Article: Eye to Eye with the Largest Fish on Earth

By Daniel Stolte, Academy of Scuba Member, Diver and Biologist
May 11, 2009 - Originally Posted to AcademyOfScuba.com
info@academyofscuba.com

An encounter with a whale shark can be the most awe-inspiring experience in your diving career

Somewhere in the Gulf of California, a dive boat slowly glides across the glassy sea. Tired from a morning of diving at a nearby island, some divers are napping on the deck, while others sit quietly in the galley, logging their dives or downloading pictures onto their laptops. Suddenly, excited shouting from the divemaster breaks the silence: “Whale shark!” – “whale shark!”

Startled, the divers congregate by the boat’s railing, squinting into the sun. The captain thrusts the engines in reverse and swiftly spins the wheel to swing the boat around. The divemaster points to a dark spot in the water, perhaps 30 feet from the boat’s starboard gunwale. A dark, diffuse shadow looms just a few feet below the surface. As the captain slowly brings the boat closer, a gaping mouth, four feet wide, breaks the surface, white flesh exposed, sucking in big gulps of seawater. Now merely inches below the surfaces, the mouth opens and closes in slow sequence, almost like a giant carp slurping duck weed from a pond’s surface. The image of an old man comes to mind, who forgot to put in his dentures.

This whale shark, basking on the surface, is at least 25 feet long. Through the clear water, the people on the boat can now see the outline of its massive body, resembling a submerged bus more than a living creature. The rays of the afternoon sun dance across the animal’s back and highlight its checkerboard pattern of bright spots.

The divers chatter excitedly while they hurry to zip up their wetsuits, strap on their masks and snorkels and slip into their fins. Within minutes of the discovery, people are back-rolling over the sides of the boat, splashing in and finning in the direction of the giant that hovers just below the surface, motionless except for the rhythmic opening and closing of its gaping mouth. An encounter with a whale shark is an experience no scuba diver will ever forget.

Mysterious giants
Named for their size and docile nature, which is more reminiscent of whales than their shark kin, whale sharks (scientific name Rhincodon typus) are not only the largest sharks, but also the largest fish in the world. Reaching lengths of up to 65 feet, they rival sperm whales in size. To put this size in perspective, picture this creature in the water, in its typical vertical feeding position with the head just below the surface, scooping in big gulps of plankton, and the massive body fading below into the blue, shimmering depths.  Technically speaking, a curious new diver who wanted to take a look at the animal’s tail would have to obtain advanced open water certification first!

Despite their unique and impressive appearance, we still know very little about these gentle giants of the seas. Even though a large whale shark’s mouth is wide enough to swallow a diver whole, these fish are in fact filter feeders, unlike most of their kin, which includes some of the world’s most feared predators. Similar to buffalo roaming the Great Plains, whale sharks cruise the high seas like grazers.  They eat plankton (microscopically small algae and animals), fish eggs and other minute marine creatures. When feeding, the whale shark typically assumes a near vertical position in the water and opens and closes its wide mouth, sucking in copious amounts of water in a rhythmic pumping motion. Specialized gill rakes trap any particle larger than a tenth of an inch or so before the animal expels the water through its enormous gill slits. In the course of a day a whale shark can process 1,500 gallons of sea water this way. Even though several thousand teeth line a whale shark’s jaws, they are tiny and serve no obvious function.

The life history of this relatively scarce but cosmopolitan species is poorly understood, but we do know that whale sharks migrate over vast distances. They inhabit a girdle of warm and temperate ocean that surrounds the globe and spans the equator between 30 degrees northern and 30 degrees southern latitude.

Surprisingly, the whale shark’s closest relative is the nurse shark (Ginglymostoma cirratum), a similarly docile, but much smaller shark that is a frequent and popular sight on shallow reef dives. In some cultures, especially of some ocean-going peoples, whale sharks occupy a mythical place in the local belief system. In Vietnam for example, the whale shark is known as “Ca Ong” or “Sir Fish,” and the fishermen erect shrines in the sand dunes along the coast in its honor. Despite their unique status, whale sharks are being hunted in many parts of the world, mainly for their fins. A single fin from a whale shark can fetch tens of thousands of dollars on the Chinese market. To satisfy the demand for shark fin soup, a highly prized delicacy in Asia, fishermen cut off a shark’s fins and throw the crippled animal overboard to die a slow and agonizing death. Each year, tens of millions of sharks are being killed and removed from their vital roles as top predators in marine ecosystems because of the cruel practice of shark finning.

So little is known about whale sharks that only fifteen years ago, nobody knew how whale sharks reproduce! Until then, it was thought that whale sharks lay eggs like most other fish, including the majority of sharks. This assumption was based on isolated finds of egg cases whose large size (over a foot in length) and the tell-tale dotted color pattern of the embryo inside gave them away as whale shark eggs. But in 1995, Taiwanese fishermen harpooned a 36-foot long female whale shark. Inside its two uteri, 300 fetuses were found, ranging from 1.5 to 2 feet in size. The discovery settled a long-standing debate: Whale sharks are ovoviviparous, meaning that the young develop inside an egg shell, but hatch in the mother and enter their watery world ready to swim and fend for themselves.

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