on its tail in the middle of the bait ball, sucking the densely concentrated fish into its mouth.  The whale sharks would not be able to feed on the small fish without the help of the tunas.

5. Each whale shark’s spot pattern is unique as a fingerprint.

Whale shark sighted at Flower Garden Banks National Marine Sanctuary. Photo credit: NOAA/Ryan Eckert 

The website maintains a database of more than 53,000 whale shark photos, and scientists and others submit more photos all the time. The database can match photos of an individual whale shark taken in different parts of the world at different times, allowing scientists to track their migrations. How does the database find matches among tens of thousands of photos? It uses a pattern-matching algorithm originally developed by NASA to identify stars in images from the Hubble Space Telescope.


6. Whale sharks give birth to live young.

A whale shark embryo, 350 mm in length. From Garrick, Proceedings of the U.S. National Museum.

For most fish, reproduction is a numbers game. They produce billions of larvae with the hope that a tiny fraction will survive. Whale sharks, like many species of shark, use the exact opposite strategy. They give birth to a very small number of live young, but maximize the chances that each will survive.

Instead of spawning, whale sharks mate one-to-one, and the females retain the fertilized eggs inside their bodies. While mammals and some species of sharks provide an ongoing supply of nutrients to their developing embryos through a placenta, developing whale sharks are entirely dependent on the single packet of nutrients that is their yolk. The embryos draw down the yolk as they develop, and when the pup finally emerges it is a small but fully formed and recognizable whale shark.

By sheltering the developing embryo inside her body, the mother helps get the young pups past their most vulnerable life stage. This strategy does have a drawback, however. Because they give birth to only a few pups at a time, whale shark populations grow very slowly. If their population numbers decline due to fishing or other threats, it can take a long time for them to recover.


Bonus fact

NOAA shark scientist Eric Hoffmayer, with whale shark. Photo courtesy of Eric Hoffmayer


This list was written with the help of NOAA biologist and whale shark expert Eric Hoffmayer. What’s the most astounding fact about whale sharks, according to Hoffmayer? “That we don’t know where their nursery grounds are. They’re the largest fish in the ocean, yet they’re elusive enough that we still can’t answer this basic biological question.” 

Hoffmayer and other scientists are using satellite tags to track whale sharks in the hopes of discovering their nursery grounds. “We could protect these animals a lot more effectively if we knew where they begin their lives.”

Read about Eric Hoffmayer's research here.

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