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Rhincodon typus, Introduction

Rhincodon typus, the whale shark, the largest living fish in the ocean. Placid and graceful grazers, eating only the smallest creatures in the sea; krill and plankton, larvae of fish, invertebrate life and algae. Only recently they have been documented ingesting dense schools of young mackerel and anchovies.

Like all the sharks and rays that make up the Class Elasmobranchia, they have no bones; their primitive cartilaginous skeletons still endure from the ancient blueprint that has proved so successful in this class of purely aquatic animals. These ocean going giants grow up to 14 meters long, around 20 tonnes in weight, and scientists claim that they have remained largely unchanged since they originated 60 million years ago, approximately 55 million years before humans arrived on the scene.

Reproduction

Very little is known about the reproduction of whale sharks, they are now known to be ovoviviparous, that is the eggs remain in the body of their mother and are hatched inside so the female gives birth to live young. There may be thousands of eggs inside a pregnant female but only a small percentage of these become embryos, the other eggs become a source of food for the developing embryos. Even fewer still will be born as pups which are between 40 and 60 centimeters in length. They can grow very rapidly in their first months gaining 150 centimeters in 6 months, around 1cm per day. That means that some of the relatively small 3 to 5 meter sharks in these waters are indeed very young, this is also reflected in the small undeveloped claspers of immature males, often mistaken by divers as females.

Sexual maturity in whale sharks does not occur until the sharks reach about 9 meters or around 30 years of age and as they are thought to have fairly low numbers of offspring they are particularly vulnerable to over fishing.

Diet

Whale sharks inhabit the world's tropical and warm-temperate oceans. While thought to be primarily pelagic and generally solitary, seasonal feeding aggregations of the sharks occur feeding at sites with abundant food.

The Thai name for whale shark is Shalam Wan which translated literally means whale shark but the Latin name is Rhincodon typus, Rhincodon meaning rasp teeth of which the whale shark has around 4000 laid down in about 300 rows. They are about 3mm long and are not used in feeding. In fact the whale shark is a filter feeder and feeds in 2 distinct ways.

1.Passive filter feeding, which is the same way basking sharks feed, swimming along with mouth agape simply allowing the water to flow over the gills catching food particles as it goes.

2.The second method is unique to whale sharks, active filter feeding whereby the shark actually sucks water in through its huge 2 meter wide jaw, closes its mouth and then squeezes the water out through the gills. During the slight delay between closing the mouth and opening the gill flaps plankton is trapped against the dermal denticles which line the gill plates and pharynx, anything above 2 to 3mm is trapped and then swallowed.

The two different forms of feeding allow the shark to have a more diverse diet which recently has been proven to include small fish which are probably sucked in by the second active method. Through these techniques a whale shark can process 1500 gallons or 6000 liters of water per hour.

Although whale sharks are largely pelagic, spending the majority of their lives in the open ocean, it is the intricate reproductive cycles of fish and corals that provide their food both over the reefs and in the open ocean so it is spectacular reefs and submerged rocky pinnacles that provide them with ideal feeding grounds and clearly play an important part in their life cycle.

Despite it's voracious appetite and enormous size whale sharks do not pose a significant danger to divers except for perhaps their tails, they can move extremely fast when they want to and that tail could do some serious damage.

Whale Shark Myth and Legend

Whale sharks are generally revered by fishermen the world over, called Ca Ong or Sir Fish in Vietnam to whom the fishermen will pray for protection and a good catch. In Japan they are known as ebisuzame, a good luck symbol and here the people revere them as guardians of the deep, said to overturn the boats of those who do not respect the sea or who take too much.

The Sixth Sense

Whale sharks like all sharks have a 6^th sense, a series of pits called the lateral line system that, as the name suggests, run down the side of the shark accompanied by the highly sensitive organs of corti situated under the sharks nose which together produce a surrounding sensory field akin to that of a magnetic field and allow the animal to detect tiny changes in pressure and virtually feel their way through their ocean world locating the movement of prey or inconsistencies in the sea water even sensing the clouds of plankton so important to whale sharks lives. They may also use this 6^th sense to navigate the seas as they migrate thousands of miles from ocean to ocean.