Nov 062009
photo courtesy of Christian Steen on Flickr Creative CommonsHomeTropical Seas
Niche large filter feeder
Favorite Foodkrill
Lengthup to 46 feet
Weightup to 12 tons
StatusVulnerable to Extinction
Threatsdepleted krill stocks, pollution, climate change

When one thinks of sharks, the words “gentle giant” don’t exactly leap to mind. That description is usually reserved for the massive whales that cruise peacefully through the waters of the vast oceans. But there is one shark that bucks the trend of its bloodthirsty relatives. The whale shark.

This massive shark is the largest living fish by a wide margin. It can grow to lengths of over 40 feet and weigh in at 11 tons, twice as much as a full-grown African elephant, the largest animal on land. Whale sharks are estimated to live up to 70 years or older due to a lack of any natural predators by virtue of their enormous size.

Everything about these sharks is huge. They sport a gaping mouth at the end of their snouts nearly 4 feet wide. Their massive gills not only provide oxygen, but also as a means of filtering tiny prey from the ocean water. The whale shark also has a massive, fatty liver that gives its huge body the buoyancy it needs to hover close to the surface, the optimal area for feeding on its favorite food, plankton. Sharks do not have buoyant swim bladders like fish, so this added adaptation has helped the whale shark to control its depth in the water.

The whale shark differs from other sharks in a number of ways besides its size. Perhaps the most important difference is the whale shark’s feeding habits. Most sharks are active predators. But the whale shark and a few other shark species have instead filled the evolutionary niche of the large whales, growing to enormous size by passively feeding on tons and tons of tiny organisms, plankton. The whale shark cruises through the water with its mouth open, filtering the microscopic critters from the surrounding waters. It seems incomprehensible for a creature to grow so large feeding on plankton, considering it takes an average of 7800 gallons of sea water to produce a shot glass full of the tiny organisms. But the whale shark’s life is devoted to eating, and as it cruises the oceans in search of plankton shoals, it does so with its mouth open, occasionally swallowing mouthfuls of fish that happen into its path.

photo courtesy of Christian Steen on Flickr Creative CommonsWhale sharks are generally not alarmed when approached by humans. Ironic, since their only external threat comes from people. These magnificent animals are disappearing in the wild and have become vulnerable to extinction in recent years. Depletion of plankton stocks due to commercial fishing, pollution, and climate change are the main culprits. As the giant whale shark continues its peaceful existence as it has always done, it’s unclear how much longer it will be able to survive if current trends continue.

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